NASA – NASA’S Kepler Mission Discovers Its First Rocky Planet. It is a little bigger than the Earth, but it orbits its star every 20 hours. This makes its temperature something like 1800 K / 3300 F, as hot as a lightbulb filament in action.
This is only the latest in a remarkable revolution in astronomy: the discovery of extrasolar planets. Since the 1990’s, over 400 of them have been discovered, and some of them have very startling properties. Several of them are “hot Jupiters”, gas-giant planets that are unexpectedly close to their primary stars. They most likely formed much farther out, then spiraled in. Even more oddly, many of them have very eccentric orbits, with eccentricities typically around 0.4 and as much as 0.7 — 2 to 6 times farther at farthest distance than at closest distance. Near-collisions between inspiraling planets? (More on this mystery) If they get close enough, their outer layers may evaporate off into outer space, leaving a rocky core, like the planet I mentioned above.
There are even planets orbiting pulsars, of all places. How did they get there?
To date, all the planets discovered orbiting “normal” stars are at least a few times more massive than the Earth, but that’s a side effect of detection sensitivity. The smaller the planet, the smaller the effect. But as astronomers improve their techniques, they may eventually detect Earth-mass planets.
Despite that, astronomers have often been able to detect more than one planet orbiting a star, and in the case of Gliese 581, at least 4 and possibly 6. Some of Gliese 581’s planets orbit at about the right distance to have liquid water on their surfaces, thus making them potentially habitable. However, Gliese 581 is a red dwarf, a relatively faint star, meaning that these planets orbit close enough to become tidally locked. So Gliesians, if any, would see their sun more-or-less fixed in the sky.
Some history and detection details: