Chris Lintott’s Universe

I’m sitting at the back of the second press conference of the day, desperately swapping between laptops to run the Astronomy Cast Live feed, catch up with press releases from the speakers and to write this.

Currently, we’re listening to the announcement of the smallest extrasolar planet to date, a 3 Earth Mass planet orbiting a brown dwarf. 3 Earth masses is small enough to all but convince me that we must be talking about a rocky or an icy planet, not a gaseous system, and it orbits its host at about the distance Venus is from the Sun. Of course, as its parent star is much fainter and smaller than the Sun, it will be freezing and there’s no hope of liquid water on the surface.

Phil, whose head is visible in the live feed at times, has all the details.

One issue that came up in the questions, and which has intrigued me for a while, is whether there is any real division between large planets and small brown dwarfs. This is something the International Astronomical Union are going to have to wrestle with as when they demoted Pluto, the declined to define what a planet is beyond our solar system.

The best supported theory of brown dwarf formation claims that these small stars are just the detritus left over from the formation of larger stars. If this is correct then there should be a theoretical minimum mass of about 5-10 times the mass of Jupiter, but as smaller and smaller brown dwarfs are found this limit is being approached.

In one of the other papers presented at the exoplanet session, Michael Liu of the University of Hawai’i talked about weighing brown dwarfs by watching binary pairs orbit each other. This isn’t easy - you need adaptive optics to give you a resolution equivalent to being able to read a magazine from a mile away, you need pointing accuracy equivalent to hitting the bullseye on a dart board in Hong Kong from an oche here in St Louis, and, perhaps most difficult of all, you need the funding and the motivation to study the slow-moving stars for years on end.

Having done all that for a pair of pairs, the results are just about perfect; our theoretical predictions are good, but not that good. The masses are supposed to depend on the energy output of the star, and on its temperature. One star was cooler than expected, and the other warmer. While this isn’t a big deal - no theories are going to be overturned, and no Nobel prizes won - it’s this kind of detailed work that’s essential in making sure our observations and understanding are as good as they can be.

The masses of the stars were well above the theoretical limit; the smallest were about 30 times Jupiter’s mass. More careful work with this technique and study of other pairs might bring us closer to the limit, though.