Do close-in giant planets orbiting evolved stars prefer eccentric orbits?

TL;DR

This study presents radial velocity measurements of giant planets orbiting evolved stars, indicating they tend to have more eccentric orbits than similar planets around main-sequence stars, suggesting a transient eccentric phase.

Contribution

It provides the first evidence that close-in giant planets around evolved stars often have eccentric orbits, highlighting a potential phase in orbital evolution.

Findings

Eccentricities are significantly non-zero in the studied systems.

Evolved star planets tend to have more eccentric orbits than main-sequence star planets.

The eccentric phase may be transient, influenced by tidal interactions.

Abstract

The NASA Kepler and K2 Missions have recently revealed a population of transiting giant planets orbiting moderately evolved, low-luminosity red giant branch stars. Here, we present radial velocity measurements of three of these systems, revealing significantly non-zero orbital eccentricities in each case. Comparing these systems with the known planet population suggests that close-in giant planets around evolved stars tend to have more eccentric orbits than those around main-sequence stars. We interpret this as tentative evidence that the orbits of these planets pass through a transient, moderately eccentric phase where they shrink faster than they circularize due to tides raised on evolved host stars. Additional radial velocity measurements of currently known systems, along with new systems discovered by the recently launched NASA TESS mission, may constrain the timescale and mass dependence of this process.