One Step Closer to Earth's Twin

Astronomers have found more than 150 planets orbiting distant stars, but they've all been so much bigger than Earth that they're totally inhospitable to life. But now, with a little help from Albert Einstein, they've spotted a world that is by far the smallest ever found — albeit still about 5.5 times the mass of our home planet — which means that many more, even smaller planets, could be discovered reasonably soon.

The new world's unfortunate name is OGLE-2005-BLG-390Lb; it orbits a dim star in the direction of the constellation Sagittarius, about 21,000 light-years from earth, close to the core of the Milky Way. At that distance, even its home star is invisible, so astronomers resorted to a sort of cosmic optical illusion, first proposed by Einstein in the 1930s, to detect it. Einstein pointed out that since massive objects bend light rays, a star could act as a sort of lens, focusing and magnifying the light of a more distant star passing behind it. In his original paper, Einstein doubted that such a thing would ever be seen— but he didn't count on modern technology. Since 1997, a Polish-led team of astronomers has been using that technology in a project called OGLE, for Optical Gravitational Lensing Experiment (hence the planet's name) to search for just such magnifications. They've seen hundreds, but one that happened last July was followed a short while later by a smaller flicker— evidence that the magnifying star had a planet trailing behind (the planet's flicker was actually detected by another group of astronomers, in the Probing Lensing Anomalies Network, or PLANET collaboration, who had been alerted by the OGLE folks).

The intensity of the flicker told the scientists how small the planet was— too small to be a gaseous blob like Neptune— and therefore probably made of rock and ice; the timing told them it's about three times further from its star than we are from the sun. Its surface temperature is probably below -360° F, much too frigid to sustain life.

The good news, however, is that it's at least relatively earthlike— and because it's only the third planet discovered this way, the proportion of earthlike planets is probably pretty high. That means Einstein's trick of light is almost certain to yield others. So while this isn't quite the discovery astronomers have been hoping for, it's a signal that finding a close twin of Earth may not be too far off — or too pie in the sky.