FIVE thousand light years away in the Sagittarius arm of the Milky Way there is a planet so hot that it rains iron. The discovery of this world, with the rather off-putting name OGLE-TR-56b, was announced last week at the American Astronomical Society meeting in Seattle. It is probably the oddest planet we鈥檝e ever come across, but the unique way it was found could be used to pinpoint planets more like Earth.
The planet was spotted by Dimitar Sasselov of Harvard University and a team based at Harvard and Caltech. They had been looking for planets outside our Solar System using a beautifully simple technique called a transit search: when a planet passes in front of its star, the star should get dimmer as a little of its light is blocked. In 1999, such a transit gave astronomers a view of Mercury in front of our own Sun (see above). If you can monitor a large number of stars for long enough with a sensitive camera you should eventually catch a planet.
OGLE-TR-56b is the first. The team used data from a project called the Optical Gravitational Lensing Experiment (OGLE), based at the Las Campanas Observatory in Chile. The OGLE data showed many stars dimming in this way, but most of them turned out to have tiny companion stars, rather than planets.
Advertisement
To confirm that OGLE-TR-56b really is a planet, the team used the Keck Telescope at Mauna Kea in Hawaii to measure wobbles in the star鈥檚 motion caused by its companion鈥檚 gravity. This is the 鈥渞adial velocity鈥� method, which has revealed about 100 extrasolar planets in the past few years. The size of the wobble depends on the mass of the companion, and in this case Sasselov and his colleagues showed that OGLE-TR-56b has about the same mass as Jupiter. A definite planet.
Orbiting at only a fiftieth of the distance of the Earth from the Sun, OGLE-TR-56b is closer to its star than any other known planet 鈥� far closer than the other 鈥渉ot Jupiters鈥� found in recent years. Until now, no planet had been found with an orbital period of less than 3 days, and it was beginning to look as though that was the lower limit, says Sara Seager of the Carnegie Institution in Washington DC. But on OGLE-TR-56b, the year lasts only 29 hours.
Because it is so close to its star, its surface is heated to a staggering 2000 K. 鈥淭his is a very interesting temperature,鈥� says Sasselov. 鈥淚t is just the right temperature for droplets of iron to form in the atmosphere.鈥�
But he is more excited that the discovery confirms the transit technique works. It is potentially much more powerful than looking for stellar wobbles 鈥� OGLE-TR-56b is fifty times farther away than any planet found before. Several transit projects are already running or are due to start soon, including STARE, Vulcan, PISCES and EXPLORE. 鈥淭here is going to be a tsunami of discoveries,鈥� says Sasselov.
The method may be sensitive enough to spot rocks as small as Earth. The transit of an Earth-like planet could be picked up by a satellite called Kepler, due to be launched by NASA in 2006. So we may not have to wait long to find a planet where the rain is more refreshing.
