Rosetta’s Day in the Sun

by Paul Gilster on August 13, 2015

Today is perihelion day for the European Space Agency’s Rosetta orbiter and the doughty Philae lander that, we can hope, may still be taking data even if we can’t talk to it. Celebrating the event, ESA has made available a new interactive viewer based on images taken with Rosetta’s navigation camera (NAVCAM). At the end of July, almost 7000 NAVCAM images were available through the Archive Image Browser, a number that will increase as the mission continues. Now we have an interactive tool that taps all those NAVCAM images.

You can have a look at the tool here. With the ability to zoom in and out, rotate the view and move across the comet, the viewer adds features like texture maps and trajectory diagrams showing where various images of the comet were taken, linking to the NAVCAM database to allow downloads of the relevant images. ESA will also be doing a Google Hangout on what it’s calling Rosetta’s Day in the Sun at 1300 UTC (0900 EDT). Hard to believe we’ve already spent a year exploring Comet 67P/Churyumov-Gerasimenko, with lengthy investigations to follow.


Image: ESA’s interactive tool for exploring Comet 67P/Churyumov–Gerasimenko. Credit: ESA.

As I’m writing, Rosetta is 185,986,924 kilometers from the Sun (265,138,407 kilometers from Earth). Perihelion was actually passed at 0203 UTC (2203 EDT on the 12th), with tweets like this announcing the news.

Leading up to the event, we have this image of some of the activity on the comet. During the approach to perihelion, Rosetta’s activity has grown, as you can see in the outburst below.


Image: A short-lived outburst from Comet 67P/Churyumov–Gerasimenko was captured by Rosetta’s OSIRIS narrow-angle camera on 29 July 2015. The image at left was taken at 13:06 GMT and does not show any visible signs of the jet. It is very strong in the middle image captured at 13:24 GMT. Residual traces of activity are only very faintly visible in the final image taken at 13:42 GMT. Credit: ESA.

The spacecraft has to maintain its distance from the comet to avoid possible damage from debris. This one was taken from a distance of 186 km from the comet’s center. The jet is estimated to have a minimum speed of 10 meters per second. It’s coming from the rugged terrain around the comet’s neck, now known as the Anuket region (here again I thrill at the process of giving names to places humans have never seen before, just as we have been doing on both Pluto/Charon and Ceres. Our unforgettable summer of exploration continues).

“This is the brightest jet we’ve seen so far,” says Carsten Güttler, OSIRIS team member at the Max Planck Institute for Solar System Research (Göttingen). “Usually, the jets are quite faint compared to the nucleus and we need to stretch the contrast of the images to make them visible – but this one is brighter than the nucleus.”

What we’re seeing on the comet is the gradual warming of the comet’s ices, causing them to sublimate and carry dust with them into space. We’re also noting that areas of the comet that have been in darkness are now getting illumination, adding to the activity. The action should peak in coming weeks, but ESA warns that the outbursts are unpredictable and can occur at any time. The July 29 outburst was the strongest yet, actually pushing away the solar wind and magnetic field from around the nucleus and affecting the comet’s gaseous coma.

The ROSINA instrument aboard Rosetta detected the changes to the coma even as its mass spectrometer showed changes in the makeup of the outflowing gases. Carbon dioxide, as compared with two days previously, increased by a factor of two, as this ESA news release relates. Methane increased by a factor of four, and hydrogen sulphide by seven.

“This first ‘quick look’ at our measurements after the outburst is fascinating,” says Kathrin Altwegg (University of Bern), ROSINA principal investigator. “We also see hints of heavy organic material after the outburst that might be related to the ejected dust. But while it is tempting to think that we are detecting material that may have been freed from beneath the comet’s surface, it is too early to say for certain that this is the case.”


Image: During an outburst of gas and dust from Comet 67P/Churyumov–Gerasimenko on 29 July 2015, Rosetta’s ROSINA instrument detected a change in the composition of gases compared with previous days. The graph shows the relative abundances of various gases after the outburst, compared with the measurements two days earlier. For example, the amount of carbon dioxide (CO2) increased by a factor of two, methane (CH4) by four, and hydrogen sulphide (H2S) by seven, while the amount of water (indicated by the horizontal black line) stayed almost constant. Credit: ESA.

As you would expect, dust hits sharply increased, reaching 30 hits per day vs. 1-2 per day earlier in July. 70 hits were recorded in one four hour period on August 1, showing the persistence of the outburst’s effects. We’ll doubtless see further demonstrations of the Sun’s influence on this comet as the mission continues to follow it. Tracking interactions between the comet and the solar wind had been high on the priority list for Rosetta researchers, so the recent outburst gives every indication that our data on the phenomenon will be abundant.



A Cosmological Fade to Black

by Paul Gilster on August 12, 2015

Some writers immerse us so deeply in time that present-day issues are dwarfed by immensity. I always think of Olaf Stapledon and Star Maker (1937) in this regard, but consider Arthur C. Clarke’s The City and the Stars (1956), in which we see the city Diaspar on the Earth of a billion years from now. And even Clarke’s story is trumped by Greg Bear, whose City at the End of Time (2008), something of an homage not just to Clarke but to William Hope Hodgson as well, takes us to the Kalpa, a place and a civilization that is trying to ward off the breakdown of physical laws one hundred trillion years hence.

With the Bear novel we enter the realm of extreme cosmology. Here spacetime itself is threatened by an entity intent on destroying it, creating a Chaos that harks back to ancient Earth myth. The human race is scattered across the cosmos, the galaxies themselves burned out husks. I also mentioned Hodgson above. The English writer (1877-1918), who would die at Ypres, produced a vast novel called The Night Land (1912) that explores a universe where the Sun has gone dark. Clark Ashton Smith, who loved purple prose and wrote his share of it, would call Hodgson’s work “…the ultimate saga of a perishing cosmos, the last epic of a world beleaguered by eternal night and by the unvisageable [sic] spawn of darkness.”

I’m drawn back to these tales today because of work out of the Galaxy and Mass Assembly (GAMA) project, an effort that itself pushes right up against the ultimate fate of the cosmos. GAMA is a multi-wavelength survey using both ground- and space-based telescopes to measure the energy output of over 200,000 galaxies. Presented at the International Astronomical Union’s 29th General Assembly in Honolulu, the results tell us that the energy produced in a section of the universe today is only half what it was two billion years ago.

A ‘world beleaguered by eternal night indeed.’ The fading, as we may conceive it, is occurring across all wavelengths from the ultraviolet to the far infrared. This is not, as it turns out, startling news, for we’ve been tracking the accelerating expansion of the universe since the late 1990s, when so-called ‘dark energy’ was invoked to explain the phenomenon. Simon Driver (University of Western Australia), who heads up the GAMA team, puts the matter rather charmingly: “The Universe will decline from here on in, sliding gently into old age. The Universe has basically sat down on the sofa, pulled up a blanket and is about to nod off for an eternal doze.”


Image: The first data from the GAMA survey examine over 200,000 galaxies, offering the best look yet at the universe’s slow fade. Credit: GAMA.

Exactly how that doze will proceed is thoroughly considered in Greg Laughlin and Fred Adams’ The Five Ages of the Universe (1999), which departs from linear time to consider logarithmic cosmological decades. We live, in these terms, in the 10th cosmological decade, approximately 1010 years after the Big Bang. We are, in fact, in the ‘Stelliferous Era,’ when galaxies and stars as we know them continue to shine, as they will through the 14th cosmological decade. But a universe ten thousand times its current age will eventually use up all nuclear fuel. What happens in the Degenerate Era to follow is well beyond our experience but not our ability to project, and if Laughlin and Adams are right, there may be a way for intelligent life to persist, perhaps even into the supremely lengthy Black Hole Era.

But back to GAMA, whose object is to map and model all the energy generated within a large volume of space. In a news release from the IAU, Driver describes that energy this way:

“While most of the energy sloshing around in the Universe arose in the aftermath of the Big Bang, additional energy is constantly being generated by stars as they fuse elements like hydrogen and helium together. This new energy is either absorbed by dust as it travels through the host galaxy, or escapes into intergalactic space and travels until it hits something, such as another star, a planet, or, very occasionally, a telescope mirror.”

The team would like to map and model this energy over the entire history of the universe, a mammoth undertaking that will involve facilities not yet online, such as the Square Kilometer Array scheduled for South Africa and Australia over the coming decade. Until then, what is being reported by the GAMA team is considered the most comprehensive assessment of the energy output of at least the nearby universe. Each galaxy is measured at 21 wavelengths from the ultraviolet to the far infrared. We can only imagine the fictional uses the universe’s slow fade will inspire as we learn more about how it occurs and how intelligence may deal with it.

Addendum: I can’t rush past William Hope Hodgson as quickly as I did, especially since I didn’t even mention his best known work, The House on the Borderland (1908). If you do get interested in Hodgson, and you should, let me recommend Michael Dirda’s comprehensive and highly readable look at the author From Out of the Depths: The Weird Tales of William Hope Hodgson, an online essay at the Barnes & Noble site.



Upcoming Interstellar Conferences

by Paul Gilster on August 11, 2015

The interstellar community has seen a surprising number of conferences since the 2011 event in Orlando, which kicked off the 100 Year Starship effort and brought unusual media attention to the idea of travel between the stars. I had thought when 2015 began that further conferences were unlikely — it seemed to be a year for consolidation and, if you will, introspection, measuring how the effort to reach the public with deep space ideas was progressing and consolidating progress on various projects like the Icarus Interstellar starship redesign.

But both Icarus and the 100 Year Starship organization have surprised me with conferences announced for this fall. Icarus pulled off a successful Starship Congress in 2013, one I remember with pleasure because of my son Miles’ work with Icarus and the chance to meet up with him in Dallas to hear interesting papers and share news and good meals. There will doubtless be much to say about Project Icarus itself at the new meeting. After all, the organization is deep into the redesign of the original Daedalus starship, applying all the changes in technology, both real and projected, that have occurred in the past 35 years.


Starship Congress 2015 is being billed as an ‘interstellar hackathon,’ one centered on “hacking the paradigm of interstellar space exploration.” The Icarus site notes that it occurs “With a nod to this year’s University setting and a Hollywood and video game-driven surge in popularity of deep space exploration (“Interstellar”, “Guardians of the Galaxy”, “EVE: Valkyrie”, “Kerbal Space Program 1.0”)…” Talks and presentations will be punctuated by workshops and speakers from the deep space community over the two days of the event — the schedule is here, as is the registration link for accommodations at the Sheraton Philadelphia University City Hotel.

The venue is Drexel University in Philadelphia for the event running September 4-5. Ticket prices are online and credit card orders are easily placed on the Icarus site. Icarus is distinguishing this event from the 2013 Starship Congress by saying “… the 2015 edition is being structured to quickly break-down status quo approaches in anticipation of reaping new results from looking at old challenges with fresh outlooks.”

I see that Rachel Armstrong has been designated “First Speaker” for the Hackathon, and Ralph McNutt, most recently in the spotlight as a co-investigator on the New Horizons mission, will give the keynote. McNutt (JHU/APL) has been involved with Icarus since its early days; I remember with appreciation his help on deep space mission concepts when I was writing Centauri Dreams (the book) back in the 2002-2004 era. And I’m delighted to see that Cameron Smith (Penn State), an anthropologist whose thinking on long-term interstellar missions has graced these pages, is also to be on hand as a special guest.

Icarus is trying to raise $20,000 through KickStarter to support Starship Congress 2015. The campaign is now live, with a page providing additional background about the organization and its plans for the event.

100 Year Starship Symposium 2015

Finding Earth 2.0 is the theme for the 2015 100 Year Starship Public Symposium, to be held from October 29 through the 1st of November at the Santa Clara Marriott in Silicon Valley. The focus draws on recent discoveries of planets either in or close to the habitable zones of their stars. While none of these can be definitively called Earth 2.0, it’s clear that we’re making progress toward that goal, finding smaller worlds around stars more like the Sun and heading for the day when a G-class star with a small rocky world in a habitable zone orbit will tantalize us with the possibility that it is as capable of developing life as our own green and blue world.

When we find such a planet, we’ll have an abundance of cross-disciplinary studies to invoke in its characterization. “The 100 Year Starship 2015 Public Symposium challenges participants to consider what specific capabilities and systems — scientific, technical and societal — will be needed over the next 5-25 years,” says the organization’s website, “not to merely suggest or catalog earth analogue candidate exoplanets, but to identify at least one definitive Earth 2.0—and to consider how such a discovery itself will impact our world and space exploration.”


You can find background on Finding Earth 2.0, the schedule, registration and hotel information here. There is also a form for submitting abstracts. The technical tracks being examined are these:

  • Designing for Interstellar
  • Propulsion and Energy
  • Interstellar Space, Stars and Destination
  • Data Communications and Information Technology
  • Life in Space – Health, Astrobiology, Earth Biology and Bioengineering
  • Interstellar Technology Enhancing Life on Earth
  • Becoming an Interstellar Civilization

Also being introduced at the 2015 symposium is the Canopus Award, designed to recognize both fiction and non-fiction works that have contributed educationally and inspirationally toward the goal of interstellar flight. The awards are to be given in two categories: Previously Published Works of Fiction, with an award made for Long Form (40,000 words or more) and one for Short Form (between 1,000 and 40,000 words), and Original Works, based on this year’s 100YSS Public Symposium theme Finding Earth 2.0. An award will be made for Short Form Fiction (1,000-5,000 words) and one for Short Form Non-fiction (1,000-5,000 words).

Jason D. Batt, a writer who is also Canopus Award program manager, explains the award’s rationale:

“100YSS is launching the awards at a particularly fortuitous time. The recent announcements of Kepler-452b exoplanet, major financial support of searches for extraterrestrial intelligence and the space probe New Horizons close encounter with Pluto and the amazing images it is generating highlight how we all look up and dream of what’s out there. The Canopus award celebrates that passion that is common to the public, researchers and science fiction fans alike.”

The award is named after a star that has been used as a navigation beacon back to the days of the earliest civilizations and forward to spacecraft like Voyager, which tracks Canopus as one way to orient itself toward Earth for data transmissions. Submissions are being accepted here through August 31 for original works and nominations for previously published works.



With the question of habitable planets on my mind following Andrew LePage’s splendid treatment of Kepler-452b on Friday, I want to turn to the interesting news out of San Diego State, where astronomer William Welsh and colleagues have been analyzing a new transiting circumbinary planet, a find that brings us up to a total of ten such worlds. Planets like these, invariably likened to the planet Tatooine from Star Wars, have two suns in their sky. Now we have Kepler-453b to study, a world that presented researchers with a host of problems.

Transits of the new world occur only nine percent of the time because of changes in the planet’s orbit. Precession — the change in orientation of the planet’s orbital plane — meant that Kepler couldn’t see the planet at the beginning of its mission, but could after it swung into view about halfway through the mission’s lifetime, allowing three transits. Clearly, this is a system we could easily have missed, says William Welsh (San Diego State), who was lead author on the study. Welsh calls the find ‘a lucky catch,’ and it’s hard to argue: The precession period is estimated to be about 103 years, with the next set of transits not becoming visible until 2066.


Image: An artist’s impression of the circumbinary extrasolar planet Kepler-453b. Credit: NASA/JPL-Caltech/T Pyle.

It was Welsh, working with San Diego State colleague Jerome Orosz, who used Kepler data back in 2012 to discover the first instance of a two-planet circumbinary system, one of the two worlds being in the habitable zone where liquid water could exist on a solid surface. Kepler-453b turns out to be the third planet identified by the mission as being a circumbinary world in the combined habitable zone of two stars. The world is a gas giant, however, and unlikely to host life as we know it (life as we don’t know it is another matter, but that’s true in our own Solar System as well).

What we’re able to deduce about Kepler-453b so far is that it has a radius about six times that of the Earth, and a mass — not readily measurable with current data — probably less than 16 times that of Earth. The planet’s orbit is 240 days, and the two stars it orbits orbit each other every 27 days. The larger star in much like our own, with 94 percent the mass of the Sun, while the smaller is about 20 percent as massive and much cooler. The Kepler-453 system is 1400 light years away in the constellation Lyra, a young system probably between one and two billion years old.

So are we going to find an Earth-like planet in a circumbinary orbit one of these days? What we’ve learned thus far is that circumbinary planets are not unusual — we’ve already found ten — and the range of configurations is wide. An Earth-class planet in the habitable zone of one of these stars would conjure up those wonderful images of twin stars at sunset, one of them going down before the other, a world of intriguing hues and shadows as the two stars moved through their own orbits, with occasional eclipses thrown in for good measure. Circumbinaries are another reminder of the diversity we’re coming to expect as we build the exoplanet catalog.

The paper is Welsh et al., “Kepler 453 b—The 10th Kepler Transiting Circumbinary Planet,” Astrophysical Journal 5 August 2015 (abstract). An SDSU news release is available.



Is Kepler 452b a Rocky Planet or Not?

by Paul Gilster on August 7, 2015

Where is the dividing line between a large, rocky planet and a ‘mini-Neptune?’ It’s a critical issue, because life is at least possible on one, unlikely on the other. But while we’re getting better at figuring out planetary habitable zones, the question of how large a planet can be and remain ‘terrestrial’ is still unresolved. As Andrew LePage explains below, our view of potentially habitable planets like Kepler-452b depends upon how we analyze this matter — clearly, just being in or near the habitable zone isn’t enough. A prolific essayist with over 100 articles in venues like Scientific American and Sky & Telescope, LePage writes the excellent Drew ex Machina site, where his scrutiny of recent exoplanet finds is intense. The work seems a natural fit given his day job at Visidyne, Inc. near Boston, where he specializes in the processing and analysis of remote sensing data.

by Andrew LePage


A couple of weeks ago, the media was filled with reports about the discovery of Kepler 452b. While NASA’s Kepler mission had found a number of potentially habitable planets earlier, all of these previous discoveries orbited dim K and M-dwarf stars which are very different than our Sun and present a number of still unresolved issues affecting habitability (see A Review of the Best Habitable Planet Candidates in Centauri Dreams for a full review of earlier finds). What made this new Kepler find unique was that Kepler 452b was a nearly-Earth-sized planet orbiting inside the HZ of a Sun-like star – the first of potentially many more such exoplanets to come from the continuing analysis of Kepler’s data set. But being a bit of a skeptic when it comes to often overhyped media reports about the potential habitability of any newly discovered exoplanet, I wanted to dig deeper into this claim.

Is It in the Habitable Zone?

According to the discovery paper submitted for publication in The Astronomical Journal with Jon Jenkins (NASA Ames Research Center) as the lead author, Kepler 452 is a G2 type star like the Sun with a surface temperature of 5757±85 K, a mass of 1.04±0.05 times that of the Sun and a radius of 1.11 +0.15/-0.09 times the Sun’s. Based on these data, it can be calculated that Kepler 452 has a luminosity about 20% greater than that of the Sun making this it a slightly heavier and brighter version of the Sun. Comparison of the known properties of this star with standard models of stellar evolution yields an age of 6±2 billion years or about 1½ billion years older than the Sun and its system of planets. Compared to the stars earlier announced with potentially habitable exoplanets, Kepler 452 was certainly quite Sun-like.

While a full assessment of the habitability of any exoplanet would require very detailed information about all of its properties, obtaining such information is simply beyond the reach of our current technology. At this early stage in our search for other Earth-like worlds, the best we can do is compare what properties we can derive to our current expectations of the range of properties for habitable worlds to determine if a new find is potentially habitable. One of those important set of properties is the orbit of an exoplanet. According to Jenkins et al., Kepler 452b is in a 384.84-day orbit with an average orbital radius of Kepler 452b is 1.046 +0.019/-0.015 AU. This orbital radius is far outside that where a planet would become tidally locked and be affected by severe stellar flare activity – two unresolved issues that call into question the potential habitability of worlds tightly orbiting much dimmer stars like those found to date.

This orbital radius combined with the stellar properties yields an effective stellar flux for Kepler 452b that is 1.10 +0.29/-0.22 times that the Earth receives from the Sun. This effective stellar flux places Kepler 452b just inside the conservative HZ of a Sun-like star as defined by the runaway greenhouse limit. Given the current uncertainties in the properties of Kepler 452b and the star it orbits, Jenkins et al. calculate that there is only a 28.0% probability that Kepler 452b actually orbits inside of the conservatively defined HZ but there is a 96.8% chance that it orbits inside a more optimistic definition of the HZ corresponding to early conditions on Venus. However, it appears that Jenkins et al. used a definition of the HZ limits for an Earth mass planet. If Kepler 452b has a mass closer to five times that of the Earth (or 5 ME), which is likely to be the case, the effective stellar flux for the inner edge of the HZ increases from 1.10 to 1.18 times that of the Earth raising the chances that Kepler 452b orbits inside of the HZ to probably better than even odds. And since Kepler 452, like all stars, would have been dimmer in its youth, Kepler 452b would have been even more comfortably inside the HZ for billions of years. Considering all these facts combined with the limitations of current models in defining the true inner boundary of the HZ, this is close enough even for this skeptic to consider Kepler 452b as potentially habitable at least in terms of its orbit and effective stellar flux.

Is It a Rocky Planet?

The other important planetary property we can measure using current detection techniques is the size of a planet. Unfortunately, it is here where many past discoveries have run into trouble. It has been suspected for some time now that somewhere between the size of the Earth (or 1 RE) and Neptune with a radius of 4 RE, planets transition from being predominantly rocky with some chance of being habitable like the Earth to being rich in volatiles such as water, hydrogen and helium becoming mini-Neptunes with little chance of being habitable in the conventional sense. Based on recent analyses of Kepler data on the radius of exoplanets smaller than Neptune combined with independently derived masses from radial velocity measurements and other techniques, we now know that this transition from predominantly rocky worlds to predominantly volatile-rich worlds occurs somewhere around 1½ to 2 RE although the precise value and nature of this transition is uncertain due to the small number of planets with measured radii and precisely determined masses in this size range as well as the measurement uncertainties of those values (see The Transition from Rocky to Non-Rocky Planets in Centauri Dreams).


Image: This artist’s concept compares Earth (left) to Kepler-452b, which is about 60 percent larger in diameter. Credit: NASA/JPL-Caltech/T. Pyle

While many earlier claims of finding potentially habitable planets have run afoul of this transition and turned out to be much more likely to be mini-Neptunes than rocky terrestrial planets, in recent months astronomers have started making some effort to address this issue in discovery papers of potentially habitable planets including Jenkins et al.. Based on the analysis of the Kepler photometric data and the properties of the star, Jenkins et al. report that Kepler 452b has a radius of 1.63 +0.23/-0.20 RE which is close to the transition value. Based on their calculations, Jenkins et al. claimed that there was a better than 50% chance that Kepler 452b is a rocky planet. But how did they arrive at this figure?

Jenkins et al. used two different distributions of probable radius values for Kepler 452b and compared them to two different published mass-radius relationships for sub-Neptune sized planets to calculate the odds that their find has a density consistent with a predominantly rocky composition. The radius value distributions were derived from the measured 1.63 +0.23/-0.20 RE radius of Kepler 452 combined with two different models used to determine the host star’s properties. The first model, SPC (Spectral Parameter Classification), determines the star’s parameters by comparing its spectrum to a collection of synthetically generated stellar spectra to find the best fit. The second model, called SpecMath, is considered more conservative and compares the star’s spectrum to a collection of 800 well-studied stellar spectra to derive the star’s properties.

To calculate the probability that Kepler 452b is a rocky planet based on those radius distributions, Jenkins et al. used two different mass-radius relationships. The first was formulated by graduate student Lauren Weiss and famed exoplanet hunter Geoff Marcy (University of California – Berkeley) which was published in March 2014. Weiss and Marcy fitted radius and mass data for 65 exoplanets to come up with a deterministic mass-radius function where a particular radius value corresponds to a single mass value. While simple, this model admittedly does not reflect the fact that exoplanets with a particular radius value can actually have a range of possible mass values reflecting a variety of bulk compositions.

The second mass-radius relationship used by Jenkins et al. was derived by Angie Wolfgang (University of California – Santa Cruz), Leslie A. Rogers (California Institute of Technology), and Eric B. Ford (Pennsylvania State University) and was submitted for publication in April of 2015. They evaluated data for 90 exoplanets using a hierarchal Bayesian technique which allowed them not only to derive the parameters for a best fit of the available data, but also to quantify the uncertainty in those parameters as well as the distribution of actual planetary mass values. Using their approach, they derived a probabilistic mass-radius relationship where the most likely mass and the distribution of likely values are determined that better reflects the uncertainties in the data and the fact that exoplanets with a particular radius value can have a range of actual masses (for a detailed discussion of this work, see A Mass-Radius Relationship for ‘Sub-Neptunes’ in Centauri Dreams).

Using the radius distributions for Kepler 452b derived from SPC and SpecMath, Jenkins et al. found that the mass-radius relationship created by Weiss and Marcy yielded 64% and 40% probabilities, respectively, that their new find has a bulk density consistent with models of rocky planets. When employing the mass-radius relationship of Wolfgang et al., they found a 49% and 62% probability, respectively, that Kepler 452b is a rocky planet. The average of these results is the origin of the quoted greater than 50% odds that the new find is a rocky planet.

Is It Really a Rocky Planet?

While this is a clever solution to a difficult problem, there are problems with this approach. First of all, while the work of Weiss and Marcy was an excellent first attempt to derive the mass-radius relationship using the newly available Kepler data set, the relationship derived by Wolfgang et al. is superior since it uses more data of higher quality that is analyzed in a mathematically more rigorous way. While Jenkins et al. recognize this and prefer the higher probabilities calculated using Wolfgang et al., they used the parameters of the mass-radius relationship derived using all 90 planets with radii up to 4 RE in the original analysis. Based on earlier work by Leslie Rogers, it was recognized that the transition from being predominantly rocky to predominantly volatile-rich takes place at radius values no greater than 1.6 RE (for a full discussion of this work, see Habitable Planet Reality Check: Terrestrial Planet Size Limit on my web site, Drew Ex Machina).

When Wolfgang et al. analyzed just the subset of exoplanets with radii less than 1.6 RE, they derived different parameters for the mass-radius relationship for these smaller planets. For a planet with a radius of 1.6 RE, for example, the most probable mass when using parameters derived from fitting all planets with radii less than 4 RE, as used by Jenkins et al., comes out to about 5 ME. If the parameters derived from just smaller planets with radii less than or equal to 1.6 RE are used, a smaller probable mass value of 4 ME is found. As a result, the probabilities derived by Jenkins et al. are biased towards higher mass outcomes with corresponding higher probabilities of finding Kepler 452b to be a rocky planet.

A better approach for determining the probability that Kepler 452b is a rocky planet would be to compare its properties directly to the population of exoplanets with known radii and accurately determined masses. Unfortunately, Rogers’ paper does not include a simple function that others can use to calculate such a probability since this was outside the scope of her work. Despite this shortcoming, the title of her paper published in March 2015 in The Astrophysical Journal really says it all: “Most 1.6 Earth-Radius Planets are not Rocky”. In other words, Kepler 452b with a radius of 1.63 RE is most likely not a rocky planet but is a mini-Neptune instead, contrary to the claims by Jenkins et al..

Other astronomers trying to calculate the odds that their finds are rocky planets or not have derived probabilities in different ways. Guillermo Torres (Harvard-Smithsonian Center for Astrophysics) on January 6, 2015 announced the discovery of eight habitable zone planets using Kepler data where they quantified the probabilities that their finds were rocky (see Habitable Planet Reality Check: 8 New Habitable Zone Planets on my web site, Drew Ex Machina). Although somewhat different from the method used by Rogers, the approach used by Torres et al. to calculate the probability that a planet with a particular radius is rocky gives qualitatively similar results. Using their model, the chances that Kepler 452b is rocky is about 45%. This is closer to the low-end 40% figure derived by Jenkins et al. than the often quoted “greater than 50%” figure.

Unfortunately, the chance that Kepler 452b is a terrestrial planet might not be as good as even 40%. Recent work by Rebekah I. Dawson, Eugene Chiang and Eve J. Lee (University of California – Berkeley) recently submitted for publication in Monthly Notices of the Royal Astronomical Society strongly suggests that planets with masses greater than about 2 ME (which would have a radius of about 1.2 RE, assuming an Earth-like bulk composition) which orbit stars with a high metallicity are more likely to be mini-Neptunes. This is because stars with higher metallicities tend to have more solid material available to form planetary embryos more quickly making it more likely for them to acquire some gas directly from the protoplanetary disk before it dissipates. Only 1% or 2% of a planet’s total mass in hydrogen and helium is sufficient to puff up its observed radius and make it a mini-Neptune. Stars with lower metallicity values tend to form planetary embryos more slowly and they might not reach the required 2 ME mass threshold fast enough to begin to acquire any more than trace amounts of gas before it has already dissipated from the protoplanetary disk. With a iron-to-hydrogen ratio about 60% higher than the Sun, Kepler 452 has a slightly higher metallicity than the Sun increasing the odds somewhat that Kepler 452b is a mini-Neptune. Taken together with Rogers work, this strongly suggests that the odds that Kepler 452b is a rocky planet are less than 50% not greater as is being claimed.


To be perfectly honest, quibbling over a couple of tens of percent probability one way or the other about the nature of Kepler 452b is most likely not all that important considering the uncertainties in its properties as well as the still substantial uncertainties in the mass-radius relationships available at this time. In the end, we will have to wait for a more definitive derivation of the mass-radius relationship and a more quantitative description of the nature of the transition from rocky planet to mini-Neptune to settle this question more accurately. Despite the outstanding issue of the nature of Kepler 452b, it still has very real prospects of being potentially habitable. But even if it proves not to be, future studies of its properties will provide scientists with vital information on the limits of planetary habitability.

While some might be disappointed by this less rosy assessment, it should be remembered that scientists are still actively analyzing the Kepler data set and performing follow up observations. There are already several potentially habitable Earth-size planet candidates found orbiting Sun-like stars that are being actively studied by members of the Kepler science team and their colleagues. It is only a matter of time before the discovery of true “Earth twins” is announced.

The preprint of the Kepler 452b discovery paper by Jenkins et al., “Discovery and Validation of Kepler-452b: A 1.6-RE Super Earth Exoplanet in the Habitable Zone of a G2 Star”, can be found here.



A Brown Dwarf ‘Laboratory’ for Planet Formation

by Paul Gilster on August 6, 2015

Detecting planets around brown dwarfs is tricky business, but it’s worth pursuing not only for its own sake but because planetary systems around brown dwarfs can tell us much about planet formation in general. A new paper from Andrzej Udalski (Warsaw University Observatory) and colleagues makes this point while noting four brown dwarf planets we’ve thus far found, all of them much larger than Jupiter. An extremely large planet well separated from a brown dwarf suggests a scaled-down binary star system rather than one growing out of an accretion disk.

Fortunately, we can use gravitational microlensing to go after much smaller worlds around brown dwarfs, a method that is not compromised by the faintness of both planet and dwarf. In microlensing we don’t ‘see’ the planet but can infer its presence by observing how light from a more distant star is affected as a brown dwarf system passes in front of it. Udalski and team have used microlensing to discover OGLE-2013-BLG-0723LB/Bb, which appears to be a Venus-mass planet orbiting a brown dwarf.


Image: A brown dwarf in relation to the Sun, a smaller star and planets. Credit: Jon Lomberg.

Andrew Tribick, a Centauri Dreams regular who passed along the link to this paper, notes that the discovery blurs the line between conventional star/planet and planet/satellite configurations. What that suggests is that similar processes are at work within the accretion disks that form around stars and those found around brown dwarfs and even planets. Here’s how the paper reports this:

OGLE-2013-BLG-0723LBb is a missing link between planets and moons. That is, its host OGLE-2013-BLG-0723LB, being a brown dwarf in orbit around a self-luminous star, is intermediate between stars and planets, in both size and hierarchical position. Moreover, the scaled mass and host companion separation of OGLE-2013-BLG-0723LB/Bb are very similar to both planets and moons in the solar system…

As the snip from the paper shows, the brown dwarf in question is itself accompanied by a low-mass M-dwarf star separated some 1.7 AU from the brown dwarf. The planet and brown dwarf, meanwhile, are separated by 0.34 AU. The system is about 1600 light years away in the direction of galactic center. The paper continues by noting similarities to planets and moons in the Solar System:

Both Uranus and Callisto are believed to have formed in the cold outer regions of their respective accretion disks, and are mostly composed of the raw materials of such regions: ice with some rock. In the case of Uranus, it is believed to have been formed closer to the current location of Saturn (10 AU) and to have migrated outward. In the table [showing the physical parameters of the brown dwarf system], the companion-host separations are scaled to the host mass. This is appropriate because the “snow line”, the inner radius at which icy solids can form (2.7 AU in the solar system) increases with host mass, probably roughly linearly. A plausible inference… is that these processes scale all the way from solar-type stars hosting planets, to brown dwarfs hosting “moon/planets”, to giant planets hosting moons.

Several issues remain to be resolved, however. While the researchers believe that the Venus-class planet is orbiting the brown dwarf, this does not guarantee that it was born in an accretion disk around it — planets in close binaries can become perturbed and move from one star to another. If this is the case here, the planet would have similarities to Triton, which is evidently a captured satellite — the authors add that the Neptune-Triton system is scaled to roughly the same parameters as OGLE-2013-BLG-0723LB/Bb.

The other issue is that there is evidence, in the form of excess light in the detection aperture, for a fourth member of this system, one that is likely more massive and luminous than the other three, and separated from them by roughly 100 AU. The presence of this fourth object would obviously affect the system dynamics at work here, complicating the issue of the Venus-class planet’s origin. What we’re left with as this question is followed up is a roughly terrestrial-mass planet orbiting a brown dwarf, a configuration that may be common, with the implication of a formation process that scales down to large planets and their own family of satellites.

The paper is Udalski et al., “A Venus-Mass Planet Orbiting a Brown Dwarf: Missing Link between Planets and Moons” (preprint).



As we close in on perihelion at Comet 67P/Churyumov-Gerasimenko, the Dawn spacecraft continues its operations at Ceres. The contrast between Dawn’s arrival at Ceres in March and New Horizons’ flyby of Pluto/Charon could not have been more striking. With Dawn’s gentle ion push, we watched Ceres gradually grow in the skies ahead, and then settle into focus as the spacecraft began orbital operations. New Horizons was a thrilling, high-velocity fling, with a sudden transition to a backlit Pluto as we settled in to wait for months of data return.

Dawn is now heading for its third science orbit, gradually descending through 1900 kilometers toward an eventual 1500 kilometer altitude above the surface — this is fully three times closer to Ceres than the previous orbit. Again, the gentle nature of ion propulsion is evident, for the spacecraft will reach the new orbit in mid-August, when data operations and imagery again flow. Bear in mind as you think about Pluto and Ceres that the latter is about forty percent the size of Pluto, another dwarf planet (if you’re willing to buy the nomenclature).

Paul Schenk, a geologist at the Lunar and Planetary Institute in Houston, sees a different comparison, however, based on the nature of the surface Dawn has revealed. We’re seeing as much as 15 kilometers difference between crater bottoms and mountain peaks, according to this JPL news release. Says Schenk:

“The craters we find on Ceres, in terms of their depth and diameter, are very similar to what we see on Dione and Tethys, two icy satellites of Saturn that are about the same size and density as Ceres. The features are pretty consistent with an ice-rich crust.”


Image: A portion of the northern hemisphere of Ceres from an altitude of 4,400 kilometers. The image, with a resolution of 410 meters per pixel, was taken on June 25, 2015. Much closer images are on the way once the new science orbit is achieved. Credit: NASA/JPL.

NASA has released a topographic map based on images from Dawn’s framing camera taken at changing viewing angles and Sun positions. It’s a striking reminder of how much we’ve clarified what’s on the surface of the tiny world.

The process of turning terrain into features and thence into names is well advanced, with the International Astronomical Union’s recent approval of names like Occator, the crater in which we find the brightest of Ceres’ mysterious spots. Occator is about 90 kilometers in diameter and 4 kilometers deep. The name comes from a Roman agricultural deity, in keeping with the agriculture theme that begins with the name Ceres itself, Ceres being the Roman goddess of agriculture. A historical footnote: When Giuseppe Piazzi discovered Ceres in 1801, his own suggestion for naming it was Cerere Ferdinandea — Cerere (in Italian) for Ceres, Ferdinandea for King Ferdinand of Sicily. The dual name, thankfully, did not catch.

We also get craters like Haulani, which now designates the smaller of the craters with bright material, a 30-kilometer crater colder than the terrain around it. Other interesting picks: The crater Dantu, after a Ghanaian god, and Ezinu, after the Sumerian goddess of grain. We also have Yalode, a crater named for a Dahomey goddess, and Kerwan, a Hopi name associated with the spirit of sprouting maze. We’re learning a good deal about these craters already.

Enter the Euphrosyne Asteroids

It was not Dawn but the NEOWISE mission (Near-Earth Object Wide-field Infrared Survey Explorer) that helped us understand the group of asteroids known as the Euphrosynes, objects with orbital trajectories that move well above the ecliptic. The eponymous Euphrosyne (you-FROH-seh-nee) is about 260 kilometers across, one of the ten largest asteroids in the main belt. According to this JPL news release, researchers believe it to be a remnant of a collision some 700 million years ago that formed the entire family of smaller asteroids.

The JPL work with NEOWISE was conducted in an attempt to learn more about their relationship with Near Earth Objects, those that have the potential to become problematic because of their close approaches to Earth. Gravitational interactions with Saturn are evidently the reason why the Euphrosynes are a source of some NEOs found on highly inclined orbits. We’re looking at a family of asteroids that can evolve into NEOs given enough time.

“The Euphrosynes have a gentle resonance with the orbit of Saturn that slowly moves these objects, eventually turning some of them into NEOs,” said Joseph Masiero, JPL’s lead scientist on the Euphrosynes study. “This particular gravitational resonance tends to push some of the larger fragments of the Euphrosyne family into near-Earth space.”


Image: The asteroid Euphrosyne glides across a field of background stars in this time-lapse view from NASA’s WISE spacecraft. WISE obtained the images used to create this view over a period of about a day around May 17, 2010, during which it observed the asteroid four times. Because WISE (renamed NEOWISE in 2013) is an infrared telescope, it senses heat from asteroids. Euphrosyne is quite dark in visible light, but glows brightly at infrared wavelengths. This view is a composite of images taken at four different infrared wavelengths: 3.4 microns (color-coded blue), 4.6 microns (cyan), 12 microns (green) and 22 microns (red). Credit: NASA/JPL-Caltech.

Remember that the WISE instrument, having surveyed the entire sky at infrared wavelengths, was put into hibernation in 2011 after cataloguing over 750 million asteroids, stars and galaxies. The spacecraft was re-purposed in the summer of 2013, focusing on asteroids. The JPL study took in 1400 Euphrosyne asteroids, finding them to be dark and in highly inclined, elliptical orbits. Because of their unique nature, says JPL’s Massiero, “…we are able to draw a likely path for some of the unusual, dark NEOs we find back to the collision in which they were born.”



Rosetta’s Comet Nears Perihelion

by Paul Gilster on August 4, 2015

With the fanfare of the New Horizons flyby of Pluto/Charon, we learned that public interest in space can be robust, at least to judge from the number of people I spoke to who had never previously seemed aware of the subject. Here’s hoping that interest continues to be piqued — as it should be — by the ongoing events at Ceres and on Comet 67P/Churyumov-Gerasimenko. With Ceres we have another exploration of a hitherto unknown surface, while the Rosetta spacecraft is watching surface activity on a comet of the kind we’ve never seen up close.

We’ve already spent a year at the comet since Rosetta’s arrival on August 6 of last year, examining the object’s frozen ices and dust as they vaporize with increasing warmth from the Sun. The gas and dust ‘atmosphere’ thus created, called the coma, can produce the kind of spectacular tails we’ve long associated with comet observations from Earth. Perihelion occurs on August 13, when the comet reaches a distance of 186 million kilometers from the Sun.


Image: The orbit of Comet 67P/Churyumov–Gerasimenko and its approximate location around perihelion, the closest the comet gets to the Sun. The positions of the planets are correct for 13 August 2015. Credit: ESA.

Comet 67P is hardly a Sun-grazer. Its orbit takes six and a half years, taking it to an aphelion of 850 million kilometers (just outside the orbit of Jupiter), while its perihelion is between the orbits of Mars and the Earth. Nonetheless, we’ve already seen increasing activity on the surface as frozen gases sublimate. No one can be sure what will happen as perihelion approaches, but this commentary on the Rosetta blog notes the 500-meter long fracture on the surface of the comet that will bear watching during peak activity. 67P is unlikely to break up (it has survived many previous orbits), but breakups do occur, as happened with Comet C/2012 S1 ISON in 2013.

It will be interesting to see how closely controllers bring Rosetta as perihelion approaches. The craft has been operating no closer than 150 kilometers in the last few months as a precaution against damage from the dust surrounding the nucleus, and because the distance depends on surface conditions, we don’t know just where Rosetta will be on August 13. We should have interesting imagery from perihelion passage relatively soon after the event. The Philae lander, meanwhile, has had only intermittent communications with controllers since regaining the link on June 13. An operational Philae at perihelion would be a bonus.

With just eight days left, keep an eye on the Rosetta blog, whose most recent entry covered the first release of pre-landing phase data from four of Rosetta’s instruments. The Archive Image Browser has also been updated with images from Rosetta’s NAVCAM and includes OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) images from the Earth, Mars and asteroid flybys that occurred enroute to the comet, between 2005 and 2010. You’ll also want to track Rosetta on Twitter @esa_rosetta and there is a Facebook page as well.

To get you in the proper frame of mind, ESA has released this animation.

Image: Images taken by Philae’s ROsetta Lander Imaging System, ROLIS, trace the lander’s descent to the first landing site, Agilkia, on Comet 67P/Churyumov–Gerasimenko on 12 November 2014. The first image was taken just over 3 km from the comet, and indicates the position of Agilkia and the area covered by the next image in the sequence, taken just 67 m away. The six images that follow were taken at approximately 10 second intervals prior to landing, with the final image of the sequence acquired 9 m above the touchdown site. The time the images were acquired, along with distance from the surface and image resolution, are marked on each image. The final slide is annotated with the estimated touchdown position and orientation of Philae, which has been calculated to within ±20 cm. Credit: ESA.

The coarse regolith seen close to touchdown shows grain sizes of 10 to 50 centimeters, and in the closest image, granules less than 10 cm across. Researchers believe the regolith extends to 2 meters deep in places and is free from fine-grained dust deposits. The large boulder visible in the early part of the animation is about 5 meters high. ESA points to its “peculiar bumpy structure and fracture lines running through it that suggest erosional forces are working to fragment the comet’s boulders into smaller pieces.” The trail of debris behind it shows us how particles move about on the surface of the eroding comet.



A ‘Rosetta Stone’ for Super-Earths

by Paul Gilster on August 3, 2015

The discovery and confirmation of the exoplanet HD 219134b give us a useful touchstone relatively close to the Solar System. At 21 light years away in the constellation Cassiopeia, HD 219134b distinguishes itself by being the closest exoplanet to Earth to be detected using the transit method. That’s useful indeed, because we’ll be able to use future instruments like the James Webb Space Telescope to learn about the composition of any atmosphere there.


Image: This sky map shows the location of the star HD 219134 (circle), host to the nearest confirmed rocky planet found to date outside of our solar system. The star lies just off the “W” shape of the constellation Cassiopeia and can be seen with the naked eye in dark skies. It actually has multiple planets, none of which are habitable. Credit: NASA/JPL-Caltech/DSS.

Too close to its star to be considered a candidate for life, the new world is a ‘super-Earth,’ sighted by the HARPS-North instrument using radial velocity techniques, which measure the pull of the planet on its host star. HARPS-N was built by researchers at the University of Geneva and installed at the 3.6-meter Telescopio Nazionale Galileo on La Palma, in the Canary Islands. Ati Motalebi (UNIGE), lead author of the paper on this work, knew the significance of a potential transit in a world this close, a transit that radial velocity methods couldn’t uncover:

“When the first HARPS-N radial-velocity measurements indicated the presence of a 3-day planet around HD 219134, we immediately asked NASA for Spitzer space telescope time,” said Motalebi. “The idea was to check for a potential transit of the planet in front of the star, a mini eclipse, that would allow us to measure the size of the planet. To do this, we needed to go to space to reach the required precision.”

Initial readings were of a planet with a mass 4.5 times that of the Earth, orbiting the star — a K-dwarf a bit cooler and less massive than the Sun — in three days. Data from the Spitzer instrument then revealed the transit, its measurements indicating the planet to be about 1.6 times larger than the Earth. Combined with the earlier mass calculation, this radius yields a density of six grams per cubic centimeter, pointing to the possibility that this is a rocky planet. Michael Gillon (University of Liege), who led the Spitzer work, calls HD 219134b “a kind of Rosetta Stone for the study of super-Earths.”

HD 219134 turned out to host other worlds as well, with three additional longer-term planets discovered from the radial velocity work. The first of these is 2.7 times as massive as Earth, orbiting in a 6.8 day orbit. A second has 8.7 times Earth’s mass and orbits in 46.8 days. There is also a Saturn-class gas giant at 2.1 AU, orbiting the star in about 3 years.

The possibility exists that the two other inner planets are also transiting worlds, a prospect that has driven planning for future observations. Stéphane Udry (also at UNIGE) considers the potential:

“In particular, the future CHEOPS satellite of the European Space Agency (ESA), developed under Swiss leadership with a strong involvement of UNIGE and of the University of Bern, will provide the perfect tool for such observations. Being able to characterise three transiting super-Earths in a single bright and close system would provide incomparable constraints for planet formation and composition models, in particular for super-Earths.”


Image: Lightcurve of HD 219134b. Credit: UNIGE/NASA/JPL-Caltech.

So HD 219134b, and perhaps its two inner-orbit cousins, could turn out to be a goldmine for future study as we turn CHEOPS as well as the JWST instrument toward them. JWST should be able to use transmission spectroscopy techniques to analyze starlight passing through any planetary atmosphere, while ground-based high-resolution spectroscopy should tell us still more, and there is the prospect of direct imaging of the outer planet in a system this close with the planned giant telescopes in the planning stage, including the European Extremely Large Telescope, the Giant Magellan Telescope, and the Thirty Meter Telescope.

From the paper:

The quality of the measurements of the radius, mass and then mean density actually foreshadows what can be expected from the future transit missions in preparation that will target bright stars (CHEOPS, TESS, PLATO). We also know from Kepler results that multi-transiting systems of small-size planets are numerous. It is then now highly suitable to search for traces of transits of the other planets in the systems. Finally, even if a potential atmosphere around the planet is expected a priori to be tiny, the brightness of the system makes it worth trying to detect features of this atmosphere in the UV, visible and NIR, from space and from the ground, especially in preparation for future measurements with larger facilities…

The paper is Motalebi et al., “The HARPS-N Rocky Planet Search I. HD219134b: A transiting rocky planet in a multi-planet system at 6.5 pc from the Sun,” accepted at Astronomy & Astrophysics (preprint). Both JPL and UNIGE offer news releases.



Envisioning Starflight Failing

by Paul Gilster on July 31, 2015

Science fiction has always had its share of Earthside dystopias, but starflight’s allure has persisted, despite the dark scrutiny of space travel in the works of writers like J. G. Ballard. But what happens if we develop the technologies to go to the stars and find the journey isn’t worth it? Gregory Benford recently reviewed a novel that asks these questions and more, Kim Stanley Robinson’s Aurora (Orbit Books, 2015). A society that reaches the Moon and then turns away from it may well prompt questions on how it would react to the first interstellar expedition. Benford, an award-winning novelist, has explored star travel in works like the six novels of the Galactic Center Saga and, most recently, in the tightly connected Bowl of Heaven and Shipstar. His review is a revised and greatly expanded version of an essay that first ran in Nature.

by Gregory Benford


Human starflight yawns as a vast prospect, one many think impossible. To arrive in a single lifetime demands high speeds approaching lightspeed, especially for target stars such as Tau Ceti, about twelve light years away.

Generation ships form the only technically plausible alternative method, implying large biospheres stable over centuries. Or else a species with lifetimes of centuries, which for fundamental biological reasons seems doubtful. (Antagonistic plieotropy occurs in evolution, ie, gene selection resulting in competing effects, some beneficial in the short run for reproduction, but others detrimental in the long.) So for at least for a century or two ahead of us, generation ships (“space arks”) may be essential.

Aurora depicts a starship on a long voyage to Tau Ceti four centuries from now. It is shaped like a car axle, with two large wheels turning for centrifugal gravity. The biomes along their rims support many Earthly lifezones which need constant tending to be stable. They’re voyaging to Tau Ceti, so the ship’s name is a reference to Isaac Asimov’s The Robots of Dawn, which takes place on a world orbiting Tau Ceti named Aurora. Arrival at the Earthlike moon of a super-Earth primary brings celebration, exploration, and we see just how complex an interstellar expedition four centuries from now can be, in both technology and society.

In 2012, Robinson declared in a Scientific American interview that “It’s a joke and a waste of time to think about starships or inhabiting the galaxy. It’s a systemic lie that science fiction tells the world that the galaxy is within our reach.” Aurora spells this out through unlikely plot devices. Robinson loads the dice quite obviously against interstellar exploration. A brooding pessimism dominates the novel.

There are scientific issues that look quite unlikely, but not central to the novel’s theme. A “magnetic scissors” method of launching a starship seems plagued with problems, for example. But the intent is clear through its staging and plot.

I’ll discuss the quality of the argument Aurora attempts, with spoilers.

Plot Fixes


The earlier nonfiction misgivings of physicist Paul Davies (in Starship Century) and biologist E.O. Wilson (in The Meaning of Human Existence) about living on exoplanets echo profoundly here. As a narrator remarks, “Suspended in their voyage as they had been, there had never been anything to choose, except methods of homeostasis.” Though the voyagers in Aurora include sophisticated biologists, adjusting Earth life to even apparently simple worlds proves hard, maybe impossible.

The moon Aurora is seemingly lifeless. Yet it has Earth-levels of atmospheric oxygen, which somehow the advanced science of four centuries hence thinks could have survived from its birth, a very unlikely idea (no rust?—this is, after all, what happened to Mars). Plot fix #1.

This elementary error, made by Earthside biologists, brings about the demise of their colony plans, in a gripping plot turn that leads to gathering desperation.

The lovingly described moon holds some nanometers-sized mystery organism that is “Maybe some interim step toward life, with some of the functions of life, but not all…in a good matrix they appear to reproduce. Which I guess means they’re a life-form. And we appear to be a good matrix.” So a pathogen evolved on a world without biology? Plot fix #2.

Plans go awry. Backup plans do, too. “Vector, disease, pathogen, invasive species, bug; these were all Earthly terms…various kinds of category error.”

What to do? Factions form amid the formerly placid starship community of about 2000. Until then, the crew had felt themselves to be the managers of biomes, farming and fixing their ship, with a bit of assistance from a web of AIs, humming in the background.

Robinson has always favored collective governance, no markets, not even currencies, none of that ugly capitalism—yet somehow resources get distributed, conflicts get worked out. No more. Not here, under pressure. The storyline primarily shows why ships have captains: stress eventually proves highly lethal. Over half the crew gets murdered by one faction or another. There is no discipline and no authority to stop this.

Most of the novel skimps on characters to focus on illuminating and agonizing detail of ecosphere breakdown, and the human struggle against the iron laws of island biogeography. “The bacteria are evolving faster than the big animals and plants, and it’s making the whole ship sick!” These apply to humans, too. “Shorter lifetimes, smaller bodies, longer disease durations. Even lower IQs, for God’s sake!”

Robinson has always confronted the nasty habit of factions among varying somewhat-utopian societies. His Mars trilogy dealt with an expansive colony, while cramped Aurora slides toward tragedy: “Existential nausea comes from feeling trapped… that the future has only bad options.”

Mob Rules

Should the ship return to Earth?

Many riots and murders finally settle on a bargain: some stay to terraform another, Marslike world, the rest set sail for Earth. The ship has no commander or functional officers, so this bloody result seems inevitable in the collective. Thucydides saw this outcome over 2000 years ago. He warned of the wild and often dangerous swings in public opinion innate to democratic culture. The historian described in detail explosions of Athenian popular passions. The Athenian democracy that gave us Sophocles and Pericles also, in a fit of unhinged outrage, executed Socrates by a majority vote of one of its popular courts. (Lest we think ourselves better, American democracy has become increasingly Athenian, as it periodically whips itself up into outbursts of frantic indignation.)

When discord goes deadly in Aurora, the AIs running the biospheres have had enough. At a crisis, a new character announces itself: “We are the ship’s artificial intelligences, bundled now into a sort of pseudo-consciousness, or something resembling a decision-making function.” This forced evolution of the ship’s computers leads in turn to odd insights into its passengers: “The animal mind never forgets a hurt; and humans were animals.” Plot Fix #3: sudden evolution of high AI function that understands humans and acts like a wise Moses.

This echoes the turn to a Napoleonic figure that chaos often brings. As in Iain Banks’ vague economics of a future Culture, mere humans are incapable of running their economy and then, inevitably, their lives. The narrative line then turns to the ship AI, seeing humans somewhat comically, “…they hugged, at least to the extent this is possible in their spacesuits. It looked as if two gingerbread cookies were trying to merge.”

Governance of future societies is a continuing anxiety in science fiction, especially if demand has to be regulated without markets, as a starship must. (Indeed, as sustainable, static economies must.) As far back as in Asimov’s Foundation, Psychohistory guides, because this theory of future society is superior to mere present human will. (I dealt with this, refining the theory, in Foundation’s Fear. Asimov’s Psychohistory resembled the perfect gas law, which makes no sense, since it’s based on dynamics with no memory; I simply updated it to a modern theory of information.) The fantasy writer China Mieville has similar problems, with his distrust of mere people governing themselves, and their appetites, through markets; he seems to favor some form of Politburo. (So did Lenin, famously saying “A clerk can run the State.”)

Aurora begins with a society without class divisions and exploitation in the Marxist sense, and though some people seem destined to be respected and followed, nothing works well in a crisis but the AIs—i.e., Napoleon. The irony of this doesn’t seem apparent to the author. Similar paths in Asimov, Banks and Mieville make one wonder if similar anxieties lurk. Indeed, Marxism and collectivist ideas resemble the similar mechanistic theory of Freudian psychology (both invented by 19th C. Germans steeped in the Hegelian tradition)—insightful definitions, but no mechanisms that actually work. Hence the angst when things go wrong with a supposedly fundamental theory.

The AIs, as revealed through an evolving and even amusing narrative voice, follow human society with gimlet eyes and melancholy insights. The plot armature turns on a slow revelation of devolution in the ship biosphere, counterpointed with the AI’s upward evolution—ironic rise and fall. “It was an interrelated process of disaggregation…named codevolution.” The AIs get more human, the humans more sick.

Even coming home to an Earth still devastated by climate change inflicts “earthshock” and agoraphobia. Robinson’s steady fiction-as-footnote thoroughness brings us to an ending that questions generational, interstellar human exploration, on biological and humanitarian grounds. “Their kids didn’t volunteer!” Of course, immigrants to far lands seldom solicit the views of their descendants. Should interstellar colonies be different?

Do descendants as yet unborn have rights? Ben Finney made this point long ago in Interstellar Migration, without reaching a clear conclusion. Throughout human history we’ve made choices that commit our unborn children to fates unknown. Many European expeditions set sail for lands unseen, unknown, and quite hostile. Many colonies failed. Interstellar travel seems no different in principle. Indeed, Robinson makes life on the starship seem quite agreeable, though maybe tedious, until their colony goal fails.

The unremitting hardship of the aborted colony and a long voyage home give the novel a dark, grinding tone. We suffer along with the passengers, who manage to survive only because Earthside then develops a cryopreservation method midway through the return voyage. So the deck is stacked against them—a bad colony target, accidents, accelerating gear failures, dismay… until the cryopreservation that would lessen the burden arrives, very late, so our point of view characters do get back to Earth and the novel retains some narrative coherence, with character continuity. Plot Fix #4.

This turn is an authorial choice, not an inevitability. Earthsiders welcome the new cryopreservation technologies as the open door to the stars; expeditions launch as objections to generation ships go away. But the returning crew opposes Earth’s fast-growing expeditions to the stars, because they are just too hard on the generations condemned to live in tight environments—though the biospheres of the Aurora spacecraft seem idyllic, in Robinson’s lengthy descriptions. Plainly, in an idyllic day at the beach, Robinson sides with staying on Earth, despite the freshly opened prospects of humanity.

So in the end, we learn little about how our interstellar future will play out.

The entire drift of the story rejects Konstantin Tsiolkovsky’s “The Earth is the cradle of mankind, but humanity cannot live in the cradle forever.” – though we do have an interplanetary civilization. It implicitly undermines the “don’t-put-all-your-eggs-in-one-basket” philosophy for spreading humanity beyond our solar system. Robinson says in interviews this idea leads belief that if we destroy Earth’s environment, we can just move. (I don’t know anyone who believes this, much less those interested in interstellar exploration.) I think both ideas are too narrow; expansion into new realms is built into our evolution. We’re the apes who left Africa.

Robinson takes on the detail and science of long-lived, closed habitats as the principal concern of the novel. Many starship novels dealt with propulsion; Robinson’s methods—a “magnetic scissors” launch and a mistaken Oberth method of deceleration—are technically wrong, but beside the point. His agenda is biological and social, so his target moon is conveniently hostile. Then the poor crew must decide whether to seek another world nearby (as some do) or undertake the nearly impossible feat of returning to Earth. This deliberately overstresses the ship and people. Such decisions give the novel the feel of a fixed game. Having survived all this torment, the returning crew can’t escape the bias of their agonized experience.

Paul Davies pointed out in Starship Century that integrating humans into an existing alien biosphere (not a semi-magical disaster like his desolate moon with convenient oxygen) is a very hard task indeed, because of the probable many incompatibilities. That’s a good subject for another novel, one I think no one in science fiction has taken up. This novel avoids that challenge with implausible Plot fix #2.

Realistically considered, the huge problems of extending a species to other worlds can teach us about aliens. If interstellar expansion is just too hard biologically (as Paul Davies describes) then the Fermi paradox vanishes (except for von Neumann machines, as Frank Tipler saw in the 1970s). If aliens like us can’t travel, maybe they will expend more in SETI signaling? Or prefer to send machines alone? An even-handed treatment of human interstellar travel could shore up such ideas.

Still, a compelling subject, well done in Robinson’s deft style. My unease with the novel comes from the stacked deck its author deals.