Gliese 581 c: Chances for Habitability

Just what might we find on Gliese 581 c, the potentially habitable planet announced yesterday? Much depends on where the planet formed in its circumstellar system. For that kind of information I listen to Greg Laughlin (University of California at Santa Cruz), whose work on planetary formation via core accretion seems to gain stature with every new planetary find. Here’s Laughlin’s take from his systemic weblog:

The planet probably migrated inward to its current location from beyond the “snowline” in GL 581’s protostellar disk, and so its composition likely includes a deep ocean, probably containing more than an Earth’s mass worth of water. Atmospheric water vapor is an excellent greenhouse gas, so the conditions at the planet’s atmosphere-ocean boundary are probably pretty steamy. It’s also possible, however, that the planet formed more or less in-situ. If this is the case, it would be made from iron and silicates and would be fairly dry. It’s unlikely, but not outside the realm of possibility, that this could be a genuinely habitable world. There’s no other exoplanet for which one can make this claim. In short, it’s a landmark detection.

Remember that the orbital period for Gliese 581 c has been determined to be 12.9 days, putting it in the heart of the star’s habitable zone. Laughlin’s systemic project has reason to celebrate this morning as we continue to digest the recent developments. The systemic collaboration is a publicly available simulation that models planetary systems using radial velocity techniques, with data for each star studied made available over the Net. Six of systemic’s users had already created models for Gliese 581 that jibe with the recently announced discovery, a testimony to the power of systemic and of collaborative science on widely distributed computers.

Here’s an ESO video on the discovery featuring Michel Mayor (Geneva Observatory):

And let’s not forget the larger trends the new planetary find highlights. The discovery paper (citation in the previous post) notes that small planets — Neptune mass and below — are more frequent than gas giants around M dwarfs. We have six very low mass detections as against three Jovian planets. “This result was significant,” the paper says, “at the 97 % level before the detection of the two new Gl 581 planets…even without accounting for the poorer detection efficiency for lower-mass planets.” We looked at those trends in a recent Centauri Dreams post.

On the frequency of detections, let me quote the paper at greater length:

The fraction of detected Neptune (and lower-mass) planets around M dwarfs is much larger than the corresponding ratio for solar-type stars… The absolute numbers of detections are similar, but the number of surveyed solar-type stars is an order of magnitude larger. This may be an observational bias due to the lower mass of M-dwarf primaries, or truly re?ects more frequent formation of Neptune-mass planets around M dwarfs. The factual conclusion remains that Neptune-mass planets are easier to ?nd around M dwarfs.

The work of Laughlin and others continues to suggest that lower mass stars like M dwarfs should produce low mass planets, which accounts for the presence of the Neptune-class and smaller worlds we’re discussing (this trend should also hold for solar mass stars that emerged from metal-poor nebulae). The new finds around Gliese 581 help to bolster these trends, while making it clear that finding more low-mass planets like this one will help firm up our theories. Obviously, all this plays into the building of target lists for future space-borne missions that will look for transits and (later) do spectroscopic analysis of planetary atmospheres.

For those of you in the UK, I’ll be discussing the Gliese 581 find on BBC radio some time between 1700 and 1800 BST today.

A Potentially Habitable Earth-like World

This is a big one, and it happens several years earlier than I had expected. A planet of about five times Earth mass, one whose radius is only 1.5 times that of our own world. Moreover, a planet that’s smack in the middle of its star’s habitable zone, with a mean temperature estimated at between 0 and 40 degrees Celsius. The models in question say that this is a rocky world, and its temperatures tell us that oceans could exist there. The first detection of a planet where carbon-based life could conceivably exist makes this one a find for the history books.

Planetary system around Gliese 581

The star is Gliese 581, already known to be home to a planet of Neptune mass and a possible third world about eight times as massive as Earth. It’s an M-class red dwarf, far smaller and cooler than the Sun. The new planet, the smallest found up to this point, orbits it in 13 days. Gliese 581, it should be noted, is comparatively close to our own Solar System, about 20.5 light years away in the constellation Libra. Radial velocity methods seem to have been made to order for this small star and the planets that circle it.

Image: Artist’s impression of the planetary system around the red dwarf Gliese 581. Using the instrument HARPS on the ESO 3.6-m telescope, astronomers have uncovered 3 planets, all of relative low-mass: 5, 8 and 15 Earth masses. The five Earth-mass planet makes a full orbit around the star in 13 days, the other two in 5 and 84 days. (c) ESO.

Xavier Delfosse (Grenoble University), a member of the discovery team, has this to say about the significance of the new world:

“Liquid water is critical to life as we know it. Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extra-terrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with an X.”

Yes, and Gliese 581, with clear evidence of a second ‘super-Earth’ in an 84-day orbit (well outside the star’s habitable zone) and its Neptune-class world (5.4 day orbit) as well, is one of the most interesting planetary systems analyzed to date. We’ve looked many a time in these pages at what a terrestrial world around a red dwarf might be like on the surface. Surely tidally locked to its parent, with that dim red sun eternally fixed in the same place in the sky. Given a thick enough atmosphere, heat transfer could occur that keeps the dark side warm enough to prevent its gases from freezing out. We’re left with the possibility of a temperate region on the day side that could, for all we know, support life.

Gl 581

Image: Gl 581. Just a dot at screen center, but perhaps home to the first habitable world ever detected. Credit: Sloan Digital Sky Survey (via systemic).

The new planet may not be anything like this, and it will take more work — and surely space-based instrumentation — to learn what its true characteristics are. But ponder a planet where infrared predominates rather than visible light, and periodic flare activity acts as an evolutionary stimulus. The consider the long lifetimes — more than a 100 times the Sun’s paltry ten billion years — that M-dwarfs have to let evolution work its wonders. There are arguments to be made for and against this scenario, but if it’s remotely true, then the number of habitable planets in our galaxy may be far higher than we’ve previously believed.

Physicist and science writer Douglas Blane recently interviewed astrobiologist Giovanna Tinetti on the subject of hunting for life-markers on exoplanets. Part of their discussion involved M-dwarfs, with Tinetti recalling a talk at Caltech by John Raven (Royal Observatory, Edinburgh), who described his work with Ray Wolstencroft on M-dwarf habitability. What kind of photosynthesis might happen on a world lit by the longer wavelengths of such a star? From the interview:

They provided a scheme for photosynthesis that uses three photons instead of two, as vegetation does on Earth. They showed that you can still have photosynthesis with a cooler star. So that started me thinking about what would happen to the red edge [photosynthesis shows high reflectance at the far red of the optical spectrum, a useful signal of the presence of vegetation]. I showed that it would be shifted, so you would need to look for a slightly different signature.

The next question of course was whether planets of such a star would have the same atmospheric characteristics as Earth. Now a scientist called Joshi had already provided a 3-D model for a terrestrial planet in the habitable zone of an M-star. He’d shown that you probably need more greenhouse gases to warm up the area not illuminated.

This is because for such a cool star the planet has to be very close. So it could be tidally locked, with one face always illuminated and the other always dark. That meant you needed a circulation of the atmosphere and a particular composition. We put that model together with my calculations on the shifted red edge, and discovered that the strength of the edge feature on an M-star terrestrial planet can exceed that on Earth, given the right conditions.

Of course, we can only speculate about the new Gliese 581 planet because thus far all we really have a read on is its orbital period, distance from the star and minimum mass. Even so, what a find, one that will surely spur yet more interest in the red dwarf category that accounts for up to 80 percent of the stars in our galaxy. Kudos to the discovery team and the amazing HARPS (High Accuracy Radial Velocity for Planetary Searcher) spectrograph, located on ESO’s 3.6-meter telescope at La Silla (Chile). HARPS detected velocity variations in this star between two to three meters per second — we’re talking about the speed of a brisk walk!

Planet hunter extraordinaire Michel Mayor (Geneva Observatory) has this to say about HARPS:

“HARPS is a unique planet hunting machine. Given the incredible precision of HARPS, we have focused our effort on low-mass planets. And we can say without doubt that HARPS has been very successful: out of the 13 known planets with a mass below 20 Earth masses, 11 were discovered with HARPS!”

All true, and you have to like where this is going. Mayor again:

“And we are confident that, given the results obtained so far, Earth-mass planets around red dwarfs are within reach.”

Oh for the dedicated (and lengthy) observing run sufficient to let HARPS do its number on Centauri B! And if you’re wondering about previous super-Earths, Gliese 876 does indeed have a planet with a minimum mass in this same range — 5.89 Earth masses — but its orbit (completed every two days) takes it too close to its star for liquid water to exist. Likewise, the icy world OGLE-05-390L weighs in at 5.7 Earth masses but is much more distant from its primary and out of the habitable zone.

The upcoming paper in Astronomy and Astrophysics is Udry et al., “The HARPS search for southern extra-solar planets : XI. An habitable super-Earth (5 MEarth) in a 3-planet system.” I’ll point you to the abstract as soon as it becomes available. Thanks to Darnell Clayton for additional information on this story.

Update: Here’s a link to the paper on this work; thanks to Malcolm Ramsay for the address.

A Huge Intergalactic Cloud

In light of yesterday’s post on black holes and their role in spreading heavy elements through the cosmos, the news out of Los Alamos provides an additional fillip of controversy re these enigmatic objects. A research team led by Philipp Kronberg has also been looking at clouds in deep space and their association with black holes, though what Kronberg’s team has identified is a distinctive object indeed. It’s a cloud of plasma more than six million light years across, one that may provide evidence for the role of black holes in triggering cosmic rays.

Here’s Kronberg on the subject:

“One of the most exciting aspects of the discovery is the new questions it poses. For example, what kind of mechanism could create a cloud of such enormous dimensions that does not coincide with any single galaxy, or galaxy cluster? Is that same mechanism connected to the mysterious source of the ultra high energy cosmic rays that come from beyond our galaxy? And separately, could the newly discovered fluctuating radio glow be related to unwanted foregrounds of the Cosmic Microwave Background (CMB) radiation?”

We’re getting more questions than answers here, which is usually the way with discoveries. The plasma cloud may contain several radio galaxies — active galaxies highly luminous in radio wavelengths — that contain black holes. If that is the case, the question becomes whether and how these black holes are converting their gravitational energies into magnetic fields and cosmic rays. The combined resources of Arecibo and the Dominion Radio Astrophysical Observatory produced this result, and an image comparable to that of a 1000 meter diameter radio telescope.

That last bit is amazing in and of itself. The paper is Kronberg et al., “Discovery of New Faint Radio Emission on 8° to 3′ Scales in the Coma Field, and Some Galactic and Extragalactic Implications” Astrophysical Journal 659 (April 10 2007), pp. 267-274 (abstract here).

A Practical Positron Rocket II

Almost exactly a year ago, I posted a story called A Practical Positron Rocket, about Gerald Smith’s work at Positronics Research on a positron reactor. Antimatter is always a hot topic, given its potential for remarkably powerful engines and its implications for deep space work, but the post in question generated responses that ranged far beyond antimatter into numerous other potential solutions to the propulsion problem.

Which is fine, but we may be encountering a bug in WordPress which is keeping more recent comments from appearing properly. I haven’t been able to confirm this, but I suspect that once comments for a given post reach a certain size limit, odd things begin to occur. In any case, I’ve had some anecdotal evidence (not just here) that this is the case.

This post, then, is for those of you who want to keep the ‘Practical Positron Rocket’ thread running. Please use the comments section here to do so, and we’ll retire the old post as a forum for comment.

Black Holes May Stir the Cosmic Broth

One of the problems of explaining the universe we live in is the presence of heavy elements. After all, the cosmos was a simple matter right after the Big Bang, with hydrogen and helium its only ingredients. Creating the heavier elements required stars, the model being that their eventual death in massive supernova explosions scattered ‘star stuff,’ as Carl Sagan liked to call it, throughout the universe, leading to the wide range of elements we see today and, of course, to life.

New research is adding black holes to that picture, seeing them as influential in spreading these same elements far and wide. The supermassive black hole at the center of the galaxy NGC 4051 is at the center of investigation. A research team led by Yair Krongold (Universidad Nacional Autónoma de México) has found that gas is escaping the black hole from a source closer to its Schwarzschild radius than previously thought. In the case of NGC 4051, that radius — the point beyond which nothing can escape the black hole — is about four million miles.

Although the accreting material in question is about 2000 times that distance, between 95 and 98 percent of it does go on to fall into the black hole. But that leaves between two and five percent that does not. The so-called ‘winds’ from black holes like this one pump heavier elements like carbon and oxygen back out of the galactic core, where they eventually become part of the clouds of dust and gas in which new stars form.

Thus we learn more about the influence of black holes on their surroundings, and their implied role in re-distributing the elements needed for life. But one puzzle remains: The fraction of escaping gas in the case of NGC 4051 is too low to account for the heavier elements found in intergalactic space. The team’s next goal, then, is to see whether more powerful active galaxies allow a greater percentage of nearby gases to escape. That work will continue using the same XMM-Newton space observatory techniques that produced the first result.

More on this in Krongold et al., “The Compact, Conical, Accretion-Disk Warm Absorber of the Seyfert 1 Galaxy NGC 4051 and its Implications for IGM-Galaxy Feedback Processes,” Astrophysical Journal 659 (April 20 2007), pp. 1022-1039 (abstract available)