Cool Stars 14 - Submitted Abstract # 361
This version created on 05 October 2006

Directly Imaged Companions:  Planets or Brown Dwarfs?


Mark Marley, NASA Ames Research Center

Outside of the solar system, the epithet 'planet' has been hung on a
motley collection of low mass objects.  Free floating 'Jupiters',
directly imaged faint companions, and of course objects detected by
radial velocity and other indirect methods have all been so marked.
But are all these objects really planets?  Like Justice Potter
Stewart, we each know a planet when we see it.  The problem, of
course, is that not everyone ultimately issues the same opinion.  In
an attempt to shed some light on the question given in the title, I
will briefly review what we know of the formation and evolution of
giant planets.  I will then argue that a 'true' giant planet forms
from accretion processes in a nebula surrounding its primary star.  It
does not form by fragmentation.  Formation processes leave
fingerprints on both the composition of the atmosphere of the planet
and on its evolution through time, at least during its youth, which
can be discerned remotely.  Thus my definition for a giant planet
would require a bulk or atmospheric composition that differs
materially from its primary and a luminosity, radius, or both that at
young ages differs from that of an object formed by fragmentation.
Both components of this definition are, for now, theoretically
motivated but can be tested when a sufficient number of objects are
directly detected and trends are apparent.  In my talk I'll apply this
definition to the directly imaged companions and will also consider
the limitations of this approach.  

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