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Habitable planet?

Gliese 581, a red dwarf star already known to have at least one other planet, has a potentially habitable planet. It's somewhat larger than Earth (about 5 times as massive), and its average temperature appears to be well within the liquid range for water.

First off, this detection is amazing. Just finding something like this is a challenge; most exoplanets are much more massive, making them that much easier to detect. And then there's the question on everyone's mind: Is this planet truly Earthlike? It's reasonably sized, and could have the right temperature, but beyond that, it's really hard to answer. For one thing, it already has a massive planet, about the mass of Neptune, orbiting even closer still. This complicates the system's dynamics significantly. The smaller planet's orbit is obviously stable in the long term, but even small orbit changes can have significant effects on a planet's climate. As far as I can tell, this is still an open question; astronomers haven't really figured out whether or not a large planet like this would make the smaller, more Earthlike planet uninhabitable. Additionally, red dwarf stars can subject planets to unusually high tides compared to larger stars. In extreme cases, this can lead to a planet having one side always face its star, much as the Moon always keeps one face towards Earth.

UPDATE: The overall dynamics may not be a severe problem. Steinn Sigurdsson writes:

In short, the planet configuration is stable and the inner Neptune mass should have neglibible influence on the middle planet (other than being a spectacular evening/morning star) - only way it would affect the climate on the middle planet is if it affected tidal locking process - the middle planet is just outside a 2:1 orbital resonance with the inner planet - hm, in fact it might be in the 12:5 resonance which is a bit weird - but their final orbital configuration might have been dynamically coupled, this could influence whether the middle planet locked to its orbital period in 1:1 rotation resonance.

Of course, we still don't know enough about the atmosphere, actual temperature, etc. to answer other questions about habitability.

If there were intelligent beings living here, what would they experience? More below.

At the moment, no SETI project on Earth has picked up any unusual signals from Gliese 581, but it's certain to draw extra SETI attention in the future. Gliese 581 isn't very far from Earth as stars go -- only 20.4 light years away. In fact, some of the major constellations look pretty much the same as they do from Earth. The Big Dipper is there, though it looks a bit squashed. So are the Pleiades and Orion, although the latter has a couple of new bright stars:

If a "Gliesean" turned a radio telescope just off Orion a bit, one somewhat dim star (4th magnitude) would be curiously radio-noisy:

Looking at this, from the perspective of the possible residents of Gliese 581, I am reminded of one of Carl Sagan's most famous essays. The possibility of distant life, even if it's not realized on Gliese 581, gives a new perspective to a certain pale yellow dot.

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