Cool Stars 14 - Submitted Abstract # 210 This version created on 05 October 2006 The Mass-Radius Relation for Low-Mass Exoplanets Sara Seager, Carnegie Institution of Washington Catherine Hier-Majumder, Carnegie Institution of Washington Marc Kuchner, NASA/Goddard Space Flight Center We present new mass-radius relationships for low-mass exoplanets with masses from 0.l to 100 Earth masses. We discuss how well exoplanetary bulk composition can be determined, considering the limiting observational error bars on transiting exoplanet masses and radii. We consider iron-type planets like Mercury, silicate-type planets like Earth, and water worlds predominantly composed of water ice. We find a universal mass-radius relation for solid planets (with masses less than 100 Earth masses), resulting from the similarity of the equations of state of solid materials. This universal mass-radius relation allows an estimate of planetary bulk composition without detailed calculations. ----------------------------------