Distant planet is second-smallest super-Earth

Astronomers using the Keck Telescope in Hawaii discovered the alien world, called HD156668b.

The planet sits in a star system about 80 light-years from Earth in the constellation Hercules and orbits its parent star once every four days or so. It is the latest extrasolar planet to join the ranks of the so-called "super-Earths," worlds slightly larger than our own.

"This is quite a remarkable discovery," said astronomer Andrew Howard of the University of California at Berkeley. "It shows that we can push down and find smaller and smaller planets."

Howard announced the planet's discovery Thursday at the 215th meeting of the American Astronomical Society in Washington.

The smallest extrasolar planet currently known is called Gliese 581 e. It has a mass that is nearly twice that of Earth and orbits a planet-filled star system about 20.5 light-years away in the constellation Libra.

Howard and colleagues discovered the new planet by observing the "wobble" in its parent star. Researchers at the California Institute of Technology, Yale University and Penn State University participated in the study.