We've posted in the past about notable discoveries of planets outside of our solar system. In every case, however, the planets identified (over 125 so far) were so-called "gas giants" -- planets like Jupiter or Saturn, not rocky "terrestrial" planets like Earth or Mars. Despite occasional anxious supposition that this may mean that Earth-like planets are vanishingly rare, the reality is that our current tools for finding planets outside the solar system work best at finding very large planets very close to their host stars.
But that doesn't mean that's all we'll find. Today, NASA announced something new -- the first sub-Neptune-sized planets found outside the solar system. One is in the system 55 Cancri, about 41 light years away, and part of a system already known to have 3 gas giants; the other is in Gliese 436, only 30 light years away. They're each only about twice the diameter of Earth, and about 10-20 times the Earth's mass (Jupiter, by comparison, is 11 times the diameter, over 300 times the mass, and 1300 times the volume of Earth). Like nearly all the other extrasolar planets found so far, these planets orbit extremely close to their parent stars, much closer than Mercury does to the Sun. Because of this, it's highly unlikely that planets that small could form gas atmospheres; therefore, these are not only the first sub-Neptune sized extrasolar planets we know of, they're also probably the first rocky -- terrestrial -- extrasolar planets found.
(NASA put together an animated "fly through" of the 55 Cancri system, available at this link.)
I know that the discovery of extrasolar planets ranks pretty low on the immediately world-changing scale. But longtime readers of WorldChanging should know by now that I like to think Big Picture and Long Term, and discovery of extrasolar planets, especially terrestrial ones, fits both patterns. Moreover, part of understanding our world is understanding its place in the universe. I, for one, am very happy to see this ongoing exploration.
Actually, NASA missed announcing the first sub-Neptune-sized planet by about a week. A European planet hunt announced the discovery of a planet that's roughly 14 Earth masses last week. It's good to see there's some old-fashioned rivalry going on here: it'll speed up the quest for the first terrestrial planet since both teams will be looking to be first.