Mini-planet almost fits Earth’s profile

Image: Extrasolar planet
Artist's conception of the newly discovered planet MOA-2007-BLG-192Lb orbiting a brown-dwarf "star" that's only 6 percent as massive as our sun. Theory suggests that the three-Earth-mass planet is made primarily of rock and ice. Observational and theoretical studies of brown dwarfs reveal that they have a magenta color due to absorption by elements such as sodium and potassium in their atmospheres.NASA
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There's a new extrasolar planet on the block: a mini-orb likely covered with a deep ocean. And it takes the record for the lowest-mass exoplanet to orbit a normal star, astrophysicists announced Monday.

The li'l planet — weighing in at three times Earth's mass ­— grabs the lightweight title from a five-Earth-mass planet just announced in April.

The super-Earth is called MOA-2007-BLG-192Lb, after its host star MOA-2007-BLG-192L, which is located about 3,000 light-years from Earth. (A light-year is the distance light travels in one year, or about 5.88 trillion miles — 9.46 trillion kilometers.)

The host star's mass is estimated to be roughly 6 percent of the sun's mass, or just below the mass needed to sustain nuclear reactions in its core, thus making it a brown dwarf. Measurement uncertainty means the host mass could be slightly above 8 percent of a solar mass, which would make MOA-2007-BLG-192L a very low-mass hydrogen-burning star. The researchers suspect that the star is indeed a brown dwarf.

"Our discovery indicates that that even the lowest-mass stars can host planets," said lead researcher David Bennett of the University of Notre Dame. "No planets have previously been found to orbit stars with masses less than about 20 percent of that of the sun, but this finding suggests that we should expect very low-mass stars near the sun to have planets with a mass similar to that of the Earth."

Bennett announced the discovery here at a meeting of the American Astronomical Society.

The planet orbits its host star at about the same distance as Venus orbits the sun. But the new planet's host star is likely between 3,000 and 1 million times fainter than the sun, so the top of the planet's atmosphere is probably colder than Pluto.

The astrophysicists suggest the tiny planet supports a thick atmosphere, which along with possible interior heating by radioactive decay, could make the surface as balmy as Earth's. (And theory suggests the surface may be completely covered by a deep ocean.)

The star-planet system was found using a technique called gravitational microlensing, in which light from the planet is bent and magnified by the gravity of a foreground object, such as a star. It marks the seventh planet to date discovered using this method.

"This discovery demonstrates the sensitivity of the microlensing method for finding low-mass planets, and we are hoping to discover the first Earth-mass planet in the near future," Bennett said.

Most of the nearly 300 exoplanets identified to date have been discovered using the radial velocity method, in which astronomers look for slight wobbles in a star's motion due to the gravitational tug of an orbiting planet. This stellar wobble technique has found mainly large, Jupiter-type planets or smaller planets that orbit too close to their host stars to harbor life.

The research, funded by the National Science Foundation and NASA, will be detailed in the Sept. 1 issue of The Astrophysical Journal.