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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Seven Planets Found in Red Dwarf System

NASA made a big announcement this week about new exoplanets found orbiting the dwarf star TRAPPIST-1 some 39 light years away.  I’ve discussed this particular dwarf star system before (1), as it was already known to have three terrestrial planets in attendance orbiting very close to this cool, fairly dim star (2,3).  The dwarf star is approximately one tenth the size of the Sun, and it’s mass places it on the border between a brown dwarf and a red dwarf star.  Unusually for a star this small, TRAPPIST-1 has a high metallicity, which actually exceeds that of the Sun (4).

Now, an international team of astronomers, using the Belgian TRAPPIST telescope in Chile and the Spitzer infra-red space telescope, have released details about a further four terrestrial planets in this mini-star system, three of which (e, f and g) are located within it’s habitable zone, where temperatures favour the presence of liquid water (5):

“Researchers led by Michaël Gillon, of the University of Liège in Belgium, have been studying the infrared light emitted by this miniature star and have detected drops in luminosity characteristic of transits, i.e. the passage of astronomical bodies moving across its face.  As early as 2015, the first three planets (dubbed b, c and d) had been identified.  Tracking the system using TRAPPIST and the space telescope Spitzer, the team was then able to identify four others planets (e, f, g and h) in 2016.  Based on the frequency of these transits and the degree of reduction in luminosity of the star, they have demonstrated that these seven planets are all comparable in size to Earth (to within 15%), and orbit very close to their star.” (6)

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Radio Bursts from Space

I recently reviewed a book about Carl Sagan’s interest in ancient aliens, written by Donald Zygutis (1).  Early on in his illustrious career, Sagan expressed scepticism about seeking E.T. life using radio telescopes, instead advocating a search through historical accounts and myths to determine whether our planet had been visited (2).  He argued that in a standard galaxy there are so many stars/planets etc, that all you’d need to do is point the radio receiver at any given galactic source beyond the Milky Way, and alien radio signals should come screaming out at you.

sagan

They generally don’t, of course, which led Sagan to the early logical conclusion that SETI’s search with radio telescopes was bound to fail.  However, this approach became the only game in town, with serious funding at its disposal, and Sagan fell into line behind it – supporting this doomed search for E.T. radio signals ostensibly from stars within out galactic neighbourhood.

vla_nm

Decades on, and SETI has come up with little of any merit.  The odd interesting blip, sure, but nothing demonstrably repetitive, or intelligent.  Other searches have also come up empty-handed, including an extensive search for highly advanced galactic civilisations using infra-red (3), based upon the theories of the physicist Freeman Dyson.  Looking for an infra-red signature from other galaxies seems like a bit of a stretch to me.  Sagan’s initial premise about radio waves emanating from other distant galaxies is more plausible.  By staring at the tiny amount of our sky that any given distant galaxy occupies, radio telescopes can cover a lot of possible stars in a very small space.  If any of them contain radio-emitting alien species, shouting for attention, then we should pick them up one would have thought.  Read More…

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Recent updates on the Search for Planet Nine

It’s a year since proposed the existence of Planet Nine (1).  Despite the fact that its discovery remains elusive, there have been a great many academic papers written on the subject, and no shortage of serious researchers underpinning the theoretical concepts supporting its existence.  Many have sought evidence in the solar system which indirectly points to the perturbing influence of this mysterious world; others have provided data which have helped to constrain the parameters of its orbit (by effectively demonstrating where it could NOT be).  Throughout 2016, I have been highlighting these developments on the Dark Star Blog.

RedPlanetX_1

At the close of 2016, two further papers were published about Planet Nine.  The first of these delves more deeply into the possibility that Planet Nine (Brown’s new name for Planet X, which seems to have caught on among astronomers keen to distance this serious search from, well, the mythological planet Nibiru) has a resonance relationship with some of the objects beyond the Edgeworth-Kuiper Belt which it is perturbing.  These kinds of resonance relationships are not unusual in planetary orbital dynamics, so such a suggestion is not that odd, even given the eccentricities of the bodies involved here.  The new research, from the University of California, Santa Cruz, bolsters the case for this kind of pattern applying to Planet Nine’s orbit:

“We extend these investigations by exploring the suggestion of Malhotra et al. (2016) (2) that Planet Nine is in small integer ratio mean-motion resonances (MMRs) with several of the most distant KBOs. We show that the observed KBO semi-major axes present a set of commensurabilities with an unseen planet at ~654 AU (P~16,725 yr) that has a greater than 98% chance of stemming from a sequence of MMRs rather than from a random distribution.” (3)

Their randomised ‘Monte Carlo’ calculations provide a best fit with a planet of between 6 and 12 Earth masses, whose eccentric orbit is inclined to the ecliptic by about 30 degrees.  They are unable to point to a specific area of the sky to search, but provide a broad-brush region which they favour as most probable.  Dr Millholland has also helpfully provided a 3D manipulable 3D figure of the cluster of extended scattered disk objects allegedly affected by the purported Planet Nine, alongside their extrapolated orbit for it (4).  Read More…

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The Galactic Core Spits out Dark Stars

A new theory about planet formation has posited that stars, placed under inordinate stress, could break apart catastrophically, flinging their smouldering remains out into the void at tumultuous speeds.  It would take quite a force to render stars apart in this way.  The supermassive black hole which lies at the centre of the galaxy creates just such an impression.  Wayward stars drifting inexorably into the depths of its immense gravitational well would not fare well, during what are termed Tidal Disruption Events (1,2).

tidal_disruption

Researchers from Harvard University (namely, undergraduate Eden Girma and James Guillochon, an Einstein fellow at the Harvard-Smithsonian Center for Astrophysics), have conducted computer simulations to model what happens to this streaming material, and the results are quite extraordinary:

Every few thousand years, an unlucky star wanders too close to the black hole at the center of the Milky Way. The black hole’s powerful gravity rips the star apart, sending a long streamer of gas whipping outward. That would seem to be the end of the story, but it’s not.  New research shows that not only can the gas gather itself into planet-size objects, but those objects then are flung throughout the galaxy in a game of cosmic “spitball.”” (3)

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Cryovolcanoes on Ceres 

It’s been a little while since the Dawn probe imaged those mysterious ‘lights’ in the craters of the dwarf planet Ceres (1).  On first impression, these seem to be impact marks where brighter materials lying below the surface were exposed following meteoritic bombardment.  But they are uncommonly bright for an asteroid, so speculation about the nature of the materials involved has been rife in the planetary science community, and what it could mean for how the dwarf planet formed in the first place (2).  The bright spots, now widely thought to be salt deposits, have recently even been given names:

“The two most famous bright spots on Ceres have been given names. These once-mysterious spots are now thought by most scientists to be salt deposits. They’re now called Cerealia Facula (for the brighter of the two spots) and Vinalia Faculae (for the cluster of less reflective spots to the east). Both names are related to ancient Roman festivals.” (3)

ceres_lights

But that’s not the only mystery on Ceres.  There is a growing consensus that there may be geophysical processes going on that are relatively recent (at least in terms of geological time periods): Read More…

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Down a Dusty Lane

Picking up on the mystery of how a massive Planet X could form beyond the outer confines of the Sun’s magnetic environment, as per my previous posts on the accretion of dust beyond the heliopause (1,2) and an exploratory scientific paper I published earlier in the year (3).  I’m searching for evidence, or at least some educated guesswork, about whether interstellar medium beyond the heliosphere of stars might be sufficient over time to build up substantial, gaseous planets loosely bound to their parent star systems.  Such planets might, I suggest, accumulate dust clouds and rings around them, undisrupted by the action of the solar wind trapped within the inner magnetic sphere of the solar system.

bd_snowdisk

Even though this kind of accumulation could be gradually taking place over billions of years, creating a meaningful adjustment to the mass of a substantial planet over these kinds of time periods, it doesn’t seem likely that this kind of effect could take place if our current interstellar environment is anything to go by (although the unexpected presence of interstellar ‘fluff’ beyond the heliopause, described by NASA (4), and the intrusion of large grain particles into the outer solar system (5) do offer some evidence of what could be ‘out there’).

Last month, I looked at evidence of massive stars being aided in their development by the dumping of immense quantities of neighbouring nebula material onto them (6).  I wondered whether a similar mechanism might also be happening in interstellar space at the planetary level, based upon globular frameworks of nebula materials (like gigantic molecular clouds, and the like).

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Niku, Drac and L91 Perturbed by Planet Nine…or Something Else?

Dr Konstantin Batygin and Dr Mike Brown argue in their latest paper that the retrograde Kuiper Belt Objects Niku and Drac could have once been extended scattered disk objects (1).  If you have been following these blogs during 2016, it will come as no surprise to you to hear that the influence which perturbed them into their anomalous current orbits was Planet Nine, the 10+Earth-mass planet lurking several hundred-plus Astronomical Units away, whose gravitational influence seems to be influencing the objects in and beyond the Kuiper Belt beyond Neptune (2):

“Adopting the same parameters for Planet Nine as those previously invoked to explain the clustering of distant Kuiper belt orbits in physical space, we carry out a series of numerical experiments which elucidate the physical process though which highly inclined Kuiper belt objects with semi-major axes smaller than a < 100 AU are generated. The identified dynamical pathway demonstrates that enigmatic members of the Kuiper belt such as Drac and Niku are derived from the extended scattered disk of the solar system.” (1) Read More…

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New Trans-Neptunian Object may add to Planet Nine Cluster

Astronomers have announced the discovery of the third most distant object in the solar system, designated 2014 UZ224 (1).  At a distance of 91.6AU, it is pipped to the title of ‘most distant solar system object’ by V774104 at 103AU (2), followed by the binary dwarf planet Eris at 96.2AU(3).  The new scattered disk object lies approximately three times the distance of Pluto away, and may be over 1000km in diameter – potentially putting it into the dwarf planet range.  Its 1140 year orbit is notably eccentric, which is becoming more expected than otherwise with this category of trans-Neptunian object.

The find is a fortunate byproduct of the Dark Energy Survey, which seems to be rather good at picking out these dark, distant solar system objects.  It was first spotted in 2014, with follow-up observations which have firmed up its orbital properties, but clearly delayed the announcement of its existence until now.  These follow-up observations were rather scatty over time, and so the Dark Energy team, led by David Gerdes  of the University of Michigan, developed software to establish its orbital properties: Read More…

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