Not so long ago, brown dwarfs (failed stars caught in an awkward in-betweener zone between stars and planets) were hypothetical bodies. Their low stellar masses allow for only a very short period of light-emission in their early years, after which they cool and darken considerably.
“[A] brown dwarf has too little mass to ignite the thermonuclear reactions by which ordinary stars shine. However, it emits heat released by its slow gravitational contraction and shines with a reddish colour, albeit much less brightly than a star.” (1)
It was recognised early on that if they existed at all, they would be very difficult to spot – and so it proved. In recent years, the ability to detect these objects has improved considerably, including more effective infra-red sky surveys. As they have become more common, the frontier of sub-stellar bodies has dropped in mass into the ultra-cool stellar bodies known as sub-brown dwarfs – many of which would equally properly be designated as rogue gas giant planets. These objects tend to have masses below 13 times that of Jupiter (13Mj) (2). These objects have always interested me greatly, and very early on in my own research efforts I was advocating the potential importance of sub-brown dwarfs in the hunt for additional planets orbiting our own Sun at great distances (3). I used the term ‘Dark Star’ to describe these ultra-cool objects; a term suggested by a friend of mine. Some can be found orbiting stars (usually beyond 50AU) while others are free-floating entities in their own right.
One of the pair of Caltech scientists who announced in January that there was a very high probability that a ‘super-Earth’, dubbed ‘Planet Nine’, exists beyond Neptune (1,2), has noted that a newly discovered eccentric Kuiper Belt Object cuts down the possibility that they were wrong still further.
“The object [uo3L91] shares some of the same behavior as the other six Kuiper Belt bodies, suggesting it has also been pushed by a large planet that is between 200 and 1,200 times the distance from the Sun to Earth. The object was discovered by the Canada France Hawaii Telescope, which is conducting the Outer Solar System Origins Survey (OSSOS); information about its movements were presented recently by astronomer Michele Bannister at the SETI Institute.” (3)