Hard Feelings Over Small Planets

Alien Neptune. A possibly rocky planet at least 14.2 times as massive as Earth orbits the star ? Cancri, in this artist's conception.

Two U.S.-led teams of astronomers have found Neptune-size planets orbiting stars beyond our solar system, bringing to three the number of such objects revealed in the last week. But the joyful glow over the new worlds--which may be the first rocky bodies known to circle other ordinary stars--was dimmed by a European team's preemptive announcement that stunned American researchers.

Of about 130 known exoplanets, astronomers think nearly all are vast spheres of gas like Jupiter. As a gas giant orbits, its gravity tugs its parent star to and fro. That periodic motion creates wobbles in the starlight, which sensitive telescopes on Earth can detect. Eager to find smaller, solid bodies, planet hunters have refined their techniques to spot ever-tinier stellar motions. For now, their quarries are planets like Neptune and Uranus, which hide major cores of ice and some rock beneath their gaseous mantles. Models show that similarly sized planets--roughly 15 to 20 times the mass of Earth--consisting mostly of rock could coalesce in the warm portions of dusty disks around newborn iron-rich stars.

Objects that might inhabit this realm popped up quickly this summer. A group led by astronomers Paul Butler of the Carnegie Institution of Washington, D.C., and Geoffrey Marcy of the University of California, Berkeley, found a planet with at least 21 Earth masses orbiting a red dwarf star. Then, a team led by astronomer Barbara McArthur of the University of Texas, Austin, detected a body of at least 14.2 Earth masses orbiting a star already known to harbor three other planets. "For the first time, it's plausible that these are mostly rocky iron balls, with surfaces enabling liquid water to puddle on them," Marcy says. "This is putting us on the doorstep of detecting other Earths." The astronomers discussed their results on 31 August at a NASA press conference in Washington, D.C.However, a European team had already trumpeted its discovery of the smallest exoplanet last week, just 5 days after its last observations (ScienceNOW, 25 August ). The release of unreviewed research struck American astronomers as a hasty rush to gain the first public acclaim. In an odd twist, several of the European scientists also are co-authors on McArthur's paper, fully refereed and originally scheduled for public release in mid-September. "I was shocked," says McArthur of the decision by her European co-authors. A colleague adds: "It's outrageous, and everyone sees it that way."

Related sites
More information on U.S. planet discoveries
Abstract of European paper, with link to full text

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