Transit Depth Variations Reveal TOI-216 b to be a Super-Puff

Abstract

The planets of the TOI-216 system have been previously observed to exhibit large transit timing variations (TTVs), which enabled precise mass characterization of both transiting planets. In the first year of TESS observations, TOI-216 b exhibited grazing transits, precluding a measurement of its radius. In new observations, we demonstrate the orbit of the planet has precessed and it is now fully transiting, so we can accurately measure its radius. TOI-216 b is a puffy Neptune-mass planet, with a much larger radius that is now well constrained to $7.84^{+0.21}_{-0.19}$ $R_{\oplus}$ and a density of $0.201\pm0.017$ g cm$^{-3}$. We numerically integrate the system across the TESS observations to update and refine the masses and orbits of both planets, finding the uncertainty in the masses are now dominated by uncertainties in the stellar parameters. TOI-216 b represents a growing class of super-puff planets in orbital resonances and with a companion in a nearly circular orbit, suggesting these planets early evolution is driven by smooth disk migration.