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Is brown dwarf new type of extreme 'whopper'?

Posted: JULY 3, 2009

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Have a team of astronomers found a new class of Jupiter-like planet with a gargantuan core of 100 Earth-masses? This is what the joint UK-Chilean-Polish team are asking themselves after first results were obtained from CHEPS (the Calan-Hertfordshire Exoplanet Search).

The object in question orbits the Sun-like star HD191760, 290 light years away. What’s more, the object, dubbed HD191760b, is in a region around the star commonly referred to as the ‘brown dwarf desert.’ The study was a joint collaboration between Hertfordshire University, Nicolaus Copernicus University, and the Universidad de Chile.

The brown dwarf HD191760b could be a new type of 'extreme-jovian' body. Image: NASA/JPL-Caltech/T.Pyle (SSC).

These are areas where very few (less than one percent) of brown dwarfs are located. For a Sun-like star this can be anything up to five times the Earth-Sun distance away from that star (though it can vary with each individual star). Brown dwarfs are bodies tens of times as massive as gas giants such as Jupiter. They’re too big to be classed as planets, but still too small to become stars. The reason the desert exists is because a brown dwarf would have to form at the same time as its companion star. If it were within a certain distance of the star it would migrate and be swallowed whole.

“If this is a brown dwarf then it kind of tallies with the desert numbers since our sample size was 100 stars.” Says Dr James Jenkins of the Universidad de Chile. “However, it is interesting that it is quite eccentric as another similar object was discovered this year with similar properties, so the desert may actually be populated a little more than we think by deuterium burning objects.” The eccentricity of these objects’ orbits could make them difficult to detect, resulting in the one-percent numbers seen in those kinds of regions.

But then where would that leave the migration theory? Jenkins says, “There is the possibility that this is a new type of ‘extreme-jovian planet’, i.e. an object that formed through core accretion [how planets like Jupiter form] but has a core mass above 100 Earth-masses.” He is at pains to point out however that this is still just speculation, and observing a transit of the brown dwarf across the face of the parent star would be the only way to be sure (HD191760b was found using the radial velocity method – detecting the slight ‘tug’ on a star caused by any orbiting companions.

HD191760 is a metal-rich star, meaning the system contains lots of complex molecules critical to planet-formation. So could there be any other orbiting bodies? “We ran simulations that showed that no planets could exist with semi major axes greater than 0.17 astronomical units (au — the distance between the Sun and the Earth) inside the orbit of the companion due to tidal forces.” Jenkins says. These are similar to tidal forces in our Solar System caused by the planet Jupiter, creating the Kirkwood Gaps in the Asteroid Belt.

However, Jenkins ends with an optimistic note: “There is the possibility that below 0.17au there could be small planets waiting to be found!” And that would certainly be worth waiting for.

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