Discovery of a new icy world

Artists impression of the newly discovered earth-like planet. © ESO

An international team of astronomers have discovered a new extrasolar planet that is thought to be more Earth-like than any other planet found so far. The planet, which has been given the rather uninspiring name of OGLE-2005-BLG-390Lb, is only 5 times the mass of Earth and orbits its parent star every 10 years. If it was located in own solar system, the planet's orbit would lie between that of Mars and Jupiter.

The planet most certainly has a rocky/icy surface and some form of atmosphere, but with temperatures around 220°C below zero, it is highly unlikely that life could exist on its surface, due to absence of liquid water - thought crucial to the presence of life. The new planet orbits a small red star located at a distance of 20,000 light years, which is fairly close to the centre of our Milky Way galaxy.

This planet was the first to be found using a technique known as gravitational microlensing, which is based on an effect noted by Albert Einstein in 1912. Einstein predicted that light passing a massive object would be bent by the object's gravitational pull. As such the gravity of a dim, intervening star can act as a giant natural telescope for us, magnifying a more distant star, which then temporarily looks brighter. Small defects in this brightening curve (see image below) can reveal the existence of a planet around the lens star. We don't see the planet, or even the star that it's orbiting, we just see the effect of their gravity.

Such an intervening star causes a characteristic brightening of the distant star that lasts about a month, whereas any planets orbiting this star can produce an additional spike in the signal lasting days for giant planets down to hours for Earth-mass planets.