Tuesday, June 3, 2008

Baby Planet Around a Baby Star

A new frontier in planet detection has now been breached! The first almost-Earth-mass planet has been discovered orbiting a sub-stellar primary object - more commonly called a 'brown dwarf' (a name coined by Jill Tarter, for all you SETI aficionados).  A decent review by Space.com can be found here, but the article makes the mistake of calling the host star a 'normal star'.  In fact, brown dwarfs are more like giant planets than normal stars - they span the mass range between gas giant planets (like Jupiter) and very-low-mass stars, with very cool surface temperatures and low luminosity.  For a long time they were considered to be an exotic stellar species - they weren't detectable in visible light, and some theorists speculated that they couldn't be formed at all in stellar clusters.  With the advent of very sensitive infrared telescopes, detecting brown dwarfs is like shooting fish in a barrel - but finding a planet around one is a rare feat.

This discovery puts an exclamation point on the idea that low-mass stars can form planets - which is both a little surprising and a very good thing for planet detection.  If we believe that the amount of protoplanetary material scales with stellar mass, and our Solar System had just enough material to form our current suite of planets, then we would end up finding very few planets around smaller stars (there just wouldn't be enough stuff).  However, this seems to not be the case - planet-hunters are finding lots of planets around low-mass stars (see the Extrasolar Planets Encyclopedia for a full count). This has been perplexing for planet formation theory (we're still working on it), but has been cause for celebration among those searching for the little buggers - since low-mass stars are much more common than high-mass stars, this means more (and smaller) planets are waiting to be found.