Although the search for extrasolar planets (planets orbiting other stars) has yielded many scientific curiosities in the form of so-called "hot Jupiters" and super-Earths, the ultimate goal of this research is to find Earth-like planets: rocky planets that are small enough, and close enough to their star (but not too close) that life might plausibly develop there. So far, no Earth-sized, Earth-like planets have ever been found, but astronomers are coming closer and closer.
- The Search Problem -
Unfortunately, searching for Earth-like planets with current telescope technology is somewhat akin to searching for bacteria with a common magnifying glass instead of a microscope: you will not be able to find any, and it's easy to conclude that therefore none exist. The problem is that planets, even ones as large as Jupiter, are far smaller than stars and do not emit light which we could observe directly. The result is that most current searches for Earth-like planets resort to a variety of "tricks" in order to find planets, like searching for the tell-tale wobble in a star's own rotation caused by its planets tugging it slightly out of its course.
Since the first confirmation of an extrasolar planet in 1992, the European-based Extrasolar Planets Encyclopedia catalogue has recorded over 440 known planets. Because the crude detection techniques available make it far easier to find large planets than small ones, the vast majority of these have been large, heavy planets, predominantly gas giants. (This does not mean the search was insignificant: astronomers have discovered, for example, that there are a large number of Jupiter-sized planets orbiting stars even closer than Mercury orbits our own Sun.)
In addition to size, a key criterion for Earth-like planets is how far they are from their star. The range for potentially habitable worlds lies roughly between Venus and Mars. So far as we can tell, whether the right sort of planet will end up at these coordinates is largely random.
- Earth-Like Planets -
As a result, relatively few of the smaller, rock-based planets have been located, and most of these are several times as large as Earth. For this reason they are unofficially called "super-Earths." These planets might resemble Earth in many respects, although because of their much greater size, we would find the gravitational forces crushing.
However, the newest entrant into the search for Earth-like planets, the recently launched Kepler Space Telescope, is far more powerful than its Earth-based predecessors. Over the next several years, it is believed that Kepler will be able to find Earth-like planets around relatively nearby stars - assuming, of course, that there are Earth-like planets to be found.
- Most Earth-Like Planets -
So far, the most astronomical interest in Earth-like planets has focused on a pair of stars: COROT-7, and Gliese 581. Circling COROT, which is about 500 light-years from the Earth, is a planet currently known only as COROT-7b, slightly less than two times as large as Earth. Estimates of COROT-7b's mass indicates that it is likely made up of a low-iron and high-water-content surface. Unfortunately, COROT-7b orbits dangerously close to its star, just a few percent as far from the surface of COROT as the planet Mercury is from our own Sun. This is far too close to be safe for life as we understand it.
A number of candidate planets have also been identified in the fascinating Gliese 581 star system, which looks like it may be a system of mixed planets a little like our own. Gliese 581 is a comparatively close 20 light-years away and, in a fit of optimism, Bebo.com paid for a message to be beamed to Earth by a Ukrainian government telescope, which will arrive in 2029. (If anyone there is listening, we could receive a reply as soon as 2050.)
Gliese 581 has four known planets. The first, Gliese 581b, is unacceptable: it is roughly the size of Neptune, and likely a gas giant. Gliese 581d comes closer, at about 10 times the size of Earth. However, Gliese 581c is five times as large, and Gliese 581e is less than twice as large. (So far, Gliese 581e is the smallest extrasolar planet that scientists have found, anywhere.) Unfortunately, Gliese 581e is believed to orbit too close to its star to support life, with a year lasting just three days. As a result, while some of these planets are Earth-like in some respects, scientists have yet to find a planet that is Earth-like in all respects.
There is always hope, however. Just a short while ago, in December 2009, a Harvard University team discovered a planet orbiting GJ 1214, about 40 light-years away, that appears to be made up mostly of water, making it the most Earth-like planet found yet in one key respect. Unfortunately, GJ 1214b is also much too close to its star, with the heat at its equator averaging over the boiling point of water. If this is true, its hypothesized oceans would survive only by being exceptionally deep as well as having an extremely cold night during which to re-condense.