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 reﬂects more frequent formation of Neptune-mass planets around M dwarfs. The factual conclusion remains that Neptune-mass planets are easier to ﬁnd 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.