Sub-Brown Dwarfs Hiding in Plain Sight
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.
These kinds of objects began to crop up with increasing frequency. Many of these early detections were at the earliest stage of their development, and so still capable of emiting visible and infra-red light (e.g. (4)), but these young sub-brown dwarfs started to give astronomers a clearer idea of the general population numbers of their older brethren lying in the galactic darkness. The WISE infra-red sky survey was partly designed to seek out nearby brown dwarfs and sub-brown dwarfs. The result of this survey was pretty disappointing in this regard, as it didn’t seem to find the kinds of populations of BDs expected (5). That was despite observing some very cool sub-brown dwarfs indeed (6).
Now, a new Canadian near-infrared (NIR) proper motion survey (known in French as the Sondage Infrarouge de Mouvement Propre (SIMP) survey) (7) has indicated that there are likely far more ultra-cool dwarfs in our solar neighbourhood than WISE had so far demonstrated. The team at WISE, many of whom were top experts on brown dwarfs, may have been surveying too narrow a compositional range for these enigmatic objects:
“Knowing the abundance and distribution of brown dwarfs provides key information on the distribution of mass in the universe, and on the mechanism of brown dwarf formation, for example, whether they form in isolation or instead are ejected from larger planetary systems. To that end, the team, led by Jasmin Robert of Université de Montréal, believed that although hundreds of ultracool brown dwarfs have already been discovered, the techniques used to identify them were overlooking those with more-unusual compositions, which would not show up in the color-based surveys generally used.
“So they surveyed 28 percent of the sky and discovered 165 ultracool brown dwarfs, about a third of which have unusual compositions or other peculiarities. When talking about brown dwarfs, ultracool means temperatures under about 3,500 Fahrenheit or 2,200 kelvin. “The search for ultracool brown dwarfs in the neighborhood of our own Solar System is far from over,” said [Carnegie’s Jonathan] Gagné. “Our findings indicate that many more are hiding in existing surveys.”” (8).
This provides great hope for those of us who advocate the existence of a sub-brown dwarf object in the outer solar system (including Nemesis, Tyche, and the Dark Star). Simply put, assumptions made about these objects may have been too restrictive, allowing a great many of them to have slipped through the net. Scientists working with data from WISE may have missed a solar system sub-brown dwarf, and then wrongly dismissed it as impossible.
Written by Andy Lloyd, 7th September 2016
European Southern Observatory “Even Brown Dwarfs May Grow Rocky Planets: ALMA sizes up grains of cosmic dust around failed star” 30th November 2012 http://www.eso.org/public/news/eso1248/
2) ‘Brown dwarf’ https://en.wikipedia.org/wiki/Brown_dwarf
3) Andy Lloyd “The Dark Star Theory” http://www.darkstar1.co.uk/ds3.htm 1999 onwards
4) Daniel Stolte “Brown Dwarf Found Orbiting a Young Sun-Like Star” 28th July 2010 https://uanews.arizona.edu/story/brown-dwarf-found-orbiting-a-young-sun-like-star
5) Ian O’Neill “Brown Dwarfs, Runts of Stellar Litter, Rarer than Thought” 12th June 2012 http://www.space.com/16112-brown-dwarf-stars-sun-rare.html
6) NASA/JPL “NASA’s Spitzer and WISE Telescopes Find Close, Cold Neighbor of Sun” 25th April 2016 http://www.nasa.gov/jpl/wise/spitzer-coldest-brown-dwarf-20140425
7) Jasmin Robert et al “A Brown Dwarf Census from the SIMP Survey” 28th July 2016, Astrophysics Journal, https://arxiv.org/abs/1607.06117
8) Carnegie Science “Brown Dwarfs Hiding in Plain Sight in our Solar Neighborhood” 6th September 2016 https://carnegiescience.edu/node/2086