• Dick writes about Earth and planetary science for Science magazine.

Youngest Extrasolar Planet Reported

27 May 2004 (All day)

Troublesome youngster A young planet around star CoKu Tau 4 could cause trouble for theories of planet formation--if its existence can be confirmed.

WASHINGTON, D.C.--Astronomers reported at a NASA press conference here today that they have found the youngest known planet circling another star. The star is so young that the putative planet would probably have formed by runaway collapse of the star's debris disk, the outer, doughnut-like part of which still circles the star. If so, proponents of an upstart theory of planet formation would get a boost.

At the press conference, astronomer Dan Watson of the University of Rochester in New York showed how infrared spectra recorded by the Spitzer Space Telescope have revealed a central hole in the disk of debris circling the nearby star CoKu Tau 4. The behavior of shorter-wavelength infrared radiation coming from the star compared to that of longer wavelengths from the dusty disk indicates that the debris has been cleared out near the star to form a gap, he said. Such gaps have been seen around other stars, but the Spitzer observations provide unprecedented clarity, Watson said. The most likely explanation for such a gap, he said, is that a planet formed there and swept up the debris. And while other examples of disk gaps are millions of years old, he said, this star “is only a million years old. That really causes problems for the standard model of planetary formation.”

The conventional way that planets like Jupiter form is by slowly accumulating icy and rocky bits until they generate enough gravity to pull in a planet's worth of gas. That's probably too slow to form the CoKu Tau 4 planet, astronomer Alan Boss of the Carnegie Institution of Washington told the press conference. He favors instead the rapid collapse of a disk's gas and dust into clumps that go on to form planets (Science, 6 June 2003, p. 1498). That would allow a planet around a star as young as CoKu Tau 4. It would also let planets form from even the shortest-lived disks, upping the number of planetary systems in the galaxy.However, other researchers are not convinced there's a planet there. “They certainly haven't detected a planet directly,” says astrophysicist Richard Durisen of Indiana University, Bloomington. A small, undetected companion star might have cleared the gap instead, for example. And “we don't know these ages very well,” he adds. The star might actually be old enough that the slower accumulation mechanism could work. Settling such questions, all agreed, may take more missions after Spitzer.

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