Numerous Nearby Brown Dwarfs?

by Paul Gilster on June 25, 2010

The space-based Spitzer telescope has performed a new study of brown dwarfs, concentrating on a region in the constellation Boötes. Fourteen of the objects, with temperatures ranging between 450 and 600 Kelvin, have been found. These are cold objects in stellar terms, and in fact are as cold as some of the planets we’ve found around other stars. 450 Kelvin works out to 177 degrees Celsius, or 350 degrees Fahrenheit, the temperature of a moderately hot oven.

In fact, it gives me pause to reflect that the focaccia I baked the night before last needed higher temperatures (500 degrees Fahrenheit) than the coolest of these brown dwarfs can supply. Most of the new objects in the Spitzer study are T dwarfs, the coolest class of brown dwarfs known, defined as being less than 1500 Kelvin (1226 degrees Celsius). One of the dwarfs in this study is cold enough that it may represent the hypothetical class called Y dwarfs, part of a classification created by a co-author of the paper, Davy Kirkpatrick (Caltech).

Concerning the brown dwarf classification system, Kirkpatrick points to the WISE (Wide-Field Infrared Survey Explorer) mission for further validation:

“Models indicate there may be an entirely new class of stars out there, the Y dwarfs, that we haven’t found yet. If these elusive objects do exist, WISE will find them.”

Image: This artist’s conception shows simulated data predicting the hundreds of failed stars, or brown dwarfs, that NASA’s Wide-field Infrared Survey Explorer (WISE) is expected to add to the population of known stars in our solar neighborhood. Our sun and other known stars appear white, yellow or red. Predicted brown dwarfs are deep red. The green pyramid represents the volume surveyed by NASA’s Spitzer Space Telescope — an infrared telescope designed to focus on targeted areas in depth, rather than to scan the whole sky as WISE is doing. The region within 25 light-years from the sun is marked by the blue sphere. Credit: JPL.

Kirkpatrick’s thoughts on WISE are fascinating in their own right (he’s also a member of the WISE science team). It has long been speculated, for example, that a cool object, perhaps a brown dwarf, could be found in nearby space, close enough to cause perturbations to the Oort Cloud. Kirkpatrick notes that his team is now calling this putative object Tyche, the benevolent counterpart to Nemesis, and says that WISE is powerful enough that it will either find Tyche or rule it out. And let me quote WISE project scientist Peter Eisenhardt (JPL) on the mission:

“WISE is looking everywhere, so the coolest brown dwarfs are going to pop up all around us. We might even find a cool brown dwarf that is closer to us than Proxima Centauri, the closest known star… We’ll be studying these new neighbors in minute detail — they may contain the nearest planetary system to our own.”

It’s worth keeping in mind that WISE is studying a volume of space 40 times larger than is covered in the recent Spitzer work. While all fourteen of the objects discovered in this survey are hundreds of light years away (and invisible to visible-light telescopes), their presence implies that there are a hundred or more brown dwarfs within 25 light years of the Sun, and the latter should be close enough to confirm with spectroscopy. The Spitzer team goes so far as to speculate that WISE may find more brown dwarfs within this 25 light year sphere around the Sun than the number of stars known to exist there.

The paper is Eisenhardt et al., “Ultracool Field Brown Dwarf Candidates Selected at 4.5 μm,” Astronomical Journal Vol. 139, No. 6 (May, 2010). Abstract available.

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{ 26 comments }

Adam June 25, 2010 at 17:39

The mind boggles at the possibilities. A BD closer than Proxima would be a very attractive as a first interstellar probe target or even a convenient way-station to systems further out if the BD has a useful magnetic field or cloud of gas surrounding it to be tapped by a ramscoop or electromagnetic drive.

Extreme far future prospects present themselves such as kindling brown dwarfs with quark-nuggets to promote fusion or imploding them to make wormholes.

Of course cosmogonic investigations will gain a bevy of new data points as nearby BDs add to the cosmic census. IMO the current front runner is gravitational instability in a heavy disk around a higher mass star. Anthony Whitworth and his collaborators have modelled this scenario extensively and it very efficiently produces BDs in similar numbers to those observed in star-forming regions.

Eric June 25, 2010 at 21:43

So when should we be hearing news from the WISE team? If nearby BW are likely to be as common as claimed in this study, what’s the probability of a detection soon?

Istvan June 25, 2010 at 22:18

“The mind boggles at the possibilities.” – Indeed!

This was quite the pins-and-needles, nothing-confirmed-yet but highly tantalizing article. I am so looking forward to the first releases of WISE data, in the hope of a game-changing target.

Elon June 25, 2010 at 23:15

So, if that “implies that there are a hundred” or more brown dwarfs within 25 LY, and Proxima is 4.2 LY away, then random distribution would give us a 4.2 over 25, cubed, times 100, implied lower bound chance of finding one closer — that’s about 50-50, right? Plus excluding nearby where’d we’d have found it already by gravitational effects, and with the mentioned upside on the 100 estimate, and it sounds like the smart bet is that WISE will indeed find a Tyche.

With the odds appearing that good, I wonder how close it could be. In particular, referencing the http://www.centauri-dreams.org/?p=13058#comment-82296 “Lily Pad” concept, how close could a Tyche reasonably be that was a useful Lily Pad, and how close to be warm enough to have a habitable zone planet outside its Roche limit, but not have been detected by other means yet? Less than 1 LY? Less than .2 LY?

Tobias Holbrook June 26, 2010 at 10:00

Finding a Habitable Planet (HB) around a nearby brown dwarf would be interesting from an astrobiological perspective, but it’s not required in order to be valuable for interstellar colonization. Certainly, we’re a lot more likely to find a Biocompatible (BP) or Easily Terraformable Planet. Expect lots of Titans, Europa’s, and quite a few Io’s around brown dwarfs. Such planets could be ripe for colonization with a few appropriately bioengineered organisms….

Cambias June 26, 2010 at 10:33

Forget about a habitable zone outside the Roche limit: some of those cooler dwarfs could have a habitable zone in their upper atmosphere!

This makes me wonder: are we dwellers on a solid planet circling a bright star the freaks? Maybe most of the life forms in the universe are reducing-chemistry organisms inhabiting brown dwarfs or supergiant planets. They’d be slow, by our standards, but there are a hell of a lot more brown dwarf stars than K through F “sun-like” stars.

Adam June 27, 2010 at 4:37

Interesting thought Cambias. Habitable brown dwarfs. As the Galaxy ages and the average metallicity increases, the opacity in the cores of proto-stellar objects will be higher and they’ll hit the Main Sequence at lower masses. According to computations by Laughlin & Adams stars as low as 0.04 solar masses will eventually form when the metallicity is ~x3 present levels. Their luminosity will be ~1 millionth of the Sun, cold enough for water clouds to form in their outer layers. Hard to imagine a nearer habitat to a Main Sequence star.

Of course the gravity will be problematic. A radius 1/10th the Sun and 0.04 the mass, means x4 gravity… ouch!

Tobias Holbrook June 27, 2010 at 8:46

Interesting. Not so sure that the habitable zone would be in the atmosphere, though – at 1 millionth of the suns luminosity, the habitable zone would be sqrt(1,000,000), or 1000th the distance that Terra is to Sol. That’s 90,000km… would that be within the Roche limit? Could there be lit stars at that mass in existence today, if they formed in an area of high metallicity? If we could get hold of a Jupiter mass of ‘metals’, could we stellarize Brown Dwarfs of that mass?

It’s looking like for for most of the Universes lit lifespan, Brown and Red dwarfs will predominate.

Carl Keller June 27, 2010 at 15:36

The salient thing is a neighborhood brown dwarf to characterize. Proximity would yield better observations and analyses. Even so, there are stellar classes, and each of our jovian worlds differ. What classes would be made of brown dwarfs? Following early categorizations, which will become Plutos: once one category, and then another (by non-BD specialists)?

Colin Weaver June 27, 2010 at 19:19

Wow, a .04 solar mass main sequence star! I wonder what the main sequence lifetime of such an object would be?

Geoff June 27, 2010 at 20:59

Since Elon brought it up, I had to do the calculation. If brown dwarfs are distributed according to a Poisson point process with a mean density of 100 per 25ly-radius-sphere, the probability of there being at least one within 4.3ly is 40%. The probability of getting two or more brown dwarfs in that volume is 8%.

Adam June 28, 2010 at 6:41

Hi Colin Weaver
One can estimate such a brown dwarf’s Main Sequence life at ~400 trillion years. Interestingly as the Galaxy ages its luminosity will remain roughly the same for ~800 billion years before going into a slow fade over 100 trillion years.

Tobias Holbrook June 28, 2010 at 11:29

“One can estimate such a brown dwarf’s Main Sequence life at ~400 trillion years. ”
400 TRILLION? Wow. And that’s only on the main sequence; how would it fare once it only has Helium to burn? Or would it be incapable of Helium fusion? Still, 400 Trillion years offers plenty of time to figure out how to survive later…

kzb June 28, 2010 at 13:30

I find the picture difficult to interpret. However, the elongated pyramid appears to contain something like seventeen brown dwarfs, and only ONE “proper” star.

I am interpreting the picture as a square-sided cone (OK I know) pointing out from the sun at the centre of the 25LY sphere. The cone goes out for some distance beyond 25LY. Its volume per unit distance increases with distance.

It seems to me that this is a random sample of a volume of space, and within that volume the ratio is about 17:1 in favour of BD’s. Is there something wrong with this interpretation?

kzb June 28, 2010 at 13:39

There is an arxiv version of the paper (no pay-wall):
http://arxiv.org/PS_cache/arxiv/pdf/1004/1004.1436v1.pdf

Also there is revision to my 17-to-1 ratio above: I neglected to count the sun, so the ratio should be 17-to-2.

Tobias Holbrook June 28, 2010 at 18:34

“Also there is revision to my 17-to-1 ratio above: I neglected to count the sun, so the ratio should be 17-to-2.”
If that’s true, then the average distance between them should be roughly 1/2 that of the average distance between lit stars. Right?

Michael Simmons June 29, 2010 at 1:19

> So when should we be hearing news from the WISE team?

From http://wise.ssl.berkeley.edu/astronomers.html

“For the current lifetime estimate of 10 months, and launch date of Dec 7, the first data release would be in April 2011. The final data release will be 11 months later in March 2012 according to current estimates.”

I’d imagine they will release their papers before those dates.

From their Facebook page on the 26 June
> We are now less than two weeks from covering the entire sky

Google “The Porcupine Survey: A Distributed Survey and WISE Followup”

kzb June 29, 2010 at 9:00

Tobias, yes what you say should be true IF the picture interpretation is correct. Which I have severe doubts about.

For one thing, it is not clear if proper stars have been deleted or not from the picture in the outer pyramid.

It is also not clear if the distance to the base of the pyramid is well-known. However, if we DO have a defined distance, that means we can calculate the volume of the pyramid, and that means we can come up with an estimate of the BD population density. We can then compare that to the better-known pop. density of proper stars (from RECONS).

The dimensions of the pyramid are probably retrievable from the paper, which I still have not had time to read properly :)

west_urn_enlightement June 29, 2010 at 12:48

The Wise team has been very slow to release data, following the old (outdated) model where university professors sit on the data until they can milk out as many discoveries as possible. The LSST effort will post data rapidly to the scientific Community ( for example) . This new model was developed in part during the Human genome project where Francis Collins had to differentiate the government effort from that of the private company -Celera.
Wise funding will run out while there are still collecting Data on Brown dwarfs(!) . This is in part due to the politics of the committee recommending funding, but might have been avoided if WISE team had posted sample data form the first month of operation. The resulting intense interest may have saved the program.
looking at some of the pictures, it will be very hard to pick out all the faint brown dwarfs and even distant solar system planets because of the background of distant nebula and galaxies. A major section of the sky will be surveyed twice , six months after the initial survey. In this section the objects closer than a parsec should stand out because of their Parallax motion. The rest of the sky will have to wait until narrow field telescopes and look at the individual objects or another wide field space telescope is launched. On the visible side, PANSTARRS may be able to sort out some objects out a couple of hundred AU. Write your congressman or call your rich friends. -Get an additional 6 months of funding for DATA COLLECTION for WISE!

kzb June 29, 2010 at 13:28

Having now attempted to read the paper, I now doubt that it could be used for a population estimate. This is because :
(1) the algorithm used is heavily weighted towards finding indisputable brown dwarfs and I think it is conceivable that many BD’s in the survey have been rejected.
(2) I can’t see any distance estimate, just one of “>70pc” for one BD.
(3) The survey is pointed at an angle of 67 degrees out of the galactic plane. The scale height of the thin disc means the stellar population is thinning out (depending on what is the range of course)

Having said that, it is worth noting the survey covered just 10 sq degrees, or about 0.024% of the sky. The implication is that if this survey could be repeated to cover the whole sky, it could discover something in the region of 58,000 (central estimate) BD’s

kzb July 1, 2010 at 7:11

I think you’re quite right west-urn enlightenment. Even now “Tyche” may have been imaged, but it’s discovery will have to wait. From what you say, even the chance of success in finding such an object has been compromised. Seems nuts when you’ve gone to all the trouble of designing, constructing and launching the instrument.

west_urn_enlightement July 1, 2010 at 23:54

I would hope that discovery of a “tyche” object (be it a drown dwarf at 0.5 light year or a neptune at 250 AU ) would ignite new interest in space exploration.
in the long run, the only practical way to the stars ( barring a new physics) is to move from small world to small world, ridding the Kuipr belt dwarf planets to the Oort cloud and beyond. Dwarf Stars would be natural congregation points in the never ending low gravity Diaspora . Frozen Volatiles = life sustaining resources

Tobias Holbrook July 2, 2010 at 8:41

“I would hope that discovery of a “tyche” object (be it a drown dwarf at 0.5 light year or a neptune at 250 AU ) would ignite new interest in space exploration.”
Possibly. A Neptune that far out may actually have a “floating ocean” on it, held up by a layer of supercritical water vapour. We don’t see them at the moment as presently Neptune and Uranus are too hot = dry.

250AU = approx. 33 light hours. If you had a ship capable of reaching a meager 1% of c, you could do the trip in far under a year. If there was a brown dwarf several light-months out, you could reach it in about 33 years travelling at the same measly velocity. Beef your propulsion system up significantly, to 1/3c, and you could get there in a year…

Adam July 3, 2010 at 4:37

Hi Guys

Geez Tobias just how fast would you be accelerating to hit 0.3c before tranversing 1/3 of an ly? Unlikely to happen for a physically reasonable propulsion system, though the unreasonable ones are the more interesting designs.

As for sitting on data… what can you do? Launch your own probe? But in WISE’s case the data acquisition time is limited by the frozen hydrogen cryo-cooling system that the designers chose. Past the current finishing date the satellite will start warming up and end up too hot for a lot of the NIR work that makes it interesting. So don’t campaign for an extension that won’t do any damned good.

Tobias Holbrook July 4, 2010 at 8:41

“Geez Tobias just how fast would you be accelerating to hit 0.3c before tranversing 1/3 of an ly? Unlikely to happen for a physically reasonable propulsion system, though the unreasonable ones are the more interesting designs.”
I don’t have the formula’s to hand for computing Brachistone trajectories (something I’ve been meaning to look for), but accelerating at a comfortable 10m/s^2 would get you to 100,000,000m/s in 10,000,000s = 2777.8h = 115 days. The average velocity during that time would be 50,000,000m/s, which means you would have travelled 500 billion kilometers = 19.3 light-days. Not bad.

west_urn_enlightenment July 6, 2010 at 1:41

Well… it is true that the 12 and 20+ micron detectors will be out of commission when the hydrogen is finished. however, the 3 an 5 micron detectors will work just fine and they are the ones that can detect T and some L brown dwarfs. there was a plan to do a warm mission, but the price was inflated by by a request to build a data center for $8MM. Really, it is a shame to turn the machine off while it is still collecting unique data.
Still, Wise is having and incredible run so .. I will be grateful. now if they would just release the data. Really, I think that policy on planning mission that hold back data this long should be changed. they are posting ateriod candidate at the minor planetcenter. still…
It will be awhile before the next mission is sent
If you are interested in the Calisto infrared survey scope , which is in the planning stages..check out this one
http://www.ipac.caltech.edu/DecadalSurvey/CALISTO_Astro2010_PPP_RFI_response.pdf
“CALISTO: The Cryogenic Aperture Large Infrared
Space Telescope Observatory
Paul F. Goldsmith”
I love this stuff. at 30 to 300 microns it will do for the trans-neptune objects what WISE is doing for asteroids.
Now guys and gals… how about power sources for ( very deep) space exploration? I am a biochemist/ synthetic biologist. give me a way to make hydrogen gas methane and O2 and I can give you a microbial factory to make the compounds needed to support a manned base on a Dwarf Planet in the Kuiper belt ( or beyond) .

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