Low-mass pair

This infrared image resolves 2M 1207 into a close pair of objects, one of them 100 times brighter than the other. Astronomers infer that their masses are 25 and 5 Jupiter masses, which would make them a brown dwarf and a planet, respectively. Both are glowing with heat left over form their formation. The image was taken with adaptive optics on one of the 8.2-meter VLT reflectors. The objects are separated by 0.78 arcsecond, which corresponds to about 55 astronomical units at the system's 230-light-year distance.

Courtesy Gael Chauvin / ESO.

A team of European and American astronomers has confirmed the first image of a planetary-mass object outside the solar system. This object orbits not a star, however, but a brown dwarf known as 2M 1207. The companion's estimated mass is about 5 times that of Jupiter.

The international team, led by Gael Chauvin (European Southern Observatory), obtained an image in April 2004 of 2M 1207 and a faint nearby object. The image was taken using adaptive optics at ESO's Very Large Telescope (VLT) in Chile. Hubble Space Telescope observations made a few months later by an independent group confirmed the cool nature of 2M 1207's companion and showed that the two objects likely have the same motion across the sky (common proper motion), which bolstered an already strong case that the two objects lie at the same, 230-light-year distance from Earth and are thus gravitationally bound.

In a new paper that has just been accepted for publication in the prestigious European journal Astronomy & Astrophysics, Chauvin's team provides more recent VLT observations that clinch the case that the two objects are bound. The team also has published a spectrum that shows that the companion's atmosphere is cool and laden with water vapor.

"This confirmation, by common proper motion, that the companion is really orbiting the brown dwarf puts the entire system on firm footing," says independent commentator Geoff Marcy (University of California, Berkeley), who leads the team that has discovered the majority of the 160 or so known exoplanets.

The companion lies at least 55 astronomical units (about twice the Neptune-Sun distance) from 2M 1207, meaning it will take more than 1,000 years to complete an orbit. Without the ability to measure the object's mass directly, Chauvin and his colleagues inferred its mass from evolutionary models that correlate a substellar object's mass with its age, temperature, and luminosity. The object also exhibits narrow spectral lines, which indicates an atmospheric pressure that one would expect in the relatively low-gravity environment of a giant planet rather than a brown dwarf.

"This object may be even lower in mass than 5 Jupiters," says discovery team member Ben Zuckerman (University of California, Los Angeles). "Two independent techniques agree that it's absolutely below the brown-dwarf threshold of about 13 Jupiter masses."

Zuckerman, Chauvin, and their colleagues have also imaged a substellar companion to the star AB Pictoris. This companion appears to be about 13 or 14 Jupiter masses — right on the borderline between a planet and a brown dwarf. Last month another team, led by Ralph Neuh√§user (University of Jena, Germany), released an image of a possible planetary-mass object orbiting the star GQ Lupi.


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