Just fifteen years ago we could only guess that plants like ours must have orbited some of the billions of stars in the cosmos. Now astronomers have pinpointed hundreds in our galaxy. A few orbit their stars at distances that could allow life. but most of these in a so-called "habitable zone" have been giant planets, around the size of Jupter. These behemoths are called gas giants because they're all atmosphere - no continents, no oceans, no islands.
Yesterday NASA announced the discovery of a new planet, Kepler-22b, which is not only orbiting in a habitable zone, but is small enough to have a solid surface and oceans. They don't know that it does have a solid surface, but with a radius 2.4 times that of Earth's, it's possible. It lies 600 light years away and orbits its sun every 290 days.
We don't know if Kepler-22b is inhabited. A skeptical take was posted on Boing Boing. One thing we don't is how likely it is that life will emerge on any planet - even a nice one that has a moderate temperature and liquid water.
Since all life on Earth is related through a common ancestor, it's not obvious how to make a universal definition of life that would cover extraterrestrials. Life on other planets may use very different building blocks from ours. NASA's defintion draws on Darwin: Life is self-sustaining chemical system capable of Darwinian evolution.
Kepler-22b was picked up by a project that NASA rejected for more than 20 years – an orbiting space telescope called Kepler. It's designed to detect planets by looking for tiny eclipses of other stars when planets pass in front of them. Those eclipses would temporarily dim these stars by a tiny amount - around a hundredth of a percent. Such precision measurements were seen as a long shot, but so far it's working.
The Kepler telescope is designed to give us an estimate of the number of planets out there by surveying 100,000 stars in a part of our galaxy toward the constellation Cygnus. Even if everything works, the satellite will detect only 1 in 100 of the Earth-size planets up there, since these tiny eclipses will be visible only if the orbit of a planet is in a plane angled toward us. So the fact that it's found hundreds of planets means that there are thousands more.