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Smallest extrasolar planet found
June 13, 2005

Artist's impression of extrasolar planet
The planet was discovered using the "wobble" technique
Astronomers have detected the smallest extrasolar planet yet: a world about seven and a half times as massive as Earth orbiting a star much like ours.

All of the 150 or so exoplanets found orbiting normal stars are larger than Uranus, itself 15 times Earth's mass.

The new find may be the first rocky world found around a star like the Sun.

The newly discovered "super-Earth" orbits the star Gliese 876, located 15 light-years away in the direction of the constellation Aquarius.

This star also has two larger, Jupiter-size planets orbiting it.

The new planet whips around the star in a mere two days, and is so close to the star's surface that its temperature probably tops 200 - 400 Celsius (400 - 750 degrees Fahrenheit) - oven-like temperatures far too hot for life as we know it.

The planet was discovered using the so-called "wobble" technique: as a planet orbits its parent star, it causes it to wobble slightly. This motion can be detected using sensitive instruments as a shift in the star's spectrum.

"We keep pushing the limits of what we can detect, and we're getting closer and closer to finding Earths," said team member Steven Vogt, professor of astronomy and astrophysics at the University of California, Santa Cruz.

The researchers have measured a minimum mass for the planet of 5.9 Earth masses. It orbits Gliese 876 with a period of 1.94 days at a distance of 0.021 astronomical units (AU), or 3.2 million km (2 million miles).

Though the team has no direct proof the planet is rocky, its low mass precludes it from retaining gas like Jupiter.

Three other supposed rocky planets have been reported, but they orbit a pulsar, the corpse of an exploded star.


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