Exoplanet orbital eccentricity: Multiplicity relation and the Solar System

Abstract

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 anticorrelation of orbital eccentricity with multiplicity (number of planets in the system) among cataloged radial velocity (RV) systems. The mean, median, and rough distribution of eccentricities of Solar System planets fits an extrapolation of this anticorrelation to the eight-planet case rather precisely despite the fact that no more than two Solar System planets would be detectable with RV data comparable to that in the exoplanet sample. Moreover, even if regarded as a single or double planetary system, the Solar System lies in a reasonably heavily populated region of eccentricity−multiplicity space. 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 and model-dependent interpretation of this relationship implies that ∼80% of the one-planet and 25% of the two-planet systems in our sample have additional, as yet undiscovered, members but that systems of higher observed multiplicity are largely complete (i.e., relatively rarely contain additional undiscovered planets). If low eccentricities indeed favor high multiplicities, habitability may be more common in systems with a larger number of planets.orbital eccentricities | dynamical evolution | Solar System | radial velocity S olar System orbital eccentricities are unusually low compared with those of exoplanets. This fact is one of the most frequently noted major surprises revealed by the discovery and early explorations of the exoplanet population orbiting Sun-like stars and has been widely interpreted to indicate that the Solar System is not a representative example of a planetary system (reviews by refs. 1-3 and references therein). Many planetary formation theories developed before the discovery of exoplanets suggested planets would have eccentricities similar to the Solar System planets (4, 5). Several attempts have been made to accurately model the dynamical evolution of planetary systems since then, with the goal of explaining the observed eccentricity distribution (6-10). These papers invoke planet−planet interactions as the primary mechanism determining the distribution of orbital eccentricities. The most recent of these papers (10) concludes that there would be a dependence of eccentricity on multiplicity (the number of planets in the system) in this scenario. We use existing radial velocity (RV) exoplanet data to test that prediction.Our dataset consists of 403 of the 441 cataloged RV exoplanets obtained since the 1990s (exoplanet.org). Of these, 127 are members of known mul...

Asteroid flux towards circumprimary habitable zones in binary star systems

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“…Exploiting the possibility to perform statistical studies for the first time thanks to the large number of discovered exoplanets, recent works (Juric & Tremaine 2008;Limbach & Turner 2015) reported the existence of an anti-correlation between the multiplicity of exosystems (i.e. the number of planets they possess) and the orbital eccentricities of their planets.…”

“…the number of planets they possess) and the orbital eccentricities of their planets. In particular, Limbach & Turner (2015) analysed a sample of 403 exoplanets showing that, once divided in bins according to the multiplicity of their host systems, the medians of their eccentricities can be fitted by a power law. These authors also verified that the same power law can be used to fit the median of the eccentricities of solar system planets.…”

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