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Red Giants and Planet Formation

This article will explore the potential for life to develop in the outer planetary systems of red giant stars.  It will then discuss the death-throes of red giant stars, and whether the subsequent outward thrust of stellar material might provide another mechanism for free-floating planets in interstellar space.

Exoplanets have already been found orbiting extremely old stars, one some 11 billion years old (1).  This star, named Kepler-444, makes our own Sun, at a mere 4.6 billion years old, seem like an infant in comparison.  The implication of this is that life could readily have got going early on in the history of the universe, long before the birth of our Sun.  Furthermore, if these exoplanets were to benefit from a relatively stable stellar environment during that long timescale, then the chances of life evolving into higher forms are statistically more probable.  Scale this up across trillions of stars, and the possibilities become clear.

Our own Sun has a shorter lifespan than this.  Its main sequence life is expected to last another 5 billion years, by which point it will have burned up all of its hydrogen fuel.  Then it will swell into a red giant star, before collapsing down into a white dwarf.  For Earth, this post-main sequence (post-MS) phase of the Sun’s life will be pretty disastrous.  The Sun’s expansion to a red giant will swallow the Earth up.  However, a less catastrophic outcome might be expected for planets in the outer solar system, beyond, say, Jupiter.  In fact, their climates might significantly improve – for a while, at least.  The habitable zone of the solar system will expand outwards, along with the expanding star.  Saturn’s largest moon Titan, for instance, might benefit greatly from a far milder climate – as long as it can hang onto its balmy atmosphere in the red heat of the dying Sun.

red_dwarf_landscape

The expansion of habitable zones, as late main sequence stars become hydrogen-starved, offers the potential for life to make a new start in previously frigid environments.  The burning question here is how long these outer planets have to get life going before the red giant then withdraws into its cold white shell.  A study published last year by scientists at the Cornell University’s Carl Sagan Institute attempted to answer this question (2), choosing to examine yellow dwarf stars whose sizes range from half that of the Sun, to approximately twice its mass.  They argue that the larger stars along this sequence could well have larger rocky terrestrial planets in their outer planetary systems than our Sun does (at least, insofar as we know it does!)  This is because the density of materials in their initial proto-planetary disks should be that much greater for larger stars (3).  Larger Earth-like planets in outer regions mean more potential for stable atmospheric conditions during the post-MS period under consideration.  In other words, the growing red giant (which is shedding its mass pretty wildly at this point) would not necessarily blast away an outer planet’s atmosphere if that rocky planet had sufficient gravity to hold onto it.

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More Dark Stars than Stars in Milky Way

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…

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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.

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