Planetary Picture? Criteria for planethood cloud object’s identity

When is a Jupiterlike object a planet and when is it just a blob of gas? That’s the question that astronomers are debating this week after two teams independently confirmed that a distant speck of light recorded a year ago is the first image of a planetary-size body beyond the solar system.

PLANET OR NOT? Planetary-mass companion (red) to the brown dwarf 2M1207. Chauvin, et al./ESO

Two to five times as massive as Jupiter, the body would seem to automatically qualify as a giant planet. But the object almost certainly doesn’t meet what many astronomers consider to be an equally important criterion for planethood: formation from a swirling disk of gas and dust that surrounds a young star.

The distant object orbits a brown dwarf rather than an actual star. Although brown dwarfs form as stars do, from the collapse of a cloud of gas and dust, they’re not massive enough to shine like stars do. Nor are they likely to have the heft to create a disk with enough material to make Jupiterlike planets.

The brown dwarf in the new studies is about 8 million years old. New estimates indicate that this object, known as 2M1207, lies some 160 light-years from Earth.

In April 2004, a team led by Gaël Chauvin of the European Southern Observatory in Santiago, Chile, used the Very Large Telescope (VLT) in Paranal to image a faint, red dot of light adjacent to 2M1207. The brightness and spectra of the dot suggested that it’s an image of a planetary-mass companion to 2M1207, orbiting at a distance greater than Pluto’s separation from the sun. However, the team couldn’t rule out the possibility that the red dot was a much heavier background object (SN: 9/18/04, p. 179: Sky Lights: Picture might show an extrasolar planet).

Follow-up observations with the VLT last February and March showed that the planetlike body indeed circles the brown dwarf, Chauvin’s team reports in an upcoming Astronomy & Astrophysics. A second team, led by Glenn Schneider of the University of Arizona in Tucson, came to the same conclusion after imaging the system with an infrared camera on the Hubble Space Telescope in August 2004 and late last month.

In announcing the findings this week in Baltimore at a symposium on extrasolar planets, Schneider said that he would leave it to the theorists to decide whether the object should be considered a planet.

While referring to the object as a planet, Chauvin’s team acknowledges that the orbiting body probably didn’t coalesce from a disk surrounding the newborn brown dwarf but instead arose at the same time and from the same cloud of gas and dust as the brown dwarf did.

The extremely low mass of the orbiting object “is an excellent discovery of a new class of object,” rather than an extrasolar planet, says Alan Boss of the Carnegie Institution of Washington (D.C.).

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