525 Davey Lab, Astronomy and Astrophysics, Eberly College of Science, The Pennsylvania State University, University Park, PA 16802, USA
To date, over 30 multiple exoplanet systems are known, and 28% of stars with planets show significant evidence of a second companion. I briefly review these 30 systems individually, broadly grouping them into five categories: 1) systems with 3 or more giant (M sin i> 0.2 MJup) planets, 2) systems with two giant planets in mean motion resonance (MMR), 3) systems with two giant planets not in MMR but whose dynamical evolution is affected by planet-planet interactions, 4) highly hierarchical systems, having two giant planets with very large period ratios (>30:1), and 5) systems of “Super-Earths”, containing only planets with (M sin i< 20 M⊕). It now appears that eccentricities are not markedly higher among planets in known multiple planet systems, and that planets with M sin i< 1 MJup have lower eccentricities than more massive planets. The distribution of semimajor axes for planets in multi-planet systems does not show the 3-day pile-up or the 1 AU “jump” of the apparently-single planet distribution.
(Online publication April 19 2010)