YOU could be forgiven for having missed the discovery of the 100th planet orbiting distant stars. But for planet hunters, reaching this milestone earlier this year symbolised a coming of age.
In 2002 alone, the number of planets astronomers have discovered by detecting wobbles in a star鈥檚 orbit has nearly doubled. Wobbles happen when a planet鈥檚 gravitational field tugs on its star鈥檚 orbit, but detecting them is a painstaking process and takes as long as the planet takes to orbit its star. No wonder, then, that most of the planets found so far are enormous gas giants in small, irregular orbits.
But after 15 years of searching, planet hunters finally announced the discovery of a planetary system with at least a passing resemblance to our own. The system does have one Jupiter-sized planet in a circular orbit that takes 15 years to Jupiter鈥檚 11 years, but two more gas giants orbit very close in to the star, equivalent to Mercury鈥檚 position in our system.
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Not satisfied, the planet hunters are plundering data from the last 15 years for signs of a more genuine analogue of our Solar System, with gas giants like Jupiter and Saturn in distant circular orbits. But finding another Earth may be tricky, as it would be too small to have a detectable effect on its star鈥檚 orbit.
There鈥檚 a similar problem with another technique pioneered this year: detecting planets as they pass in front of their star, blocking out some of its light. But a more sensitive method that could detect smaller planets, planned for 2014,will cancel out the light from a star to leave only the weak infrared signature of any planets.
Besides the rising tally of known planets, there have been plenty of surprises for astronomers to get their teeth into. Infrared maser emissions from three separate planetary systems suggest water is present in the atmospheres of their gas giants, although the planets are almost certainly uninhabitable. Nearer home, planetary scientists are still squabbling over whether the huge underground ice deposits on Mars, confirmed this year by NASA鈥檚 Mars Odyssey, have any implications for the existence of primitive life.
There鈥檚 also renewed controversy among astronomers over data on the atmosphere of Venus. Some claim that the presence of the gases hydrogen sulphide and sulphur dioxide must be due to microbes in the planet鈥檚 atmosphere. Both discussions are unlikely to be resolved until the end of the decade, however, when the European Space Agency plans to send a probe to Venus to bring back a sample of its atmosphere, and NASA hopes to bring back soil from Mars.