Brown Dwarf Companion: Planet or Star?

by Paul Gilster on April 7, 2010

Our knowledge of brown dwarfs is expanding rapidly, and with the help of the WISE mission, we will be able to build a much more complete catalog of such stars in our neighborhood. But look what the Hubble Space Telescope, in conjunction with the Gemini Observatory, has produced: A companion to a brown dwarf that gets us right back into the debate about how to define a planet. When Pluto was in question, we were faced with a true imbroglio. Now the question involves not a small but a large object, and forces us to consider whether its origins can make an object of acknowledged planetary mass something else instead.

But first, the imagery (this is, after all, a direct detection). The primary brown dwarf is 2M J044144, images of which were obtained as part of a survey of 32 young brown dwarfs in the Taurus star-forming region 450 light years away. Both objects are visible below:

Image: Hubble Space Telescope (top) and Gemini North (bottom) images of the 2M J044144 system showing the smaller companion at 8:00 position. The companion has an estimated mass of between 5-10 times the mass of Jupiter. In the right panel of both the HST and Gemini images the brighter light from the brown dwarf has been removed to show the companion more clearly. Credit (top): NASA, ESA, and K. Todorov and K. Luhman (Pennsylvania State); (bottom): Gemini Observatory/AURA and K. Todorov and K. Luhman (Pennsylvania State University).

The primary brown dwarf weighs in at about 20 Jupiter masses, separated from the smaller body by 3.6 billion kilometers, which would put the latter between the distances of Saturn and Uranus in our own system. The question about the planetary status of the smaller object comes up because while its mass is consistent with other gas giants we’ve discovered, its age tells us that it may not have formed like a planet. Kevin Luhman (Pennsylvania State) has this to say:

“This is the youngest planetary-mass companion that has been found so far, and its extreme youth provides constraints on how it could have formed. The formation mechanism of this companion in turn can tell us whether it is truly a planet.”

Core accretion models of planet formation have a planet gradually forming within a circumstellar disk, a rocky core forming before the accumulation of a gas envelope. An alternative model relies on instability within the same disk, causing a clump of gas to collapse and form a gas giant on a short time-scale. But neither of these methods seems to apply here. The companion object did not have time to form in this young system by core accretion, and gravitational instability relies on having enough material to make an object of this mass.

We’re left with a third option, that this smaller object formed from the collapse of the cloud of dust and gas in much the same way that the primary star did. This is from the paper on this work, and refers not only to the system in question, but to the earlier discovered companion of the brown dwarf 2M J12073346, which was found in 2004:

This result is consistent with the rapid formation expected from both gravitational instability in disks and fragmentation of cloud cores. The latter is more likely to have produced these two secondaries given their relatively large masses compared to the primaries.

The formation models are shown below:

Credit: NASA, ESA, and A. Feild (STScI).

This would mean that the same process that makes binary stars can produce planetary-mass objects. If we define planets as objects that build up inside disks, then 2M J044144′s companion is not a planet. The idea gains weight by the observation that a nearby M-dwarf seems to have a brown dwarf companion of its own. Again quoting the paper:

Indeed, if 2M J044144 A and B are members of a quadruple system, then its hierarchical configuration further suggests that 2M J044144 B formed by cloud core fragmentation. Additional data are needed to determine more definitively whether 2M J044144 A/B and 2M J044145 A/B comprise a quadruple system.

In this presumed quadruple system, all four objects would have formed in the collapse of the same cloud. Objects that push up against our categories are cause for celebration. They force us to ponder the nature of our definitions, and to modify them to revise theory. Meanwhile, brown dwarfs will remain much on the minds of those interested in the prospects of future interstellar missions. They seem to exist in abundance, and we may yet find one or more closer to us than the Alpha Centauri stars. And who knows what kinds of companions we may turn up around them?

The paper is Todorov et al., “Discovery of a Planetary-Mass Companion to a Brown Dwarf in Taurus.” In press at Astrophysical Journal Letters (preprint).

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{ 13 comments }

andy April 7, 2010 at 12:23

It’s definitely becoming more and more clear that the star formation process extends down below the fusion threshold, and there seems to be quite a bit of evidence suggesting the planet formation process can produce objects which undergo core nuclear fusion.

Non-fusing stars and deuterium-burning planets, who would have thought it?

Lionel W April 7, 2010 at 14:41

Does the cloud collapse model imply a lower limit for formation of bodies?

Would a jupiter sized body, an earth sized body, or even a ceres style body be able to form from cloud collapse, or would the mass of these bodies be too low for gravity to pull the cloud material together?

I have relatively little specialist knowledge into astrophysics, but am wondering whether planet size bodies may be forming/formed in space (outside of the gravitation influence of a solar system) and whether such bodies are in abundance, floating freely between stars.

I am made to think of Sedna, with its unusual different color – I wonder whether it formed not by core accretion in the solar system’s disk, but by cloud collapse far off in space, and then became captured by the sun’s gravity into its unusual elliptical orbit

But is the mass of Sedna too low for cloud collapse to be a possible formation process?

Adam April 7, 2010 at 15:29

Hi andy

Core accretion would produce a deuterium burner that burns the stuff in a shell around the heavier element core.

NS April 7, 2010 at 15:57

“Deuterium-burning planets”.

I hadn’t heard about this. Do you have a link?

Terraformer (a.k.a Tobias Holbrook) April 7, 2010 at 17:53

Wow, a quadruple system, with a red dwarf, two brown dwarfs, and a superjovian. I’d love to be there. The inhabitants of the red dwarf would be to the brown dwarfers what red dwarfers would be to the inhabitants of solar analogs, and the ones around the superjovian… you get the picture.

What’s the relative abundance of brown to red dwarfs observed?

andy April 8, 2010 at 2:13

Adam: depends on how much mixing is going on really. In the interior of giant planets the heavy elements can dissolve in the hydrogen, so there can be substantial quantities of hydrogen in the core. For example, in Uranus and Neptune, which may well be something like gas giant cores, the model of separate, distinct layers (core, mantle, atmosphere) fails to reproduce the moment of inertia for the planet, but a mixture model with continuously increasing heavy element content towards the centre of the planet works a lot better.

David April 8, 2010 at 3:27

NS,
For more about “Deuterium burning planets”, see
http://arxiv.org/abs/0802.1810

A good, very recent review paper on planetary interiors can be found at
http://arxiv.org/abs/0802.1810

James M. Essig April 8, 2010 at 18:31

Hi Folks;

These brown dwarfs can be an excellent source of hydrogen fuel for fusion powered relativistic rockets, fusion runway powered craft, and nuclear pulse type of craft such as those of Project Orion, Project Medusa, and now Project Icarus.

I can imagine large pumping stations that could mine the hydrogen via some form of conveyence conduits for stations in orbit around such stars. Perhaps some form of beanstalks that are centered at stellar stationary orbits would work for fuel pumping or conveyence.

The materials required would need to be superstrong to say the least. The best known materials that might approach the required strength might be carbon graphene which is about 200 times stronger than structural steel, carbon nanotubes, and boron nitride nanotubes.

Hydrogen and Helium seem to be a dominant real baryonic cosmic mattergy energy source, at least in our universe, and so from a philosophical perspective, any minable sources of hydrogen or helium is great news to me.

David April 8, 2010 at 21:18

Mistake above, the review paper on the interiors of exoplanets is at
http://arxiv.org/abs/1001.3577v1

Adam April 10, 2010 at 8:06

Hi All
David, those are the papers, especially that first one. Speaks for itself, but it helps to have been written by some major researchers in exoplanet/brown-dwarf modelling.

Such a strange concept – a deuterium fusion planet – but the modelling doesn’t lie! Stars with cores really can exist. I do often wonder if there can’t be hefty objects significantly enriched with deuterium that last as long as hydrogen-fusing stars… but deuterium fuses too easily. Too much and an object is liable to actually explode in a runaway reaction.

Incidentally most of a lower mass Main Sequence star’s energy comes from deuterium fusion – the proton-proton fusion reaction makes the stuff, which fuses almost as soon as it is made from protons. A star with a hotter core fuses more via the CNO cycle, which is only a minor player in our Sun at present. As the Sun’s core heats up the CNO cycle will take over and it’ll drive the evolution away from the Main Sequence and ramp up significantly as the Sun becomes a Red Giant. Interestingly, by that stage, the CNO burning happens in a shell around an essentially inert non-fusing core… which is where we started this conversation.

ljk April 12, 2010 at 22:27

http://www.technologyreview.com/blog/arxiv/25034/?ref=rss

‘Potato Radius’ To Define Dwarf Planets

Posted: 11 Apr 2010 09:10 PM PDT

Astronomers propose the first objective definition of a planet that separates potato-like objects from spherical ones

Deciding what is and isn’t a planet is a problem on which the International Astronomical Union has generated a large amount of hot air. The challenge is to find a way of defining a planet that does not depend on arbitrary rules. For example, saying that bodies bigger than a certain arbitrary size are planets but smaller ones are not will not do. The problem is that non-arbitrary rules are hard to come by.

In 2006, the IAU famously modified its definition of a planet in a way that demoted Pluto to a second class member of the Solar System. Pluto is no longer a full blown planet but a dwarf planet along with a handful of other objects orbiting the Sun.

The IAU’s new definition of a planet isan object that satisfies the following three criteria. It must be in orbit around the Sun, have sufficient mass to have formed into a nearly round shape and it must have cleared its orbit of other objects.

Pluto satisfies the first two criteria but fails on the third because it crosses Neptune’s orbit(although, strangely, Neptune passes).

Such objects are officially called dwarf planets and their definition is decidedly arbitrary. In its infinite wisdom, the IAU states that dwarf planets are any transNeptunian objects with an absolute magnitude less than +1 (ie a radius of at least 420 km).

Today, Charles Lineweaver and Marc Norman at the Australian National University in Canberra focus on a new way of defining dwarf planets which is set to dramatically change the way we think about these obects.

The problem boils down to separating the potato-shaped objects in the Solar System from the spherical ones. What Lineweaver and Norman have done is show from first principles how this dividing line falls naturally between objects that are larger and smaller than 200 kilometers in radius.

Their approach is simply to look for a potato-sphere threshold in images of bodies in the Solar System. The empirical evidence suggests that the threshold lies at about 200 km.

Lineweaver and Norman then work out the material strength of these bodies in their early years when their shape was being determined. They calculate the other forces at work on these bodies, such as the gravitational forces and the forces associated with rapidly spinning bodies.

It turns out that when viewed from this point of view, the 200 km threshold fits pretty well. Anything smaller than this would almost certainly not have been squeezed by forces large enough to mould it into a sphere. Anything larger, on the other hand, is sufficiently squeezed t form a sphere.

Lineweaver and Norman’s conclusion is that dwarf planets are essentally anything larger than 200km in radius that have not cleared their own orbit of other bodies.

Such a definition fits most objects in the Solar System but there are one or two oddities that don’t fit the bill. the asteroid Vesta, for example, is both potato-shaped and larger than 200km across. Lineweaver and Norman explain this away by suggesting that it may have been deformed by a collision relatively late on in life.

The 200km threshold looks to be a sensible criteria that the astronomical community can rally around. The trouble is that it dramatically increases the number of objects that count as dwarf planets and that may not please everyone, particularly those who hanker for special treatment for Pluto.

On the other hand, it makes Pluto the main representative of the dwarf planets, an important but poorly studied subgroup of bodies in the Solar System. That can only increase interest in this icy object.

As astronomers are only too keenly aware, interest is more or less synonymous with funding. And, of course, that is the unspoken issue at the heart of the debate over what is and isn’t a planet.

Ref: http://arxiv.org/abs/1004.1091: The Potato Radius: a Lower Minimum Size for Dwarf Plan

James M. Essig April 15, 2010 at 8:43

Hi Folks;

The cool thing or should I say the warm thing about these brown dwarfs is that for planets locked in close orbit around them such that the appearent angular diameter of the brown dwarf on the planet would be simmilar in relation to a human person standing next to a large indoor heating vent, the brown dwarf might provide heat for hundreds of millions if not billions of years to the planet thus promoting the development of life, perhaps even intellegent life.

Imagine that the potential gravitational energy of a fully formed brown dwarf is equivalent to 0.01 percent of the potential energy of usable nuclear fusion fuel for a star similar to the Sun while the star is in its main stable portion of its lifetime, a period of about 10 billion years in total. Since the Sun has a black body surface temperature of about 5,800 K, perhaps a brown dwarf having a surface temperature of 580 K or about 310 degrees C could radiate at this level for (0.0001)(10 EXP 10)[(580/5,800) EXP - 1] years or for about the same lifetime as that of our Sun. This assumes that the surface area of a large brown dwarf is roughly that of our Sun. The mathematical reasoning is thus that the brown dwarfs total EM energy radiated power is proportional to the 4th power of its surface temperature while holding its surface area constant.

I find the notion of visiting such a world if they exist, and basking in the dark night time warm glow of a brown dwarf along an ET beach with tiki tourches and exotic ET fruit drinks quite appealing.

We can only imagine that brown dwarfs may be associated with a whole new qualitative dimension of vacation experiences for generations of our decendents to follow.

Melvyn B. Lenton June 25, 2011 at 20:21

Planet or Solar System?
Eureka! I have finally found a discussion group that does not pass a Kidney Stone at the mention of “free floating” planetary mass objects or of Deuterium burning planetary mass objects.
My question, How does one classify these systems ?
If a Brown dwarf is orbiting a parent star and has 3 planetary mass objects, approx. 7-12 Jupiter masses, orbiting it.
If a Brown dwarf orbits no parent star, “free floating”, and has 3 planetary mass objects, approx. 7-12 Jupiter masses, orbiting it.

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