Centauri Dreams
Imagining and Planning Interstellar Exploration
Exomoons: A Direct Imaging Possibility
It’s good to see that David Kipping’s work on exomoons is back in the popular press in the form of A Harvest of New Moons, an article in The Economist. Based at the Harvard-Smithsonian Center for Astrophysics, Kipping’s Hunt for Exomoons with Kepler (HEK) culls Kepler data and massages the information, looking for the tug of large moons on transiting exoplanets. The basic method will by now be familiar to Centauri Dreams readers:
Dr Kipping’s technique relies on the fact that moons do not simply revolve around their host planets; planets also revolve around their moons—or, rather, the two bodies both revolve around their common centre of mass. If a planet is large and its moon small the distinction is trivial. But if the planet is small and the moon is large, it is not. In the case of Earth and its moon, for example, the common centre lies only around 1,700km (1,100 miles) beneath the Earth’s surface. Someone looking from afar at the movement of Earth would thus be able to deduce the moon’s existence without having to see it directly.
And as we’ve discussed in previous articles, the need for a large moon is significant. Kipping has recently reckoned that a moon about one-fifth as massive as the Earth should be in range of detection, but Ganymede, the biggest moon in our Solar System, is only 1/40th as massive. The first exomoon detected, then, will likely be a very large object, big enough that its signal won’t be masked by the presence of other planets in the same system. I’ve always had a fascination with exomoon studies and thus am looking forward to the first presentation and data analysis by the HEK team, slated to occur at the American Astronomical Society meeting in January.
Image: Exomoon hunter David Kipping. Credit: CfA.
But I want to focus in on something else in this article, namely the work of Mary Anne Peters and Edwin Turner, who have asked in a recent paper whether a large enough exomoon orbiting close enough to its planet (and far enough from its star) might produce an infrared signature detectable from Earth. Think Io, and ponder how tidal heating churns the insides of such a moon, creating heat-generating friction and, in the case of Io, active volcanoes.
Peters and Turner, both at Princeton, produce an acronym I had never encountered before: THEMs, or Tidally Heated Exomoons. In terms of direct imaging, a THEM is quite interesting. For one thing, a tidally heated moon may remain hot and bright for the lifetime of its star, making it visible in solar systems both old and young. For another, such a moon may orbit its planet far away from the primary star while remaining hot because of tidal heating. It is thus a luminous target at a large separation from the star whose light would otherwise drown out its signal.
The researchers calculate that tidal heating could produce terrestrial-planet-sized moons with effective temperatures as high as 1000 K, moons as much as 0.1% as bright as the system’s primary (if the central star were low in mass). This is interesting stuff: Io has the highest measured temperatures of any body in the outer Solar System because of the tidal heating effect. Now imagine the system of Galilean moons orbiting Jupiter were scaled down to orbit Neptune in more or less the same configuration. If this were the case, Io would be more luminous than Neptune itself, and if it were as massive and dense as the Earth, the authors say it would be the brightest Solar System object beyond 5 AU, outshining even Jupiter at some wavelengths.
Given all this, there is a case to be made that tidally heated exomoons may actually be easier targets for direct imaging than the kind of hot, young gas giants at large separations from their star that are most likely to be found by current imaging efforts:
Direct imaging detection of physically plausible, tidally heated exomoons is possible with existing telescopes and instrumentation. If tidally heated exomoons are common, for example if typical gas giant exoplanets are orbited by satellite systems broadly similar to those found in the Solar System, we are likely to be able to image them around nearby Sun-like stars in the midst of their main sequence lifetimes with current or near future facilities.
The paper suggests that existing instruments can detect exomoons at temperatures of 600 K or above and radii of Earth-size or larger, while future mid-infrared space telescopes like the James Webb Space Telescope will be able to directly image heated exomoons with temperatures greater than 300 K and radii of Earth size and larger as long as they are orbiting at 12 AU or more from their star. Perhaps we’ve imaged our first exomoon already: The authors think it is possible that Fomalhaut b, whose identity as a planet remains controversial, is actually a tidally heated exomoon, or a blend of the emissions from a hot, young gas giant and a heated moon.
In one figure, the researchers look at the performance of JWST’s Mid-Infrared Instrument (MIRI), charting exomoons with temperatures similar to those found on Earth. The results:
…it is plausible that some of the exomoons JWST is capable of detecting, could potentially be habitable, in the sense of having surface temperatures that would allow liquid water to be present. Some of these exomoons have comparable irradiance to the gas giants in our solar system. At ~ 14µm and 300K, an Earth-radius exomoon would be as luminous as Jupiter. However, if [the] Jupiter were colder due to being less heated by its primary and/or being older, the Earth-like moon would be much brighter than the planet.
The paper is Peters and Turner, “On the Direct Imaging of Tidally Heated Exomoons” (preprint).
Possible Habitable World in a Six-Planet System
At 42 light years from Earth, the star HD 40307 is reasonably within the Sun’s neighborhood, so the news of a potentially habitable planet there catches the eye. HD 40307 is a K-class dwarf, one previously known to be orbited by three super-Earths — with masses between the Earth and Neptune — that are too close to the star to support liquid water on the surface. Now we have the discovery, announced in a new paper in Astronomy & Astrophysics, of three more super-Earth candidates found by digging into data from HARPS (the High Accuracy Radial Velocity Planet Searcher) and HIRES (the High Resolution Echelle Spectrograph).
Mikko Tuomi (University of Hertfordshire) and team put a new software tool called HARPS-TERRA to work on the archival data that allowed them greater precision in filtering out false positives from stellar activity. Says Tuomi:
“We pioneered new data analysis techniques including the use of the wavelength as a filter to reduce the influence of activity on the signal from this star. This significantly increased our sensitivity and enabled us to reveal three new super-Earth planets around the star known as HD 40307, making it into a six-planet system.”
The outermost planet of the six is the one to watch. HD 40307’s habitable zone lies between 0.43 and 0.85 AU. With an orbital semi-major axis of 0.60 AU, candidate planet HD 40307 g is in the sweet spot, receiving about 62 percent of the radiation the Earth receives from the Sun. The paper notes that while that level of radiation may seem low as compared to the Earth, our planet actually lies fairly close to the inner boundary of the Sun’s habitable zone.
Image: An artist’s impression of HD 40307 g. Credit: University of Hertfordshire.
Unfortunately, we aren’t likely to get any help from transits here, as previous attempts to detect the inner candidate HD 40307 b have been unsuccessful, and the likelihood of a transit by HD 40307 g is considered to be about 0.6 percent. But the planet is enticing. From the paper:
As is the case with the previously reported planets in the HZs of nearby stars, additional observational information is necessary – on top of con?rming the planetary nature of the three new signals we report in this work – to decide if HD 40307 g indeed can support water on its surface. In the meantime, and given that it would be a rather massive object for a telluric planet, detailed climatic (e.g. Barnes et al., 2012) and planetary interior simulations (e.g. Korenaga, 2010; Stein et al., 2011) can be used to estimate its suitability for supporting liquid water and, perhaps, life. Unlike the other previously reported candidate habitable planets around nearby M dwarfs (e.g., GJ 581 and GJ 667C), HD 40307 g is located farther away from the central star and, like the habitable zone planet candidate Kepler 22b (Borucki et al., 2012), it should not su?er from tidal locking either (Kasting et al., 1993; Barnes et al., 2009b).
HD 40307 g has a minimum mass of about 7 Earth masses, and according to the researchers, it is so deeply nested inside the habitable zone that even with a slightly eccentric orbit, it would remain inside the HZ for a full orbital cycle. In a University of Hertfordshire news release, researcher Guillem Angla-Escude notes that “The star HD 40307, is a perfectly quiet old dwarf star, so there is no reason why such a planet could not sustain an Earth-like climate.” Meanwhile, planet formation theorists will have plenty to work with as they add the system around HD 40307 into the exoplanet mix. Its architecture is different enough from our own — with the tight packing of six planets inside 0.60 AU — that a range of formation possibilities come into play as we ponder how Earth-mass objects eventually emerge in their star’s habitable zones.
Moreover, whereas the interesting and potentially habitable Kepler 22-b is located about 200 pc from Earth, HD 40307 is close enough that the star-planet angular separation should be workable for future missions like ESA’s Darwin or NASA’s Terrestrial Planet Finder, assuming these find their way through budget hurdles and eventually get built. If HD 40307 g can be confirmed, we could be talking about the first habitable planet to be observed at this level.
A Digression on Delayed Missions
Let’s pause on this for a moment as I recall Michael Lemonick’s recent op-ed in the Los Angeles Times. Lemonick, whose new book Mirror Earth is just out (Walker & Company, 2012), runs through what was supposed to have happened with space-based observatories. By the early 2000s, the Space Interferometry Mission was going to bring new precision to exoplanet finding, while the James Webb Space Telescope, then planned as bigger and more powerful than the one under construction, was supposed to be launched and operational by 2007. Terrestrial Planet Finder, designed to image those ‘mirror Earths,’ was banked for 2020.
Says Lemonick:
As of today, however, SIM has been canceled; the smaller, less powerful Webb will launch by 2018 perhaps; and the TPF has been put on the back burner, maybe permanently. These disappointments have partly to do with NASA’s ever-shrinking science budget, but SIM and TPF were also torpedoed by internal squabbling among scientists who disagreed about the best designs and about whether SIM was vital or unnecessary.
To be sure, Lemonick credits the ‘scrappy ingenuity’ that has produced not just Kepler but refinements to Earth-bound radial velocity methods of the kind that produced Centauri B b, not to mention the citizen scientists pooling their transit skills on planethunters.org. We can hope the setbacks are only temporary, but it gives me pause to think how many discoveries we could be making — including not so far off imaging of a planet like HD 40307 g — if missions like these and ESA’s Darwin had a stable line of development.
Comparing Habitable Zone Planets
But back to HD 40307 and its planets. The researchers review current thinking on the few habitable zone planets we’ve found thus far and measure HD 40307 g against them. GJ 581 d and HD 88512 b orbit at the outer edge of the habitable zone of their stars, although mechanisms involving clouds of CO2 or water could create habitable models [NOTE: See andy’s comment below, correcting me about HD 85512 b]. Both Kepler-22 b and GJ 667C c orbit well within the habitable zone, with Kepler-22 b possibly a small version of Neptune rather than a rocky planet with a solid surface. The paper continues:
…the precise natures of GJ 667C c and HD 40307 g are unknown and they could also be scaled down versions of Neptune-like planets with thick atmospheres and solid cores. Compared to the candidates orbiting nearby low-mass stars (GJ 581 d, GJ 667C c, HD 85512 b), HD 40307 g is not likely to su?er from tidal-locking – which improves its chances of hosting an Earth-like climate. Like for the other HZ candidates, it is not yet possible to determine its physical and geochemical properties and a direct-imaging mission seems to be the most promising way of obtaining observational information on these properties.
So the work ahead is first to verify the existence of the three new planets and get a better fix on the orbits and minimum masses of each. The paper points out that the dense dynamical packing in this system means that the planet candidates around HD 40307 are likely to have masses close to their minimum values, but additional measurements are clearly in the cards.
The paper is Tuomi et al., “Habitable-zone super-Earth candidate in a six-planet system around the K2.5V star HD 40307,” accepted at Astronomy & Astrophysics, available online.
Re-Envisioning the Telescope
An inventor named Tom Ditto has been casting a serious look at diffraction gratings as large primary collectors for telescopes, work that has been getting a bit of a buzz on the Internet. See, for example, An Old Idea Gives Telescopes a New Twist, and ponder how much the eponymous Dittoscope sounds like something out of a Tom Swift story. Nonetheless, an instrument based on a diffraction grating as its primary light-gathering source may prove useful in a variety of astronomical settings, including the ability to produce extremely high quality spectroscopic information for radial velocity exoplanet searches.
Diffraction happens when a small obstacle or opening causes a wave of light to interfere with itself, creating patterns that depend on the size of the diffracting object and the size of the wave. A diffraction grating, in this case a flat surface with a regular pattern of grooves, can be used to separate different wavelengths of light, which will interfere at different angles. The use of diffraction gratings in astronomy dates from 1786, when the American clockmaker and astronomer David Rittenhouse ran experiments on the behavior of diffracted light. Joseph von Fraunhofer would later use dispersion by diffraction grating to resolve atomic lines of sunlight and starlight. Spectroscopy has made use of diffraction gratings to study the stars ever since.
Image: Diffraction at work. The grooves of a compact disc can act as a grating and produce iridescent reflections. Credit: Wikimedia Commons.
In a presentation to the NASA Institute for Advanced Concepts back in 2007, Ditto showed that his Dittoscope could be conceptualized as a standard telescope capable of spectroscopy aimed at a flat grating. The standard telescope’s spectrometer has its own grating and slit. The use of the second spectrometer eliminates the overlapping spectra from the flat primary grating. Each object is thus imaged at a single wavelength at any unique angle of incidence. There are no moving parts other than the rotating Earth, with the instrument oriented east to west.
The ground based Dittoscope, then, takes advantage of the Earth’s rotation, as described in the presentation for Ditto’s Phase I study: “The precession of objects in the night sky causes their incident angles to rotate. For any incident angle there is a corresponding wavelength, so an entire spectrogram can be assembled over the course of a night.” With this enormous field of view — a 40 degree arc — millions of stars are placed within view simultaneously:
Since the output of the telescope is spectrographic, multiple object spectrometer problems are no longer the vexing issue of conventional telescopy. Every object has its spectrum taken. Stars do not need to be localized in advance of taking their spectra. There are spectral signatures for all objects in sight from first light.
Rather than tracking individual stars, the Dittoscope would simply let stars pass over the grating to detect the spectral changes of different wavelengths. As an online paper on this work notes: “There are expenses associated with the mounts, but the mechanical complexity is reduced to a single axis which is static during observation runs…” From a practical standpoint, small early experiments can be expanded in incremental steps until a full observatory is constructed.
Image: The roof is coming off the observatory. Gone are the domes, the sliding hatch doors and the rotating walls. A Dittoscope can lay flat to the ground. Its roof may be the primary objective. Wind resistance is negligible. The secondary optics are buried in a trough, and the ray paths can be protected within a pacified atmosphere, even a vacuum. Credit: Tom Ditto.
Note the other advantages that accrue from Ditto’s implementation of the idea. Because the diffracting grating primary collector is flat, many of the size constraints imposed on standard telescopes are eased, especially the requirement to contain heavy mirrors. Ditto’s paper argues that tolerance specifications for flatness in the axis of diffraction may not be prohibitive — he even talks about plate glass as a possible medium. All this points to the ability to construct enormous collecting surfaces at relatively low cost. One possibility Ditto mentions is a lunar observatory at the Moon’s equator that has no moving parts, returns detailed spectra for all objects along the zenith, and operates with a service life of decades.
Now think of this in terms of other space-based platforms. Theoretically, it would be possible to develop a collecting surface that can be stowed on board a launch vehicle in the form of rolls of kilometer-length membrane — Ditto evidently experimented in his prototypes with methods for printing diffraction gratings out on various kinds of industrial material packaging. Once in space, the rolls of collector membrane could be deployed by induced centrifugal forces.
Moreover, the Dittoscope can be segmented so that collectors of an arbitrarily large size can be constructed. As the system is deployed, it can be added to incrementally to achieve the needed specifications. Ditto explains the construction of a large, ground-based instrument: :
A very large collector can be constructed from thousands of identical smaller gratings on individual supports… Each table could support glass substrates for the grating elements. Piezo positioners or other micro-adjusters would be used to slide gratings into alignment relative to each other. Alignment of disparate segments could be achieved using a laser that targeted a test patch on each grating. The laser would work like a collimating laser. The adjusters would be used so that a calibration wave length appeared superimposed at the same position of the secondary sensors.
Huge projects thus become possible:
This suggests planet finder projects where telescopes grow incrementally, starting with demonstration models in the 100 sq. meters, followed by working versions for bright stars in the 10,000 sq. meter scale, and finally imaging faint targets using square kilometers of collector built from modules as small as 1 sq. meter.
Ditto sees his instrument as an exoplanet detection tool that would provide the spectrographic information needed to do radial velocity work of extraordinarily high quality. Because the secondary can be a conventional telescope, the primary collector — the diffraction grating — can be located in a different place. We can envision a secondary telescope based on the ground being fed by a primary collector in Earth orbit, a system that would allow the collector to be shared among many existing ground-based telescopes.
Ditto’s intriguing proposal to the NASA Institute for Advanced Concepts was funded in 2006, allowing the construction of early prototypes. A Phase II grant proposal is now in the works. The Phase I proposal is “Primary Objective Grating Astronomical Telescope,” available at the NIAC site. Tom Ditto’s paper “The Dittoscope” explains the concept online. Thanks to John Kilis and Jason Wentworth, who provided invaluable pointers to this work.
Vesta: A Protoplanet’s Mutable Surface
I remember having a particularly strong ‘sense of wonder’ moment when reading Poul Anderson’s “The Snows of Ganymede” when I was a kid. Anderson was good at this kind of thing, but really my reaction was not just to this story but to the whole notion of taking a distant astronomical object and placing people in it. A bright point in the telescope suddenly becomes a landscape and you feel your sense of scale – the sheer immensity of things – beginning to shift under your feet. These thoughts are triggered by the latest news from the Dawn mission about Vesta, and a UCLA news release commenting on the asteroid’s steep topography, which often leads to landslides.
Immediately I was thinking of stark drops and boulder-strewn regolith with no friendly blue/green Earth in the sky and wondering what it would be like to see Vesta in person. Thus dreams accumulate. Ray Bradbury found that attaching names to unknown places is a distinctively human enterprise, and one that when abandoned suggested deep change in the psyche (see “Dark They Were and Golden-Eyed,” one of Ray’s superb Mars stories, for more on this). We’re well along in naming features on the worlds our space probes have revealed to us, but standing in the imagination amidst these forbidding landscapes is what really makes some of the deep space imagery sing to me. That feeling is also, of course, a prime reason for reading science fiction.
Image: Vesta’s surface as imaged by the Dawn spacecraft. Credit: NASA/JPL.
The frisson of deep space brings many into the field, but thankfully the science holds just as deep a sway on the imagination. About Vesta we’ve learned the unexpected fact that the asteroid — or protoplanet, as you choose – has a range of brightness that is about as extreme as any rocky object yet found in the Solar System. The tiny world’s variation in dark and light areas has been under intense scrutiny, with the finding that the widely distributed dark areas — apparently not correlated with underlying geological features — are likely material from carbon-rich asteroids. Thus Christopher Russell (UCLA, and Dawn principal investigator):
“Ever since Dawn arrived at Vesta [in July 2011] and we saw the bright and dark streaks across the surface, we have wondered how the zebra got her stripes. Now we know that the bright streaks and spots are due to very pure early Vestan material, and the dark patches are deposits on the surface most probably due to collisions with material from the dark outer reaches of the asteroid belt.”
The estimate is that to arrive at what we see on Vesta today, approximately 300 dark asteroids between 1 and 10 kilometers in diameter have struck the protoplanet during the last 3.5 billion years. The researchers, whose work appeared on November 1 in Nature, believe that enough accumulated to produce between 1 and 2 meters of material on the surface. Because Vesta is thought to have formed at the very beginning of the Solar System, it is a place that records the system’s formation some 4.5 billion years ago, and a surface that has had plenty of time to accumulate debris from impacts.
Meanwhile, it’s interesting that while we have observed a form of ‘weathering’ on the Moon and on various asteroids, Vesta’s surface seems to be continually renewed. The weathering is thought to be the result of the accumulation of metallic particles containing iron, and we would expect these to dull the outer layers of the surface of Vesta. The researchers say, however, that Dawn’s visible and infrared mapping spectrometer (VIR) and framing camera find no such accumulation. Here the steep topography mentioned above comes into play, leading to landslides that mix surface materials. Impacts large and small should also mix the outer layers over time.
Now that Dawn is on its way to Ceres, the largest object in the main belt, keep in mind that this is one of the darkest of the asteroids, meaning we’re likely to learn more there about the dark materials that have had such an effect on Vesta. Ceres also beckons because it is thought to hold water or ice in substantial amounts under its crust, adding to its astrobiological interest. Operations around Ceres begin in 2015, at which time our imaginations will be able to seize on yet another set of deep space images showing us terrain features in need of names, out of which we can fashion a new set of dreams.
The papers are Pieters et al., “Distinctive space weathering on Vesta from regolith mixing processes,” Nature 491 (01 November 2012), pp. 79-82 (abstract) and McCord et al., “Dark material on Vesta from the infall of carbonaceous volatile-rich material,” Nature 491 (01 November 2012), pp. 83-86 (abstract).
Astrobiology: The Necessity of Asteroids
Let’s talk this morning about the ‘snow line,’ the boundary in the Solar System beyond which volatiles like water ice remain cold enough to keep intact. Rebecca Martin (University of Colorado) and Mario Livio (Space Telescope Science Institute) have been running simulations using models of planet-forming disks around young stars. The idea: To calculate the location of the snow line in these disks as measured against the mass of the central star. Their hypothesis is that asteroid belts in other solar systems will be located at the snow line, with implications for life.
Here’s the thinking on this. We know that asteroids, in addition to creating impact threats that can trigger world-changing events, may also have had a crucial role delivering water and organic compounds to the early Earth. Occasional asteroid impacts, says the theory of punctuated equilibrium, may have accelerated biological evolution, forcing species to adapt to rapidly changing conditions. And there are still other possibilities, as the paper notes (I’ve removed the internal citations for brevity):
The formation of large moons may also require an asteroid collision…although here again, a different origin for the impactor has been considered…Our Moon, for instance, stabilises the rotation axis of the Earth and prevents weather extremes that would have resulted from chaotic motion. This process may not be universal, since it depends on initial conditions. According to some hypotheses, life itself may have been delivered to Earth by an asteroid… Heavy elements, including some that are essential for life, were also probably delivered to the Earth’s crust through collisions… During the early times of formation the Earth was molten and its gravity pulled heavy elements to its core leaving the crust depleted of elements such as iron, gold and platinum.
The authors point out that if even one of these ideas is true, then the formation and evolution of asteroid belts may play a major role in the development of complex life.
But gas giants are also a major part of this picture. In our system, the gravitational influence of Jupiter plays a role in the development of the asteroid belt. The planet’s presence just beyond the snow line meant that nearby material inside its orbit could not form into planets, but instead settled into the belt of fragmented rocks that we see today. Livio explains it like this:
“To have such ideal conditions you need a giant planet like Jupiter that is just outside the asteroid belt [and] that migrated a little bit, but not through the belt. If a large planet like Jupiter migrates through the belt, it would scatter the material. If, on the other hand, a large planet did not migrate at all, that, too, is not good because the asteroid belt would be too massive. There would be so much bombardment from asteroids that life may never evolve.”
Image (click to enlarge): This illustration shows three possible scenarios for the evolution of asteroid belts. In the top panel, a Jupiter-size planet migrates through the asteroid belt, scattering material and inhibiting the formation of life on planets. The second scenario shows our solar-system model: a Jupiter-size planet that moves slightly inward but is just outside the asteroid belt. In the third illustration, a large planet does not migrate at all, creating a massive asteroid belt. Material from the hefty asteroid belt would bombard planets, possibly preventing life from evolving. Illustration Credit: NASA, ESA, and A. Feild (STScI). Science Credit: NASA, ESA, R. Martin and M. Livio (STScI).
Back to the simulations. Martin and Livio put their models to work to test their proposition that asteroid belts in other solar systems would be located in the vicinity of the snow line. They examined 90 stars known from observations with the Spitzer Space Telescope to have warm dust that could indicate the presence of an asteroid belt, finding that the temperature of the dust was consistent with the snow line around these stars. The researchers also studied 520 giant planets outside our Solar System, finding only 19 of them residing outside the snow line.
The suggestion is that most of the giant planets that originally formed outside the snow line have migrated too far inward to preserve the kind of asteroid belt that would enhance evolution on inner, terrestrial worlds. The ideal circumstance is a gas giant placed just outside the snow line, like Jupiter, where the planetesimals of an early solar system are too gravitationally disturbed to coalesce into planets. Only four percent of the observed gas giants are found in this position.
If Martin and Livio are right, then our best chance to find living worlds is to find systems where gas giant planets exist outside the snow line. But even here, the conditions are narrowly defined:
Inward migration disrupts the asteroid belt. Terrestrial planets may still form from scattered planetesimals in the habitable zone after the migration, but if asteroid belts are indeed necessary for complex life to evolve, then such evolution is unlikely in these systems. On the other hand, a small amount of giant planet migration may be necessary to remove a significant fraction of the initial belt mass because otherwise there would be too many devastating impacts on the planet
for life to evolve. Consequently, there appears to be a very narrow “window of opportunity” of time during which the giant planet should form, in order for the correct amount of migration to take place – potentially making our solar system even more special.
In other words, too populous an asteroid belt would lead to too many impacts on inner worlds, while the lack of an asteroid belt could mean complex life would be unlikely to develop on these worlds in the first place. The placement of the gas giant becomes a critical parameter.
The authors are quick to note that selection effects may be at play in their data. While our radial velocity techniques are improving all the time, it is still difficult to observe planets in long-period orbits, which makes it likely that, over time, we will find more gas giants outside the snow line than the four percent referred to above. Whatever the actual percentage of gas giants outside the snow line turns out to be, the authors argue that systems with a gas giant outside an asteroid belt will still be the best places to look for complex life.
The paper is Martin and Livio, “On the formation and evolution of asteroid belts and their potential significance for life,” accepted for publication in Monthly Notices of the Royal Astronomical Society (preprint). A Royal Astronomical Society news release is also available.
G-Class Outliers: Musings on Intelligent Life
Because I had my eyes dilated yesterday afternoon en route to learning whether I needed new reading glasses (I do), I found myself with blurry vision and, in the absence of the ability to read, plenty of time to think. Yesterday’s post examined a paper by a team led by Jack T. O’Malley-James (University of St Andrews, UK), addressing the question of how our planet will age, and specifically, how life will hang on at the single-cell level into the remote future. It’s interesting stuff because of its implications for what we may find around other stars and I pondered it all evening.
Have a look at one of the figures from the O’Malley-James paper, which shows the stages a habitable Earth-like planet (ELP) will pass through as it ages around main sequence stars. I also clip the caption directly from the paper.
Image: Time windows for complex and microbial life on Earth analogue planets orbiting Sun-like stars (F(7), G and K(1) stars) during their main sequence lifetimes. Assuming that the processes leading to multicellular life are the same as on Earth (i.e. 1 Gyr for life to emerge and 3 Gyr for multicellularity to evolve), the potential lifespans of a more complex, multicellular biosphere are estimated. Multicellular life was assumed able to persist until surface temperatures reach the moist greenhouse limit for an Earth analogue planet in the continuously habitable zone of the star-type in question. Microbial life is then assumed to dominate until either the maximum temperature for microbial life is exceeded, or until the star leaves the main sequence (whichever happens first). The average age of Earth-like planets was found by Lineweaver (2001) to be 6.4 +/– 0.9 Gyr based on estimates of the age distribution of terrestrial planets in the universe. This average age falls within microbial and uninhabitable stages for G and F type stars respectively, but falls within the multicellular life stage for K stars. Credit: Jack T. O’Malley-James.
Note the comparatively brief window for multicellular life in planets orbiting F-class stars when compared to a G-class star like our Sun, and note too that K-class stars (Alpha Centauri B is the nearest example) offer a much longer period of clement conditions for any planets in their habitable zone. The figure does not display M-class red dwarfs, but there the picture changes entirely because these stars can burn for trillions of years depending on their mass. If we do discover that life is possible on planets orbiting red dwarfs, then the time frame for intelligence to develop is proportionately extended.
With the universe now thought to be some 13.7 billion years old, is it possible that most intelligent species simply haven’t had time to appear on the scene? With up to 80 percent of the stars in our galaxy being red dwarfs, we may exist early in the overall picture of living intelligence, and most of it may evolve around stars far different than our own. Yesterday I talked about our gradual tightening of the number for eta-Earth (?Earth), the percentage of Sun-like stars with planets like ours in their habitable zone. What we are still in the dark about is eta-Intelligence (?Intelligence), the percentage of habitable zone planets with life that evolve intelligent species.
But back to F, G and K-class stars and what we do know. The O’Malley-James paper makes the significant point that G-class stars have a window for multicellular life that, based on the solitary example of our own planet, appears roughly the same length as the developmental period needed to produce it. And after the era of multicellular life, as the parent star swells toward red giant status, the era of microbial life returns for a still lengthy stretch, though shorter than the one that began it. Thus the reasonable statement “It is entirely possible that some future discoveries of habitable exoplanets will be planets that are nearing the end of their habitable lifetimes, i.e. with host stars nearing the end of their main sequence lifetimes.”
Without any knowledge of ?Intelligence, we can’t know what happens as the multicellular window begins to close. But if intelligence is not rare, then we can conceive of advanced civilizations taking the necessary steps to ensure their survival, either through migration to other star systems or massive engineering projects in space, perhaps remaining near the parent star. The kind of ‘interstellar archaeology’ championed by those who search for Dyson spheres and other massive constructs is an attempt to find projects like these, a form of SETI that is not reliant on the intent of a civilization to make contact and one that does not assume radio or optical beacons.
Which would be harder to detect: 1) the biosignature of single-celled life on a planet orbiting a dying star or 2) the infrared or visual signature of large-scale engineering near or around the same kind of star, as conducted by Kardashev Type II civilizations or higher?
I don’t have an easy answer for that because we know so little about what an advanced civilization might do to save itself, but it seems reasonable to pursue ‘interstellar archaeology’ with as much vigor as other forms of SETI. Certainly the boundaries of the discipline are expanding. Meanwhile, the O’Malley-James paper (citation in yesterday’s entry) points to the beginning of a much larger project that looks at extremophile biosignatures as a way of preparing us for the day when detailed information about terrestrial planetary atmospheres becomes available.
And the feeling persists: If M-class dwarfs are the wild card, what sort of hand has the universe dealt us? Are we, with our bright yellow sun that moves across the sky, the real outliers?
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