Population-level Eccentricity Distributions of Imaged Exoplanets and Brown Dwarf Companions: Dynamical Evidence for Distinct Formation Channels*

Abstract

The orbital eccentricities of directly imaged exoplanets and brown dwarf companions provide clues about their formation and dynamical histories. We combine new high-contrast imaging observations of substellar companions obtained primarily with Keck/NIRC2 together with astrometry from the literature to test for differences in the population-level eccentricity distributions of 27 long-period giant planets and brown dwarf companions between 5-100 AU using hierarchical Bayesian modeling. Orbit fits are performed in a uniform manner for companions with short orbital arcs; this typically results in broad constraints for individual eccentricity distributions, but together as an ensemble these systems provide valuable insight into their collective underlying orbital patterns. The shape of the eccentricity distribution function for our full sample of substellar companions is approximately flat from e=0-1. When subdivided by companion mass and mass ratio, the underlying distributions for giant planets and brown dwarfs show significant differences. Low mass ratio companions preferentially have low eccentricities, similar to the orbital properties of warm Jupiters found with radial velocities and transits. We interpret this as evidence for in situ formation on largely undisturbed orbits within massive, extended disks. Brown dwarf companions exhibit a broad peak at e ≈ 0.6-0.9 with evidence for a dependence on orbital period. This closely resembles the orbital properties and period-eccentricity trends of wide (1-200 AU) stellar binaries, suggesting that brown dwarfs in this separation range predominantly form in a similar fashion. We also report evidence that the "eccentricity dichotomy" observed at small separations extends to planets on wide orbits: the mean eccentricity for the multiplanet system HR 8799 is lower than for systems with single planets. In the future, larger samples and continued astrometric orbit monitoring will help establish whether these eccentricity distributions correlate with other parameters such as stellar host mass, multiplicity, and age.

SCExAO/CHARIS Direct Imaging Discovery of a 20 au Separation, Low-mass Ratio Brown Dwarf Companion to an Accelerating Sun-like Star *

“…HD33632Ab's eccentricity posterior probability distribution has a 68% upper limit of 0.29. This is smaller than the eccentricities of most brown dwarfs studied in Bowler et al (2020), who argue that systematically higher eccentricities of brown dwarf-mass companions relative to exoplanets point to a The orbit fits used Hipparcos and Gaia absolute astrometry, Lick Observatory precision radial-velocity measurements for HD 33632 Aa, and relative astrometry of HD 33632 Ab. The posterior on HD 33632 Aa's mass is dominated by our adopted prior of 1.1±0.1M e .…”

The SPHERE infrared survey for exoplanets (SHINE)

“…For the eccentricity distribution, we considered the recent results derived by Bowler et al (2020) for directly imaged exoplanets and brown dwarf companions. For this parameter we adopted a Beta distribution with parameters [α = 0.95, β = 1.30], which corresponds to the best fit to the full sample of wide substellar companions studied in Bowler et al (2020). For each simulated companion, we then computed the corresponding projected separation from the drawn orbital elements and the semimajor axis a of that grid point.…”

The Habitable Zone Planet Finder Reveals a High Mass and Low Obliquity for the Young Neptune K2-25b

“…Both the stellar obliquity and orbital eccentricity are clues about the formation and subsequent dynamical history of planetary systems. For example, based on direct imaging data, Bowler et al (2020) reported a difference in the eccentricity distributions of planets and brown dwarfs, evidence that these objects form in different ways. Planets are expected to form on circular and coplanar orbits within protoplanetary disks, although they can develop nonzero eccentricities via planet/ planet interactions (Rasio & Ford 1996), secular von Zeipel-Lidov-Kozai cycles (Naoz 2016;Ito & Ohtsuka 2019), planetdisk interactions (Goldreich & Sari 2003), or other dynamical processes.…”