K2-97b: A (Re-?)Inflated Planet Orbiting a Red Giant Star

TL;DR

The discovery of K2-97b, a planet orbiting a red giant star, suggests planetary inflation can be caused directly by stellar radiation rather than delayed cooling, challenging previous assumptions about planet inflation mechanisms.

Contribution

This paper presents the first evidence that stellar irradiation can directly cause planetary inflation, based on detailed characterization of K2-97b around a red giant star.

Findings

K2-97b is highly irradiated with flux 170 times Earth's during star’s main sequence phase.

Planet radius measurements vary up to 30% depending on lightcurve reduction methods.

Stellar irradiation likely causes planetary inflation, not delayed cooling.

Abstract

Strongly irradiated giant planets are observed to have radii larger than thermal evolution models predict. Although these inflated planets have been known for over fifteen years, it is unclear whether their inflation is caused by deposition of energy from the host star, or inhibited cooling of the planet. These processes can be distinguished if the planet becomes highly irradiated only when the host star evolves onto the red giant branch. We report the discovery of K2-97b, a 1.31 $\pm$ 0.11 R$_\mathrm{J}$, 1.10 $\pm$ 0.11 M$_\mathrm{J}$ planet orbiting a 4.20 $\pm$ 0.14 R$_\odot$, 1.16 $\pm$ 0.12 M$_\odot$ red giant star with an orbital period of 8.4 days. We precisely constrained stellar and planetary parameters by combining asteroseismology, spectroscopy, and granulation noise modeling along with transit and radial velocity measurements. The uncertainty in planet radius is dominated by systematic differences in transit depth, which we measure to be up to 30% between different lightcurve reduction methods. Our calculations indicate the incident flux on this planet was 170$^{+140}_{-60}$ times the incident flux on Earth while the star was on the main sequence. Previous studies suggest that this incident flux is insufficient to delay planetary cooling enough to explain the present planet radius. This system thus provides the first evidence that planets may be inflated directly by incident stellar radiation rather than by delayed loss of heat from formation. Further studies of planets around red giant branch stars will confirm or contradict this hypothesis, and may reveal a new class of re-inflated planets.