Brown Dwarf Results Promising for Planets

by Paul Gilster on December 7, 2012

Do planets form easily around brown dwarf stars? Are they actually common? We’re getting a glimpse of the possibilities in new work at the Atacama Large Millimeter/submillimeter Array (ALMA), where a brown dwarf known as ISO-Oph 102 (also called Rho-Oph 102) is under investigation. In most respects it seems like a fairly run-of-the-mill brown dwarf, about 60 times the mass of Jupiter and thus unable to ignite hydrogen fusion. It’s also tiny, at 0.06 times the mass of the Sun, a dim object in the constellation of Ophiuchus.

The work suggests that in the outer regions of a dusty disk surrounding Rho-Oph 102 there exist the same kind of millimeter-sized solid dust grains found around the disks of young stars. That’s intriguing because astronomers have thought that earlier finer grains would not be able to grow into these larger particles in the cold, sparse disks assumed to be around brown dwarfs. Those that did form were thought to disappear quickly toward the inner disk, where they would be undetectable. The fact that these larger grains are here has interesting implications, according to CalTech’s Luca Ricci, who led the international team performing the study:

“We were completely surprised to find millimetre-sized grains in this thin little disc. 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.”

Image: This wide-field view shows the star-forming region Rho Ophiuchi in the constellation of Ophiuchus (The Serpent Bearer), as seen in visible light. This view was created from images forming part of the Digitized Sky Survey 2. Credit: ESO/Digitized Sky Survey 2/Davide De Martin.

The disks around brown dwarfs may thus be more similar to those around young stars than we had realized, suggesting that rocky planets may not be uncommon here. We do have a few brown dwarf planets — 2M1207b, MOA-2007-BLG-192Lb and 2MASS J044144 — already in the book, one of them relatively small at 3.3 Earth masses. And a study by Andrey Andreeschchev and John Scalo (University of Texas) has indicated that terrestrial-mass planets should form around these objects as long as there is enough disk material to work with.

Image: 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, known as cosmic dust, are similar to very fine soot or sand. Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have for the first time found that the outer region of a dusty disc encircling a brown dwarf — a star-like object, but one too small to shine brightly like a star — also contains millimetre-sized solid grains like those found in denser discs around newborn stars. Credit: ALMA (ESO/NAOJ/NRAO)/M. Kornmesser (ESO).

The ALMA work also found carbon monoxide around the brown dwarf, its first detection in a brown dwarf disk. All this is promising stuff if we want to construct exotic scenarios about what may happen around these dim objects. Lacking the mass to fire hydrogen fusion, a brown dwarf is nonetheless going to emit heat because of its slow contraction due to gravity. Any brown dwarf with terrestrial planets around it is going to be doing a slow fade as it gives up gravitational potential energy. That means that if there is a habitable zone there, it gradually moves inward.

I’ve quoted him on this before, but Centauri Dreams reader Andy Tribick may not mind if I put this comment of his out there again. He’s talking about life around brown dwarfs:

It’d be interesting to come up with some scenarios for evolution on such a planet whose star decreases in luminosity as it ages (as opposed to more conventional stars that brighten as they age) – perhaps life might begin in the cloud layers of an initially Venus-like planet, moving to the surface as the atmosphere cools and the oceans rain out of the atmosphere, and finally moving to a more Europa-like state with the oceans frozen under an ice layer.

The 2004 study by Andreeshchev and Scalo I referred to above concluded that a brown dwarf with the mass of 0.07 solar masses, not all that far off Rho-Oph 102’s 0.06, could produce a habitability duration of a billion years, presumably long enough to get basic life functioning. And because the two scientists are talking about a classic habitable zone definition involving liquid water on the surface, you can see that Andy Tribick’s idea would extend the duration of possible habitability at both ends. Andreeshchev and Scalo themselves can extend habitability out to a surprising four billion years depending on how far they push the Roche limit, which governs how close a planet can be to its host star before it’s torn apart by tidal forces.

So maybe there’s a case for astrobiology around brown dwarfs, objects about which we still have much to learn. Just how common are they? While some scientists have suggested that brown dwarfs might be as common as the M-dwarfs that make up as much as 80 percent of the stars in the Milky Way, the results from WISE — the Wide-field Infrared Survey Explorer — turn up one brown dwarf for every six stars. That’s still a significant number of objects, and it includes 33 brown dwarfs known to be within 26 light years of the Sun, but the idea of interstellar targets closer than Alpha Centauri, at least stellar targets, is evidently fading from view.

But back to Luca Ricci and team and their work with ALMA. The great news embedded in the story is that ALMA is only partially up to strength. This array of radio telescopes in Chile’s Atacama desert operates at millimeter wavelengths and is intended to consist of 66 instruments when complete, but Ricci’s team worked with just a quarter of the final complement of antennas. In terms of observing planetary system formation, ALMA is already proving its worth, and a completed ALMA installation should be able to give us a close look at Rho-Oph 102:

“We will soon be able to not only detect the presence of small particles in discs, but to map how they are spread across the circumstellar disc and how they interact with the gas that we’ve also detected in the disc,” Ricci said. “This will help us better understand how planets come to be.”

The paper is Ricci et al., “ALMA observations of rho-Oph 102: grain growth and molecular gas in the disk around a young Brown Dwarf,” accepted at the Astrophysical Journal (abstract). The Andreeshchev and Scalo paper is “Habitability of Brown Dwarf Planets,” Bioastronomy 2002: Life Among the Stars. IAU Symposium, Vol. 213, 2004 (full-text).

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Anthony Mugan December 7, 2012 at 10:33

An extremely tentative question inspired by the interesting thoughts around habitability in such a system in the article…
Could the upper layers of a cooler (e.g. Y class) brown dwarf contain enough metals to make it a possibly habitable environment for micro-organisms (e.g. if some were to survive ejection from a nearby planet and the journey over to the brown dwarf) or are these objects fundamentally different in their chemistry to very large giant planets?

jkittle December 7, 2012 at 15:59

Wow ! I read this article and realized that we may be misinterpreting the Data form WISE! What if Brown dwarfs never get warm enough to clear away the dust the enshrouds their birth, and that most brown dwarfs are therefore cloaked by gas and dust? This might shift there heat signatures as they cool out of the range that Wise was interpeting as Brown Dwarf signature, and maybe disguise them as nebula. Most brown dwarfs would be hard to see and only those whose envolope was removed by other means ( a second gas giant planet or a near encounter by another star) would show up in the Clean infrared where we expect them. getting more proper motion data form a second WISE data collection rum might thus identify mis- categorized objects because of thier high proper motion.

Rob Henry December 7, 2012 at 18:04

Jkittle, I know little about this but I am wondering if dusty nebula that are entirely without gas would have already stood out as conspicuous anomalies?

andy December 7, 2012 at 19:28

Regarding MOA-2007-BLG-192Lb, it seems the host star is more likely to be a red dwarf star, albeit one right up against the hydrogen-1 fusion limit: see Kubas et al. (2012).

As regards the scenario I mentioned in my comment, I admit the suggestion of life arising in the clouds on a “humid Venus”-type planet is a bit fanciful, though surprises would perhaps not be too surprising (!). Hopefully the initial hot state would not result in the desiccation of the planet: I’m hoping the UV/XUV output of young brown dwarfs would be too feeble to split too much hydrogen out of the planet’s water reservoir but there is not enough observational data to be sure.

johnq December 7, 2012 at 21:19

Wow, indeed. As I was thinking about this today, I wondered where is Hal Clement when we need him? Just kidding. Of course, we always need him. I couldn’t stop thinking about the kind of life forms that might evolve on this planet, large furry creatures with huge saucer eyes, half tiger/squid, or some equally outlandish flying thing that possesses extremely sensitive sonar-type detectors.

Though the “brown dwarf” gives off heat (How much? What would be the temperature range on a hypothetical planet of a brown dwarf in the inhabitable zone? Air pressure? Wind speed? ), presumably the BD doesn’t give off much light (not in our wavelengths anyway!) My sense is that the prospects for intelligence are dim, but one has to wonder. Over a couple billion or so years, there would be time for some strangeness in the proportion.

Anyway, the complaint, point, I am trying to make is this: how can something 60 times the mass of Jupiter (some half-million km in diameter. Am I right?) possibly be termed a “dwarf?” Dwarf compared to what? Too late now, admittedly, but I would have liked to propose that these objects, because of their gravitational heat radiating mechanism, be termed “gravitars.” (Might have had to pay off a few lawyers, however.)

Phil December 7, 2012 at 23:00

A 0.06 solar mass BD is going to start hot, certainly with a late M type spectrum,and so will give off a fair bit of visible light. Actually, it will burn Deuterium for several million years as well.

Ive always wondered at what temperature/spectral class a brown dwarf would cease to be visible to human eyes. Cloudiness and metalicity are going to complicate things, but I would have thought that by late T class, BDs would cease to be self luminous to human eyes.

As far as close BDs are concerned – we still have a big gulf between the early Y class / 400 degree objects WISE has found and Jupiter, and I remain hopeful that objects closer than Alpha cen might eventually be found, although what instrument will be used to do it is anybody’s guess!

P

jkittle December 8, 2012 at 14:08

John Q
Because of a quirk of gravity / pressure / volume… it turns out all large gas giants and brown dwarfs ( from around jupiter to about 60 time jupiter) have approximately the same size. The increased mass also increases gravity and thus compresses more gas into the same space. Once the hydrogen burning limit is reached the energy of fusion expands the sphere though still the volume grows a bit slower than the mass.
I am hopeful that a scope equivalent to WFIRST will be flown. I have also posted comments elsewhere on the web describing whey we need to fly TWO infrared survey scopes at the same time to better identify and filter signals from the solar system objects perhaps the NRO scopes that are in surplus can be used for this purpose. Posting two scopes at DIFFERENT la grange points and flying a multi-year all sky survey mission is sort of the next step. Hope we can also get mid infrared coverage not available on earth ( 3 to 5 and also 5 to 12 micron) – These can be detected with a passively cooled scope. one of the big challenges is actually data communications. This is another area where NASA needs a new approach. An few orbital receivers for deep space missions with a data processing capability would be really achievable and give FANTASTIC results. it would let distant probes use new wavelengths that cannot get though our atmosphere for communications and only the processed data would them be downlinked to earth on a relatively short , high powered shared communications channel.

johnq December 8, 2012 at 21:30

>jkittle December 8, 2012 at 14:08

Thank you for your response. Great information!

spaceman December 9, 2012 at 2:27

I remember reading that planet formation was considered unlikely around BDs for the reasons mentioned above. This struck me as odd given that the giant planets in our solar system all have their own “planets” which we call moons; however, then I remember hearing that the giant planet moons probably did not form via accretion rather they were most likely captured. If they were not captured and formed via accretion it would only lend support to the idea that BDs, even large objects, should easily form bodies the size of Earth. Is my understanding of the origin of our system’s giant planets correct–that is, how certain are we that the moons of, say, Jupiter, were captured as opposed to formed in situ via a miniature version of the way the planets form around stars??

Scott Gordon December 11, 2012 at 9:33

Spaceman:

Not sure where your information comes from. Almost universal consensus is that the large moons of the giant planets (with the probably exception of moon Triton) formed by accretion around their current parent planets.

There is no widely accepted proposal that (Triton excepted) anything but the smaller irregular moons in odd orbits, were captured.

Triton is the probable exception because it could have become attached to Neptune (in a very odd retrograde orbit) after starting off as half of a binary planet, the members of which exchanged energy as they passed close to Neptune so one was captured and the other flung off. Such an event cannot explain the extensive systems of coplaner orbitting moons (eg. of Jupiter) and is not needed to explain them.

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