A small planet just seven or eight times as massive as the Earth has been found circling a nearby star. Astronomers say it is the most Earth-like world we have ever seen beyond our solar system.
“For the first time, we are beginning to find our planetary kin among the stars,” says Geoff Marcy of the University of California at Berkeley, US. His team announced the discovery at a news conference in Arlington, Virginia, US, on Monday.
Over the past decade, astronomers have discovered about 150 extrasolar planets circling ordinary stars like the Sun. The vast majority have been gas giants larger than Jupiter.
In 2004, Marcy’s team reported discovering smaller Neptune-sized planets which were 18 to 25 times as massive as the Earth, while another team in Europe reported a Uranus-sized world about 14 times the mass of Earth.
But the new “super-Earth” is by far the smallest planet seen circling a commonplace star. The team discovered it while observing a star called Gliese 876 from the Keck Observatory in Hawaii. Gliese 876 is a red dwarf, one-third of the mass of the Sun, and lies just 15 light years away in the constellation Aquarius.
A slight “wobble” of the star had already revealed that it is being tugged by the gravity of two gas giants as they orbit around it with periods of 30 and 61 days.
But careful monitoring of the star over three years has now confirmed that there is also a small planet six to nine times as massive as the Earth whipping around the star every 1.94 days. Fly around Gliese 876 in this streaming movie (requires RealPlayer).
It might well be a rocky planet with a dense atmosphere. However, it is impossible to know for sure because no example planet of this mass exists in our own solar system. Its mass lies mid-way between that of Earth and Uranus – two very different worlds.
“We actually have no direct information about the chemical or mineral composition of the planet, nor do we know if it’s primarily rocky, like Earth, or a combination of rock, ice and gas,” says Marcy.
But it is certain that the planet is swelteringly hot. It lies only about 3.2 million kilometres from its star – just 2% of the Earth-Sun distance – and its surface temperature must be higher than 200ºC. “Because the planet is in a two-day orbit, heated to oven-like temperatures, we do not expect life,” says Marcy’s colleague Paul Butler of the Carnegie Institution of Washington, US.
The team says the new planet is unlikely to remain a record-breaker for long. At the Keck Observatory, it is now possible to measure extremely subtle star wobbles, so even smaller planets should soon turn up. And space-based missions such as NASA’s Terrestrial Planet Finder, due for launch around 2014, should spot habitable Earth-like planets in droves.
Marcy’s team has submitted the results to The Astrophysical Journal.
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