The Compositions of Small Planets
The NASA Kepler Mission has demonstrated that planets with radii larger than Earth yet smaller than Neptune are common around Sun-like stars. Although Kepler has determined the physical sizes of hundreds of such planets, we know virtually nothing about their masses and, by inference, their compositions. HARPS-N is an ultra-stable fiber-fed high-resolution spectrograph optimized for the measurement of very precise radial velocities. Using HARPS-N we have measured to high precision the masses of several of these so-called super-Earths. I will report on the resulting constraints on the planetary compositions, and address the transition from rocky planets, composed of rock and iron, and Neptune-like worlds, which have accreted and retained an envelope of primordial H/He gas. I will then explain the future role of the NASA Transiting Exoplanet Survey Satellite (TESS), scheduled for launch in 2017.