Planet hunters are getting closer to finding an alien version of home. Astronomers have identified the smallest planet known to orbit a normal star, with a mass just 7.5 times that of Earth--half the size of the previous record-holder. Among the 155 known exoplanets, the new world joins a tiny subset that may consist mainly of rock.
Gravity betrays the existence of most exoplanets. As a world circles its sun, the planet tugs back and forth on it. Telescopes can detect those tugs as slight wobbles in the star's light. This technique easily reveals titans the size of Jupiter, but smaller planets exert tiny tugs. Last year, rival European and U.S. teams used sensitive new measurements to discover three hot worlds roughly the size of Neptune (ScienceNOW, 31 August 2004:), which theorists regarded as plausible rocky bodies.
Now, the U.S. team has found a yet smaller world, so close to its star that its orbital "year" takes a mere 1.94 days. The planet circles a dim red dwarf called Gliese 876, just 15 light-years from Earth. Astronomers already knew that two large gaseous planets orbited Gliese 876, one in 30 days, the other in 60. By monitoring this resonant pattern, a team led by astronomer Geoffrey Marcy at the University of California (UC), Berkeley, noticed a distortion that pointed toward a third planet. A model created by theorist Eugenio Rivera of UC Santa Cruz, combined with more observations, pinpointed the new world's small orbit and its mass. Marcy's team described the work today at a press conference at the National Science Foundation in Arlington, Virginia.The planet is just 3 million kilometers from its star and probably roasts with a temperature of 200 to 400 degrees Celsius, says theorist Jack Lissauer of NASA's Ames Research Center in Mountain View, California. Team members lean toward a solid nickel-iron rock composition for the hot planet, possibly with a thick atmosphere. "But it could be more akin to a small Neptune," notes Lissauer, with a high proportion of gases and interior ices. Astronomers will resolve that issue when they find a planet of similar mass that eclipses its star, says theorist Alan Boss of the Carnegie Institution of Washington, D.C. That observation would yield the planet's size and density. For now, Boss thinks the new world is not substantively different from the three planets announced last year. "I view these as members of a single class," says Boss. "They are possibly rocky worlds."Related sites:
Images and animation from NSF
California-Carnegie planet search team
Background on Gliese 876
Extrasolar planets encyclopedia