Here's some timely news, about a week after I attended a Secret Science Club lecture about the search for earthlike planets around distant stars, the Kepler team found a rocky planet orbiting in the habitable zone of another star. Phil Plait describes the planet in Slate thus:
This doesn’t mean the planet is Earthlike, though. For one thing, it’s bigger than we are: Its diameter is 1.6 times that of Earth. We don’t know its mass, unfortunately, and without that we can’t know its density. The density is what gives us our first clue about what the planet’s made of; water has a density of 1 gram per cc, but iron is 8. Rock is 2–3.
If the planet has the same stuff in it as Earth does, it’ll be more massive; four times Earth’s mass*. In that case, its surface gravity would be 1.6 times Earth. If you weighed 100 pounds on Earth, you’d weight 160 pounds there. But only if it’s rock and metal like we are. If it’s less dense (more rock) than, the surface gravity will be lower; if it’s denser (more metallic), it’ll be even higher.
Sounds like Jack Vance was onto something... at any rate, it's great to read that Kepler has had such a resounding success. How soon before we're watching alien sitcoms, picked up by radio telescopes?