Centauri Dreams
Imagining and Planning Interstellar Exploration
Micro ‘Bots’ to the Stars?
Debra Fischer (Yale University) takes a brief look at the next thirty years as part of a Discover Magazine 30th anniversary section, an appearance notable more for what Fischer doesn’t say than what she does. Any hint of how her radial velocity studies of the Alpha Centauri system are proceeding? I wouldn’t have expected any, I’ll admit, and Fischer says nothing about it, but the betting here is that we’ll have an announcement within the next year either by Fischer or Michel Mayor’s team either giving us a planetary discovery or sharply constraining the alternatives.
What Fischer does speculate on beyond the notion that we’ll detect life in exoplanetary atmospheres is that interstellar probes will eventually fly. You may recall Robert Freitas’ notion of interstellar probes loaded with artificial intelligence and as tiny as sewing needles, scattered into the galaxy in their hordes to investigate potentially habitable worlds. Fischer, too, likes miniaturization, which does so much to mitigate the huge propulsion issues:
Outside the gravitational field of Earth, we could launch robotic spacecraft to other destinations in our solar system. Further ahead I’d like to see tiny spacebots – smaller than your cell phone—travel outside our solar system to the nearest star system, Alpha Centauri. By keeping the mass of those spacebots low, we could more easily accelerate them. We could launch an army of these tiny bots and have them do what your cell phone does: take pictures and phone home.
Yes, and just maybe we could use them to create the kind of communications station at the Alpha Centauri gravitational lensing distance that Claudio Maccone envisages, using it to communicate at extremely low power with a comparable robotic relay at our Sun’s gravitational focus. That would set up tremendous bandwidth opportunities for tiny transmitters and allow valuable scientific studies to flourish.
Meanwhile, we’ll all keep speculating on the big question for the immediate future — when will the first habitable extraterrestrial planet be discovered? Greg Laughlin (UC-Santa Cruz) and Samuel Arbesman (Harvard University) have a go at this with a new paper that attempts a sociometric analysis of the question. The researchers create a metric of habitability that can be applied to already discovered planets and use a boostrap analysis to extrapolate discoveries into the immediate future. The prediction that emerges from this is near-term: The first Earth-like planet will be discovered (with high probability) by mid-year 2011. The method will likely be planetary transit or radial velocity (Debra Fischer’s Alpha Centauri work again comes to mind).
Will Kepler find the first habitable planet? Don’t count on it. For one thing, the next Kepler results we get will be of planets that are probably too hot to sustain life:
While the initial results of Kepler were released on June 15, 2010, the Kepler team has delayed publication of 400 of the most promising extrasolar planetary candidates until February 2011. Within this large pool of withheld candidates, it is virtually certain that some have radii that are observationally indistinguishable from Earth’s radius. It is likely, however, that because of the limited time base line of the mission to date, the Kepler planet candidates to published in February 2011 may be too hot to support significant values for H [habitability].
Laughlin and Arbesman re-ran their analysis using only those planets discovered via the transit method, learning that the method cannot determine a likely date of discovery because we have relatively few planets found by transits, and all rank very low on the habitability scale. But the authors’ habitability metric curve deployed on a larger population of 370 well-characterized known exoplanets continues to point to as early as May of 2011 and very likely by the end of 2013 for that first habitable planet. The method, fully described in the paper, is fascinating. What’s more, with target dates this close, we’ll have an early read on how prescient its authors really are.
Interestingly enough, the authors note that the habitability factor for most of the 370 planets in their study is zero, but of course Gliese 581d is an exception, recently examined by other authors and found to be potentially habitable. Laughlin and Arbesman disagree with the assessment, pointing out that the planet’s mass should be close to ten Earth masses. The paper describes Gl 581d’s ‘…possibly water-dominated composition more akin to an ice giant planet such as Uranus or Neptune than to a terrestrial planet like the Earth.’
The paper is Arbesman and Laughlin, “A Scientometric Prediction of the Discovery of the First Potentially Habitable Planet with a Mass Similar to Earth,” accepted by PLoS ONE (preprint). Re Gliese 581d, the paper is Wordsworth et al., “Is Gliese 581d Habitable? Some Constraints from Radiative-Convective Climate Modeling” (preprint).
Detecting (and Understanding) Life Signals
A symposium celebrating the first fifty years of NASA’ exobiology program takes place on October 14 in Arlington, Virginia. ‘Seeking Signs of Life’ looks all the way back to 1959, when NASA funded its first exobiology investigation, an experiment for a future spacecraft to detect life on Mars. The actual exobiology program was established in 1960, and led to the three Viking experiments that eventually flew. Exobiology has these days morphed into ‘astrobiology,’ as we look at topics as diverse as chemical evolution in interstellar space and planetary formation.
For those in range of Arlington, more information is available here. Be aware as well of a workshop on SETI that is now taking place at the National Radio Astronomy Observatory in Green Bank, WV, marking the 50th anniversary of Frank Drake’s first search for extraterrestrial signals. Webcasts begin at 0830 EDT (1230 UTC), and will include Drake’s views on ‘SETI in 2061 and Beyond’ at that time on September 15. Further information is available from NRAO.
Thinking about astrobiology has me turning to an interesting notion put forward by Caleb Scharf last week on his Life, Unbounded site. It has to do with what we mean by habitability, a necessary term in searching for life on other worlds that has to be defined to help us narrow our search, but one that may be misleading. Scharf (Columbia University) is wondering whether habitability is only part of a template that may be just a bit too tidy to be truly descriptive:
If there is one inevitable thing about life it is that particular variants, species, modes of existence, are all prone to extinction. A new work by Drake & Griffen in Nature this week makes this point rather succinctly. They show, by subjecting water flea populations to a series of unfortunate events, how the population dynamics of a species can fundamentally shift due to environmental changes. Fluctuations in population numbers occur even in stable environments, but the character and size of these fluctuations changes in degrading environments, and beyond a certain point there is no recovery. Long before it all goes down the tube there are clear statistical indicators that things are not well – population sizes drop as the tree begins to fall.
We do indeed know that extinction events are an unfortunate fact of life on planets like ours — at least, they have been on this one. Right now we’re looking to a near-term future when we can go to work on planetary atmospheres, eventually subjecting terrestrial planets to scrutiny with space-based spectroscopy. But if we do find biomarkers in an alien atmosphere, what will they mean? Scharf argues that any planet we find with these methods will most likely be one in which life is moving toward instability, a fertile but dangerous period pointing to catastrophe:
We are most likely to be able to sniff out the signs of life on a terrestrial-type planet when it’s in full swing. Suppose a world is having a particularly fertile episode, chock-a-block with organisms, but not a stable situation. It’s prime for collapse. Relative populations will swing high and swing low. At the high point for some, a planet may show the greatest bio-signatures, and make itself far more tasty for our prying telescopic eyes. Without running the numbers it’s impossible to give a precise answer, but it would seem that the odds will be shifted. We may be most likely to find not the signs of normality, but the signs of a system approaching some kind of biological collapse – just like the stock market, it’s all about the fluctuations.
Evolutionary success may wind up creating all the conditions for sudden, catastrophic change, a delicate balance that, once put out of whack, quickly degrades. We may detect, then, signs of planets overrun by particular kinds of life, with populations in a state of fluctuation over timescales as low as thousands or even hundreds of years. Must it always be so? Of course not, but what Scharf is saying is that our limited detection sensitivity will hamper us in the early going, and we’d better be careful about the kind of conclusions we draw from the evidence.
Which brings me to the closing panel at the upcoming astrobiology conference in Arlington. It’s titled “Homing in on ET Life: Where, and How, To Look.” With our one example of planetary life to go on, the ‘how’ gets increasingly important. We aim for the most recognizable life scenarios but must keep in mind that what Scharf calls ‘convenient truths’ can mislead us. Stable, long-term life may not show as strong a signal as biological collapse, so that as is the case with ‘hot Jupiters,’ we start off by seeing extreme examples of a much more evenly distributed phenomenon. We’ll want to learn from that lesson if it’s one we observe in spectroscopy, and avoid drawing too many conclusions from the outliers that our early instrumentation may reveal.
Next Gen NEAR: Targeting an Asteroid
A manned mission to an asteroid sounds, on first hearing, like a true deep-space venture, and in the days when we thought of the asteroids as largely confined to a belt between Mars and Jupiter, so it would have been depicted. But today we know that a large population of near-Earth objects (NEOs) is out there, close enough to make one of them the most obvious target for a mission beyond the Earth-Moon system. Moreover, they’re a necessary target given our need to understand their composition in case we ever have to change an asteroid trajectory.
Even so, you don’t send a human team to a completely unknown destination, which is why robotic asteroid exploration continues to loom large. Two missions — Japan’s Hayabusa and NASA’s Near Earth Asteroid Rendezvous (NEAR) — have actually orbited and landed on an asteroid. Now the Applied Physics Laboratory at Johns Hopkins University is proposing a follow-on to the NEAR mission that would give us the needed insights for later human visits.
James Garvin is chief scientist at NASA’s Goddard Space Flight Center, which is working with APL and the Johnson Space Flight Center on what the trio are calling ‘Next Gen NEAR,’ a robotic precursor mission to a near-Earth asteroid. Garvin sizes up Next Gen NEAR this way:
“We’ve learned a lot about NEOs using telescopes, Earth-based radar and two robotic missions, but we’d have to get up close and personal with a specific asteroid again, and learn much more about its environment, before we could send human explorers. But there is nothing intuitive about operating at an asteroid; in fact, sending humans to an asteroid would be one of the most challenging space missions ever. So to make sure we really understand that challenge, we’ve paired NASA experts in small-body robotic and human spaceflight with the only team in the U.S. to design, build and operate an asteroid-orbiter mission.”
Image: Artist’s impression of the Next Gen NEAR spacecraft approaching a near-Earth object, or NEO. A concept based on the successful Near Earth Asteroid Rendezvous mission, Next Gen NEAR could serve as a robotic ‘precursor’ for a human visit to a near-Earth asteroid. Credit: Johns Hopkins University Applied Physics Laboratory.
If Next Gen NEAR lives up to its predecessor’s standards, we’ll be doing well. NEAR was able to produce more than 160,000 images of asteroid 433 Eros, studying its geology, geophysics and composition. Next Gen NEAR would be what APL is calling a ‘workhorse of a mission’ that can launch in 2014 and return a similar windfall of data at a cost lower than a Discovery-class mission. As proposed, the spacecraft would run on commercially available subsystems, carrying lightweight scientific instruments including a surface-interaction experiment and composition-measuring spectrometers, and would be launched by a medium-class rocket.
Next Gen NEAR is an interesting and evidently cost-effective mission concept that takes us another step toward meeting the goal of a manned mission to an asteroid. The more experience the better with this kind of operation — landing on a body with infinitesimal gravity and no atmosphere is a different kind of operation than putting a payload on a planetary surface. The operations in close orbit and in contact with the surface that NEAR and Hayabusa have already demonstrated can be tuned up further in a mission like this. We’ll see how this concept is greeted at a time when expanding our knowledge of Earth-crossing asteroids is becoming a more visible priority.
Of ‘Hot Jupiters’ and Short Lifetimes
Globular clusters held an early fascination for me, and I guess anyone who encounters these rich cities of stars for the first time wonders what it would be like to be on a planet deep inside one of them. The clusters appear to be distributed in a spherical halo around the galactic center, ancient collections of stars much lower in heavy elements than stars in the galactic disk (although globular clusters in some other Local Group galaxies seem younger). The thought of the night sky on a planet embedded in such a place makes the mind reel, star upon star upon star filling the view.
Image: The globular cluster 47 Tucanae, the second brightest globular cluster orbiting the Milky Way (behind Omega Centauri). Imagine the night sky deep within such a cluster. Credit: South African Astronomical Observatory.
But a new paper suggests that at least one category of planets may be rare in such clusters. It follows up on an earlier survey of the cluster 47 Tucanae which examined some 34,000 stars and came up empty. The theory here is that the high density of stars in globular clusters disrupts planetary orbits. Even more significant, tidal effects upon planets much closer to their star than Mercury is to our Sun eventually cause ‘hot Jupiters’ to experience orbital decay and an early demise. Add this to the low metallicity (few elements heavier than hydrogen and helium) in globular clusters and you have an environment not conducive to planet building in the first place.
Brian Jackson (NASA GSFC) puts it this way: “Globular clusters turn out to be rough neighborhoods for planets, because there are lots of stars around to beat up on them and not much for them to eat.” Working with colleague John Debes, Jackson argues that any ‘hot Jupiters’ in globular clusters would be destroyed quickly by tidal effects if nothing else, their orbits gradually moving closer to their star until the planet is torn apart by the star’s gravity or crashes into it.
Thus we can explain the dearth of planets in 47 Tucanae. The researchers’ simulations using tidal effects and the proximity of nearby stars showed that hot Jupiters would be unlikely to survive even when metallicity is left out of the equation. In fact, Debes and Jackson have found that approximately one third of hot Jupiters won’t survive the first billion years of a cluster’s life, not to mention the eleven billion years 47 Tucanae has been in existence. The simulations suggest that at 47 Tuc’s age, at least 96 percent of the hot Jupiters would have perished.
Here Kepler becomes a relevant tool. The mission has four open clusters (much less dense than globular clusters) in its survey field, in a range of ages from less than half a billion to nearly 8 billion years old. All of these clusters appear to have the raw materials from which planets are formed, making them a good test for the tidal decay model. The new work suggests that Kepler should find up to three times more Jupiter-class planets in the youngest cluster than in the oldest one. Fewer expected hot Jupiters, in other words, as cluster age increases and, as a corollary, increasingly tight orbits for detected planets. We should have some answers soon.
And this is interesting, from the paper on this work, on the results of a planet being consumed by its star:
If planets are engulfed, one would expect a signature of pollution in the stellar atmosphere… or an increase in stellar rotation rates… If the process strips just the envelope but leaves a dense core, there might be an excess of ?5-10 ME planets with short periods above that expected through orbital migration alone. There might even be a mass-period relationship for the remnant cores, analogous to that observed for tidally-stripped white dwarfs… Although transit surveys may have difficulty detecting these small stranded remnant cores, their detection would provide an important clue to the fate of close-in gaseous planets.
As to other planets in globular clusters, we’re forced to look for smaller worlds in far more distant orbits. Says Debes: “The big, obvious planets may be gone, so we’ll have to look for smaller, more distant planets. That means we will have to look for a much longer time at large numbers of stars and use instruments that are sensitive enough to detect these fainter planets.” Personally, I hope we start finding them, if only to validate those spectacular sky scenes I’ve long imagined.
The paper is Debes and Jackson, “Too Little, Too Late: How the Tidal Evolution of Hot Jupiters affects Transit Surveys of Clusters,” accepted by The Astrophysical Journal (preprint).
ExoClimes 2010: Exoplanetary Atmospheres
The ExoClimes 2010 conference (“Exploring the Diversity of Planetary Atmospheres”) is well in progress in Exeter (UK) as I write, with its talks now being posted online and the hope that video of the presentations will soon be available on the conference site. Already the latest lingo is in the air, as in ‘Hermean,’ a term used by Brian Jackson (NASA GSFC) to describe hot, rocky exoplanets with tenuous atmospheres. The analogy is with Mercury, though these are even hotter places with magma oceans and melted surfaces, leading to what Jackson calls a ‘rock vapor atmosphere’ that just might be visible given sufficient spectral resolution.
But what catches my eye this morning, as I survey the ongoing conference buzz online from an ocean away, is Franck Selsis (Laboratoire d’Astophysique de Bordeaux) and his work on the atmospheres of short-period terrestrial exoplanets. Selsis is interested in the habitability of planets around M-dwarfs, noting their strong tidal interactions with their primary, the likelihood of tidal locking for planets in circular orbits, and the problems of atmospheric freeze-out on the dark side, calling for a heat redistribution mechanism to produce habitable surface conditions.
The broader issue, obviously of interest for this conference, is how a terrestrial world in the habitable zone of an M-dwarf would maintain an atmosphere in the first place. What Selsis argues in his presentation (I’m looking at his slides) is that finding and characterizing dense atmospheres on super-Earths is a major objective for understanding how such atmospheres form and survive. Active M-dwarfs show flare activity longer than the more sedate K and G-class stars, so we need to understand how atmospheres act here, and for that we need statistics.
That’s where it gets tricky. Transiting worlds within 10 parsecs aren’t going to offer the statistics we need, leading Selsis to speculate on whether we can measure the phase curves of non-transiting terrestrial exoplanets. If so, we can increase the number of targets by a factor of 10, but only if we can work out ways to detect and measure the infrared phase curve based on reflected light from the planet, as opposed to the primary and secondary transits of more established methods. A large, rocky planet around a low-mass M-dwarf is a good test case. Make it hot enough (0.05 AU) and you get a surface temperature that’s not too hot to hold an atmosphere, and you also get the highest planet/star contrast available for such a planet.
Other Exeter news: Sushil Atreya (University of Michigan) is interested in another kind of hot world, a version of Saturn’s moon Titan. Imagine a nitrogen-rich atmosphere like Titan’s in a temperature regime where chemical reactions are accelerating rather than moving in ultra-slow motion. A hot Titan would be a world much more like Venus than the Earth. But given the right migration scenario, such a world should be out there, its atmosphere filled with carbon soot and sulphur, in the grip of heat and abundant greenhouse gases. Atreya’s presentation lays out the case, and I’m looking forward to the video of it and other talks as ExoClimes 2010 continues.
Detecting Exoplanet Volcanoes
We’re entering the era of the ‘super-Earths,’ when rocky planets larger than our own will pepper the lists of new discoveries. These smaller worlds will occasionally make a transit of their star, as does CoRoT-7b, and that’s when things really get interesting. After all, we know that secondary eclipses, in which a transiting exoplanet swings behind its star as seen from Earth, can be used to study distant atmospheres. The method collects light from both star and planet and, when the planet is hidden, subtracts the starlight to get the planetary signature.
Now Lisa Kaltenegger (Harvard-Smithsonian Center for Astrophysics) and colleagues Wade Henning and Dimitar Sasselov are advancing the idea that we can use near-term instrumentation like the James Webb Space Telescope to spot volcanic eruptions using these same methods. Their model is based on eruptions on an Earth-like planet, extrapolating from what happens on our world to suggest that sulfur dioxide from a major volcanic event on an exoplanet is measurable because it can be produced in huge quantity and is slow to wash out of the air.
Mount Pinatubo, which erupted in the Philippines in 1991, accounted for 17 million tons of sulfur dioxide blown into the atmosphere in a layer between 10 and almost 50 kilometers above the Earth’s surface. But we also have the example of the Tambora eruption of 1815, which was as much as ten times more powerful than Pinatubo. Tambora is the largest observed eruption in recorded history, an explosion that could be heard 2600 kilometers away. Fine ash particles stayed in the atmosphere for a period of years, and the summer of 1816 became known as ‘the year without a summer’ in the northern hemisphere , a climatic anomaly evidently related to the release of vast amounts of sulfur.
Eruptions of this magnitude don’t occur frequently on Earth, but there is no reason to think that young, rocky exoplanets would not be more volcanically active than our more mature world. And ponder the effects of tidal heating, not only on planets but also on their moons. From the paper:
Tidal heating may contribute significantly to volcanism for eccentric exomoons or eccentric planets in heliocentric periods of 20-30 days or less… Such tidal heating has the potential to generate from thousands to millions of times more internal heating than in the modern Earth. However, at the upper end of this heat range, magma is more likely to escape in a non-explosive pattern, or may simply emerge into lava lakes or magma oceans partly sustained by the high insolation values near stars, such as the magma ocean suggested for Corot 7b. Such extreme worlds may also be more likely to have a reducing atmosphere. Significant tidal activity can be stimulated in multi-body systems by secular perturbations, secular resonance crossings, or a deep and stable mean motion resonance, analogous to the Galilean moon system.
So the most extreme heating may well work against detectable volcanic activity, but tidal effects may be pronounced in more moderate scenarios:
Modest tidal heating cases may easily supply some exoplanets with both a 10x increase in the size and frequency of large eruptions, while simultaneously enhancing nonexplosive activity.
And that, of course, is only part of the picture. Volcanic activity may also be keyed to planetary age, with younger planets expected to have more residual accretion heat and higher radionuclides, while plate tectonic activity can increase the frequency of explosive volcanoes. The paper is careful to examine other mechanisms for heat escape in non-explosive events and notes that we don’t know whether hotter planets would necessarily show more explosive eruptions. Nonetheless, an Earth-like world less than thirty light years from the Sun should be a fair candidate for JWST studies that can help us put constraints on some of these variables.
Although it’s true that secondary eclipse studies give us a relatively crude picture of a planetary atmosphere, they do help us find particularly abundant molecules and provide us with a basic model that can be developed over time. This new work shows that in the best case scenario, volcanic features become visible at values between 1 to 10 times the Pinatubo eruption — the probability of observing an eruption of Pinatubo class is about 1 percent if four Earth-like planets are observed for one year, while a Tambora-size event could be detected with a 10 percent probability by observing roughly 50 such planets for two years. The paper concludes:
These observations becomes a very interesting option to characterize rocky planets, especially if one assumes larger, and/or more frequent eruptions than on Earth, or smaller host stars, where a planet in the HZ orbits closer to their stars, increasing the transit probability., or longer SO2 residence times than on Earth.
The paper is Kaltenegger et al., “Detecting Volcanism on Extrasolar Planets,” in press at The Astrophysical Journal (preprint).
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