Studying the Interior Structure of an Extremely Eccentric Hot Jupiter via Deep VLT Imaging

I will describe how our group at Exeter has used the VLT-SPHERE instrument to place constraints on the internal structure of HD 20782b, a Hot Jupiter with the most extreme eccentricity known to date (e 0.96). In the dynamically-driven migration scenario (e.g. Kozai-Lidov cycles combined with tidal dissipation), a Jupiter mass planet is dynamically excited to a high eccentricity by a third body, and its orbit subsequently shrinks and circularizes through tidal dissipation. Our deep observations of the HD 20782 system rule out any additional (third) companions with masses in the range 20-60 Jupiter masses at orbital separations 10-60 AU that might be responsible for exciting the extreme eccentricity of the inner planet. Our lack of detections of any additional companions in the system indicates that the eccentricity of the planet was gained early on and has persisted until the present. The apparent failure of the tidal dissipation mechanism in this system means that we can place strong constraints on the tidal quality factor "Q" of HD 20782b. Specifically, our models of planetary tidal evolution suggest a remarkably high value for the planet's tidal Q factor of 10⁷ - 10⁸: two to three orders of magnitude higher than that measured for other extrasolar planets, as well as members of our own solar system. Our result suggests a possible structural difference between HD 20782b and other giant planets inside and outside our solar system. If time allows, I will discuss how our approved 52-hour JWST Early Release Science Program will pave the way for future observations of additional systems with extremely eccentric planets starting in 2021. JWST will illuminate the interior structures of many more eccentric Jovian mass planets going forward, or possibly even image the extremely eccentric planets themselves.