Brown dwarfs: sub-stellar and super-planetary

I am fascinated by a kind of object that is known as a “brown dwarf“. Brown dwarfs are called substellar objects because they are less massive than stars, and do not produce energy by fusing hydrogen in their cores. Instead, they shine by radiating the heat from their initial formation, just as a tiny ember from a fire cools and fades.  Due to their inherently faint nature, the first ones were only discovered in the mid-1990s when new infrared cameras opened a new window on the Universe.  Brown dwarfs are now known with masses and temperatures that range from those of the coolest stars down to those of gas giant planets. They exhibit an amazing range of properties and behaviours, from dramatic flares like those seen on active young stars, to phenomena more familiar to planetary scientists such as clouds, weather and aurorae. By studying these amazing objects we provide crucial insights to our understanding of how stars and planets form, the physics and chemistry of planetary atmospheres and, even now, still find previously unseen neighbours right on our Galactic doorstep.

Brown dwarfs – failed stars?

I became interested by brown dwarfs as a result of work I did as part of my PhD thesis on low-mass star formation. Brown dwarfs represent the low-mass extreme of the star formation process, and whether there is a minimum mass of object that can be created by the same process that makes stars is an important unanswered question in astronomy. The fine detail of how brown dwarfs are formed is still debated, but broadly speaking most form in a similar manner to cool, low-mass, stars from clouds of interstellar gas collapsing and fragmenting under their own gravity. As a young star forms from a collapsing gas cloud,  it contracts and heats up until its core is so hot that nuclear fusion takes place, fusing hydrogen to helium. This releases energy, heat, in the core which prevents the star from collapsing further. In a brown dwarf, this does not happen. Although they also heat up as they collapse and contract,  their lower mass (less than 75 times the mass of Jupiter or 75 M_Jup) means they never achieve the high core temperature and pressure required to fuse hydrogen at a rate sufficient to halt their contraction. Instead, they eventually reach a point where the core can no longer contract, and then they begin to cool off. As a result, young brown dwarfs can be relatively hot, with similar temperatures to cool stars, but as they age they cool, and older brown dwarfs are increasingly cold, with similar temperatures to planets that have now  been observed in other star systems.

Super-planets?

Many brown dwarfs orbit stars, in fact my research has found that at least approximately 7% of  brown dwarfs orbit stars on wide orbits (100s-1000s  AU). Many of these brown dwarfs are far more massive than planets such as Jupiter, and share more characteristics with stars. At lower masses though, they share many characteristics with giant planets, so it is not obvious how best to distinguish between planets and brown dwarfs that orbit stars, or if such a distinction is meaningful or helpful. Although brown dwarfs never fuse hydrogen, those that are more massive than 13 Jupiter-masses (13 M_Jup) go through a brief phase of burning hydrogen’s heavier isotope, deuterium. Because deuterium is much less abundant than hydrogen, this phase is very quick, and only slows a young brown dwarf’s contraction for a short time.  None-the-less, the 13 M_Jup deuterium burning limit is used by many to define the brown dwarf-planet boundary for objects that are orbiting a star, and objects below this mass that are not orbiting a star are often referred to as free-floating planets. In practice though, the distinction is often somewhat blurred.

A UKIDSS image of Ross 458ABC, with Ross 458C indicated. Ross 458AB is the bright star in the top left.

For example, Ross 458C was discovered in the UKIDSS infrared sky survey and orbits a young double star known as Ross 458AB at an orbital distance of over 1000 AU. It appears to have a mass of between  5 and 20 M_Jup. In fact, it is highly likely that the mass of this object is below the deuterium burning limit. Despite the fact that, by the definitions above, we should call this object a planet, many astronomers would describe Ross 458C as a brown dwarf. This is probably due to its wide orbit, which is much wider than the orbits of planets like Jupiter (~5 AU), and suggests that this object has a very different origin. It thus appears, that the difference between what we call a planet and what we call a brown dwarf  is a little more complicated than a simple mass-limit.