Different Planetary Eccentricity-Period (PEP) Distributions of Small- and Giant-Planets

TL;DR

This study analyzes the eccentricity-period distributions of small and giant exoplanets, revealing distinct patterns likely caused by different tidal and dynamical processes, with implications for planetary formation and evolution.

Contribution

It provides the first detailed comparison of PEP distributions for small and giant exoplanets, highlighting the role of tidal circularization and dynamical interactions.

Findings

Giant planets show an increasing eccentricity with period, reaching 0.8 at 100 days.

Small planets exhibit a flat eccentricity distribution between 0.1 and 0.5.

The observed distributions are not due to detection biases.

Abstract

We used the database of $1040$ short-period ($1 \leq P < 200$ days) exoplanets radial-velocity (RV) orbits to study the planetary eccentricity-period (PEP) distribution. We first divided the sample into low- and high-mass exoplanet sub-samples based on the distribution of the (minimum) planetary masses, which displays a clear two-Gaussian distribution, separated at $0.165M_J$. We then selected $216$ orbits, low- and high-mass alike, with eccentricities significantly distinct from circular orbits. The $131$ giant-planet eccentric orbits display a clear upper envelope, which we model quantitatively, rises monotonically from zero eccentricity and reaches an eccentricity of $0.8$ at $P \sim 100$ days. Conversely, the $85$ low-mass planetary orbits display a flat eccentricity distribution between $0.1$ and $0.5$, with almost no dependence on the orbital period. We show that the striking difference between the two PEP distributions is not a result of the detection technique used. The upper envelope of the high-mass planets, also seen in short-period binary stars, is a clear signature of tidal circularization, which probably took place inside the planets, while the small-planet PEP distribution suggests that the circularization was not effective, probably due to dynamical interactions with neighboring planets.