A newly discovered planet light-years from Earth is just four times the mass of our home planet, making the second smallest extrasolar planet to be found to date.
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.
Their tried-and-true planet detection method measures the effects of a planet's gravity on its parent star. As the planet passes behind its stellar parent as seen from Earth, its gravity tugs slightly on the star, causing the starlight to shift to a redder wavelength in the light spectrum. When the planet passes in front of the star, the effect causes the light to appear bluer.
More than 400 extrasolar planets have been discovered using the same method. But most of those planets are giants similar in mass to Jupiter.
Howard said it has been the long-standing goal among astronomers to find low-mass planets similar to Earth, though their size makes them difficult to detect.
Planet hunter Geoff Marcy, who is also of the University of California at Berkeley and participated in Howard's study, created the Eta-Earth Survey for Low Mass Planets to hunt for super-Earths. So far, the survey has found two such planets, Howard said.