Thanks for the comment… I would agree that the “super-jovian” planets are indeed likely forming via gravitational instability. For an object like 2M1207b (where the primary-to-secondary mass ratio is of order 5 to 1) it’s effectively impossible for the core-accretion process to operate in the standard way. The disk around 2M1207 was likely massive enough and cold enough to easily encounter gravitational instability. For the other objects you mention, the primary-to-secondary mass ratio is considerably larger, but I would nevertheless suspect that in these cases we’re seeing the extreme mass ratio tail of the process that gives rise to brown dwarfs.

It’s been pretty clear for a number of years, now, that a “brown dwarf desert” exists between roughly 10 and 20 Jupiter masses. Of the objects that do fall in this region, some of them are likely forming through the low-mass tail of the gravitational instability mechanism, while other are coming from the high-mass tail of the core-accretion process.

best,
Greg

Greg, do you think that gravitational instability hypothesis may explain the presence of “super-jovian” planets(HD 222582b, 14Herb, 2M1207b etc..) ?

Can the core-accretion model can explain the forming of these super-jovian planets ?

The “average” star forms in a stellar group of roughly 300 stars, which is not a sufficiently dense environment to provide the UV radiation required to whittle a gas giant down to a super-Earth. For a good overview of this issue, see the recent paper by Adams et al. 2006.

Because the average red dwarf star does not form in a dense enough environment to experience extreme photoevaporation, one would expect to find many more Jupiter-mass planets in orbit around red dwarfs if the primary formation mechanism is disk gravitational instability. This is not what is observed.

A year ago, I thought that the outer two planets orbiting the nearby red dwarf GJ 876 might have formed through disk gravitational instability. I’m less inclined to believe this hypothesis now, however. (1) At the Aspen workshop, Nunos Santos told me that their measurements of the metallicity of GJ 876 indicated that it was quite metal rich. Red dwarf stars are extremely hard to get good metallicity estimates for, and formerly, the star had been believed to be metal poor. (2) Eugenio’s discovery of GJ 876 “d”, which almost certainly contains 7.5 Earth masses of rock and ice, indicates that there was a large amount of solid material in GJ 876′s protoplanetary disk. I think that the GJ 876 system is therefore largely an outcome of the core-accretion process.

I do, though, think that these issues need a lot more investigation.