As has been demonstrated repeatedly, the boundaries between a planet and another object are often hard to define. We’re all aware of the trouble in creating a definition at the lower end of the mass scale, but at the upper end it is even worse as the nearest clear cut brown dwarfs (the next step between a planet and a star) are quite a distance away and pretty dim.
The definition of a brown dwarf is that it is capable of fusing deuterium, but not quite able to deal with hydrogen. The lower end of the brown dwarf mass scale is around 7-13 times the mass of Jupiter and the upper end around 85 Jovian masses. This can only be confirmed by actually seeing brown dwarfs, high mass exoplanets and low mass stars and measuring their masses. The trouble with astronomy is that observations do sometimes tend to through up new conundrums – like a planetary candidate or two at brown dwarf masses. Now another spanner has been thrown into the works.
A binary system has been found that doesn’t seem to be two stars orbiting their common centre of mass, like other binaries. One of them appears to be an exoplanet. This is all fine and well, you might think, the planet merely formed quite a distance away from its host star – and the distance this companion is from its host star is but 3.6 billion km (ie between the distance that Uranus and Neptune orbit the Sun). However, the star it shares a centre of mass with is itself a brown dwarf, and the planetary disc it could support isn’t quite that wide. Also, the age of the system is just too young for the normal method of planetary formation to have taken place.
This method is known as core accretion and results from instabilities in the disc of matter surrounding a star creating cores around which gravitational collapse can occur. Over time, the core accretes matter onto it and a planet is the result.
This planet is however 5-10 times the mass of Jupiter. It is likely too low mass to fuse deuterium and so cannot be a Brown Dwarf on that criterium. However, the discoverer of the system says it should be called a Brown Dwarf based on the fact that the evidence points toward it forming directly from core collapse in the molecular cloud that formed the companion Brown Dwarf. Furthermore, observations suggest that the binary system is gravitationally connected to a second binary system of low mass objects – a red dwarf star and another Brown Dwarf. The discoverer, Dr Kevin Luhman of Penn State University, believes that this shows the same processes that form stars can be responsible for objects all the way down to planetary size.
UPDATE: Even as I was writing, a report came through of the discovery of the closest ever brown dwarf to Earth. UGPSJ0722-05 was found through looking at surveys of the sky in the infrared light these objects glow brightest in. The closeness of the object means it is relatively bright compared to others of its ilk as observed from Earth, making it a good thing to study. It was discovered by the UK Infrared Telescope in Hawaii, which the STFC has decided must shut to save money. As well as being the closest brown dwarf, this one is the coolest at a mere 400 K (the next closest being a rather warm 500K). The article also gives a third definition of brown dwarfs – it has a homogeneous composition due to the heat in its core convecting through the body, whereas planet’s differentiation their chemical composition (similar to the definition that works in the lower end of the planetary mass). The brown dwarf also has water, methane and an unexplained chemical absorbing at 1.25 microns.