The (Potentially) Habitable Worlds of TRAPPIST-1

by Paul Gilster on February 27, 2017

When the news about the seven planets of TRAPPIST-1 broke, I immediately wondered what Andrew LePage’s take on habitability would be. A physicist and writer with numerous online essays and a host of articles in magazines like Scientific American and Sky & Telescope, LePage is also a specialist in the processing and analysis of remote sensing data. He has put this background in data analytics to frequent use in his highly regarded ‘habitable planet reality checks,’ which can be found on his Drew ex Machina site. Having run a thorough analysis of the TRAPPIST-1 situation the other day, Drew now gives us the gist of his findings, which move at least several of the TRAPPIST-1 planets into a potentially interesting category indeed.

By Andrew LePage


Like so many other people interested in exoplanets, I made it a point to watch NASA’s press conference live on February 22. Based on the list of participants released by NASA a couple of days earlier, a number of people (myself included) suspected that this was going to be an announcement about new findings of the TRAPPIST-1 planetary system. Back in May of 2016, a team of scientists led by Michaël Gillon (University of Liège – Belgium) had announced the discovery of three Earth-size exoplanets orbiting TRAPPIST-1 – a very small red dwarf star known as an ultracool dwarf named after ESO’s ground-based TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) telescope which had spotted the transits of these exoplanets during an observing campaign in 2015. This star and its system of transiting planets was a natural target for follow up observations by ground and space-based instruments.

As it turned out, NASA’s press conference did involve an announcement of the results of new observations of TRAPPIST-1. A total of 1,333 hours of new photometry including 518 hours of data from NASA’s Spitzer Space Telescope had been acquired since the original discovery paper about TRAPPIST-1 had been submitted by Gillon et al.. Most helpful of all was a virtually uninterrupted 20-day observation run by Spitzer from September 19 to October 10, 2016 which allowed a thorough evaluation of the system. In the end, Gillon et al. had identified the transits of a total of seven exoplanets orbiting TRAPPIST-1 – the largest number of exoplanets found so far orbiting a star. Most exciting of all was the claim that three of these Earth-size exoplanets were potentially habitable.

As my published work over the past couple of decades can testify, I am a long-time believer that the galaxy is filled with habitable planets (and moons!). However, I have also been quite skeptical of frequently dubious claims made by some in recent years that various new exoplanetary discoveries are potentially habitable. Back in May 2016 when Gillon et al. originally announced the discovery of the first three exoplanets found orbiting TRAPPIST-1, the ESO press release and other sources claimed that they were all potentially habitable. My published review of the data available at the time showed no support for this claim: two of the new exoplanets were much more likely to be slightly larger and hotter versions of Venus while the orbit of the third exoplanet was so poorly constrained that nothing meaningful could be said yet about its potential habitability. Naturally I was quite skeptical about this new claim being made by some of the same scientists about TRAPPIST-1. With a copy of the new discovery paper by Gillion et al. in hand along with other peer-reviewed papers on this system published in recent months, I performed a fresh review of the potential habitability of the exoplanets in this system.


Image: This diagram shows the changing brightness of TRAPPIST-1 over a period of 20 days in September and October 2016 as measured by NASA’s Spitzer Space Telescope and various ground instruments. The dips in brightness caused by transiting exoplanets are clearly seen. (ESO/M. Gillon et al.)

Definition of the Habitable Zone

A thorough assessment of the habitability of any extrasolar planet would require a lot of detailed data on the properties of that planet, its atmosphere, its spin state and so on. Unfortunately, at this very early stage, the only information typically available to scientists about extrasolar planets is basic orbit parameters, a rough measure of its size and/or mass and some important properties of its sun. Combined with theoretical extrapolations of the factors that have kept the Earth habitable over billions of years (not to mention why our neighbors are not), the best we can hope to do at this time is to compare the known properties of extrasolar planets to our current understanding of planetary habitability to determine if an extrasolar planet is “potentially habitable”. And by “habitable”, I mean in an Earth-like sense where the surface conditions allow for the existence of liquid water on the planet’s surface – one of the presumed prerequisites for the development of life as we know it. While there may be other worlds that might possess environments that could support life, these would not be Earth-like habitable worlds of the sort being considered here.

One of the important criteria which we can use to determine if a planet is potentially habitable is the amount of energy it receives from its sun known as the effective stellar flux or Seff. According to the work by Ravi Kopparapu (Penn State) and his collaborators on the limits of the habitable zone (HZ) based on detailed climate and geophysical modeling, the outer limit of the HZ is conservatively defined as corresponding to the maximum greenhouse limit of a CO2-rich atmosphere where the addition of any more of this greenhouse gas would not increase a planet’s surface temperature any further. For a star like TRAPPIST-1 with a surface temperature of 2559 K, this conservative outer limit for the HZ as defined by Kopparapu et al. (2013, 2014) has an Seff of 0.22 corresponding to a orbital semimajor axis of 0.048 AU. This Seff value for the outer limit of the HZ is lower than the 0.36 for a planet orbiting a more Sun-like star because ultracool dwarf stars emit so much of their energy in the infrared part of the spectrum where atmospheric absorption is important.

The inner limit of the HZ is conservatively defined by Kopparapu et al. (2013, 2014) by the runaway greenhouse limit where a planet’s temperature would soar even with no CO2 present and lose all of its water in a geologically brief time in the process. For an Earth-size planet orbiting TRAPPIST-1, this happens at an Seff value of 0.91 which corresponds to a distance of 0.024 AU. Once again, this Seff value for the inner edge of the HZ is lower than the 1.11 for a Sun-like star because TRAPPIST-1 radiates so much of its energy in the infrared.

Because of the tight orbits of these exoplanets and the constraints placed on their eccentricity, it is likely that they are synchronous rotators with the same side perpetually facing their sun. Detailed climate modeling over the last two decades now shows that synchronous rotation is probably not the impediment to habitability as it was once thought. In fact, it has been shown that slow or synchronous rotation can actually result in an increase of the Seff for the inner edge of the HZ. According to the recent work by Jun Yang (University of Chicago) and collaborators, the inner edge of the HZ for a slow rotator orbiting a star like TRAPPIST-1 would have an Seff of 1.44 corresponding to an orbital distance of just 0.019 AU.


Image: This diagram shows a comparison of the properties of the newly discovered planets of TRAPPIST-1 with the inner planets of our solar system. (NASA)

The Exoplanets of TRAPPIST-1

The first two exoplanets in this system, TRAPPIST-1b and c, have radii of 1.09 RE (or Earth radii) and 1.06 RE, respectively. While it was claimed back in May 2016 that these two exoplanets were potentially habitable, their Seff values of 4.3 and 2.3 are higher than the 1.9 value for Venus, which is most definitely not a habitable planet. With their Venus-like sizes, Venus-like rotation states and Seff values in excess of Venus’, these are most likely to be non-habitable, Venus-like worlds contrary to the original claims made in May 2016. Fortunately, Gillon et al. have now adopted the more conservative definition of the HZ of Kopparapu et al. (2014, 2014) so this dubious claim was not repeated in the new discovery paper.

As we move outward from the parent star of this system, things begin to become a bit more interesting. What is now designated TRAPPIST-1d has a radius of 0.77 RE which is intermediate between Earth and Mars in size and is therefore likely to be a rocky planet. With an Seff of 1.14, TRAPPIST-1d would seem to be comfortably inside the HZ for a slow rotator as defined by Yang et al.. However, as Gillon et al. mention in their new paper, more recent work by Kopparapu et al. (2016) has shown that Coriolis effects for synchronous rotators with short orbital periods will alter the global circulation pattern in a way which affects cloud formation on the dayside – clouds which help to reflect away much of the energy the planet receives from its sun moderating the surface temperature in the process. With an orbital period (and presumably a period of rotation) of just four days, TRAPPIST-1d is probably rotating too quickly to maintain sufficient cloud cover on its dayside to keep from experiencing a runaway greenhouse effect. While it is certainly worthy of continued detailed study, it would seem that the chances that TRAPPIST-1d is potentially habitable are not very promising and Gillon et al. do not categorize this new find of theirs in that way.

The situation with TRAPPIST-1e is substantially better and it has been identified in the new work by Gillon et al. as being potentially habitable. With an Seff of 0.66, this exoplanet is comfortably inside the conservatively defined HZ of TRAPPIST-1. With a radius of 0.91 RE, it is only slightly smaller than Earth and is not expected to be a volatile-rich mini-Neptune with poor prospects of being habitable. If it were not for the still unresolved issues associated with orbiting so close to an ultracool dwarf and how that affects the volatile inventories of such worlds, TRAPPIST-1e could be considered one of the best candidates currently known for being a potentially habitable exoplanet. Undoubtedly, detailed climate modeling of this exoplanet will help to determine the range of water and other volatile content values which could yield a habitable world much as is being done for our Earth-size neighbor, Proxima Centauri b, as well as the growing list of other potentially habitable red dwarf exoplanets.

The next planet out, TRAPPIST-1f, was also identified as being potentially habitable in the new work by Gillon et al.. Its Seff value of 0.38 is comparable to that of Mars but, since so much of the energy emitted by TRAPPIST-1 is in the infrared, it is still comfortably inside the conservatively defined HZ for this star. While the radius of 1.05 RE would suggest that TRAPPIST-1f is a rocky world like the Earth, other data hint otherwise and raises some possible problems.

Because of the packed nature of this planetary system with its orbits near resonance, it is expected that they would strongly interact with each other gravitationally producing variations in their transit timings. Gillon et al. performed an analysis of these transit timing variations (TTV) derived from all of their photometry data and found them to be on the order of tens of seconds to more than a half an hour – more than sufficient to estimate the masses of the inner six planets. Unfortunately, the uncertainties associated with current TTV-derived mass values are still rather large while the calculated densities (which can be used to help constrain the bulk compositions of these exoplanets) are even more uncertain still. What can be said is that all of these exoplanets are approximately Earth-mass (or ME) objects. The calculated densities with their large uncertainties are also not inconsistent with a rocky composition… the one exception being TRAPPIST-1f.

TRAPPIST-1f has a TTV-derived mass of 0.68±0.18 ME – the most accurately known mass in this system so far. This yields a density that is 0.60±0.17 times that of Earth’s which is suggestive of a volatile-rich bulk composition. It could be that TRAPPIST-1f is a mini-Neptune with a deep hydrogen-rich atmosphere overlaying layers of high temperature/pressure phases of ice rendering it non-habitable. It might also be more of an ocean planet with a CO2-rich atmosphere a few times denser than the Earth’s capping a deep ocean of liquid water. Hubble observations might help to eliminate the former possibility by searching for hydrogen in an extended atmosphere although observations by JWST and other future instruments will be required to begin to explore the latter possibility.

But before too much is read into the apparent low density of this exoplanet, it should be remembered that TTV-derived masses are notorious for changing by rather large amounts as new data become available. NASA’s Kepler spacecraft is currently wrapping up Campaign 12 of its extended K2 mission where it observed a star field which includes TRAPPIST-1. With a virtually continuous photometric data set running from December 15, 2016 to March 4, 2017, it should be possible to calculate more accurate TTV-derived masses in the coming months.

It may turn out that the uncertainties in the mass and density of TRAPPIST-1f have been underestimated and it is actually a denser rocky world like the Earth. But even if the low density of TRAPPIST-1f is confirmed and it is unlikely to be potentially habitable, it nevertheless strongly suggests that small planets orbiting ultracool dwarfs can retain substantial amounts of their water and other volatiles contrary to some of the less optimistic predictions that have been made. This would markedly improve the habitability prospects of many red dwarf planets. For now, TRAPPIST-1f is a reasonable candidate for being potentially habitable – definitely better than TRAPPIST-1d but maybe not as good as e.


Image: This diagram shows the relative sizes of the orbits of the seven planets orbiting TRAPPIST-1. The shaded area shows the extent of the habitable zone (HZ) with alternative boundaries indicated by dashed lines. (ESO/M. Gillon et al.).

The last of their discoveries identified by Gillon et al. as being potentially habitable is TRAPPIST-1g. With a radius of 1.12 RE, it is unlikely to be a mini-Neptune but its currently ill-defined density as well as the fact that the smaller and closer TRAPPIST-1f may be volatile-rich makes it impossible to exclude the possibility. With a Seff of 0.26, TRAPPIST-1g is towards the outer edge but still comfortably inside the HZ for such a cool star. Once again, the claim made by Gillon et al. that this is a potentially habitable exoplanet it a reasonable one given what we currently know about this world. The final planet in this system, TRAPPIST-1h, still has an ill-defined orbit but it seems likely that it is outside of the HZ.


Contrary to my initial reservations, it does appear that the claim that the TRAPPIST-1 system contains three potentially habitable exoplanets has merit given what we currently know about them. There are obviously unresolved issues about how much of their original volatile inventories these exoplanets have managed to retain despite the higher luminosity of their parent star during its earliest history as well as its subsequent bouts of chromospheric activity like flares not to mention the relatively high flux of X-ray and extreme ultraviolet radiation that have already been observed. While losses of volatiles are expected, it is still not known with any certainty how this will ultimately affect the habitability of these and a growing list of similar red dwarf exoplanets. The fact that the initial TTV analysis of this system implies that TRAPPIST-1f has a volatile-rich bulk composition is a hopeful sign that exoplanets in the HZ of small red dwarfs can retain their volatiles, which improves the habitability prospects of such worlds.

Fortunately, TRAPPIST-1 with its seven transiting, Earth-size exoplanets is an ideal laboratory for exploring the question of how such worlds evolve and whether they can be habitable. New observations from NASA’s Hubble Space Telescope are already working their way through the peer-review process which may help constrain the properties of these exoplanets. We should also expect an analysis of the new Kepler data to provide more information in the next few months on the properties of these exoplanets especially better TTV-derived mass (and density) estimates. It is also possible that additional exoplanets will be found orbiting TRAPPIST-1, although it is unlikely that more will be found in the already tightly packed HZ. The commissioning of NASA’s James Webb Space Telescope and other instruments in the years to come also promises to shed much light on the properties of these exoplanets and their potential habitability. The excitement generated by these new finds is definitely well deserved.

A more detailed discussion of the history of TRAPPIST-1 observations, the properties of its exoplanets and their potential habitability can be found at “Habitable Planet Reality Check: The Seven Planets of TRAPPIST-1” (

Selected References

Michaël Gillon et al., “Temperate Earth-sized Planets Transiting a Nearby Ultracool Dwarf Star”, Nature, Vol. 533, pp. 221-224, May 12, 2016

Michaël Gillon et al., “Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1”, Nature, Vol 542., pp. 456-460, February 23, 2017 (preprint of paper is available from the ESO at

R.K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013

Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014

Ravi Kumar Kopparapu et al., “The Inner Edge of the Habitable Zone for Synchronously Rotating Planets around Low-mass Stars Using General Circulation Models”, The Astrophysical Journal, Vol. 819, No. 1, Article ID. 84, March 2016

Jun Yang et al., “Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate”, The Astrophysical Journal Letters, Vol. 787, No. 1, Article id. L2, May 2014



SPECULOOS: Nearby Red Dwarfs

by Paul Gilster on February 24, 2017


Let’s turn the clock back a bit on the TRAPPIST-1 discoveries with a reminder of Hubble work on this system announced last July. A team led by Julien de Wit (MIT) used the Hubble Space Telescope’s Wide Field Camera 3 to look for atmospheres on TRAPPIST-1b and 1c, two of the three planets then known around this star. The researchers were able to take advantage of a rare simultaneous transit, when both planets crossed the star within minutes of each other, an event that has been calculated to occur only every two years.

The result: No sign of the kind of hydrogen-dominated atmospheres we would expect on gaseous worlds. That was good news, for reasons that Nikole Lewis (Space Telescope Science Institute) explained:

“The lack of a smothering hydrogen-helium envelope increases the chances for habitability on these planets. If they had a significant hydrogen-helium envelope, there is no chance that either one of them could potentially support life because the dense atmosphere would act like a greenhouse.”

Image: NASA’s latest exoplanet ‘travel poster.’ From the JPL caption: “Some 40 light-years from Earth, a planet called TRAPPIST-1e offers a heart-stopping view: brilliant objects in a red sky, looming like larger and smaller versions of our own moon. But these are no moons. They are other Earth-sized planets in a spectacular planetary system outside our own. These seven rocky worlds huddle around their small, dim, red star, like a family around a campfire.” The poster can be downloaded here. Credit: NASA-JPL/Caltech.

This was, of course, before we knew there were seven planets in this system, but it was clear at the time that future observations would be needed to tell us what kind of atmospheres these worlds had, if any, and what their surface conditions might be. The paper in Nature also noted the need for spectroscopic analysis to look for methane or water features, all part of estimating the depth of any atmospheres on the two worlds.

I hark back to this story because we’re proceeding with exactly the kind of focused work the de Wit team was calling for in the summer of 2016. For it turns out that the discovery of TRAPPIST-1’s first three planets, and the four subsequent ones, was part of a larger project called the Search for habitable Planets EClipsing ULtra-cOOl Stars (SPECULOOS), whose goal is to search for planets in the habitable zones of the nearest 500 ultracool stars and brown dwarfs.

Its acronym created as a nod to a Flemish spiced shortbread, SPECULOOS is in the early stages of its work. At the European Southern Observatory’s Paranal Observatory in Chile, four robotic telescopes make up the observing infrastructure, each of them housing a one-meter primary mirror and cameras sensitive in the near-infrared. The project involves scientists from the University of Liège (Belgium) as well as other universities, and is under the leadership of Michaël Gillon, who has led the TRAPPIST-1 planetary discovery effort.

Thus the TRAPPIST effort (TRAnsiting Planets and PlanetesImals Small Telescopes) is actually folded into the larger SPECULOOS survey, as is a second ESO effort at Oukaïmden Observatory in Morocco. But SPECULOOS is designed to survey ten times as many red dwarfs as TRAPPIST does, and according to this ESO news release, it is expected to discover a number of systems similar to TRAPPIST-1, at least in terms of the number of planets involved, if perhaps not as fortuitously angled to give us seven transits.

Thus the Hubble work of 2016 can be placed within a larger context, the ongoing effort to survey nearby red dwarfs and brown dwarfs and determine which of these are most suitable for studying their atmospheres. By examining the mass, radius and orbital parameters of such worlds and analyzing possible atmospheres, SPECULOOS will feed future observatories like the James Webb Space Telescope and the 39-meter European Extremely Large Telescope with the target list they need.

The excitement of this ‘golden age’ of exoplanet discovery can only build as we realize that these upcoming observatories, along with missions like TESS (Transiting Exoplanet Survey Satellite) and CHEOPS (CHaracterising ExOPlanets Satellite) are not that far in the future (TESS is scheduled for launch in 2018, and CHEOPS should be ready for launch by then). Having made a statistical analysis of the broad exoplanet population with Kepler, we now turn to stars closer to home, and the possibility of finding biomarkers in their atmospheres.


Image: Planets in a compact red dwarf system. Credit: ESO.

In addition to the paper on TRAPPIST-1’s seven planets cited yesterday, I also want to cite the de Wit et al. paper referenced above, “A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c ‘,” Nature 537, (01 September 2016), 69–72 (abstract) and Gillon et al., “Temperate Earth-sized planets transiting a nearby ultramool dwarf star,” Nature 533 (12 May 2016), 221–224 (abstract).



Further Thoughts on TRAPPIST-1

by Paul Gilster on February 23, 2017


In yesterday’s news conference on the seven planets around TRAPPIST-1, exoplanet scientist Sara Seager (MIT) pointed to the discovery as accelerating our search for habitable worlds. “Goldilocks,” Seager said in a finely chosen turn of phrase, “has many sisters in this system.” I think she’s exactly correct, even though we don’t yet know if any of these particular worlds house life. For as Seager went on to point out, we now need to study the atmospheres of these planets to find out what’s really going on, especially on potentially habitable TRAPPIST-1e, f and g.

Seager’s enthusiasm for TRAPPIST-1 is based on the fact that, whatever we eventually learn about its planets, we’re seeing such an abundance of possibilities here that similar, possibly life-bearing systems are doubtless commonplace. And with this system, we have transiting worlds in the solar neighborhood whose atmospheres can be analyzed by upcoming missions like the James Webb Space Telescope, or via installations on the ground like the soon to be available European Extremely Large Telescope. We’re swiftly moving to a place where the active search for biosignatures in distant atmospheres becomes a regular activity.


Image: This artist’s concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets’ diameters, masses and distances from the host star. Credit: NASA-JPL/Caltech.

Those atmospheres, of course, are a cause of concern when planets are in such proximity to a young star. Yesterday I pointed to a paper, one of whose co-authors was Michaël Gillon (STAR Institute, University of Liège), a key player in all of the TRAPPIST-1 planetary findings. Noting the opportunity these planets present for atmospheric analysis, the authors discussed the X-ray and ultraviolet radiation that could damage planetary atmospheres. I notice that Lisa Kaltenegger (director of the Carl Sagan Institute at Cornell), shares radiation concerns.

Kaltenegger has already homed in on the problem with two papers currently in the works on how life would fare in a high ultraviolet radiation flux environment. Her take:

“How good or bad would such a UV environment be for life? Our paper, currently under review at Monthly Notices of the Royal Society, discusses just this scenario for the Trappist-1 system, examining the consequences of different atmospheres for life in a UV environment. We find that if the star is active, as indicated by the X-ray flux, then planets need an ozone layer to shield their surface from the harsh UV that would sterilize the surface. If the planets around Trappist-1 do not have an ozone layer (like a young Earth), life would need to shelter underground or in an ocean to survive and/or develop strategies to shield itself from the UV, such as biofluorescence.”

Notice that Kaltenegger is not ruling out life even under strong radiation constraints. We know from our own world how adaptable life can be, and we can’t rule out the possibility that even a young flare star could serve as an evolutionary spur to forms of adaptation and protection. We also know nothing of possible magnetic fields on these worlds. So I come back to Seager’s point: We now need to study these atmospheres to go beyond theory to observation.

About the worlds themselves, I noticed in the paper that Michaël Gillon and colleagues wrote for Nature that the planetary orbital inclinations found at TRAPPIST-1 are all very close to 90°, which tells us we’re looking at a system that is ‘dramatically co-planar,’ and we’re seeing it nearly edge-on. What a spectacular opportunity this gives us. The authors tell us that the six inner planets are moving in resonance with each other, which can be deduced from transit timing variations (TTVs). This is of more than theoretical interest because it goes to the origin of these planets, and hence to their possible composition. From the paper:

Orbital resonances are naturally generated when multiple planets interact within their nascent gaseous discs. The favoured theoretical scenario for the origin of the TRAPPIST-1 system involves accretion of the planets further from the star, followed by a phase of disc-driven inward migration — a process first studied in the context of the Galilean moons around Jupiter. The planets’ compositions should reflect their formation zone, so this scenario predicts that the planets should be volatile-rich and have lower densities than Earth, in good agreement with our preliminary result for planet f…

In other words, we’re dealing with worlds that likely migrated in from much further out in the planetary system, meaning they formed beyond the ‘snowline,’ where ice should have been plentiful. That bodes well for possible water on the surface, depending on what we find through our atmospheric studies. TRAPPIST-1 gives us what Seager referred to as a ‘laboratory’ to study planets around red dwarfs. Here we can test out questions of tidal lock and radiation as we perform follow-up work with Hubble, Spitzer and other instruments, all the while awaiting the chance with the James Webb instrument to analyze planetary atmospheres. Transmission spectroscopy, after all, can detect water, methane, oxygen.


Image: An artist’s conception of the view from the surface of TRAPPIST-1f like this might easily have run as the cover on a 1950s issue of Hans Stefan Santesson’s Fantastic Universe, or maybe one of the large-size Analogs from the early 1960s. Now we’re learning that places like this are really out there. Credit: NASA/JPL-Caltech.

Meanwhile, I can’t help but think in terms of science fiction, and how well the image above would have fit on many a novel dust jacket or magazine cover. From one of these planets, we can look up and see other planets close enough to spot geographical features and cloud patterns. If life emerges on one of these worlds, it can surely move between them, as we know from the presence of so many Martian rocks that have made their way to the Earth. And if a civilization might arise with views like these, what a spur toward the development of technologies to go into space and visit not one but a host of compelling neighbors!

The paper is Guillon et al., “Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1,” Nature 542, 456–460 (23 February 2017). Abstract.



Seven Planets around TRAPPIST-1

by Paul Gilster on February 22, 2017

The red dwarf known as TRAPPIST-1 could not have produced a more interesting scenario. Today we learn that the star, some 40 light years out in the constellation Aquarius, hosts seven planets, all of which turn out to be comparable to the Earth in terms of size. Moreover, these worlds were discovered through the transit method, meaning we have mass and radius information for all of them. Today’s report in Nature tells us that three of the planets lie in the habitable zone, and thus could have liquid water on their surfaces.


TRAPPIST-1 b, c, d, e, f, g and h are the worlds in question, and all but TRAPPIST-1h appear to be rocky in composition, based on density measurements drawn from the mass and radius information. Drawing on existing climate models, the innermost planets b, c and d are probably too hot to allow liquid water to exist, while h may be too distant and cold. But the European Southern Observatory is reporting that TRAPPIST-1e, f and g orbit within the star’s habitable zone, leaving us with the possibility of oceans and the potential for life.

Caution compels me to home in on the word ‘potential’ in the above sentence, and also to remind readers that we’ve seen many planets described as being in the habitable zone for which later study made a much less compelling case. Thus I appreciate lead author Michaël Gillon (STAR Institute, University of Liège), whose enthusiasm is evident when he says “This is an amazing planetary system — not only because we have found so many planets, but because they are all surprisingly similar in size to the Earth!” But I also look forward to the close analysis the community gives habitable zone issues and what it will reveal. In particular, let’s see what Andrew LePage comes up with in his own Habitable Zone Reality Check.

My own reservation about habitability: The age of TRAPPIST-1, thought to be in the range of 500 million years, points to a young dwarf of the kind given to flare activity. Here I note a paper from Peter Wheatley (University of Warwick), with Michaël Gillon as one of the co-authors. In “Strong XUV irradiation of the Earth-sized exoplanets orbiting the ultracool dwarf TRAPPIST-1,” Wheatley and team present XMM-Newton X-ray observations of TRAPPIST-1, finding ‘a relatively strong and variable coronal X-ray source with an X-ray luminosity similar to that of the quiet Sun.” A snip from the paper:

The TRAPPIST-1 system presents a fabulous opportunity to study the atmospheres of Earth-sized planets as well as the complex and uncertain mechanisms controlling planet habitability. Whatever the mechanisms at play, it is clear that these planets are subject to X-ray and EUV irradiation that is many-times higher than experienced by the present-day Earth and that is sufficient to significantly alter their primary and any secondary atmospheres. The high energy fluxes presented here are vital inputs to atmospheric studies of the TRAPPIST-1 planets.

None of that is to downplay the significance of this discovery, but simply to put it in context (it also should remind us how many factors come into play in the word ‘habitability’). Even so, with seven planets in a compact system around this dim red star, we certainly have some interesting real estate to work with. And we’ll certainly have plenty to investigate in a system with multiple transits. TRAPPIST-1 has about 8 percent the mass of the Sun. To be in the habitable zone here, a planet needs to be close to the parent star — indeed, the planetary orbits around TRAPPIST-1 are not much larger than what we find among Jupiter’s larger moons, and much smaller than the orbit of Mercury in our own system.

That means that transits are deep, as the planets are close to a very small star. Gillon and co-author Amaury Triaud (University of Cambridge) worked with space and ground instruments to make this detection, which follows up their original discovery of three Earth-sized planets there, announced in 2016. The TRAPPIST-South telescope at La Silla produced data complemented by the Very Large Telescope (Paranal) and the Spitzer Space Telescope and several other ground based instruments in the course of these observations.

The news conference on the TRAPPIST-1 findings goes online just as I publish this, and I’m sure we’ll have more to say about this fascinating system in short order.



Exoplanet News Conference

by Paul Gilster on February 22, 2017

You’ll want to see the news conference scheduled by NASA at 1300 EST (1800 UTC) today, an exoplanet finding of considerable interest to Centauri Dreams readers (I’ll have more on this later in the day). The event will air live on NASA Television and the agency’s website. Links available here.

Briefing participants:

* Thomas Zurbuchen, associate administrator of the Science Mission Directorate at NASA Headquarters in Washington

* Michael Gillon, astronomer at the University of Liege in Belgium

* Sean Carey, manager of NASA’s Spitzer Science Center at Caltech/IPAC, Pasadena, California

* Nikole Lewis, astronomer at the Space Telescope Science Institute in Baltimore

* Sara Seager, professor of planetary science and physics at Massachusetts Institute of Technology, Cambridge

A Reddit AMA (Ask Me Anything) about exoplanets will be held following the briefing at 1500 EST (2000 UTC) with scientists available to answer questions in English and Spanish.



Interstellar Conference News

by Paul Gilster on February 21, 2017

Registration is now open for the 2017 Tennessee Valley Interstellar Workshop, which will be held in Huntsville, AL on October 4-6. The title for this year’s conference is “Step By Step: Building a Ladder to the Stars.” The registration page is here, and if you’re thinking of attending, I recommend registering right away, as spaces filled up swiftly the last time around. This year’s TVIW will take place in partnership with the Tau Zero Foundation as well as Starship Century, which has already produced two successful symposia of its own.

Despite its regional name, the Tennessee Valley Interstellar Workshop has become a well received forum for interstellar discussions on a global scale, with speakers and workshop participants well known to Centauri Dreams readers. Registration at this year’s event costs $175, with discounts available for students. Pre-symposium seminars for an additional fee are to be held on Tuesday October 3. This year’s topics are Conflict in Space; Laser Propulsion: An Introduction to Laser Propulsion and Assessment of Relevant Current Technologies; and Human Life in Space – Separating Reality from Wishful Thinking.

I’ve been pleased to attend all of the previous symposia except the last one, which I had to miss because of an untimely bout of the flu. The recent call for papers jogged me into getting my registration in early, as I don’t want to miss two in a row. The text below is taken directly from the submissions page on the TVIW site:


TVIW 2017 Call for Papers, Workshop Tracks, and Posters: The Tennessee Valley Interstellar Workshop (TVIW), in collaboration with Starship Century and Tau Zero Foundation, hereby invites participation in its 2017 Symposium to be held from Wednesday, October 4 through Friday, October 6, 2017, in Huntsville, Alabama. Our Program Committee is seeking proposals for Plenary Papers/Talks, Working Tracks, and Sagan Meetings as well as other content such as posters.

Plenary Papers include traditional papers, lectures, and presentations that will be selected from submitted abstracts. See Appendix 1 for details and abstract submission guidelines.

Working Tracks are collaborative, small group discussions around a set of interdisciplinary questions on an interstellar subject. See Appendix 2 for details and abstract submission guidelines.

Sagan Meetings are new for TVIW 2017. Carl Sagan famously employed this format for his 1971 conference at the Byurakan Observatory in old Soviet Armenia, which dealt with the Drake Equation. Each Sagan Meeting will invite five speakers to give a short presentation staking out a position on a particular question. These speakers will then form a panel to engage in a lively discussion with the audience on that topic.

Invited Talks are presentations that contain significant results or describe major activities in the field and will be solicited by sponsoring organizations or the conference organizers.

Discussion Groups are for those not participating in the working tracks or Sagan Meetings and offer opportunities for free form discussion of subjects of mutual interest to attendees. They are unstructured and no specific output is expected although it is hoped that these groups might generate teams and/or topics that would lead to future Working Tracks and possible collaborative efforts in the interstellar field. Coffee, pen, and paper will be provided. An expanded list of possible topics will be available each day of the Symposium and anyone wishing to propose a topic is free to do so. Contact David Fields ( to suggest topics.

Other Content includes, but is not limited to, posters, displays of art or models, demonstrations, panel discussions, interviews, or public outreach events. Please refer to Appendix 1 for more information and the abstract submission guidelines.


Full information on formats and other structural matters can be found on the Submissions page. If you’re wondering about ‘working tracks,’ TVIW has used these in the past to engage up to four parallel tracks on issues of interstellar import, such as mission targets, propulsion systems, life support and the human factors needed for interstellar exploration. Each working track will be allocated two-hour blocks each day. Proposals for working tracks are still open, with the letter of intent deadline coming up on March 3, and deadline for complete proposals on March 31. TVIW hopes to have four to six working tracks in the 2017 symposium.


I notice that Andrew Siemion (UC-Berkeley), who serves as director of the UC Berkeley Center for the Search for Extraterrestrial Intelligence (SETI) will be speaking in Huntsville. Siemion is also one of the leaders of the Breakthrough Listen Initiative, which under the aegis of the Breakthrough Prize Foundation is conducting the most sensitive search yet for signs of extraterrestrial technology. At TVIW 2017, Siemion will be discussing “The Search for Ourselves Among the Stars,” a look at the past, present and future of SETI activities.

Foundations of Interstellar Studies Workshop

I’ve also recently heard from Kelvin Long, who heads up the Initiative for Interstellar Studies, about a workshop to be held at City Tech, CUNY in New York from June 13-15. The group’s goal is “facilitating real progress on existing problems related to interstellar studies.” This year’s session is to have a propulsion focus, but according to the workshop’s web page, the focus will change with successive meetings as issues arise and concepts change.


Sponsored by the i4IS and the Center for Theoretical Physics (CTP) at City Tech, the workshop is intended as a small gathering with informal conversations and social interactions designed to promote discussion. The deadline for ‘extended abstracts’ is March 15, though this can be extended to the 25th. Early bird registration begins April 1, with regular registration beginning April 17. The advantage of early registration is a discounted fee for attendance ($200 per person); the fee goes up to $250 once regular registration begins.

Here is the group’s overview of the event:


At the start of this new millennium we are faced with one of the greatest challenges of our age Can we cross the vast distances of space to visit other worlds around other stars? At the end of the last century the idea of interstellar travel was considered one of science fiction. In recent times that has changed and interstellar flight has received much interest. This is particularly since the discovery of many planets outside of our Solar System around other stars. Indeed, we now know that an Earth sizes mass planet orbits one of our closest stars, Proxima b. In addition, national space agencies and private commercial industry are beginning to turn their attention to the planets and beyond. It is time to start considering the bold interstellar journey and how we might accomplish it. Yet, this challenge presents many difficult problems to solve and who better to address them than the global physics community.

The Institute For Interstellar Studies (I4IS) and the Center for Theoretical Physics (CTP) at City Tech have partnered to bring together some of the best minds in the fields of physics to address some of the fundamental problems associated with becoming an interstellar capable civilisation.


The first day of the workshop is devoted to ‘energetic reaction engines,’ i.e., engines that involve the ejection of matter or energy rearward from the vehicle to generate thrust. This could be electric, plasma, nuclear thermal, fission, fission-fragment, fusion, antimatter catalyzed fusion, antimatter. Day 2 focuses on sails and beamed energy via photons or particle beams, covering laser sails, microwave sails, particle beamers, stellar wind pushers. Day 3 is given over to breakthrough propulsion topics, “an area of technology development that seeks to explore and develop a deeper understanding of the nature of space-time, gravitation, inertial frames, quantum vacuum, and other fundamental physical phenomena.”

Harold ‘Sonny’ White is to chair day 3 of the workshop, with Kelvin Long taking day 1 and CUNY’s Roman Kezerashvili taking day 2. White’s presence will give the opportunity for those interested in his latest EmDrive work to learn and ask questions. I haven’t seen him since we ate cheeseburgers sitting around the swimming pool at the Dallas Starship Congress meeting some years back. I’ve enjoyed being at several conferences with Kelvin, and remember Roman Kezerashvili from the Aosta conference in Italy where I first met him. He’s a rigorous scholar and an engaging conversationalist. It will be interesting to see how this crew finalizes the lineup of presentations for the upcoming event.

Submissions to the workshop are open, and those accepted are to appear in the Journal of the British Interplanetary Society. On the nature of submissions, the group says this:

All submissions should be attempted solutions of existing problems, or at least a strong discussion on the pathway towards a solution. This is a working meeting and audience participation and discussion should be expected in any results. The rule for the workshop is “no solution, no presentation”. Some spaces will be reserved for ‘special observer status’ participation.

For further information on submission format, see Foundations of Interstellar Studies
Workshop at City Tech, CUNY



Martian Civilization

by Paul Gilster on February 17, 2017

What kind of civilization might eventually emerge on Mars? Colonies of various kinds have been examined in science fiction for decades, but as we close in on the possibility of actual human arrival on the planet, perhaps in the 2030s, we can wonder how living on a different world will change the people who eventually choose to call it home. The prolific Nick Nielsen likes to take the long view, arguing in the essay below that while there are contrasting definitions of civilization itself, we may yet learn through experiment and experience how a ‘central project’ emerging from local conditions may define the future of colonies on other worlds. Human history offers guidance, but it may be that a successful Martian colony will see its position as a gateway to the exploration of the Solar System. You can follow Nick on his Grand Strategy: The View from Oregon site, as well as his Grand Strategy Annex.

by J. N. Nielsen

Mars 5

Settling Mars


Suppose that one or several planned large-scale missions to Mars come to fruition over the next few decades. Perhaps the first mission or missions are temporary scientific visits that endure a few weeks or months and then Mars is left vacant again. Even if it is only a handful of individuals temporarily on Mars for a few weeks or a few month of exploration, the camaraderie unique to these early Mars missions will be the first intimation of a distinctively Martian social milieu.

Beyond the transient exploration of a scientific mission, the vision of several Mars mission planners includes settlement, and these plans, if realized, will mean that eventually there will be large numbers of human beings living and working on Mars. We may see a patchwork of multiple settlements and multiple temporary scientific missions, existing side-by-side, each pursuing their own ends in their own ways. Some of the early explorers may chose to return and to remain, their lives having been touched and irrevocably changed by their initial encounter with the Red Planet.

In the case of an ongoing human presence, the numbers of human settlers will grow, eventually also they will become self-supporting and self-sustaining. Whether or not they formally declare their independence, they will be independent for all practical purposes. Given these eventualities, at some point we will need to recognize that an independent and distinctive Martian civilization exists. At what point in its development would we recognize a Martian civilization? What will be the character of this civilization?

A lone explorer on Mars by Alberto Vangelista

Image: A lone explorer on Mars by Italian digital artist Alberto Vangelista.

A Martian Perspective

In the classic science fiction film Forbidden Planet there is a striking scene early in the film in which two characters discuss the color of the sky, and one says, “I think a man could get used to this and grow to love it.” Will Martian settlers get used to the red skies of Mars and grow to love it?

Whether or not they love the red skies of the Red Planet, these red skies will be a fact of life on Mars no less than the red sands under foot. These Martian facts of life will collectively shape a distinctive Martian perspective, and a Martian civilization will grow out of a uniquely and distinctively Martian perspective. In What will it be like to be a Martian? I have already discussed that there will be something that it is like to be a Martian (borrowing from Thomas Nagel’s famous formulation that there is something that it is like to be a bat [1]), and in The Martian Standpoint (and Addendum on the Martian Standpoint) I discussed the emergence of a distinctively Martian perspective.

This perspective will be marked by properties in common with terrestrial civilization (such as being human) as well as properties not shared with terrestrial civilization (living life under a red sky, being able to pick out Earth in the night sky, having to wear a pressure suit outside, and so on). Most of that which is in common with terrestrial life will pass unnoticed, but the differences will be prominent in the minds of Martian settlers precisely because the differences will stand out against the background of unnoticed similarity.

Mars itself, its gravity, its weather, its seasons, the length of its day and coolness of the sun in the sky, as well as the adaptations that the settlers will have to make in order to live on Mars, will become selection pressures that will shape the social life of these communities. An individual human being who experiences what it is like to be a Martian, and who, as a consequence of living on Mars, has a Martian perspective, will be an individual participating in a community, all of whom are experiencing what it is like to be a Martian and to have a Martian perspective. Pride in being first on Mars will be mixed with equal parts homesickness, and, just so, every aspect of human moral psychology will find itself tested by the tension between old and new. From this dialectic will emerge an outlook unique to Mars, the Martian perspective, and this Martian perspective will inform all aspects of social life, from the most intimate introspection to the most public debates on what kind of society the Martians should build for themselves.

As the Martians go about building the economic infrastructure of Martian civilization, an intellectual superstructure will come into being in parallel with the built environment, and infrastructure and superstructure will be inseparably joined by the central project of Martian civilization, which at first will simply be the attempt to build a self-sustaining and self-supporting human presence on Mars. [2] What form the central project of Martian civilization will take after this initial goal is achieved cannot now be known. Martian society will be sufficiently small that it could be comprehensively motivated and unified by a central project, and the population of Mars will be sufficiently self-selected for scientific acumen and practical ability that whatever central project naturally grows out of the combined exertions of this population is likely to be as distinctive as the self-selected conquistadors who came to South America and the self-selected Puritans who came to North America.

Earth and Mars

A Tale of Two Planets: Terrestrial Civilization and Martian Civilization

Civilization on Earth has already passed through many stages of development, and it is at least arguable that at least some terrestrial civilizations have reached maturity, but a nascent civilization on Mars, while an heir to these mature traditions of terrestrial civilization, would be an entirely novel enterprise. The Martian civilization will be a new civilization, and as a new civilization it will begin its social development at its inception; it will not be a mature civilization of long-established institutions, but a tentative experimentation in institution building and in ways of life possible on Mars.

When a civilization originates in a given historical epoch, that historical epoch is expressed in that civilization, so that the civilization of classical antiquity expressed the world of the ancient Mediterranean Basin and the civilization of medieval Islam expressed the world of seventh century Arabia and the civilization of the industrial revolution expressed Enlightenment era northern Europe. Martian civilization, coming into being in the twenty-first civilization, would emerge from a radically different social context than any of these previous civilizations, and so it would express a radically different world than civilizations of the past. Martian civilization, then, could be a new civilization in more than one sense. It would also be a civilization de novo.


Image: Workshop of Filippe Maëcht and Hans Taye. Constantine Directing the Building of Constantinople. 1623-1625. Wool, silk, gold and silver. 484 × 480 cm (190.6 × 189 in). Philadelphia.

De novo civilization

For quite some time I have been planning to write about the possibility of what I call de novo civilization, i.e., civilizations that are newly constituted, but are distinct from those civilizations with which civilization began on Earth. The earliest civilizations in the world—the West Asian Cluster (Anatolia, Mesopotamia, Egypt, etc.), Mesoamerican, Peruvian, Chinese, and Indian civilizations, at a minimum—were all de novo civilizations, originating as something entirely new in the history of the planet. These original civilizations might be called “founder” civilizations, as they were the founders of all civilizations to subsequently follow.

Descended from these “founder” civilizations were a greater or lesser number of subsequent civilizations—depending upon the principles we adopt to individuate and therefore count civilizations—that were derived from the founder civilizations through descent with modification, through idea diffusion, through allopatric speciation, and so on. By identifying de novo civilizations as new civilizations distinct from this small, finite class of founder civilizations, I am suggesting that a new civilization can come into being through a new foundation (or a re-foundation) of some existing civilization. What particularly interests me most are those civilizations that “suddenly” come into being as the result of some relatively rapid historical change. Martian civilization would be such a de novo civilization arising from a new foundation.

The best example I can offer of de novo civilization is that of Byzantium. The Byzantine Empire is typically identified as becoming a distinct entity sometime between Constantine’s foundation of Constantinople (on the site of the earlier Greek city of Byzantium) in 330 and the reign of Justinian during the sixth century AD. Constantine spared no expense in furnishing his new Christian capital city, endowing it with art and sculpture essentially looted from other much older cities. An urban proletariat was even imported to populate the new metropolis. Eventually Greek speaking, and eventually Orthodox in its Christianity, Constantinople and the distinctive Byzantine civilization over which the city presided had inherited the traditions of Roman civilization, and as the city grew in size and influence there was no “breakdown” of trade or communication that isolated the region. When the last legal emperor of the western Roman Empire, Romulus Augustulus, surrendered control of Rome to the barbarian king Odoacer, the imperial insignia were sent to Constantinople for safekeeping. Thus Byzantium, still in touch with its parent civilization, nevertheless speciated and became its own distinctive civilization, different from Rome even while continuing to self-identify as Roman.

So it will be, I think, with Martian civilization, which will become its own distinctive civilization even while continuing to self-identify with essential elements of terrestrial civilization. The selection pressures upon terrestrial and Martian civilization will be so markedly different that the speciation of Martian civilization from its parent terrestrial civilization is nearly inevitable, although there will be ongoing commerce, communication, and conflict between Earth and Mars. Martian civilization will emerge as a de novo civilization even in the absence of a rupture between Earth and Mars; the transfer of some portion of terrestrial civilization to a human population on Mars will be sufficient for a new foundation of civilization, even if this is not what is intended.

V. Gordon Childe

Image: Prehistorian V. Gordon Childe at Skara Brae, Orkney.

V. Gordon Childe’s “urban revolution” on Mars

One of the most influential accounts of the origin of civilization is that of V. Gordon Childe, and, ironically, it was not explicitly cast as an account of civilization, but rather of the “urban revolution,” i.e., the origin of cities. [3] There is a vast literature on Childe’s “urban revolution” and it has become a commonplace among archaeologists, especially those archaeologists formulating theories about the origins of civilization, to employ Childe’s ten criteria for the urban revolution as a definition of civilization: something is a civilization if it possesses most of the items on Childe’s list. [4] Subsequent prehistorians have tinkered and tampered with Childe’s model, but for the most part it remains intact and continues to influence archaeological thought about civilization even today.

While Childe does not himself assert that the properties he identifies as characterizing the urban revolution constitute a definition of civilization, he may as well have said so, as this is the lesson that has been taken from the paper. In so far as “urban revolution” implies the revolutionary appearance of many cities, the lesson is justified. A rough characterization of civilization could be a network of cities actively engaged in cooperation and conflict with each other. [5] We see this pattern clearly in Mesopotamia, in Mesoamerica, in the Indus Valley, and will probably find it wherever civilization independently emerges.

Following this example, when there are a network of settlements on Mars actively engaged in cooperation and conflict with each other (as in the suggestion above that Mars may be a patchwork of settlements both temporary and permanent), we could at that point identify a Martian civilization. As Martian civilization grows, it will unify itself as a planetary civilization, all of which evolves under the uniform physical selection pressures of the planet, just as terrestrial civilization has evolved under the uniform selection pressures of Earth. On Mars, communication between regions of the planet will be nearly instantaneous, as is communication on Earth today, and the immediate neighborhood of Mars, its satellites and space stations, will also be a part of this instantaneous communications network. Mars will have its own internet, which will presumably be updated on a regular basis, much like a backup system where Mars and Earth each back up the other. Martian social media will be dominated by “Martian issues” just as terrestrial social media will be dominated by terrestrial issues.

MARS from the Moon PHOBOS by Jack COGGINS

Image: Mars may come to be the origin of a spacefaring civilization. (Mars from the Moon Phobos by Jack Coggins, 1951).

Two planetary civilizations projected onto the cosmos

A planet is a natural unit for civilization, which I have expressed elsewhere by saying that planetary civilization is the natural teleology of civilization. [6] Beyond the scope of a planetary civilization communication will experience relativistic delays that become longer the more distant the parties to the communication. There will be communication between Mars and Earth, of course, but of a stilted and somewhat awkward variety, as there will be trade, probably a trickle of luxury goods (rather than staples) as once slowly moved along the Silk Road tenuously connecting the ancient east to the ancient west. Communication and commerce, however, will underscore rather than unify the natural planetary units of Earth and Mars. Exactly what is communicated and what is traded (as well as what is not communicated and what is not traded) will define a system of meanings and values, and these systems will be different on Earth and Mars. [7]

We can always formulate a more comprehensive conception of civilization that includes both terrestrial civilization and Martian civilization—presumably this more comprehensive conception will be “human civilization” as this conception will of necessity be based on those properties shared in common between terrestrial and Martian civilization—much as we can today speak of a planetary civilization that encompasses the many regional civilizations that have grown together as human transportation, communication, and commerce networks have come to integrate the planet entire. Perhaps this more comprehensive conception of civilization could also be called a de novo civilization. With planetary civilization converging on totality, the next stage of emergence in large-scale social organization will be the interaction of these distinct planetary civilizations—the civilizations of Earth, Mars, the moon, and elsewhere, including clusters of artificial habitats.

The expanding scope of large-scale social organization, from a network of cities involved in cooperation and conflict to a network of planets involved in cooperation and conflict and eventually a network of planetary systems engaged in cooperation and conflict, define stages in the development of a cosmological civilization. The civilization that we may yet build within our own solar system will be a model in miniature of an interstellar civilization in which it is a network of planetary systems engaged in cooperation and conflict that defines large-scale social organization. In this context, the different between terrestrial and Martian civilization may become significant.

In the settlement of the New World it is interesting to note the difference between those regions settled directly by European peoples and those regions settled not from the Old World, but from earlier settlements. Thus while New England was settled by Puritans from England, the Carolinas were settled by Caribbean planters. [8] Sugar cane was such a lucrative crop that every scrap of available ground on the Caribbean islands was planted in sugar cane plantations, but these plantations in turn needed to be supplied with foodstuffs and building materials, and so the Carolinas were settled in order to produce the sustenance and material goods required by the export-oriented monoculture of sugar plantations in the Caribbean. [9] The cultural differences between these regions persists to the present day, and is likely to continue to persist into the foreseeable future.

It would be reasonable to expect that a similar pattern will reveal itself in the settlement of the solar system, with some colonies being established directly from Earth, while other colonies may be established by Martian and Lunar settlements, once these latter have reached a sufficient state of development that they can mount outward colonization efforts themselves. [10] In this way, the characteristic differences between terrestrial and Martian civilization will be perpetuated throughout the solar system, and perhaps even throughout the galaxy, and may persist long after any rivalry between Earth and Mars is politically relevant.

But will it ultimately be terrestrial or Martian civilization that leaves the greatest imprint on the universe? The fact that Martians will have already made the leap from Earth to Mars, representing the first spacefaring diaspora, and the likely disproportionate scientific and technological knowledge and expertise in the Martian population to come, will predispose Martians to a central project for their civilization based on spacefaring. Once the Martians have assured their survival and independence, the solar system will be at their doorstep. Mars is the perfect base for a spacefaring civilization, with the lower gravity making the construction of a space elevator easier than on Earth, and being positioned close to the asteroid belt Thus even if a scientific and spacefaring civiization does not fully emerge on Earth, social conditions on Mars may be more favorable to such a development.



[1] Thomas Nagel, “What is it like to be a bat?” The Philosophical Review, LXXXIII, 4 (October 1974): 435-50.

[2] I sometimes define civilization as an economic infrastructure joined to an intellectual superstructure by a central project. I regard this formulation as tentative. Mass societies may be too large and too diverse to be defined by a single central project, so a mass society may have several central projects, but no single, dominant project—or it may have no central project at all. Prior to the advent of mass society, regional civilizations (not yet having converged on planetary scale) were almost always strongly marked by a central project, which almost always was soteriological or eschatological in nature.

[3] V. Gordon Childe, “The Urban Revolution,” The Town Planning Review, Vol. 21, No. 1 (Apr., 1950), pp. 3-17. (Careful observers of the Indiana Jones films will notice that the archaeologist protagonist of the films cites V. Gordon Childe.)

[4] In brief, Childe’s list includes, 1) extent and density of settlements, 2) division of labor, i.e., craft specialization, 3) surplus value transferred to social elites (which might also be called “capital accumulation”), 4) monumental architecture, 5) social stratification, 6) writing, 7) science, 8) art, 9) trade, and 10) prioritizing residence over kinship. I briefly touched on Childe’s conception of civilization in terms of the urban revolution in my talk at the 2015 Starship Congress, “What kinds of civilizations build starships?” in which I also gave an exposition of my understanding of economic infrastructure and intellectual superstructure (cf. note [2]).

[5] Above in note [2] I said that I sometimes define civilization as an economic infrastructure joined to an intellectual superstructure by a central project; I also sometimes define a civilization as a network of cities bound by relationships of cooperation and conflict. I regard all of these formulations as tentative; the definitive definition of civilization has yet to be formulated. The definition of civilization in terms of a network of cities is obviously a practical characterization that could be established by means of archaeology; a definition of civilization as a central project linking infrastructure and superstructure is much more abstract, and for the same reason it is much more likely to be adaptable to unforeseen developments in the future.

[6] The assertion that planetary civilization is the natural teleology of civilization may be true for only one historical stage in the development of civilization (I explored this idea in Counterfactual Suboptimal Civilizations of Planetary Endemism and Addendum on Civilizational Optimality). It could be argued that the natural extent of a civilization grows over time, so that the earliest manifestation is a city-state with a surrounding region, then an empire, then a regional civilization, then a planetary civilization, then a system-wide civilization, and so on.

[7] These systems of meanings and values constitute part of the intellectual superstructure.

[8] Similarly, in South America Chile was settled for purposes of supply rather than monoculture export.

[9] New England also came to rely on export-oriented monoculture, but of tobacco rather than sugar, especially the “tobacco colonies” of the Chesapeake Bay region. While Caribbean islands were not large enough both to produce sugar for export and to produce their own food, there was sufficient land in New England for both export and staple crops.

[10] It is to be expected that most if not all of the earliest settlement enterprises will be financial failures, if historical analogy holds: “Many early colonial adventures—like Cartier’s voyages, the Panfilo de Narvaez expedition, and Raleigh’s Guiana and Roanoke projects—were characterized by gigantic losses. By the end of the 1620s every single English colonial company had failed both financially and organizationally, and every single early French trading company had been dissolved; by 1674, the Dutch West Indies Company had gone bankrupt for the first of two times.” (A Companion to the Literatures of Colonial America, edited by Susan Castillo and Ivy Schweitzer, p. 64)



Deep Space Projects for Citizen Scientists

by Paul Gilster on February 16, 2017

I’m always interested in ways readers can dig directly into data from our telescopes, and this morning I can point to two. I’ll begin with the Lick Carnegie Exoplanet Survey, which has just released 60,949 precision Doppler velocities for 1,624 stars. The data draw on observations using HIRES (the High Resolution Echelle Spectrometer) on the Keck 1 telescope on Mauna Kea (Hawaii). As exoplanet hunter Greg Laughlin (UC-Santa Cruz) explains on his systemic site, the data contain hundreds of possibly planetary signals.

If you’d like to dig into this material, which includes hints of a super-Earth around the fourth closest star to the Sun (Lalande 21185), I’ll remind you of Stefano Meschiari’s Systemic Console, developed with Laughlin as a way of exploring exoplanetary data. The latest version completely reworks the older Console and provides the tools needed to study the Lick Carnegie material. Versions of this open source software are available here, and a visit to the Earthbound Planet Search website will yield links to the Lick Carnegie data release.

As to the stars under examination, they are primarily the nearest and brightest F, G, K and M-class stars visible from Mauna Kea. All are within 100 parsecs (326 light years), though some include a relatively short range of data, while others offer a deeper dataset. Accompanying the data release is an upcoming paper, “The LCES HIRES/Keck Precision Radial Velocity Exoplanet Survey,” slated for publication at The Astronomical Journal (preprint available).

Discussing the data release on systemic, Laughlin noted an interesting fact as he placed the Lick Carnegie work in context:

…as the breakthroughs rolled in, the Keck I Telescope was gradually accumulating Doppler measurements of hundreds of nearby Sun-like stars with HD designations and magnitudes measured in the sevens and eights. This data is as important for what it shows (scores of planets) as for what it doesn’t show (a profusion of planets with Jupiter-like masses and orbits). There are several reasons why our Solar System is unusual, and Jupiter is one of them.


Image: From Greg’s systemic discussion, drawing on Rowan 2016.

Just how unique is our Solar System? It’s an intriguing question to keep in mind, and one that provides additional motivation for digging into the data in classrooms and in the home. Laughlin has spoken before about ‘democratizing’ the search for planets, which is all about making not just the data but the software tools available for trying out possible planetary orbits that would explain what we’re seeing. Expect much more as this dataset is examined.

Backyard Worlds: Planet 9

NASA’s Wide-field Infrared Survey Explorer (WISE) mission captured a lot of attention in these pages as it scanned the entire sky between 2010 and 2011. Although its primary mission ended in 2011, WISE was reactivated in 2013 with the charter of finding near-Earth objects that could be potentially hazardous. The latter mission was named the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE).

A lot of us were keeping an eye on M-class dwarfs and, especially, brown dwarfs, wondering whether a dim object might not exist much closer to the Earth than the Alpha Centauri stars. If WISE had found something like this, it would have given us a much closer target for an interstellar mission, but no such target has yet emerged. Even so, we’re not quite through with the WISE data. There may be brown dwarfs waiting to be found therein, not to mention distant objects in our own Solar System including, if it exists, the as yet undetected Planet Nine.


Image: This diagram shows the orbits of several Kuiper Belt objects that were used to infer the existence of Planet 9. If it exists, Planet 9 may reveal itself in WISE infrared images. Credit: ASU.

Making such finds now comes down to looking for moving objects in WISE images, an effort in which human eyes have a role to play, because computers can trip over image artifacts in crowded parts of the sky. Thus we get Backyard Worlds: Planet 9, a new website that puts computer users — citizen scientists — to work doing what Clyde Tombaugh did in the search for Pluto, studying imagery to pick out the moving object while disregarding the artifacts.

The site works through ‘flipbooks’ — brief animations that show the changes in small parts of the sky over time. Objects spotted by sharp-eyed observers will be prioritized for future investigation. The image below illustrates the process with a real WISE discovery. (3)

Image: Previously known brown dwarf WISE 0855-0714 is seen here in this Backyard World flipbook as a moving orange dot at upper left. Citizen scientists will be asked to inspect images just like this to search for new objects in the solar neighborhood. Credit: NASA.

Working in the infrared, WISE produced data on nearly 750 million individual sources in the sky, working at infrared wavelengths that could reveal objects emitting at low temperatures. In other words, there’s plenty of work here for those patient enough to study these images. “There are just over four light-years between Neptune and Proxima Centauri, the nearest star, and much of this vast territory is unexplored,” said the lead researcher for Backyard Worlds: Planet 9, Marc Kuchner, an astrophysicist at NASA’s Goddard Space Flight Center.

Bringing the human ability to spot movement against a fixed background, and bringing in large numbers of volunteers to work the project, allows all of us to get involved with these ongoing investigations. Can we find what the computers have missed amidst the virtual noise?



Stellar Pulsations Induced by Planet

by Paul Gilster on February 15, 2017

It’s no surprise that planets can affect the stars they orbit. We’ve used that fact for several decades now, relying on radial velocity studies that showed the movement of a star toward us and then away again as it was tugged on by the planet under investigation. But now we’re hearing about another kind of planetary effect, one whose future uses may be intriguing. We’re seeing a star’s brightness change in evident synchrony with a planetary orbit.

The star is some 370 light years away from the Earth. The planet in question is HAT-P-2b, a ‘hot Jupiter’ in a highly elliptical orbit that makes its closest approach to the star every 5.6 days. The planet, discovered by the automated HATNet project (Hungarian Automated Telescope Network), is about 8 times Jupiter’s mass. The temperature changes its orbit should induce in its atmosphere led indirectly to the brightness discovery, for researchers led by Julien de Wit (MIT) wanted to learn about the circulation of energy in the planet’s atmosphere, causing them to turn to the Spitzer infrared telescope for data.

The dataset de Wit and colleagues consulted contained some 350 hours of observation between July of 2011 and November of 2015. The planet’s passage close to and then behind the star as seen from Earth was the key window of interest, as changes to the star’s brightness can reveal how much energy it is delivering to the planet. The method has been used with success before on planets like HD 149026b, likewise studied with Spitzer data.

What the researchers found was a bit of a surprise. When the planet passed behind the star, oscillations in the star’s light with a period of 87 minutes became visible. The signals were tiny — de Wit compares them to the sound of a mosquito next to a jet engine — but they demanded attention because their period was an exact multiple of the planet’s orbital frequency.

The paper recounts the team’s exhaustive analysis of possible effects in the Spitzer instrument itself, and explains why the only conclusion they could reach was that this was a stellar, not an instrumental, effect. Bear in mind that while HAT-P-2b is a massive planet, it is dwarfed by its host star, which is more than 100 times more massive. A pulsation effect at this scale seems unusual, showing how much we have to learn about planet/star interactions.

“This is really exciting because, if our interpretations are correct, it tells us that planets can have a significant impact on physical phenomena operating in their host-stars,” says co-author Victoria Antoci, a postdoc at Aarhus University in Denmark. “In other words, the star ‘knows’ about its planet and reacts to its presence.”


Image: For the first time, astronomers have observed a star pulsing in response to its orbiting planet. The star, HAT-P-2, pictured, tracks its star in a highly eccentric orbit, flying extremely close to and around the star, then hurtling far out before eventually circling back around. Credit: NASA (edited by MIT News).

The find is interesting indeed, but fraught with issues. For one thing, pulsations on a star are far from unusual. Could what de Wit and company observed simply be a natural phenomenon unrelated to the planet? The answer is almost certainly no, because the oscillations are only observed during the planetary occultations, and the correspondence between the pulsations and the planet’s orbital frequency implies a strong connection between the two.

Pulsations caused by tidal effects have been seen before in binary star systems (these are known as ‘heartbeat’ stars), and the researchers turn to tidal effects as an explanation. From the paper:

The photometric observations also show no sign of orbit-to-orbit variability nor of orbital evolution but reveals high-frequency low-amplitude stellar pulsations that correspond to harmonics of the planet’s orbital frequency, supporting their tidal origin.

But the explanation only goes so far:

Current stellar models are however unable to reproduce these pulsations. HAT-P-2’s RV [radial velocity] measurements exhibit a high level of jitter and support the orbital evolution of HAT-P- 2 b inconsistent with the ultra-precise eclipse times. The inability of current stellar models to reproduce the observed pulsations and the exotic behavior of HAT-P-2’s RV indicate that additional observations and theoretical developments are required to understand the processes at play in this system.

We have, in other words, to gather more data, presumably through future missions like TESS, PLATO and CHEOPS, to understand these intriguing interactions. The star’s pulsations are the tiniest variations of light from any source that Spitzer has ever measured. But on a broader front, are we seeing the development of a new kind of exoplanet discovery tool, one that would work regardless of the orbital inclination of an undetected planet? The paper speculates on the possibility, noting that such detections could flag stars that we would then want to submit to examination with direct imaging and radial velocity techniques.

The paper is De Wit, “Planet-Induced Stellar Pulsations in HAT-P-2’s Eccentric System,” Astrophysical Journal Letters 14 February 2017 (preprint).



A KBO-like Object around another Star?

by Paul Gilster on February 14, 2017

We’re beginning to find evidence of objects like those in the Kuiper Belt beyond our own solar system. In this case, the work involves a white dwarf whose atmosphere has been recently polluted by an infalling object, giving us valuable data on the object’s composition. The work involves the white dwarf WD 1425+540, whose atmosphere has been found to contain carbon, nitrogen, oxygen and hydrogen. The findings are unusual because white dwarfs are the dense remnants of normal stars, with gravitational fields strong enough to pull elements like these out of their atmospheres and into their interiors, where they are immune from detection by our instruments. And that implies a relatively recent origin for these elements.

Lead author Siyi Xu (European Southern Observatory) and team worked with spectroscopic observations from HIRES (the High Resolution Echelle Spectrometer) on the Keck Telescope and included data from the Hubble instrument. The researchers believe the white dwarf’s atmosphere has been enriched by the breakup and eventual spiral into the star of a minor planet, whose composition mimics what we find in Kuiper Belt objects. WD 1425+540, some 200 light years away in the constellation Boötes, thus absorbed a body that is calculated to have been composed of 30 percent water and other ices and 70 percent rocky material.

The event, which would have involved the gravitational disruption of the object’s orbit, causing its infall, disintegration and the eventual absorption of its elements by the star, may have occurred as recently as 100,000 years ago. We could be seeing a process that has implications for the existence of life on planets in the inner system like our own. The Earth may well have been dry when it first formed, with life’s building blocks delivered as the result of collisions with other objects. Siyi Xu sees this as a process that can occur anywhere:

“Now we’re seeing in a planetary system outside our solar system that there are minor planets where water, nitrogen and carbon are present in abundance, as in our solar system’s Kuiper belt. If Earth obtained its water, nitrogen and carbon from the impact of such objects, then rocky planets in other planetary systems could also obtain their water, nitrogen and carbon this way. We would like to know whether in other planetary systems Kuiper belts exist with large quantities of water that could be added to otherwise dry planets. Our research suggests this is likely.”


Image: Rendering of a white dwarf star (bright white spot), with rocky debris from former asteroids or a minor planet that has been broken apart by gravity (red rings). Credit: University of Warwick.

Temperatures in the protoplanetary disk of our own system are thought to have determined the distribution of water, with dry rocky worlds inside the snow line, and water ice available beyond it, much of it still to be found in asteroids, comets and Kuiper Belt objects. Nitrogen ice can also be found in comets from the outer regions of the Kuiper Belt and in the Oort Cloud. The assumption has been that similar conditions prevailed around other stars, but our knowledge of analogs to Kuiper Belt objects in other systems has remained scant.

This makes white dwarfs like WD 1425+540 a useful tool, for the detection of heavy elements in their atmosphere has to point to an external source, giving us a way to measure the broad composition of objects in the system. And indeed, the paper notes that there are at least a dozen ‘polluted’ white dwarfs whose accreted material has been measured in detail. Excess oxygen that can be attributed to water-rich objects has been detected in only three.

From the paper:

The accreting material observed in WD 1425+540 provides direct evidence for the presence of KBO analogs around stars other than the Sun. In addition, WD 1425+540 has a K dwarf companion at 40.0 arcsec (2240 au) away (Wegner 1981). The presence of a distant stellar companion can impact the stability of extended planetary systems and, thus, enhance the chances of perturbing objects – that initially orbit far from a white dwarf – into its tidal radius via the Kozai-Lidov mechanism (Zuckerman 2014; Bonsor & Veras 2015; Naoz 2016).

Interestingly, the authors have worked parameters for the disrupted object in the system. When it was on the main sequence, WD 1425+540 would have been about twice as massive as the Sun and 10 times more luminous. Its nitrogen-bearing KBO analogs would have been found at about 120 AU, or three times further from the star than the Sun’s Kuiper Belt objects. Add in the star’s red giant phase and its Kuiper Belt moves out by a factor of 3. Thus the object that was accreted by WD 1425+540 likely orbited beyond 300 AU before experiencing the gravitational disruption that drove it inward toward the star and its eventual destruction.

Most material accreted onto white dwarfs in previous studies of polluted atmospheres has come from dry minor planets. This paper argues that the spectroscopic work on WD 1425+540 gives us strong evidence for volatile-rich planetesimals around other stars:

With this new dataset, we can conclude that extrasolar terrestrial planets could have volatile element and water abundances provided by KBO analogs that are comparable to those on Earth.

The paper is Xu et al., “The Chemical Composition of an Extrasolar Kuiper-Belt-Object,” Astrophysical Journal Letters 836, L7 (2017). Abstract / preprint.