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)

It should be said that the numbers of free-shooting planets theoretically generated in this way would make up perhaps only 0.1% of the Milky Way’s free-floating worlds.  Many of the others are ejected by standard star systems during their chaotic early periods of planetary formation.  This is particularly true of binary star systems, and other complex stellar arrangements (4).   It’s likely that free-floating planets actually outnumber their ‘normal’ bound counterparts (5).  So, given the extraordinary number of free-floaters out there in interstellar space, this 0.1% is still equivalent to hundreds of millions of these cold, gaseous objects – many of which would be equivalent to the Dark Star class of object I have written about.  Their fate is varied:

“Of those newly born objects about 95 percent were flung from the galaxy out into the outer spiral arm hinterlands. A much smaller percentage remained bound to Sagittarius A*, destined to endlessly circle the dark monster that ripped the original star apart. The smallest set of planetoids, fewer than one percent of the total, are now wandering the outskirts of the Milky Way, perhaps within about six hundred light-years of Earth.”” (6)

The streams of gas strewn out from the shredded star are capable of clumping into gas giant-sized planets within just one year.  This contrasts with the millions of years that gas giants generally take to form, according to current models of planet-formation.  So how do they manage to form so quickly in such a high velocity environment?

freefloater_spitballs

I wonder whether this could provide a template for the clumping of material within interstellar space – interstellar medium is also subject to a high-velocity environment within galactic streams and currents.  This is an area of research that interests me greatly at the moment, as I attempt to explain how planets could form in interstellar domains beyond the heliopause, and perhaps continuing to accumulate materials around them for billions of years (7).  Recent work has been carried out by the Hubble Space Telescope as it images the interstellar space that the two Voyager spacecraft are now traversing beyond the heliopause.  The data indicates that the environment beyond contains zones of ‘clumpier’ material than expected (8).  This finding, shoring up NASA previous descriptions of ‘interstellar fluff’ lurking beyond the heliopause, provides more evidence that accumulation of interstellar medium can occur in interstellar space; at least locally.  The mechanism for this remains unclear.

In the case of the ‘spaghettified’ streams of star matter created by the supermassive black hole Sagittarius A*, the residual internal gravitational pull of the gaseous clumps is enough to cause planetary formation within the stream of matter:

“Hydrodynamical simulations of this process have revealed that within this stream, the local self-gravity dominates the tidal field of Sgr A*. This residual self-gravity allows for planetary-mass fragments to form along the stream that are then shot out into the galaxy at velocities determined by a spread of binding energies.” (9)

If true, then this is quite extraordinary.  The monster back hole at the centre of the galaxy is routinely chewing up stars which get too close, and spitting out their innards across the galaxy in the form of bite-sized planetary chunks.

The other aspect of this is that the composition of these high-velocity second generation planets would vary depending upon what part of any given star they might have accrued from.  This is an important point, because it lends itself to the possibility of quite exotic Dark Stars, whose gaseous and chemical signatures could be quite different from those forming ‘naturally’ out of stellar nurseries, or from rotating proto-planetary disks.  The sub-stellar properties of such objects might vary accordingly.  Monster dwarf sub-stars?

 

Written by Andy Lloyd,  9th January 2017

References:

1)  Lee Mohon “Tidal Disruption” 21st October 2015 https://www.nasa.gov/mission_pages/chandra/tidal-disruption.html

2)  Bogdanović, Tamara et al “Tidal Disruption of a Star By a Massive Black Hole: Observational Signature” The Astrophysical Journal, Volume 610, Issue 2, pp. 707-721,http://adsabs.harvard.edu/abs/2004ApJ…610..707B

3)  Harvard-Smithsonian Center for Astrophysics Press Release 2017-01 “Our Galaxy’s Black Hole is Spewing Out Planet-size “Spitballs”” 6th January 2017 https://www.cfa.harvard.edu/news/2017-01

4)  Ramin Skibba “Binary stars shred up and shove off their newborn planets” 13th January 2017 https://www.newscientist.com/article/2117948-binary-stars-shred-up-and-shove-off-their-newborn-planets/

5)  Mike Wall “Lonely Rogue Worlds Surprisingly Outnumber Planets with Suns” 18th May 2011 http://www.space.com/11699-rogue-alien-planets-milky-common.html

6)  Daily Galaxy ‘The Planetary Spitball Machine’ –Milky Way’s Supermassive Black Hole Spews Out Millions of Star Fragments –“Large as Jupiter and Neptune” 6th January 2016http://www.dailygalaxy.com/my_weblog/2017/01/the-planetoid-spitball-machine-milky-ways-supermassive-black-hole-spews-out-millions-of-star-fragmen.html

7) Andy Lloyd “The Cumulative Effect of Intermittent Interstellar Medium Inundation Upon Objects In The Outer Solar System” February 2016, DOI: 10.13140/RG.2.1.5112.5526,https://www.academia.edu/21700220/The_Cumulative_Effect_of_Intermittent_Interstellar_Medium_Inundation_Upon_Objects_In_The_Outer_Solar_System

8)  Hubble News Release number: STScI-2017-01 “What will the Voyager Spacecraft Encounter Next? Hubble Helps Provide a Roadmap” 6th January 2017 http://hubblesite.org/news_release/news/2017-01

9)  Eden Girma “Modeling the spatial distribution of fragments formed from tidally disrupted stars” 4th January 2017, Presentation at 229th AAS Meeting, Grapevine, TX,https://guidebook.com/guide/85338/event/14966513/

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