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Thoughts on Brown Dwarfs, Disks and Planets

Planetary systems around dim brown dwarfs are a fascinating thing to contemplate, and for a vivid imagining of future human activities on such planets, I’ll send you to Karl Schroeder’s Permanence. The 2002 novel posits ingenious engineering to sustain bases on such worlds, and even comes up with an interstellar propulsion method powered up by their energies that sustains an expanding starfaring culture. A brief sample of Schroeder’s universe (not enough to be a spoiler):

…the brown dwarfs each had their retinue of planets — the halo worlds, as they came to be called. And though they were not lit to the human eye, many of these planets were bathed in hot infrared radiation. Many were stretched and heated by tidal effects, like Io, a moon of Jupiter and the hottest place in the Solar System. And while Jupiter’s magnetic field was already strong enough to heat its moons through electrical induction, the magnetic field of a brown dwarf fifty times Jupiter’s mass radiated unimaginable power — power enough to heat worlds. Power enough to sustain a population of billions; enough to launch starships.

Speculative fiction has new wonders to mine as we learn more about brown dwarfs, and our discoveries are coming faster all the time. One reason is that we’re getting better at detecting them. In 2006, Katelyn Allers (University of Texas at Austin) and colleagues published a list of nineteen candidate brown dwarfs, all of them young and all but one now confirmed as either low-mass stars or sub-stellar objects. Allers’ team was able to use Spitzer data to search for infrared excesses, which is where the tale gets intriguing. The excess in the infrared is presumably due to circumstellar disks, leading us to wonder how small an object has to be to form with an accretion disk.

Current thinking has it that such disks probably don’t form around central objects smaller than a few Jupiter masses, but the fact that young brown dwarfs are more luminous, and hence easier to detect, than older ‘field’ brown dwarfs means we can find them by homing in on the places where stars are being born to study disk formation around these cool, low-mass objects.

This is what Paul Harvey, also at UT-Austin, has done, working with Allers and team to extend the early Spitzer results to luminosity levels that should allow the detection of objects as faint as two Jupiter masses. Their work used deeper Spitzer imaging of an area in the Ophiuchus star-forming region studied in the earlier work, where the youngest objects under investigation are evidently about one million years old.

The result: Eighteen new brown dwarf candidates with the near-infrared magnitudes and colors we would expect from such objects and the possible infrared excess that is consistent with a circum-object disk. From the paper:

Contamination by background field dwarfs in the brighter magnitude range of our sample and by extragalactic objects at the fainter magnitudes is likely to be significant. It is certainly possible that at least half of our candidates are such contaminants. Narrow-band filter photometry in progress, however, has shown that at least several of our candidates are likely to be low-mass BD’s with circum-object disks. It is likely that further candidates exist in our data set, though problems with diffuse 5.8 and 8μm emission in the region make it difficult to clearly confirm many more disk candidates.

If even the smallest brown dwarfs normally tend to form with accretion disks, the case for planets forming around these objects is strengthened, and given that brown dwarfs are found to be increasingly common (WISE will help us greatly in assessing their numbers), we may be looking at hosts of planetary systems of a kind we have only recently begun to imagine. The new work extends the earlier Allers study to luminosities a factor of ten below its limits, giving us new data about how smaller objects form with the accretion disks that can create companions.

The paper is Harvey et al., “A Spitzer Search for Planetary-Mass Brown Dwarfs with Circumstellar Disks: Candidate Selection,” available online. Thanks to Antonio Tavani for the tip on this paper.


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  • Carl Keller August 16, 2010, 15:25

    Karl’s other thoughts have been mentioned before here. http://www.kschroeder.com/my-books/permanence/interstellar-cyclers is thought-provoking and was mentioned in https://centauri-dreams.org/?p=10891 (which had a link to Permanence as well).

    Cyclers remind me of ski lifts, or perhaps escalators. Riders ‘time’ to accommodate their movements to board or disembark. The entire affair is akin to trains which make a huge three-dimensional circuit, and each of the huge vessels enter into their various ports of call on schedule, as if linked by some chain. That the energies may be tapped from these not-so-exotic objects for navigation would be a tremendous bonus.

    Schroeder wrote “the idea that you should expend billions or trillions of dollars to accelerate a starship, only to decelerate it again, is pure lunacy. 90% of the ship’s mass is support structures–either power or life support systems. The key to viable interstellar transport, in my view, is simple: if you’ve got it all in motion, keep it in motion and re-use it. The only thing you want to decelerate at your destination is your cargo.”

    Upper practical speed limits, debris near targets that must be fended or avoided, likely necessity of developing induced hibernation technologies are just a few of the other problems the coming generations will have to tackle. Of the current ideas of starships, I like this and the Enzmann model.

  • Phil August 17, 2010, 4:23

    And if that’s not remarkable enough, there are possibly even more exotic animals in the ‘brown dwarf zoo’.

    I was pretty astonished to read recently Perets’ paper on ‘Second Generation Planets’ (involving the formation of protostellar disks in old evolved binary systems where mass is transferred from an evolved star to its companion). He notes that in a second generation circumstellar disk, surviving first generation planets might act as ‘seeds’ for new planets, and that this might be an efficient process that could “lead to a core accretion formation of exceptionally massive planets, effectively becoming brown dwarfs”!

    Who would have thunk it?

    Paper at http://arxiv.org/abs/1001.0581


  • andy August 17, 2010, 17:06

    Phil: I’d come across that one a while back, had a suspicion this might be a workable hypothesis after the announcement of the disk around Mira B. Of the systems listed in the paper you linked, Gliese 86b is probably the prime suspect for being an observed second-generation planet: the current binary configuration would make it difficult for gas giants to form, and things would be much worse before the white dwarf progenitor lost mass and expanded its orbit.

  • Phil August 18, 2010, 1:02

    Fascinating! I didnt know that – thanks Andy.