Exoplanet 'more likely to be brown dwarf'

Nice pictures, but don't get too excited

By Lucy Sherriff, 6th April 2005

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A European team of astronomers claims to have taken the first direct image of an extra-solar planet. Their announcement comes just weeks after two US teams made a remarkably similar boast, and has prompted a sceptical response from some parts of the astronomical community.

The possible exoplanet is orbiting a star known as GQ Lup, thought to be similar to our own Sun, when it was younger. The BBC has an image here.

The youth of the system was a crucial factor in identifying the possible planet. Because it is so young, the planet, which is between one and 42 times the mass of Jupiter, is relatively bright. The researchers were also helped by the fact that its orbit is also quite wide: it circles the star at around 100 times the distance that Earth orbits the sun.

The team made the discovery using image data from the European Southern Observatory's (ESO) Very Large Telescope (VLT) in Chile, the Hubble Space Telescope and the Japanese Subaru Telescope in Hawaii.

But not everyone is convinced. Jean Schneider, who maintains a catalogue of exoplanetary candidates at the Observatoire de Paris, notes: "The value of the mass, resting on planet models, is very uncertain and different with different models".

In other words: if the planet is one or two times Jupiter's mass, then we could be onto something. But if it is 42 times Jupiter's mass, we are more likely to be looking at a failed star, or brown dwarf, which would make GD Lup a binary system.

Indeed, only a few weeks ago (in March), scientists reported discovering a miniature solar system, whose central star was no more than 96 times more massive than Jupiter. A body is considered a star if it is 75 times as massive as Jupiter, while objects more than 13 times as massive are generally considered brown dwarfs.

Professor Mark McCaughrean, from Exeter University, told the BBC: "Almost all current theoretical models would peg the mass of this object at somewhere between 15 and 40 Jupiters: it's much more likely to be a brown dwarf (a failed star), not a planet."

He added that there was "no evidence whatsoever" that the planet actually orbits the star.

"Evidence for a co-moving sub-stellar companion of GQ Lup" has been published in the journal Astronomy and Astrophysics. ®