WISE: Brown Dwarf Hunter Extraordinaire

by Paul Gilster on November 13, 2009

Friday is a travel day for me, so be aware that comment moderation will be slow and sporadic. I just have time to get in word about the upcoming launch of the WISE mission, slated for December 7. NASA is planning a media briefing next Tuesday (November 17) to discuss the mission, which is designed to scan the entire sky at infrared wavelengths, spotting perhaps hundreds of thousands of asteroids and studying a wide range of stars and galaxies.

The technology is fascinating in and of itself. WISE will image the entire sky in the infrared, using detectors kept below 15 Kelvins (which is only 15 degrees C above absolute zero) by a solid hydrogen cryostat. The telescope will be oriented to look out at right angles to the Sun, always pointing away from the Earth, so that its observations sweep out a circle in the sky. After six months, the instrument will have observed the entire sky, producing nearly 1.5 million images and creating, ultimately, an atlas of the entire celestial sphere.

This is exciting stuff. For one thing, WISE should be able to measure the diameters of more than 100,000 asteroids. For another (and this may be of the most interest to Centauri Dreams readers), WISE will be able to detect stars much dimmer than the Sun. These brown dwarfs, many of which have yet to be discovered, should be readily apparent to the WISE instrument, and of course we hope for one that ranks as the closest star to the Earth. And beyond all this, WISE will be able to produce a global map of the galaxy and its associated dust.

But back to the brown dwarf issue. WISE principle investigator Ned Wright refers to the chart below. WISE is sensitive to radiation with wavelengths of five microns, useful for our purpose because from brown dwarfs down to Jupiter-class gas giants, a large fraction of the emitted radiation appears at five microns, as the figure shows:

bd-only

About which Wright has this to say on a page of his Web site devoted to the brown dwarf hunt:

These low mass stars are expected to be more numerous than the more massive stars like red dwarfs, and thus there should be brown dwarf stars closer to the Solar system than Proxima Centauri [italics mine]. The green curve shows a 200 K model atmosphere calculation from Burrows et al. (1997) for an object with the radius of Jupiter at the distance of Proxima Centauri. WISE will easily be able to detect these nearby brown dwarfs.

Image: Plot showing nearby objects compared to the sensitivity of WISE. A free floating Jupiter at 1 light year (FFP), and a 200 K brown dwarf at the distance of Proxima Centauri (BD). Credit: Edward Wright/UCLA.

So if we are dealing with a brown dwarf closer than any other star, WISE ought to be the mission to find it. Surveys like the Two-Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) have discovered numerous brown dwarfs, but have been unable to locate any cooler than 750 K. We can expect WISE to see 450-K brown dwarfs out to a distance of 75 light years, and brown dwarfs as cool as 150-K out as far as ten light years. All eyes may be on Kepler and CoRoT for terrestrial exoplanets, but a nearby brown dwarf would be huge, putting WISE on the front pages.

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{ 23 comments }

keith November 13, 2009 at 12:27

I find this very interesting, it will be incredible if we suddenly find out there are lone brown dwarfs all around. Also there could be isolated planets, planets that got ejected from their host systems during the violent early days of system evolution or because of close encounters with passing stars. Of particular interest is comparing the data from WISE and the microlensing surveys.

kurt9 November 13, 2009 at 15:56

Yeah, WISE will be as interesting as Keplar. It appears that the entire mission, or at least the data collection part of it, requires less than 8 months, following getting the probe in the correct location. Perhaps WISE will find Nemesis, if it exists.

Adam November 13, 2009 at 17:00

As Keanu Reeves put it so well “Whoah!”

That’s incredible sensitivity. Does make me wonder just how many BDs we’ll discover roving free of the stars since the number estimates vary so widely

andy November 14, 2009 at 11:56

This would be very interesting for attempting to determine the transition between spectral type T and the (still hypothetical) spectral type Y. This would give quite a bit of insight into the atmospheres of giant planets located further out in their planetary systems than the hot Jupiters.

Wonder how useful this mission will be in constraining the numbers of massive planets on wide orbits, like those around HR 8799 and Fomalhaut.

Hans Bausewein November 14, 2009 at 13:11

Looks like an excellent spacecraft to find NEO’s.
From the FAQ:
“Some patch of the sky (e.g. Asteroids) will be observed about a dozen times over about 2 days as they move through the sky along with the telescope’s survey pattern. Objects fixed in the sky will be observed a minimum of 8 times (at the ecliptic equator) over 12 hours before the survey pattern moves past them. Objects at the ecliptic poles will be repeatedly observed every 90 minutes throughout the entire 6 months. A second pass on half of the sky is possible during the extended mission, fixed-sky objects will be seen twice as much and 6 months apart. ”

http://wise.ssl.berkeley.edu/mission_faq.html

Daniel November 14, 2009 at 17:20

I wonder if the WISE mission, could detect planets far out as neptune in our solar system,something like planets of neptune mass that could produce a internal heart like neptune or jupiter ,and i wonder if the mission could detect earth mass planet for out as neptune,

could WISE be capable of such detections?

i´m glad if someone could answer me this question thank you

Adam November 15, 2009 at 6:25

Hi Daniel

Your question is hard to interpret, but I’ll attempt a reply based on what you seem to be saying.
1) Could WISE detect a Neptune as far away as Neptune?
Neptune is much, much closer than the brown dwarf stars it is designed to see, so even though Neptune is much cooler it would still be seen as it produces about 160% more heat than it receives from the Sun.
2) Certainly a Neptune as hot as Jupiter would stand out and would be detected within several thousand AU of the Sun. WISE will probably be sensitive enough to detect any brown dwarf companions of the Sun.
3) Earth mass planets would probably be too cold to emit significant infra-red radiation in WISE’s spectral range, but WISE will be very sensitive to anything warmer than 15 K – its operating temperature. Earth emits roughly 65 milliwatts of heat per square metre, on average, from its internal sources – that’s an effective temperature of 32 K. That might be detectable out to a considerable distance. A planet of Earth mass, but made of more ice than rock, won’t emit as much heat and probably won’t be detectable, but a rocky planet might be.

andy November 15, 2009 at 12:23

According to this Planetary Society blog post, WISE would be able to detect a Jupiter-analogue (i.e. same mass, size, temperature) out to about a light year, and a Neptune-analogue out to about 700 AU.

Michael Spencer November 15, 2009 at 16:40

When I came across Ned Wright’s name in Paul’s summary, of course I remembered the incredible cosmology ‘short course’ maintained by Prof. Wright for many years– I can’t remember when I first encountered his lively and approachable summary, which I have recommended to many of my friends. And as the higher math I learned in college has somehow seeped through cranial crevices, these essays are all the more useful.

What a pleasure to see Prof. Wright as the PI on this!

keith November 16, 2009 at 8:50

Methods for detecting cool BD’s up to now have been INFERENTIAL. That is, they rely on things like microlensing of background stars to detect the brown dwarf, and not direct observation.

WISE could validate these inferential methods, and in all probability will do this. BUT I think there is always the possibility of a “Woops !” moment, and that would be provided if WISE detects a huge population of BD’s.

It’s interesting that the WISE scientists state that BD’s should be more common than stars. I’m not sure if this is accepted dogma, I believe that most think the mass distribution function turns down towards the lower end of the red dwarf mass distribution. However I personally think this could be observational bias, and I can’t think why the mass distribution peak should coincide with the hydrogen-fusion threshold.

Anyway, hopefully with WISE we are about to find these things out.

Ronald November 16, 2009 at 11:05

Keith:

I know this has been suggested before, but, if you are right, this could possibly explain at least part of the dark matter content of the universe. That is, if indeed the mass distribution function keeps going up exponentially towards the lower end.

Anyway, if indeed, as you suggest in your previous post, there are ‘lone brown dwarfs all around’, they might pose a risk for (relativistic velocity) interstellar travel, like underwater rocks for ships.

keith November 16, 2009 at 14:36

Hi Ronald, yes I do wonder if this portion of the dark matter has been underestimated, and WISE should clarify that one way or the other. Don’t forget though, the number of BD’s IS constrained by other observations, not the least of which is, we are still here !

I don’t think it’s likely the population density would be high enough to pose a risk to relativistic interstellar travel, at least at modest tau factors. Surely by the time we were advanced enough to have relativistic travel, we would also have the capability to detect BD’s on our proposed route ?

Another thing to bear in mind, connected with this point, is that lone BD’s are likley to have a mini solar system of their own. So it’s not just the BD itself we have to think about, it’s also what is orbiting around it. If a BD nearer than Centauri IS discovered, think of the race to image it optically, detect its planets and resolve its whole system. Rich pickings for astronomers there.

Mike November 16, 2009 at 15:32

To Keith and Ronald, these questions about Brown Dwarf populations,their formation and their possible Dark Matter signature are,I think, the main reason the WISE mission was funded. Stellar formation certainly favors
lower mass stars. Does this continue into the Brown Dwarf range?
Hopefully be this time next year we’ll be WISE about it.

kurt9 November 16, 2009 at 18:50

Some of you will probably flame on this, but I just have to say it. I seems to me that stuff like Keplar and WISE is where the space budget should be spent on, not the shuttle or ISS. All of the discoveries in the past 20 years have been from automated probes, everything from Galileo to Keplar, than from the manned space program. Yet the manned space program (shuttle and ISS) gets the bulk of NASA and space science funding. I don’t this is either appropriate, or terribly cost-effect. I would rather NASA spent money on the TPF than on the Aries rocket. Private companies can develop space transportation.

If government funding is necessary for manned space flight, it should be as a series of “X” prizes leading the development to make commercial space development possible.

Ronald November 17, 2009 at 6:00

kurt9:
flame you? No, cheer you! 3 times hooray, this is straight from my heart.

Incredibly much more science missions, such as SIM and TPF (both interferometer and coronagraph), would be possible for the budget of ISS and other prestige oriented manned missions.

My impression as a European is that an important reason for this non-scientific spending within NASA is the military/prestige/space-race history of the US space program. European ‘space history’ is quite different, largely non-military, and hence ESA is now fast on its way to become a scientific world leader in its field for a considerably lower total budget. Just consider the recent European exoplanetary dicoveries, its ESO in Chili and planned Darwin en E-ELT. No chauvinism intended, I rather see cooperation than competition in this field, particularly for budgetary reasons.

The future of commercial space flight will probably be increasingly private, government (and scientific) institutions should focus on science.

keith November 17, 2009 at 8:47

Thanks Mike. I always have this difficulty with the concept that the stellar mass distribution should peak at, or just above, the hydrogen burning threshold. That smacks of intelligent design or creation by a deity.

I do not know of any reason why the peak population should coincide with this limit, other than an APPARENT coincidence caused by observational bias. Does anyone have any thoughts on this?

keith November 17, 2009 at 8:55

Hi kurt9 -you’re on the wrong site for this comment :) However you ARE right, the science per dollar is much more favourable for unmanned exploration. The ISS, that was just a black hole for money. It is not even going to be retained and built up as a staging post, it’s going to be crashed back to Earth.

Tell you what, if the governments have to keep “stimulating the economy” and bailing out banks, I’d much rather that economic stimulus was directed at manned spaceflight than protecting massive bonuses in the financial sector and keeping house prices out of the reach of ordinary people. How about you?

kurt9 November 17, 2009 at 13:35

Of course, I’m an advocate of manned space exploration and eventual human settlement of space. However, I think this can be accomplished better by private industry. Space transportation is like any other transportation. You have a competitive industry and the costs tend to decrease over time. Having a government funded space launch capability is a lot like having a government owned airline. A fair number of start-ups have come into existence in recent years. One of them, Space-X, has successfully launched satellites into orbit for paying customers. Their heavy launcher will soon be available. Some of the other start-ups are getting near viability as well. I think we will see the emergence of a competitive space launch industry with 3-4 players during the next decade. This, in turn, will drive the development of better and cheaper launch capabilities in the decade after that.

NASA should be reformed to focus exclusively on space science. Science is not easily paid for by private industry. No private company is ever going to finance something like the TPF. The discoveries of space science provide the motivation for the human expansion into space. Thus, space science can be considered a public good, worthy of public finance. I think freed from the space transportation role, NASA would become a far more effective agency. Out of the 8 NASA centers, only two (Kennedy and Johnson) are focused on space transportation. I think this results in the other 6 centers being somewhat ham-stringed in their effectiveness. If Johnson and Kennedy were privatized into commercial space ports, the other 6 research centers would be free to operate more like the national labs (Los Alamos, Sandia) and would be able to increase their productivity. JPL’s role as the mission control for the space probes would be similarly enhanced as well. Space transportation is a service that can be provided better by private industry.

This kind of reform would benefit both NASA and commercial space industry.

andy November 17, 2009 at 18:57

One of the interesting things regarding the brown dwarf mass function is that it may well be strongly environment dependent. Whatever the details of the process that forms binary stars, it seems to disfavour extreme mass ratios, hence the observed “brown dwarf desert” where both binary star formation and planetary accretion find it difficult to produce objects orbiting solar-type stars. Outside binary systems, there are indications that in (at least some) open clusters, the mass function keeps rising even as you get below the deuterium fusion threshold. Then there’s the issue of what the situation is like outside the clusters… Clearly the mass function must turn over somewhere (or we’d be overrun with vast numbers of tiny “gas giants”), the question is where.

ljk November 17, 2009 at 22:09

Nov. 17, 2009

George H. Diller
Kennedy Space Center, Fla.
321-867-2468
george.h.diller@nasa.gov

STATUS REPORT: ELV-111709 EXPENDABLE LAUNCH VEHICLE STATUS REPORT

Spacecraft: Wide-field Infrared Survey Explorer (WISE)

Launch Vehicle: Delta II 7320

Launch Pad: SLC-2, Vandenberg Air Force Base, Calif.

Launch Date: No earlier than Dec. 9, 2009

Launch Window: 6:10 – 6:23 a.m. PST

The launch of WISE aboard the Delta II is being rescheduled by approximately two days. A launch date of Dec. 9 currently is under review.

This additional time will allow sufficient time for a review of flight data from a Delta IV to be launched from Cape Canaveral Air Force Station planned early in the first week of December.

The Delta II flight simulation was conducted successfully on Nov. 11. The first stage was loaded with liquid oxygen on Nov. 12 for leak checks and associated launch vehicle testing. There were no issues or concerns with the Delta II rocket.

At the Astrotech payload processing facility on north Vandenberg, work is complete to load and then freeze the cryogenic hydrogen. This hydrogen will be used to supercool the infrared telescope during its six months of observations.

There is one final week of spacecraft testing which is under way. Work will then begin to prepare WISE to move to the launch pad on or about Nov. 20 for integration with the Delta II.

keith November 18, 2009 at 14:48

Thanks Andy, like you say the mass function must turn down somewhere, but I find it difficult that the turning point would be near the hydrogen fusion limit, as seems to be assumed in some quarters.

Also it’s common to put the “Nemesis” concept into the tin-hatters area, but when you look at the possibilities for cool BDs and isolated planets, I’ve started wondering again.

Here’s hoping WISE finds perhaps just 1-10 brown dwarfs nearer than Centauri. It would be cool if there were stepping stones to the stars, but not too cool if there were thousands of the things !

andy November 18, 2009 at 17:54

Yes it definitely seems that hydrogen fusion is not the turning point for the stellar mass function, nor apparently is deuterium fusion: observations of the sigma Orionis cluster suggest the mass function is still rising even when you get down to about 6 Jupiter masses, though the rate of increase seems to be falling off. So much for deuterium fusion being a useful way to distinguish between brown dwarfs and planets.

One of the problems with the Nemesis idea is that it is very far from proven that the various mass extinctions were caused by bolide impacts. Sure, that hypothesis seems to be promising for the K/T (or K/Pg now that various authorities are trying to phase out the usage of the Tertiary), it is very unclear that impacts caused the other ones. Without the requirement for periodic impact events, you lose the motivation for invoking the idea of Nemesis in the first place.

keith November 19, 2009 at 14:29

That’s an interesting link, andy. It’s also worth noting that about half the BD’s detected in that cluster survey have circumstellar discs, which implies mature BD’s will have planetary systems of their own.

The RECONS mass function for stellar mass down to 0.1M(sun) is M^-1.20, whereas this study has M^-0.6 ( in the range 0.11 to 0.006M(sun)) So possibly some turn-down in the rate of increase is occurring in the BD region, but the frequency is still inversely related to mass. The less massive a class of object, the more common it is, down to 0.006 of the mass of the sun.

So it looks like a real possibility, as I’ve long suspected, that BD’s outnumber proper stars, by several to one.

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