Expanding the search for life to so-called super-Earths

Thursday, December 18, 2008

Article here.

This will be a more and more prominent topic in the next little while as we have just started to be able to discover planets a few times larger than Earth, and the question after finding them (since they have a solid surface) is whether life can exist on them, or underneath the surface. These super-Earths have qualities that could enable life to exist even outside a star's habitable zone:

Most such efforts focus on finding planets in another solar system's "habitable zone" -- the distance from a star where temperatures are just right for supporting liquid water on the surface and thus life as we know it.

But water is much more plentiful beyond the habitable zone, in the outer reaches of a solar system, Gaudi explained. It's most often found as ice -- at the heart of gas planets such as Jupiter, on frozen moons such as Europa, and on super-Earths. In fact, Earth's water probably originated elsewhere, and found its way here on comets or asteroids.

So rather than looking for warm planets like Earth that happened to acquire water, Gaudi and his colleagues decided to look at cold super-Earths that formed with water already in place.

They examined the likelihood that some internal heat source might enable liquid water to form under the ice. As Gaidos and Seager modeled scenarios for heating the interior of super-Earths, Gaudi modeled whether the planets they hypothesized would be detectable.

Gaidos and Seager found that very big super-Earths, ones around 10 times the mass of Earth, could retain enough heat from their formation to form a liquid ocean beneath the ice -- even though those planets would be located some five times farther from their star than the Earth is from its sun.
This was shown after the discovery of the planet Gliese 581 c; it was originally thought that it might have a temperature similar to ours, but after a few more calculations it turned out that given the atmosphere it's likely to have that it would be way too hot:
Gliese 581 c generated interest because it was initially reported to be the first potentially Earth-like planet in the habitable zone of its star, with a temperature right for liquid water on its surface, and by extension, potentially capable of supporting Earth-like life. However, further research on the potential effects of the planetary atmosphere casts doubt upon the habitability of Gliese 581 c and indicates that the third planet in the system, Gliese 581 d, is a better candidate for habitability.
Gliese 581 d is much farther out from its star than Gliese 581 c, but thanks to its atmosphere it seems like life would be more likely to exist there:
Although Gliese 581 d orbits outside the theoretical habitable zone of its star, scientists surmise that conditions on the planet may be conducive to supporting life. Scientists originally believed that Gliese 581 d would be too cold for liquid water to exist, and therefore could not support life in forms as existing on Earth. However, since Earth's temperature would be about -18°C without any greenhouse gases, and due to a theorized greenhouse effect of Gliese 581 d, research now suggests that atmospheric conditions on the planet could create temperatures at which liquid water can exist, and therefore the planet may be capable of supporting life.
Now, planet c has a mass of 5 Earths and d a mass of 7.7. What is interesting to speculate about once we have a possible candidate for life is what sort of environment is most conducive to creating a spacefaring civilization. Earth for example has always had the Moon right up in the sky to watch and wonder about, and I expect that without a moon we would have had less of a drive to make it out into space as soon as we did. On the other hand, we may have done a better job at colonizing low Earth orbit (LEO) instead, and could have had a permanent foothold in space already if there was no moon to aim for.

A planet with a mass 7.7 times that of Earth also means its that much harder to reach orbit compared to the Earth. To leave Earth's orbit one needs a velocity of over 11 km per second, so you can imagine how much harder it would be to achieve orbit around this planet. I would think that a planet most likely to have a spacefaring civilization would:
1) Have a relatively low gravity, perhaps something similar to Mars
2) Have a fairly large moon, perhaps around the size of Ceres, closer than our moon but still relatively far away. Moons similar to Phobos and Deimos I would think would be hazardous for a young spacefaring civilization given that with their low gravity anyone standing on the surface could use a sling-like implement to launch rocks onto the surface with incredible force. Any country that first reached one of these moons would be able to strike at the surface of the planet whenever they wanted. Then again, we also have nuclear weapons that can launch at any time and we haven't used them since the first time in WWII.

This is all leading up to a question: is there any research on what would be ideal conditions for a spacefaring civilization? Low gravity, a moon or several moons worth exploring nearby and prominently featured in the night sky, perhaps relatively friendly planets nearby as well, and a star system within one to three light years would seem to be ideal to me. Alpha Centauri for example has Proxima Centauri, which is technically in the same system but so far away that it would be a perfect candidate for a long-distance probe if you happened to be from that system. I suspect we would have already sent a probe out to explore the area around that star if we were from the Alpha Centauri system.

(Proxima Centauri is some 13 000 AU away from the other two stars, and Voyager 2 has reached 87 AU. Still, that's much closer than the Alpha Centauri system, at some 270 000 AU from us, and a dedicated mission with our technology could reach it.)


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