Hunting brown dwarfs | University of Oxford
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Hunting brown dwarfs

Pete Wilton

They're stars that never quite made the big time: mysterious cosmic objects known as 'brown dwarfs'.

Today the team behind the CoRoT space telescope report that they have found a rare example of a brown dwarf tightly orbiting its star.

I asked CoRoT team member Suzanne Aigrain of Oxford University's Department of Physics about 'brown dwarf deserts', the gap between giant planets and stars, and what would happen if our solar system had its very own brown dwarf...

OxSciBlog: What is a brown dwarf?
Suzanne Aigrain: A brown dwarf [BD] is a celestial object intermediate in mass between a planet and a star. It's helpful to recall the definition of a star: a star is a ball of gas held together by its own gravity and which radiates light produced by thermonuclear reactions in its core, mainly burning Hydrogen to produce Helium. A brown dwarf is an object very much like a star, but which is not massive enough to burn Hydrogen in its core.

As such, brown dwarfs are faint and radiate mainly in the infrared, slowly releasing the heat they accrued during their formation. On the other hand, according to the International Astronomical Union's definition, a planet is also held by its own gravity but it is a) in orbit around a star or brown dwarf and b) not massive enough to burn Deuterium (Deuterium is an element which burns even more easily than Hydrogen). Any object which has a mass below the Hydrogen limit but above the Deuterium limit is thus a brown dwarf. This is the case for CoRoT-15b.

The definitions I have given above leave a rather fuzzy area for the case of object which are below the Deuterium burning mass limit but are not in orbit around a star or brown dwarfs - these are sometimes called sub-brown dwarfs or free-floating planets.

OSB: What is the significance of CoRoT finding a BD? Are they rare?
SA: Brown dwarfs are not rare in themselves, on the contrary. It is difficult to detect and study them, because they are faint compared to stars, so we don't know as many of them as we know stars, but over the past 20 years, with the advent of better and better infrared detectors, we have been discovering many of them.

What is extremely rare, however, is to find one in a tight orbit around a star, as in the CoRoT-15 system. Until a few years ago, we knew of none at all, and this absence was called the 'brown dwarf desert'. Now we know of a handful, but CoRoT-15b has the shortest orbital period of any known brown dwarf. The very existence of CoRoT-15b in its tight orbit is interesting (see below), but the fact that it transits across the disk of its parent star makes it even more useful, because it enables us to measure its radius.

OSB: What can they tell us about how planets & stars evolve?
SA: Systems like CoRoT-15 are very important to understand star and planet formation as well as evolution. The majority of brown dwarfs are thought to be the result of the same process which forms stars. Stars form from giant clouds of gas and dust. Regions in these clouds which are marginally more dense than their surroundings attract more material onto themselves, and these over-densities grow and grow until thermonuclear fusion ignites in the core, and a star is born.

If a clump never grows large enough for that to happen - because the material within its gravitational influence runs out - you get a BD. So, from the formation point of view, there is nothing fundamentally different between a star and a brown dwarf, but whilst tight binary stars are quite common, tight binary systems involving a star and a brown dwarf are rare.

Why is CoRoT-15b different? Did it get kicked into its current orbit by a close encounter with another star? Could it have formed like a planet, which forms in the disk of material accreting onto a star, instead?

There are very few BDs in close binaries, and even fewer which transit their parent star. These are the only BDs whose radii we can measure, so they are very valuable. If you make a plot of radius versus mass for stars and planets, stars all more or less fall on a single line, which basically is the line you expect for a self-gravitating ball of Hydrogen. This line flattens out at low masses - from 0.1 solar masses to a Jupiter mass, the expected radius is about 1 Jupiter radius.

However, the measurements for exoplanets are scattered, with a range of radii observed for a given mass. This is because the radius of a planet is affected not just by its mass, but also by its composition (how much solid versus gaseous material it contains) and by the amount of light it receives from its parent star. CoRoT-15b fills an important gap in this diagram, between low-mass stars and planets. It's also extremely close to its star, so extremely hot, and hence a particularly strong test of just how much intense irradiation can affect the radius of an object of that mass.

OSB: How might our solar system be different if it contained a BD?
SA: We know that having a binary companion does not prevent planet formation, since we know of stars which have both one or more planets and a binary companion. If the Sun had a wide BD companion, the solar system would not necessarily be very different. We would definitely know about it, however: BDs are faint compared to stars, but a BD that close to us would not be missed.

On the other hand, if the BD was very close-in like CoRoT-15b, things would be very different. I'll consider two possibilities: If the BD formed in-situ, there would be no disk, or very little of it, around the Sun, for the planets of the solar system to form out of. There might have been a disk around the binary (we have seen such disks around other binaries) and it's conceivable that this disk might form planets. We currently know of no such circum-binary planets, but this is at least in part because it is harder to detect them.

But if the BD was captured (or kicked into a close orbit from a wider one) after the Sun had formed its planets, then that would most probably have a very dramatic impact, as the gravitational influence of the inbound BD would wreak havoc on the planets and most likely eject them from the solar system!

OSB: What do we hope further CoRoT finds could reveal about BDs?
SA: CoRoT already found another transiting BD, CoRoT-3b. It is less massive and less close in than CoRoT-15b, but the fact that CoRoT found two of these very rare systems shows that it is well-suited to detecting them. Along with the NASA mission Kepler, which is also searching for transiting planets, CoRoT can hope to discover several more systems like these in the next few years. They will tell us more about how these rare systems form, about what forms the difference between a massive planet and a BD, and about how BDs evolve when very close to their host star.

Dr Suzanne Aigrain is based at Oxford University's Department of Physics.