Eccentricity-Multiplicity Correlation

Not particularly surprising, but it seems that systems where planets are in circular orbits have lower eccentricities. The Solar system fits well into a correlation of multiplicity vs eccentricity.

The Solar System and the Exoplanet Orbital Eccentricity - Multiplicity Relation
http://arxiv.org/abs/1404.2552

Quote :

The known population of exoplanets exhibits a much wider range of orbital eccentricities than Solar System planets and has a much higher average eccentricity. These facts have been widely interpreted to indicate that the Solar System is an atypical member of the overall population of planetary systems. We report here on a strong anti-correlation of orbital eccentricity with multiplicity (number of planets in the system) among catalogued RV systems. The mean, median and rough distribution of eccentricities of Solar System planets fits an extrapolation of this anti-correlation to the eight planet case rather precisely. Thus, the Solar System is not anomalous among known exoplanetary systems with respect to eccentricities when its multiplicity is taken into account. Specifically, as the multiplicity of a system increases the eccentricity decreases roughly as a power law of index -1.20. A simple and plausible but ad hoc model of this relationship implies that approximately 80% of the one planet and 25% of the two planet systems in our sample have additional, as yet undiscovered, members. If low eccentricities favor high multiplicities, habitability may be more common in systems with a larger number of planets.