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
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