You've probably heard by now, but scientists at the European Southern Observatory have announced that they have found the smallest planet yet around another star (other than around pulsars, which would be completely inhospitable to life). It's still bigger than Earth, about five times our mass, which means it would have a surface gravity about 1.6 times ours.
Most exciting, though, is that this planet is relatively temperate. It orbits a weak, red dwarf star (Gliese 581), that doesn't give off much heat. But it orbits close enough--about 6.7 million miles--to its parent star that its surface temperature is estimated to be between 0 and 40 degrees Celsius (32 to 104 degrees Fahrenheit).
It's too early to tell if the planet would be hospitable to life or--and how exciting is this?--if it already has life on it. All scientists know for sure is the planet's mass and distance from its parent star... the planet could be icy, with a much larger diameter and lower temperature. Or it could be shrouded in a very dense atmosphere, like Venus, in which case it would be extremely hot. But so far indications are good for the planet to be capable of sustaining liquid water, and thus capable of supporting life.
Gliese 581 is one of the one hundred closest stars to Earth, at only about 20.5 light years distant, in the constellation Libra. More resources should be put into probing stars in our stellar neighborhood, as it is quite possible that there are many, many more Earth-like planets to be found.
Over the next several years, several space-based missions will join the terrestrial telescopes that are already searching. Earlier this year, the ESA's COROT went into service studying the accoustical waves that ripple across the surface of stars as a result of planets orbiting around them, similar to tidal effects in our oceans here on Earth due to the orbit of the moon. Astronomers expect to find between 10-40 rocky worlds, together with tens of new gas giants, in each star field that COROT will observe. Every 150 days COROT will move to a new field and begin observing again.
Next to launch will be NASA's Kepler mission, currently scheduled for launch in October 2008. Kepler will essentially be a super-powerful photometer that measures the brightness of stars. When a planet passes between Kepler and its parent star, it will block a portion of the star's light, and Kepler will register this difference. It will do this for 100,000 stars over the course of its 4-6 year mission.
No sooner than 2015 or 2016, NASA will launch SIM Planetquest (formerly known as the Space Interferometry Mission), which will use interferometry to amplify the light of any planets and negate out the glare of the parent star to look for planets. The launch date for this mission has already been pushed back five times, and with NASA's science budget continuing to be squeezed, it's likely that it will be pushed back farther, or even cancelled.
NASA is also planning a pair of missions, called the Terrestrial Planet Finder missions. The first of these, the Visible Light Coronagraph (TPF-C), will collect starlight and the very dim reflected light from the planets. The telescope would have special optics to reduce the starlight by a factor of one billion, thus enabling astronomers to detect the faint planets. TPF-C could conceivably launch as early as 2014, but I wouldn't count on it.
The second TPF mission, the Infrared Optical Interferometer (TPF-I), will small telescopes on a fixed structure or on separated spacecraft floating in precision formation would simulate a much larger, very powerful telescope. The interferometer would utilize a technique called nulling to reduce the starlight by a factor of one million, thus enabling the detection of the very dim infrared emission from the planets. TPF-I could conceivably launch by 2020, but I doubt NASA will let it get that far.
Also on the far horizon is the ESA's Darwin mission, which would use three space telescopes flying in formation as an interferometer, similar to TPF-I. In addition to detecting Earth-sized planets, Darwin would be capable of determining their atmospheric content. Darwin is currently slated for a launch no sooner than 2020.
Some of these missions will never be launched for political or economic reasons, but the ones that do manage to make it to launch will bring potentially hundreds or thousands of discoveries of worlds outside our solar system, many of which may be capable of supporting human life.
Then we'll just need to find a way to get there.