For some time, astrophysicists have argued over how many Dark Stars there might be in the galaxy, with varying opinions. (Note that astronomers use several different names for these objects: sub-brown dwarfs, Y Dwarfs, ‘planemos’). In this short article, I argue that new evidence presented about the stellar populations of open star clusters point towards there being more Dark Stars than stars in our galaxy.
When I use the term ‘Dark Star’ in my book (1) and internet articles, I’m generally referring to gas giant planets/ultra-cool dwarf stars which are several times more massive than Jupiter, up to perhaps ~13 times as massive (at this point, the gas giant begins to burn deuterium and is reclassified as a brown dwarf). Most examples of these objects (perhaps more than a few million years old) are essentially dark. By contrast, very young examples light up more brightly, because they still retain some heat from their formation. It’s a curious quirk of nature that these sub-brown dwarfs are actually smaller in size than Jupiter, despite being heavier. Because these objects are so small, and so dim, they are extraordinarily difficult to observe. Some have been found, but they are usually either extremely young (and therefore still burning brightly), or are exoplanets discovered orbiting parent stars (and so detectable through gravitational ‘wobble’ effects, or other means of finding massive exoplanets).
It has been my contention for some time that the populations of these objects are significantly underestimated. It is recognised generally that these ultra-cool dwarf stars may be free-floating objects in inter-stellar space, often as a result of having been ejected from young star systems as the fledgling planets in those systems jostle for position. Opinions about their numbers vary greatly among astrophysicists. There may be twice as many of these objects as stars, according to studies involving gravitational microlensing surveys of the galactic bulge (2). Other studies conflict with this conclusion, arguing that there may be as few as 1 object of 5-15 MJup size per 20-50 stars in a cluster (3). This discrepancy is important because the difference is perhaps as high as two orders of magnitude, and this ultimately affects our understanding of how many free-floating Dark Stars we can expect to find out there.
Their mass, lying between that of Jupiter and the deuterium-burning limit at about 13 MJup (4) seems to single Dark Stars out as rather special objects:
“An abrupt change in the mass function at about a Jupiter mass favours the idea that their formation process is different from that of stars and brown dwarfs. They may have formed in proto-planetary disks and subsequently scattered into unbound or very distant orbits.” (2)
Therefore, if the number of free-floating sub-brown dwarfs (also sometimes known as “planemos”) is on the high end of expectation, then it means that there are also likely to be far more of these objects in wide, distant orbits around their parent stars. This, in turn, increases the likelihood of there being a similar Dark Star object (or more) in our own immediate solar neighbourhood. Read More…
I’ve spent many years extolling the virtues of life on a cold brown dwarf moon. Similar to the Galilean moons of Jupiter, a moon orbiting a sub-brown dwarf would be warmed internally by the tidal forces generated by its proximity to such a powerful gravitational force. Additionally, the sub-brown dwarf itself might provide some local heating, or at least an abundance of charged-particle strewn local magnetic fields to energise the sub-stellar environment. So, a habitable environment on a moon seems a likely scenario. If a cold, dark sub-brown dwarf were to be found orbiting the Sun at a great distance, then it neatly provides the grounding for extraterrestrial life on our doorstep (1).
This seems to me to be the simplest scenario for life in a sub-brown dwarf system. There are complexities – tidally-locked moons (2), lack of light, and so on. But the basics are there.
Another exotic possibility is that the sub-brown dwarf itself might harbour life. The complex cloud systems in these failed stars can contain layers which are at room temperature, and abundant in water and other chemical goodies which could form the building blocks of life. A team of astronomers from Edinburgh University have been considering this very point, wondering whether very simple life might be able to get going in the clouds of a cold brown dwarf (3). This life might arise in two ways – either somehow evolving from scratch in the cloud environment, or originally being seeded into it by an impacting asteroid or comet. Either way, conditions for life might be good, except for the lack of a solid surface to dwell on:
“Floating out by themselves in the Milky Way galaxy are perhaps a billion cold brown dwarfs, objects many times as massive as Jupiter but not big enough to ignite as a star. According to a new study, layers of their upper atmospheres sit at temperatures and pressures resembling those on Earth, and could host microbes that surf on thermal updrafts...Observations of cold brown dwarf atmospheres reveal most of the ingredients Earth life depends on: carbon, hydrogen, nitrogen, and oxygen, though perhaps not phosphorous.“ (4)
These ideas build upon work done by the late, great Carl Sagan (with his Cornell colleague E. E. Salpeter) on the potential for life in the clouds of the gas giant Jupiter, first considered back in the 1970s (5). They envisioned giant ‘floaters’ filled with hydrogen bobbing through the Jovian atmosphere, tiny ‘sinkers’ and self-propelled ‘hunters’ which had evolved from the lazy floaters (6). All very speculative, but presented in Dr Sagan’s inimitably compelling fashion. Read More…