Thursday, November 13, 2008

First Light from Planets!

It seems like they've finally done it:  an unambiguous image of an extrasolar planet (actually, several of them) around a main-sequence star (HR8799).  They've been getting close for several years - the image of 2M 1207b was an excellent start, but was often disparaged because 1) the imaged object (b) orbits a brown dwarf (a) rather than a "normal" star and 2) the mass of (b) may be large enough to place it in the brown dwarf range as well.  These new detections (as well as the detection of a planet around a well-known young star named Fomalhaut announced at the same time) have none of these uncertainties - the planet masses are well below the mass limit for a brown dwarf (~13x the mass of Jupiter), and the stars are both A-type main-sequence stars (1.5 - 2x the mass of our Sun).  Unless there turns out to be a major screw-up, the era of direct detection of exo-planets has begun!

Now let's turn to what's interesting about these two systems.  First, both stars are quite young (less than 100 Myr) - this means their planets are still hot from their formation, making them easier to detect.  It also means that much of the debris from planet formation still remains in the system, in the form of dust rings surrounding the planets.  These dust rings are much, much brighter than the planets themselves due to the much higher surface area available to radiate (look at the picture of Fomalhaut for a good idea of this), and this has lead many researchers to design ways to search for the presence of planets around young stars by looking for patterns in the dust created by their orbits.  These new detections corroborate these techniques - patterns can be seen in the dust that can be directly attributed to the planet, and help constrain the mass of the planet.

The second (and I think more interesting) fact is that all the planets are at very large distances from their parent stars - the three planets around HR8799 are between 25 and 70 AU (astron. units - Earth-Sun distance), while the planet around Fomalhaut has an orbital radius of 100 AU!!  For reference, the farthest giant planet in our own Solar System (Neptune) is thought to have formed at 15 AU from the Sun.  How do these massive planets form so far out?  Is it because the stars are larger?  This seems like it might explain the planet at 25 AU, but the others are really pushing it.  Do they form like stars - by collapsing out of the young protostellar cloud?  These planets form the opposite endpoint of the "Hot Jupiter" planets first discovered by radial velocity surveys - and they look like they will have the same mind-blowing effect on the planet formation community.

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