Jan 18, 11:11 p.m. | Updated [Illustrating that science on the edge of understanding is implicitly tentative, the case against Gliese 581g appears to be gaining strength.]
Talk about timing. Months ago, Rory Barnes, an astronomer at the University of Washington with a talent for finding planets, agreed to give a plenary talk at the northwest sectional meeting of the American Physical Society on the subject of “The Hunt for Habitable Exoplanets.”
The talk takes place today in Walla Walla, Wash., but Barnes, in an e-mail exchange last night, said he “had to rush through some quick changes,” adding that “I’m not complaining.”
The reason, of course, was the news this week that other planet hunters had identified a planet, Gliese 581g, with attributes suitable for harboring life (as we know it) orbiting a red dwarf star about 20 light years from Earth. I asked Barnes for his impressions of the planet and he said it looks like “the real deal.” Among the critical characteristics, of course, is greenhouse warming (yet another reminder that there really is such a phenomenon).
So here we are, sitting on a “real deal” planet, regarding evidence hinting — as has long been presumed — that there are quite a few other potential havens for biology out there. But simple logic, whether or not you subscribe to the precautionary principle, says we would do well to take extra care in making sure we don’t screw up this one in ways that are hard to reverse. As Carl Sagan noted in his “pale blue dot” speech in 1996, “there is no hint that help will come from elsewhere to save us from ourselves.”
Here’s Barnes’s abstract as it read before the news broke:
Fifteen years after the discovery of planets around other stars, the discovery of rocky, 1 Earth-mass, 1 Earth-radius planets around other stars is imminent. Of particular interest are planets that receive similar amounts of stellar radiation as the Earth, as they could support liquid water, probably the most critical requirement for habitability. Current technology favors the discovery of these planets around low-mass stars, about one-tenth the mass of the sun. In that case, habitable planets need to orbit about ten times closer to these cooler star. Such proximity introduces hazards that are of negligible importance for life on Earth: increased exposure to (time-varying) short wavelength radiation, stronger magnetic fields, tidal effects, and atmospheric removal by the stellar wind. Nevertheless, there are no obvious reasons to presume these planets are sterile. I discuss observational campaigns to discover rocky exoplanets, theoretical methods to predict their potential habitability, and prospects for the remote detection of life.
He said he’ll send a note describing the buzz about Gliese 581 from the meeting. In the meantime, here’s his detailed reaction to the findings:
My first impression of this discovery is that it is the real deal. There have been other planets that have flirted with potential habitability, but this one is a very good candidate. I see five issues for habitability: 1) greenhouse warming, 2) composition (water?), 3) internal energy, 4) stellar flaring, and 5) tidal effects. None of these appear to be obvious impediments to habitability.
1) Although it receives slightly less radiation from its host star than the Earth, that is not really an issue. The Earth needs about 20 degrees of greenhouse warming to reach the freezing point of water, whereas this planet requires about 50. A little more water, CO2 or methane than in the Earth’s atmosphere would take care of that, and that is entirely plausible.
2) Does it have the right ingredients for life, specifically water? I suspect yes. That there are 6 planets within 1 AU of the host star suggest the planets migrated from colder environs of the protoplanetary nebulae. Models of terrestrial planet formation of low-mass stars find that if planets form only from local material, they don’t get much bigger than 1 Earth mass. But this system has tens of Earth masses close to its star. Therefore the most likely origin scenario for these planets is that they formed at larger distances where ice was very prevalent, so probably this planet accreted a substantial amount of water.
3) Habitable planets require communication between the surface and the interior. This can only occur if there is sufficient internal energy to drive resurfacing on the planet (like plate tectonics on the Earth). Otherwise nasty molecules like carbon dioxide can build up in the atmosphere. On the Earth, the internal energy comes from radioactive decay and the ongoing differentiation of the Earth. Larger planets will probably have more energy from these sources, but the radiogeneic inventory of the planet is a wild card (it’s hard to know the isotopic ratios of exoplanets). Being close to its star, the planet also probably receives some tidal heating, similar to on Io. The level of tidal heating is difficult to estimate without better information about its orbit (specifically its eccentricity), but my preliminary calculations lead me to believe it’s about 1-10% of the Earth current level of internal heating. Not much, but it can help. And it’s certainly not at Io’s level. So all in all, there’s probably enough energy for tectonic activity.
4) One of the concerns raised for planets around low-mass stars is that the habitable zone is closer to these more active stars. In other words, atmospheres could be removed by larger stellar winds and coronal mass ejections. Vogt et al. point out that this particular star is actually very quiescent. So at least at present that doesn’t seem to be an issue.
5) Tidal effects are also of concern for planets in the habitable zones of lows mass stars. In additional to the heating described above, tides can also lock the rotation rate and fix the orientation of the rotation. There seems to be a lot of concern regarding the possibility of synchronous rotation (often mistakenly assumed to be the same as “tidally locked”). I study tides, so I could go on about this for hours, but here are the salient points. Planets can get caught in spin-orbit resonances, such as Mercury, and rotate faster than synchronous. Or if the orbit is modestly eccentric, the planet will probably rotate faster than synchronous. We can’t tell if these phenomena have occurred, but they are certainly possible. If the tilt goes to zero, that could also be bad, but the tilt could be forced by interactions with the other planets (this occurs on Mars). If the tilt is non-zero and the planet rotates synchronously, then at least certain regions of the planet will oscillate through day and night. And even if it is synchronously rotating, certain regions of the planet can still be habitable.