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Brown Dwarfs Sparser than Expected

Nobody has been anticipating the results from WISE — the Wide-field Infrared Survey Explorer — any more than I have. Speculations about the number of brown dwarfs in the galaxy have been all over the map, with some suggesting they might be as plentiful as M-dwarfs, which make up perhaps 80 percent of the stellar population. But the latest results from our infrared scan of the sky argue a much different result: Brown dwarfs turn out to be considerably more rare than stars, with an initial tally of the WISE data showing just one brown dwarf for every six stars.

Thus Davy Kirkpatrick, a member of the WISE science team at NASA’s Infrared Processing and Analysis Center at Caltech:

“This is a really illuminating result. Now that we’re finally seeing the solar neighborhood with keener, infrared vision, the little guys aren’t as prevalent as we once thought.”

Ouch. The nice thing about a sky full of undiscovered brown dwarfs was that it might serve up interstellar destinations closer than the Alpha Centauri stars. None has been discovered yet, and we’ll apparently have to travel a lot farther to move between average-spaced brown dwarfs than we once thought. Not that these stars — or ‘failed stars’ as they are called because they cannot sustain hydrogen fusion at the core — are not interesting in their own right. We’d like to learn how they form, and how they tread the fine line between planet and star.

Image (click to download a larger version): Our own back yard, astronomically speaking, from a vantage point about 30 light-years away from the sun. The image highlights the population of tiny brown dwarfs recently discovered by NASA’s Wide-field Infrared Survey Explorer, or WISE (red circles). The image simulates actual positions of stars. All brown dwarfs known within 26 light-years are circled. Blue circles are previously known brown dwarfs, and red circles are brown dwarfs identified for the first time by WISE. The slightly larger M-dwarf stars, which are the most common type of star in the solar neighborhood, are shown with enhanced brightness to make them easier to see. Image credit: NASA/JPL-Caltech.

WISE has already identified the class of brown dwarfs known as Y dwarfs, one example of which is more or less at room temperature (25 degrees Celsius), making it the coldest of all star-like bodies found. This JPL news release says that in its survey to this point, the WISE team can identify about 200 brown dwarfs in the Sun’s vicinity, including 13 of the Y dwarf category. 33 of these brown dwarfs as measured by parallax methods are within 26 light years of the Sun, while 211 other stars also exist within the same volume of space.

Chris Gelino, also at the Infrared Processing and Analysis Center, puts a positive spin on the matter:

“Having fewer brown dwarfs than expected in our celestial backyard just means that each new one we discover plays a critical role in our overall understanding of these cold objects. These brown dwarfs are fascinating objects that are bridging the gap between the coldest stars and Jupiter.”

True enough, but the chances of a brown dwarf closer than Proxima Centauri are rapidly fading, although discovering additional Y dwarfs could adjust the ratio of brown dwarfs upwards slightly, if not as high as once thought. And as the WISE data continue to be sifted for further brown dwarf information, we should also keep in mind that we’re still pretty much in the dark about the number of ‘rogue’ planets out there, moving through the interstellar night without any stellar companion. Planets as much as several times the mass of Jupiter could still be out there, too faint to show up in the WISE data but possible targets for future deep space probes.

For more on rogue stars, see Island-Hopping to the Stars, which examines the work of Louis Strigari (Stanford University). Here you’ll find his estimate that there may be up to 105 compact objects per main sequence star in the galaxy that are greater than the mass of Pluto. It’s an extraordinary figure, but a big part of Strigari’s work relies upon the idea that rogue planets (he calls them ‘nomads’) in open clusters follow the brown dwarf mass function, so it will be interesting to see how the WISE data may eventually affect his calculations.


Comments on this entry are closed.

  • Tulse June 11, 2012, 9:27

    What would the benefit of closer brown dwarfs be for future interstellar exploration? Would they actually have any resources that could make them useful as a way station, or would they just be a target for pure exploration?

  • Ronald June 11, 2012, 11:00

    In fact, this was previously suggested by the fact that, although star abundance seems to increase exponentially with decreasing mass, there was already a sharp drop-off noticed within the smaller red dwarf population, the peak being somewhere around spectral class M4 or M5, dropping off beyond that (M6, etc.).

    I also dare doubt the recently suggested outrageous ‘100,000 rogue planets for every star’, for fundamental reasons: most probably there simply isn’t the planetary building material in the form of heavier elements, metalicity, for all those speculated multitudes of rogue planets. This in turn for logical reasons of stellar nulear fusion processes, elements generally become less abundant the heavier they are, because they are formed from lighter elements by the nuclear fusion processes within stars, and beyond iron only in supernovae, if I am not mistaken. Planets are really the small children of stars.

  • Ronald June 11, 2012, 11:09

    Tulse, your critical question is very justified: I think that BDs as a kind of interstellar refuelling station make hardly or no sense at all, once you are coasting along at maximum attainable speed, crew in suspended animation or something similar.
    Only as a serious in-between colonization target by itself would a BD make sense. But BDs don’t seem to be very attractive for settlement, even if they have any planets, which still remains to be seen. I would suggest: let’s go the extra few light years to a real (solartype) star.

  • Stan June 11, 2012, 13:37

    Island hopping sounds like a nice idea…if we were rowing a boat. But the benefits of stopping would have to be tremendous to outweigh the cost of decellerating and acellerating. Presumably if we started out on that trip we would have the ability to get all the way in one go.

  • Curt Wohleber June 11, 2012, 14:09

    Tulse, I’m just a layperson, but I think it was they would provide a closer target for interstellar exploration. The idea that there might be a brown dwarf not too far beyond the Oort Cloud was pretty tantalizing.

    Another good SF plot device down the tubes. Karl Schroeder’s “Permanence” posited a civilization of colonies in closely spaced brown dwarf systems. Then someone invents a hyperdrive, but it only works in proximity to a steep gravity well.

    I don’t know if the WISE data precludes the possibility that brown dwarfs might be abundant elsewhere, just not in our immediate stellar neighborhood.

  • Gregory Benford June 11, 2012, 14:09

    A stepping stone can provide early trials of AI directed sails, etc.

  • andy June 11, 2012, 14:12

    I never quite understood why brown dwarf way-stations would be useful for interstellar travel: given the enormous costs involved in slowing down from reasonable velocities for interstellar travel and then speeding back up again, a way-station does not seem like a particularly appealing prospect. Surely it would be much better to just head straight for the destination?

  • Abelard Lindsey June 11, 2012, 14:54

    Here you’ll find his estimate that there may be up to 105 compact objects per main sequence star in the galaxy that are greater than the mass of Pluto.

    The WISE results definitely calls into question Strigari’s ideas.

  • Michael June 11, 2012, 15:52

    I think worlds with densities of around platinium and the interaction of any magnetic fields with surrounding bodies would be of a valuable exploratory nature although everything would have to be done in infrared as there would be very little visible light. Resources would be pretty thin unless there are orbiting bodies as the G forces on these world are very very high.

  • Mike June 11, 2012, 16:03

    This new data will help improve theories of stellar and planetary formation.
    Interesting isn’t it? As much as twice as many red dwarfs are postulated by other new work but WISE is observing relatively few brown dwarfs.
    I think that stellar formation usually will not grow stars below a certain mass and planetary formation will usually not grow planets above a certain mass.
    On the other hand brown dwarfs may be rare compared to red dwarfs but at the suggested population numbers here they are much more numerous then the massive O, B and A stars.
    Still, considering that the stellar formation process favours low mass stars the comparative rarity of brown dwarfs to red dwarfs indicates some cut-off process at work in those stellar nurseries.
    I hope that the gravitational lensing projects may give us a better idea of the numbers of rogue planets as well.

  • andy June 11, 2012, 17:09

    It would be interesting to know what range of masses this is valid for: it will be interesting to see what happens when you get down to masses closer to those of the solar system planets, where we might expect to see a population of ejected planets as well as low-mass brown dwarfs. There are several ways to eject planets from a planetary system, probably the two main times this occurs in the history of a planetary system are after the removal of the stabilising influence of the protoplanetary disc, and during the strong mass loss at the end of the star’s evolution (this doesn’t mean that a planetary system may not undergo catastrophic events in the intervening time of course…)

  • Colin Weaver June 11, 2012, 18:45

    While not as common as hoped, this result shows that (at least in our neighborhood), brown dwarfs have a similar abundance to K class stars, around 13%. Not exactly rare either, but probably kills off the chance of a brown dwarf within a light year or two.

  • Adam Crowl June 11, 2012, 18:59

    Strigari et al’s extrapolation was to Pluto-masses, so the integrated total of heavy elements is quite small. Of course we’ve no solid idea of the numbers yet of micro-lensing objects, so observational programs make a lot of sense.

    If the mass between the stars is ~30 Jupiter masses per cubic parsec and 1% is astrophysical “metals”, then ~90 Earth masses of planetary objects could exist. Several thousand Pluto-like objects could be mixed in and not noticed.

  • magne June 11, 2012, 19:25

    For an generation type interstar ship refueling makes sense, they travel slow so it’s pretty cheap to stop, however they will consume recourses they can get from planets or asteroids around the brown dwarf, don’t make sense for other and more realistic types of transport.

  • Phil June 11, 2012, 19:56

    I wonder if star abundance reverses at the point where they become fully convective? Isnt that around M4? I can’t imagine WHY that would happen though. This is disappointing news for me, but hey thems the breaks…

    We still dont know what happens forther down the ‘Y’ pole – maybe abundance reverses again, perhaps as objects cease to be products of the tail of normal star forming processes and start to be the results of dynamical ejections from solar systems?


  • Brett Bellmore June 11, 2012, 21:52

    I don’t think the idea was ever that you’d get going fast enough to make it to Proxima in one go, and then stop to resupply at a brown dwarf, or even a largish comet. The idea was that you’d island hop from small body to small body because you *couldn’t* get going fast enough to make it to the next star in a reasonable time. That if your intermediate destination is a .4 light years away, instead of 4, you can get there at a tenth the speed, which means an incredibly easier trip given the exponential nature of rocketry.

  • Brasidas June 11, 2012, 22:56

    Could a nearby dwarf be used for a mid-flight gravity assist to boost speed? The lower temps mean you could potentially get close in to a relatively massive body for a signifcant acceleration. Even if you don’t do a lot of science, this might make it worth a flyby. Plus, you wouldn’t necessarily slow down during the passage, so you wouldn’t need to spend extra fuel for it, although you may want to bring along some extra fuel to get bigger push from the gravity assist.

  • William June 12, 2012, 0:43

    This in fact doesn’t affect the idea of island-hopping to the stars that much. It just means that the “islands” will all be comet-sized to small-planet sized, instead of brown-dwarf sized.

    Someone did a study some time ago (I wish knew the source) that outside the Oort Cloud, there are comets wandering around in true interstellar space. These are comets were ejected from ours and others’ star systems. They estimated them to be far less per cubic space than in the Oort Cloud, though, and the average space between them is on the order of 100 A.U., instead of the 1 A.U. (?) apart as in the Oort Cloud. These “comets” can be anything in size up to kuiper-belt-sized.

    The scenario of “island-hopping” to the stars is not to be dismissed or taken lightly. It might very well be how interstellar travel/migration happens. There may be an interstellar speed limit on how fast ships realistically can travel through the interstellar gulfs, because of all the stray material — ranging in size all the way from atoms to gas giants — that could pose a huge hazard if the ships collide with it. This would mean that interstellar travel and migration might/would have to happen slowly and gradually.

  • spaceman June 12, 2012, 1:26

    I would think this new WISE BD revelation actually strengthens the claim of microlensing searchers who report that the large number of free-floating objects they are finding are in fact actually planets rather than small brown dwarfs.

    The microlensing teams 2011 announcement of free-floating objects implies that they are comparable to or greater in number to main sequence stars. The WISE data now imply that BDs are significantly less common than MS stars– that is, the number of objects that form like stars, as BDs presumably do, falls off at masses well above several jupiter masses. This means that objects reported by microlensing searches are most likely actual planets as opposed to BDs.

  • Tempus June 12, 2012, 1:50

    Brown dwarfs DO have one resource that might be very useful to interstellar probe missions, and it might not even require them to slow down. Quite the opposite, in fact.

    In the same manner as the giant planets of our solar system, brown dwarfs are likely to have powerful magnetic fields. If we went the micro-probe route and used the magnetic fields of the Jovian planets in our solar system to accelerate swarms of tiny probes like railgun projectiles outward into interstellar space, then might not interstellar brown dwarfs be used for extra acceleration and course changes?

  • tesh June 12, 2012, 6:09

    Does anyone else get the feeling that interstellar travel is going to be like the wild-west. Sort of like outposts being established in small watering holes in the middle of nowhere and ships travelling in slowboat fashion across the stars/BDs/rogue planets, stopping off for a refill/R&R.

    I doubt that interstellar travel is only going to involve colony settlement ships. There will also be explorer class of missions (with people hopefully) as well as industry and wierdos wanting to settle in a quiet backwater corner.

  • Ronald June 12, 2012, 6:43

    @Adam: your comment answers my point about metallicity and planetary mass, thanks. yes, this would reconcile a relatively small total amount of ‘metals’ with a large number of objects: dwarf-planets.
    Interesting, but for colonization purposes not very appealing.

  • Ronald June 12, 2012, 7:09

    @Mike: “Still, considering that the stellar formation process favours low mass stars the comparative rarity of brown dwarfs to red dwarfs indicates some cut-off process at work in those stellar nurseries.”

    Exactly, as I mentioned in a previous comment above, there was already a drop-off detected in the exponentially increasing abundance with lower mass beyond about M5 orso (i.e. about 15% of solar mass).

    Apparently then there is not a continuum from planets via BDs to stars and the formation processes of planets and stars are really different.

    Remarkably, this also seems to be confirmed by the recently observed fact (Kepler), that the middle-class planets (Super-Earths and Neptune class sub/ice giants) are the most common among planets.

    So, if these results combined will hold in the future, we may expect a real and large population gap between the masses of the largest planets and the smallest stars. Again, confirming different formation processes.

  • kzb June 12, 2012, 7:50

    I’m disappointed, but I don’t think it was unexpected at this point. If the sky had lit up with new brown dwarfs when the turned the WISE cameras on, we would’ve heard rumours long ago.

    On the bright side, the press release notes that Jupiter mass objects can be detected out to several light years, but the data processing to date would not have found these. So it’s still conceivable that one or more of these will be found nearby, especially given the recent estimate that there is 1.8 of these per star.

    Also note they say it is highly likely that further Y-classs BDs will turn up in the data, which could raise the ratio from 1:6 to 1:5 or 1:4.

  • Joy June 12, 2012, 16:20

    Ok, now I am going to put my money where my mouth is. I have long been a “Planet X” skeptic. With the null result from WISE, I am happy to put the nail in that speculative coffin. (Not to say that there are not trans Neptunian objects in Solar orbit larger than Pluto.) But I will ship a bottle of 42 Below NZ vodka to the first person to notify me of a TNO larger in diameter than Mars confirmed in a peer reviewed publication. Paul knows how to find me.

  • Rob Henry June 12, 2012, 18:13

    It is true that microlensing surveys that have indicated that the density of large planets in a line from here to the galactic core is about one per main sequence star. Since the law of thermodynamics’ itself leads to equilibrium average star/rouge planet velocities wrt each other that scale inversely as the root of mass – we know the we can’t confine planets to the galactic core, or they would gain hundreds of km/s and have to *cool off* by moving to the galactic suburbs where Sol is. Thus if anyone was serious about using these figures to determine planet density in our neighbourhood and modelled it properly, they would find a far larger figure.

  • Brett Bellmore June 12, 2012, 21:05

    To be useful for island hopping, intermediate bodies would have to provide much needed native resources. What would they be?

    Certainly not fuel, stopping to pick up fuel would be a total wash.

    I’m thinking there are basically three candidates:

    1. Volatiles to restock life support systems. Over hundreds or thousands of years you might need to replace leaks.

    2. Disposable mass to serve as ablative shields.

    3. Someplace to settle down, and let the *next* generation make the next hop.

    The third is probably the most important.

  • Eniac June 13, 2012, 0:37


    yes, this would reconcile a relatively small total amount of ‘metals’ with a large number of objects: dwarf-planets.
    Interesting, but for colonization purposes not very appealing.

    Why not? The larger the planet, the less accessible the resources, due to the gravity well. The dwarfer, the better, I’d say.

  • Lazy June 13, 2012, 9:31

    Pretty sure the meaning of “island hopping” is that we make permanent settlements around those brown dwarf with some humans stay there (maybe making nuclear-powered city on planets around it?) , before jumping to further star. It will help the next exploration -both who want to go to other star or who from other star want to back to earth- to become easier

    It’s like in Sim/Civilization game, honestly.

  • kzb June 13, 2012, 13:06

    Joy -take note of what Rob Henry says above. Also, the WISE press release makes it clear that planetary mass objects would not be detected in the data processing algorithms used so far.

    It’s still just about possible for one or more of these objects to be closer than the AC system.

  • jkittle June 14, 2012, 8:45

    I would be dishonest if I did not say i am a bit disappointed by the wise data. I guess the world does not work quite the way I thought and it certainly does not seem to want to cooperate with me on this point. Very unscientific of me to have a preference for more brown dwarfs, not less. The universe is what it is.
    I wonder if these objects may cool a bit faster than we think? if they heat up earlier and burn out faster maybe they are just harder to detect. The equations/models on this are not all that well understood. If the older objects are cooler than we expect then the detection by wise my only be picking up the younger ones(?) .

  • kzb June 14, 2012, 13:39

    jkittle: I’ve read some of the arxiv paper. I DO wonder about “Type 2 errors”, i.e false-negatives, and I wonder what other people think about this?

    There are 534 candidates within 20pc, only a minority of which have been followed up so far. But to get on the candidate list an object has to jump through quite a number of tight hoops.

  • ljk November 30, 2012, 13:14

    Brown Dwarfs Might Host Planets Too

    by Nancy Atkinson on November 30, 2012

    Brown dwarfs inhabit a kind of fuzzy line between stars and planets: their mass is seemingly too small for them to be full-fledged stars and yet they are too large to be planets. These dim stars were only discovered in 1995 but current estimates say that brown dwarfs could be as numerous as normal stars in our galaxy.

    Now, astronomers have found a brown dwarf that has a dusty disc encircling it, just like the discs encircling regular, young stars. It contains millimeter-sized solid grains, and around other newborn stars, these discs of cosmic dust are where planets form. Astronomers say this surprising find challenges theories of how rocky, Earth-scale planets form, and suggests that rocky planets may be even more common in the Universe than expected.

    Rocky planets are thought to form through the random collision and sticking together of what are initially microscopic particles in the disc of material around a star. These tiny grains are similar to very fine soot or sand.

    However, in the outer regions around a brown dwarf, astronomers expected that grains could not grow because the discs were too sparse, and particles would be moving too fast to stick together after colliding. Also, prevailing theories say that any grains that manage to form should move quickly towards the central brown dwarf, disappearing from the outer parts of the disc where they could be detected.

    “We were completely surprised to find millimeter-sized grains in this thin little disc,” said Luca Ricci of the California Institute of Technology, USA, who led a team of astronomers based in the United States, Europe and Chile. “Solid grains of that size shouldn’t be able to form in the cold outer regions of a disc around a brown dwarf, but it appears that they do. We can’t be sure if a whole rocky planet could develop there, or already has, but we’re seeing the first steps, so we’re going to have to change our assumptions about conditions required for solids to grow,” he said.

    Full article here: