New way to open Aladdin's Cave

A week ago it was announced that the second definite discovery of a planet orbiting a distant star has been made using a method called "gravitational microlensing". The method uses the gravitational field of one star to amplify the light of a more distant one. It was an astronomical milestone for a number of reasons.

While nearly 160 extra-solar planets (exoplanets for short) have now been found, most have been what astronomers call "hot Jupiters". By this they mean massive planets comparable with Jupiter but orbiting very close to the parent star. These are very unfamiliar worlds and quite unlike the planets of our own solar system.

The new planet is several times heavier than Jupiter but it lies further from its star than Mars is from the Sun. This means that it will be a cold, gaseous world and more like Jupiter than most of the previous exoplanet discoveries.

And that is the real advantage of the gravitational lensing technique. It is more likely to find planets in orbits similar to the Earth, Mars and Jupiter than anything else, so the method is more likely to discover planetary systems similar to our own. In fact, the detection method is so sensitive that in favourable cases it may even be possible to identify Earth-sized planets within the "habitable zone". This is a very important issue in astronomy today - exactly how common are Earth-sized planets? We only know of one - Earth - so it will be a major achievement to find one more.

Beyond the discovery of this "cool Jupiter", astronomers are also excited about other things they are able to learn from these new observations. For example, the size of the distant background star can be measured with amazing precision considering it is over 20,000 light years away. The measurement of the star's diameter is equivalent to seeing a 50c coin, edge on, standing on the surface of the Moon. In comparison, the Hubble Space Telescope would have trouble spotting Eden Park on the Moon.

Even details of the structure of the star's atmosphere can be derived, an extremely rare piece of stellar intelligence that has almost never been obtained before.

Right now, a team of astronomers scattered around the world - mostly the ones who use computers rather than telescopes - are have begun mining these new observations to extract all the amazing details they contain. In time this may well become routine but for the present, astronomers are just starting to glimpse the riches of a new Aladdin's Cave.