Evidence for Reflected Light from the Most Eccentric Exoplanet Known

TL;DR

This study presents new observational data and analysis of the highly eccentric exoplanet orbiting HD 20782, detecting potential reflected light signals and refining its orbital parameters, thereby advancing understanding of such extreme planetary systems.

Contribution

The paper provides the first combined orbital and astrometric analysis of HD 20782b, constrains its inclination, and reports possible phase variations caused by reflected light, which are novel insights for such eccentric exoplanets.

Findings

Refined orbital parameters and inclination of HD 20782b.

No transit detected during predicted times.

Evidence of phase variations possibly due to reflected light.

Abstract

Planets in highly eccentric orbits form a class of objects not seen within our Solar System. The most extreme case known amongst these objects is the planet orbiting HD~20782, with an orbital period of 597~days and an eccentricity of 0.96. Here we present new data and analysis for this system as part of the Transit Ephemeris Refinement and Monitoring Survey (TERMS). We obtained CHIRON spectra to perform an independent estimation of the fundamental stellar parameters. New radial velocities from AAT and PARAS observations during periastron passage greatly improve our knowledge of the eccentric nature of the orbit. The combined analysis of our Keplerian orbital and Hipparcos astrometry show that the inclination of the planetary orbit is $> 1.22\degr$, ruling out stellar masses for the companion. Our long-term robotic photometry show that the star is extremely stable over long timescales. Photometric monitoring of the star during predicted transit and periastron times using MOST rule out a transit of the planet and reveal evidence of phase variations during periastron. These possible photometric phase variations may be caused by reflected light from the planet's atmosphere and the dramatic change in star--planet separation surrounding the periastron passage.