A Survey of Multiple Planet Systems

TL;DR

This survey reviews known multiple exoplanet systems, categorizing them into five groups, and discusses their orbital characteristics, eccentricities, and distribution patterns, highlighting differences from single-planet systems.

Contribution

It provides a comprehensive classification and analysis of multiple exoplanet systems as of 2008, summarizing their dynamical properties and orbital distributions.

Findings

Eccentricities are not higher in multiple systems.

Lower-mass planets (<1 M_Jup) have lower eccentricities.

No 3-day pile-up or 1 AU jump in multiplanet systems.

Abstract

As of August 2008, 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 (Msini > 0.2 M_Jup) 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 (Msini < 20 M_Earth). It now appears that eccentricities are not markedly higher among planets in known multiple planet systems, and that planets with Msini < 1 M_Jup have lower eccentricities than more massive planets. The distribution of semimajor axes for planets in multiplanet systems does not show the 3-day pile-up or the 1 AU "jump" of the apparently-single planet distribution.