Stellar Question: Extrasolar planet or failed star?

Astronomers have dreamed of photographing a planet orbiting a star outside the solar system. Last week, researchers announced that a tiny dot of light next to a young, sunlike star might be that long-sought image. But the discovery could be sinking under its own weight.

MYSTERY OBJECT. Tiny dot of light to the right of the star GQ Lupi in this image is either a brown dwarf or the first image of a planet orbiting a sunlike star. ESO

The body lies near the star GQ Lupi, 450 light-years from Earth. Last year, Ralph Neuhäuser of the University of Jena in Germany and his colleagues used the Very Large Telescope in Paranal, Chile, to obtain several images and spectra of the object. Archived images reveal that the object was first seen by the Hubble Space Telescope in 1999. The old and new images indicate that the body is bound to GQ Lupi but lies much farther from the star than Pluto does from the sun.

Other astronomers agree with those conclusions but debate the team’s estimate of the body’s mass. The standard way of determining the mass of a proposed extrasolar planet is to record the wobble that it induces in the motion of its parent star. But because the recently imaged body lies so far from GQ Lupi, the wobble isn’t detectable, and researchers must rely on models that use the temperature, size, and brightness of the body to estimate its mass. Such models are unreliable for extremely young objects such as the roughly 2-million-year-old companion to GQ Lupi, notes Didier Saumon of the Los Alamos (N.M.) National Laboratory.

Using two models developed by members of their own team, Neuhäuser and his colleagues calculated that the body probably has a mass between one and two times the mass of Jupiter. However, when the team used another model, which they claim is less applicable, they found that the body could tip the scales at a weight up to 42 times that of Jupiter. The researchers will report their findings in the May Astronomy & Astrophysics.

The high-end estimate places the object beyond the widely accepted 13-Jupiter-mass limit for a planet. Instead, the object would qualify as a brown dwarf, a body that forms the way stars do but quickly fizzles out. Astronomers have captured several images of brown dwarfs orbiting stars.

Mark McCaughrean of the University of Exeter in England says that the models the discovery team favors are unpublished and largely untested. Relying on a published and tested model of the brightness and temperature of young planets, he calculates that the object is a brown dwarf weighing between 15 and 40 Jupiters.

Gibor Basri of the University of California, Berkeley says that the object could be a planet after all. He bases his conclusion on models that his own research team has used to estimate the weight of tiny bodies seen in a separate region of the sky.

If the object around GQ Lupi is indeed a planet, it must have formed closer to the star than it now resides, some 60 to 130 times the Earth-sun distance. Planets coalesce from clouds of dust, gas, and ice that swaddle newborn stars, and the density of such material isn’t enough at great distances from the star to form a Jupiter-mass body. An unseen companion planet might have flung out the planet to its current location, notes Neuhäuser.

Saumon notes that the object’s large distance from GQ Lupi makes it even more likely to be a brown dwarf, not a planet. In contrast, the recently announced detection of infrared light from a closely orbiting planet with a well-characterized mass, even though it doesn’t constitute an image (SN: 3/26/05, p. 195: Alien Light: Extrasolar planets are detected in new way), is far more intriguing, he says.

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