An artist's impression of the Gliese581 planetary system.
Credit: The Planetary Society
People have reasoned for hundreds of years that at least some of the other stars in the universe must have planets orbiting them.
After all, the Sun has eight planets, over 160 known natural moons, and thousands of asteroids. But scientists only discovered the first planets in orbit around a star other than our Sun in 1991.
Today, more than 443 planets have been found (in 351 planetary systems). Most of these are giants like Jupiter, in short-period orbits, resulting in extremely hot surface temperatures.
None of the planets discovered so far are as small as Earth. The smallest planet to date orbits a star called Gliese 581, with a mass of 1.9 Earth masses.
On 6 March 2009 NASA launched Kepler, a mere 2.7-metre-long spacecraft that will provide a rough estimate of how many Earth-like planets there are in our galaxy.
Kepler is looking at just one large area of the sky, in the constellations Cygnus and Lyra, and will simultaneously measure the variations in the brightness of more than 100,000 stars, searching for the tiny 'winks' in light output that happen when a planet passes in front of its star.
A spacecraft designed to last 3.5 years, Kepler is not searching for life, but it expected find the planets that may harbour life.
In January 2010, it was announced that Kepler had detected five exoplanets, all 'hot Jupiters' - an extrasolar planet that is similar in size to Jupiter but orbit much closer to their parent stars - these planets are unlikely to harbour life.
"The Kepler spacecraft is expected to detect a handful of Earth-sized planets within the next few years. If it doesn't we will have to revise our thinking about the way stars and protoplanetary disks of gas and dust actually form planets," says Jill Tarter, research director of the SETI Institute.
Unfortunately, the worlds that Kepler is likely to discover are too far away for the next generation of space telescopes to study more closely and look for evidence of life in the mix of their atmospheres.
Find out more in the latest issue of Cosmos magazine, with a 39-page special on SETI!
Jacqui Hayes is the deputy editor of Cosmos magazine and editor of Cosmos Online.
