News from the world of maths: Small, cool planet found (27/01/2006)

Scientists have found the smallest extrasolar planet yet. The inspiringly named OGLE-2005-BLG-390Lb orbits a red dwarf about 22,000 light years away, close to the centre of the Milky Way. Its mass is between 2.8 and 11 times that of the Earth's mass, possibly making it the least massive planet that has ever been found outside our own solar system. It's about 2.6 times as far away from its star than the Earth is from the Sun. This distance, together with the fact that the star is a lot smaller and less bright than our Sun, means that temperatures on this icy, rocky planet are somewhere around -220° — too cold for life as we know it.

The planet was discovered using a new technique called gravitational microlensing. Until now, planets were spotted by the way their gravitational pull makes their host star wobble, a technique which uses the Doppler effect (see Plus article Brave young worlds). But for this to work, the planet has to be massive enough or close enough to its star to exert the necessary pull. Consequently, all planets found so far are much more massive than OGLE-2005-BLG-390Lb.

Gravitational microlensing uses a different approach: as a star passes in front of a more distant reference object, its gravity acts like a lense and increases the object's brightness for a period of a few weeks. If the moving star has a companion planet, then this gives the brightness an extra spark. Although this is the third exoplanet discovered using gravitational microlensing, it's the first one of such low mass. Scientists think that these small worlds are much more common in the universe than was previously assumed. To find out more and see some pictures read this article from the European Southern Observatory.