Cool Stars 14 - Submitted Abstract # 206
This version created on 05 October 2006

A Decreased Probability of Forming Habitable Terrestrial Planets
around Low-Mass Stars


Sean Raymond, University of Colorado
John Scalo, University of Texas
Victoria Meadows, Caltech

Planets smaller than about 0.3 Earth masses are less likely to retain
the substantial atmospheres and ongoing tectonic activity required to
support life.  A key element in determining if sufficiently massive
"sustainably habitable" planets can form is the availability of solid
planet forming material.  The protoplanetary disk mass is thought to
scale roughly linearly with the stellar mass, with a large scatter.
We use simple scaling arguments and dynamical simulations of
terrestrial planet formation from planetary embryos to explore the
relationship between terrestrial planet mass, disk mass, and the mass
of the parent star.  We consider systems without Jovian planets, which
observations imply should be common.  Our analysis suggests that the
fraction of systems capable of forming >0.3 Earth-mass planets in the
habitable zone (HZ) decreases for low-mass stars, for virtually every
realistic combination of parameters.  We estimate that less than 20%
of M stars can form >0.3 Earth-mass planets in the HZ, with a
decreasing fraction for lower-mass stars.  We also introduce a ``water
mass limit'':  below a critical disk mass, there is not enough radial
mixing to deliver water to HZ planets.  This disk mass limit
corresponds to a HZ planet mass of about 0.3-0.5 Earth masses.  The
HZs of low-mass stars are likely to contain dry, roughly Mars-sized
planets.  Thanks to the Virtual Planetary Laboratory, a NASA
Astrobiology Insitute lead team, for funding.

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