Given all the fervor over the definition of Pluto (planet? dwarf planet? snowball?), let’s hope the debate over the discovery of a planet that lies in an equally hazy area of classification is a little calmer. The COROT satellite recently discovered an extrasolar planet named Corot-exo-3b. It’s quite a curiosity as far as exoplanets are concerned, and its characteristics – such as a density twice that of lead – may force astronomers to rethink the distinction between massive planets and low-mass brown dwarfs.
Corot-exo-3b is orbiting close to its star, and takes 4 days and 6 hours to complete one orbit. For comparison, Mercury orbits the Sun every 88 days. It’s also roughly the same size as Jupiter, but far more dense, totaling a whopping 21.6 times Jupiter’s mass. This makes classification of the object a bit tricky.
“COROT-exo-3b might turn out to be a rare object found by sheer luck. But it might just be a member of a new-found family of very massive planets that encircle stars more massive than our Sun. We’re now beginning to think that the more massive the star, the more massive the planet,” said Dr Francois Bouchy, from the Institut d’Astrophysique de Paris (IAP), a member of the team that discovered the object.
Because of its extreme density, Corot-exo-3b lies in the shady area of classification between planet and brown dwarf. Brown dwarfs are massive bodies (between about 13 and 80 times the mass of Jupiter) that don’t make the cut for fusing hydrogen in their cores – and thus don’t shine in optical wavelengths – yet are much more massive that what is normally classified as a planet. Brown dwarfs can fuse deuterium even at lower masses (above 13 Jupiter masses), and lithium in masses above 65 that of Jupiter.
Planets generally form out of a disk of dust and gas that surrounds the early star they orbit, and then are pulled in closer due to friction with the debris that lies in their orbit. The close orbit and very short orbital period of Corot-exo-3b was likely caused by this effect.
The COROT satellite initially discovered the planet by measuring the change in the brightness of the host star as the planet passes in front of it. As the planet moves in front of the star, it slightly darkens the visible light, and then the star brightens once again as the planet moves behind it. The bigger the planet, the more it will darken the light coming from the star. The pull of a planet as it moves around its star can also redshift or blueshift the light coming from the star, and this shift can give information as to the mass of the planet.
Follow-up observations of the planet were done by a collaboration of scientists from around the world, led by Dr. Magali Deleuil from the Laboratoire d’Astrophysique de Marseille (LAM). Their results will be published in the journal Astronomy and Astrophysics.
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