Astroblog: A Transiting Neptune-Like World!

Thursday, May 17, 2007

A Transiting Neptune-Like World!

Gliese 436b as rendered in Celestia (it's listed as HIP 57087 in the star browser)

While we are waiting for MOST and a host of amateur transit watchers to complete data gathering on Gliese 581 to see if Gliese 581c transits, a new report comes of the smallest ever world to be seen transiting its star.

Gliese 436b was discovered back in 2004, by Marcy and Butler, and lies 10.27 parsecs (about 33 light years) away in Leo. However, a European team has just reported observing a transit of this world (PDF of paper at arxiv). The observations significantly constrain the mass and radius of Gl 436b. The planet is roughly 23 Earth masses, with a radius 3.95 times Earths radius. Its mass and radius is consistent with the main component of this world being water "ice". It is also likely, according to the authors, that it has an atmosphere consisting of hydrogen and helium (accounting for roughly 10% of the planets mass), to properly account for the mass and radius.

Note the scare quotes around "ice". This world will be hot, it orbits its Sun about 13 times closer than Mercury's orbit around our Sun, so its temperature must be in the range of 520-670K (around 300 deg C, that's hot). Hot and Ice don't normally go together, but the high pressures involved in a Neptune sized world may keep the ice stable at high temperature. Alternatively, Gl 436c may be an ocean world, with an atmosphere largely of steam and a surface of superhot water kept liquid by the high atmospheric pressure.

Either way, this is an important and exciting find. By observing transiting "Hot Jupiters", we have been able to determine some components of their atmosphere, and make a crude surface map of one. Now we have a new class of exoplanet to study, one that is not a Jovian, and that will expand our knowledge of the structure (and possibly formation) of planets enormously.

Also, the fact that Gl 436b is an ice world, means that it formed far from its Sun, and migrated to its present position. This makes it more likely that "Earth-like" Gliese 581c is an ice world that migrated as well, rather than a rocky world.

For more information, see the paper. Also, the world is discussed at the exoplanet web log, at Centauri Dreams (man I have to update my blogroll) , there is an article at New Scientist, Scientific American and an account of the teams discovery at Swissinfo. Predictably, Wikipedia already has updated info on Gliese 436b.

Labels: Astronomy, celestia, exoplanet, transit