Are imaged planets really failed stars?

BY KEITH COOPER

ASTRONOMY NOW

Posted: 22 April, 2009

Most of the images of exoplanets released towards the end of 2008 may not be planets at all, according to new research presented at the National Astronomy Meeting being held during the European Week of Astronomy and Space Science at the University of Hertfordshire. Ralph Neuhaeuser from the Astrophysical Institute and University Observatory in Jena, Germany, claims that the evidence is edging towards them being brown dwarfs instead, although a lot will depend on what is decided upon for the official definition of the lower mass of a brown dwarf.

The image of three planet-like objects around the star HR 8799. Image: Christian Marois and Bruce Macintosh.

Images of seven planets orbiting five stars were released to the public last year. These included three planets of 10, 10 and 7 Jupiter masses around the star HR 8799, a three Jupiter-mass planet orbiting in a protoplanetary debris disc around Fomalhaut at a distance 119 times that of the Earth-Sun distance, and a planet around the young T Tauri star CT Cha with a mass between 11 and 23 Jupiter masses. The latter was discovered by Neuhaeuser's own team.

However, Neuhaeuser points out that there are a number of assumptions being made that may not necessarily be correct. First, because these planets take hundreds of years to orbit their stars we have barely seen a fraction of their orbital movement, meaning that we cannot really apply Kepler's Law to ascertain their masses. Instead, we derive their masses from their age and brightness; younger planets that are still contracting and forming are generating more heat and are brighter than older planets. However, this calculation relies on assuming initial conditions for the formation of the planets and their parent stars. If any of those assumptions are wrong, their masses may be way off. And even if we do have their masses correct, it's not clear where the boundary between planets and brown dwarfs begins and ends, and indeed there is probably some overlap between the two populations.

To find out more about the three planets around HR 8799, watch our exclusive video with their co-discoverer Dr Jenny Patience of the University of Exeter.

Secondly, young stars are formed in clusters, and they share identical proper motions through space with all their cluster siblings. An object seen close to a young star with the same proper motion may be a planet, but Neuhaeuser points out an alternative: "When we see a star and its companion with the same proper motion, it is possible that they orbit around one another, but it is also possible that they are independent members of the same cluster, but just a few light years in front or behind each other."

The only planet that is in the clear, in Neuhaeuser's eyes, is Fomalhaut b. This is because if the planet was any larger than three times the mass of Jupiter, it would destabilise the protoplanetary dust disc around the star that it orbits.

Vague definitions of brown dwarfs aside, Neuaeuser's comments have certainly provoked some discussion amongst astronomers involved in these direct detections of claimed planets, with no one really willing to commit either way. However direct imaging is a burgeoning field and it seems certain that there will be more detections in the future.