Stormy Outlook for Brown Dwarfs

by Paul Gilster on January 9, 2014

“Weather on Other Worlds” is an observation program that uses the Spitzer Space Telescope to study brown dwarfs. So far 44 brown dwarfs have fallen under its purview as scientists try to get a read on the conditions found on these ‘failed stars,’ which are too cool to sustain hydrogen fusion at their core. The variation in brightness between cloud-free and cloudy regions on the brown dwarf gives us information about what researchers interpret as torrential storms, and it turns out that half of the brown dwarfs investigated show these variations.

Given the chance nature of their orientation, this implies that most, if not all, brown dwarfs are wracked by high winds and violent lightning. The image below could have come off the cover of a 1950’s copy of Astounding, though there it would have illustrated one of Poul Anderson’s tales with Jupiter as a violent backdrop (“Call Me Joe” comes to mind). Brown dwarfs are, of course, a much more recent find, and in many ways a far more fascinating one.

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Image: This artist’s concept shows what the weather might look like on cool star-like bodies known as brown dwarfs. These giant balls of gas start out life like stars, but lack the mass to sustain nuclear fusion at their cores, and instead, fade and cool with time. Credit: NASA/JPL-Caltech/University of Western Ontario/Stony Brook University.

Storms like these inevitably suggest Jupiter’s Great Red Spot, too, but we want to be careful with analogies considering how much we still have to learn about brown dwarfs themselves. What we can say is this: Brown dwarfs are too hot for water rain, leading most researchers to conclude that any storms associated with them are made up of hot sand, molten iron or salts.

The idea that brown dwarfs have turbulent weather is not surprising, but it is interesting to learn that such storms are evidently commonplace on them. Even more interesting is what the Spitzer work has revealed about brown dwarf rotation. Some of the Spitzer measurements found rotation periods much slower than any previously measured. Up to this point the assumption had been that brown dwarfs began rotating quickly shortly after they formed, a rotation that did not slow down as the objects aged. Aren Heinze (Stony Brook University) had this to say:

“We don’t yet know why these particular brown dwarfs spin so slowly, but several interesting possibilities exist. A brown dwarf that rotates slowly may have formed in an unusual way — or it may even have been slowed down by the gravity of a yet-undiscovered planet in a close orbit around it.”

Whatever the case, brown dwarfs do seem to be opening a window into weather systems in exotic places, systems that can be studied and characterized by their variations in brightness. Heinze presented this work at the 223rd annual meeting of the American Astronomical Society in Washington for principal investigator Stanimir Metchev (University of Western Ontario).

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Tarmen January 9, 2014 at 19:17

Crazy to think about all the stunning storm vistas that right this second churn and krak at Jupiter and Saturn and Venus and all the numbered brown dwarf skies. Such raw power rolls and crashes where no one is to witness the thundering and the unearthly flashes and beautiful cloudscapes.

Michael January 10, 2014 at 3:53

‘Up to this point the assumption had been that brown dwarfs began rotating quickly shortly after they formed, a rotation that did not slow down as the objects aged. Aren Heinze (Stony Brook University) had this to say:

“We don’t yet know why these particular brown dwarfs spin so slowly, but several interesting possibilities exist. A brown dwarf that rotates slowly may have formed in an unusual way — or it may even have been slowed down by the gravity of a yet-undiscovered planet in a close orbit around it.”’

It is possible that they all rotate slowly because they find it harder to get rid of angular momentum in their formation process, this gives more time for the disc to be disrupted instead of forming the central mass. It is possible that higher temperatures and more mass available in a solar systems formation process allows angular momentum to be more easily removed and speeds up the formation process. This is a possible reason, inefficient angular momentum removal processes, why there are fewer Brown Dwarfs out there.

David Cummings January 10, 2014 at 8:53

Big news on the red dwarf front:

http://www.scientificamerican.com/article.cfm?id=a-star-at-the-edge-of-eternity

If this is, indeed, a red dwarf only 40 light years away, it’s a good site for a future home for our progeny. With trillions of years of output still remaining.

kzb January 10, 2014 at 9:13

In Asimov’s books, I remember the rotation speed of stars being discussed in relation to the existence (or not) of planetary systems. Stars can be divided into fast rotators and slow rotators. The fact that most are slow rotators was seen as powerful evidence that planetary systems are commonplace.

Most of the angular momentum of the solar system is in the planets, despite them being a tiny proportion of the total mass. During development, angular mometum gets transferred from the central body to the planetary system.

(As an aside it would be interesting to see the stellar rotation speed correlated with the exo planetary systems, maybe there are things to be learned in that). Anyway, if slow rotation was a powerful argument in favour of planetary systems for stars, and it has now been confirmed, surely that same reasoning applies to brown dwarfs. The slow rotation is surely due to the possession of planetary systems ?

Harry R Ray January 10, 2014 at 10:22

The recently discovered Luhman 16 binary brown dwarf system was not one of the 44 in the survey. I assume that Spitzer will observe it soon (if it has not done so already, with data in hand but not yet fully analysed. If massive close orbiting planets slowdown rotation rates, and one of these two brown dwarfs has a signifigantly slower rotation rate than the other, that brown dwarf would most likely be the one that hosts the putative planet possibly detected recently via astrometry!

ljk January 10, 2014 at 23:23

http://www.astrowatch.net/2014/01/new-kind-of-planet-or-failed-star.html

FRIDAY, JANUARY 10, 2014

New Kind of Planet or Failed Star? Astrophysicists Discover Category-Defying Celestial Object

An object discovered by astrophysicists at the University of Toronto nearly 500 light years away from the sun may challenge traditional understandings about how planets and stars form. The object is located near – and likely orbiting – a very young star about 440 light years away from the sun, and is leading astrophysicists to believe that there is not an easy-to-define line between what is and is not a planet.

“We have very detailed measurements of this object spanning seven years, even a spectrum revealing its gravity, temperature, and molecular composition. Still, we can’t yet determine whether it is a planet or a failed star – what we call a ‘brown dwarf’. Depending on what measurement you consider, the answer could be either,” said Thayne Currie, a post-doctoral fellow in U of T’s Department of Astronomy & Astrophysics and lead author of a report on the discovery published this week in Astrophysical Journal Letters.

Named ROXs 42Bb for its proximity to the star ROXs 42B, the object is approximately nine times the mass of Jupiter, below the limit most astronomers use to separate planets from brown dwarfs, which are more massive. However, it is located 30 times further away from the star than Jupiter is from the sun.

“This situation is a little bit different than deciding if Pluto is a planet. For Pluto, it is whether an object of such low mass amongst a group of similar objects is a planet,” said Currie. “Here, it is whether an object so massive yet so far from its host star is a planet. If so, how did it form?”

Most astronomers believe that gas giant planets such as Jupiter and Saturn formed by core accretion, whereby the planets form from a solid core that then develops a massive gaseous envelope. Core accretion operates most efficiently closer to the parent star due to the length of time required to first form the core.

An alternate theory proposed for forming gas giant planets is disk instability – a process by which a fragment of a disk gas surrounding a young star directly collapses under its own gravity into a planet. This mechanism works best farther away from the parent star.

Of the dozen or so other young objects with masses of planets observed by Currie and other astronomers, some have planet-to-star mass ratios less than about 10 times that of Jupiter and are located within about 15 times Jupiter’s separation from the sun. Others have much higher mass ratios and/or are located more than 50 times Jupiter’s orbital separation, properties that are similar to much more massive objects widely accepted to not be planets. The first group would be planets formed by core accretion, and the second group probably formed just like stars and brown dwarfs. In between these two populations is a big gap separating true planets from other objects.

Currie says that the new object starts to blur this distinction between planets and brown dwarfs, and may lie within and begin to fill the gap.

“It’s very hard to understand how this object formed like Jupiter did. However, it’s also too low mass to be a typical brown dwarf; disk instability might just work at its distance from the star. It may represent a new class of planets or it may just be a very rare, very low-mass brown dwarf formed like other stars and brown dwarfs: a ‘planet mass’ brown dwarf.

“Regardless, it should spur new research in planet and star formation theories, and serve as a crucial reference point with which to understand the properties of young planets at similar temperatures, masses and ages,” Currie said.

The discovery is reported in a study titled, “Direct imaging and spectroscopy of a candidate companion below/near the deuterium-burning limit in the young binary star system, ROXs 42B”.

The observational data used for the discovery was obtained using the telescopes of the Keck Observatory and Subaru Observatory on Mauna Kea, Hawaii and the telescopes of the European Southern Observatory in Chile.

The international research team includes scientists from: the Space Telescope Science Institute in Baltimore, MD; the University of Montreal; the University of Hyogo in Kobe, Japan; the Universitats-Sternwarte Munchen and the Ludwig-Maximilians-Universitat, Munchen, Germany; and the University of Hawaii.

Credit: utoronto.ca

torque_xtr January 11, 2014 at 20:22

The one thing that blurs planet-stellar object division is the formation by fragmentation of circumstellar/protoplanetary disk. There were hints that in the disks around the most massive stars, especially in the early universe, full-fledged stars several tenths of the solar mass could form by this or very similar mechanism (http://www.space.com/10801-stars-early-universe-loners.html), and it’s possible that fragmentation could provide objects from around 1 M(jup) (http://www.mpia-hd.mpg.de/homes/ppvi/posters/2H028.pdf and the similar works) and maybe lower all the way to several hundreds M(jup)s. Should these be counted as planets? If not all of them, which should, and which should not? How should be called the objects like CoRoT-3b, or ROXs-42Bb, or stellar-mass objects formed as planets around O-type stars? The dividing line may be fuzzy…

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