Absence of extended atmospheres in low-mass star radius-gap planets GJ 9827 b, GJ 9827 d and TOI-1235 b

TL;DR

This study investigates the atmospheres of three low-mass star planets in the radius gap, finding no significant atmospheric escape signals, which supports models involving photoevaporation and core-powered mass-loss over gas-poor formation.

Contribution

First high-dispersion infrared spectroscopic observations of low-mass star planets in the radius gap, providing constraints on atmospheric escape and mass-loss rates.

Findings

No significant He I absorption detected, setting upper limits.

Mass-loss rates consistent with photoevaporation and core-powered models.

Supports atmospheric evolution theories over gas-poor formation models.

Abstract

\textit{Kepler} showed a paucity of planets with radii of 1.5 - 2 $\mathrm R_{\oplus}$ around solar mass stars but this radius-gap has not been well studied for low-mass star planets. Energy-driven escape models like photoevaporation and core-powered mass-loss predict opposing transition regimes between rocky and non-rocky planets when compared to models depicting planets forming in gas-poor environments. Here we present transit observations of three super-Earth sized planets in the radius-gap around low-mass stars using high-dispersion InfraRed Doppler (IRD) spectrograph on the Subaru 8.2m telescope. The planets GJ 9827 b and d orbit around a K6V star and TOI-1235 b orbits a M0.5 star. We limit any planet-related absorption in the 1083.3 nm lines of triplet He I by placing an upper-limit on the equivalent width of 14.71 m{\AA}, 18.39 m{\AA}, and 1.44 m{\AA}, for GJ 9827 b (99% confidence), GJ 9827 d (99% confidence) and TOI-1235 b (95% confidence) respectively. Using a Parker wind model, we cap the mass-loss at $>$0.25 $\mathrm M_{\oplus}$ Gyr$^{-1}$ and $>$0.2 $\mathrm M_{\oplus}$ Gyr$^{-1}$ for GJ 9827 b and d, respectively (99% confidence), and $>$0.05 $\mathrm M_{\oplus}$ Gyr$^{-1}$ for TOI-1235 b (95\% confidence) for a representative wind temperature of 5000 K. Our observed results for the three planets are more consistent with the predictions from photoevaporation and/or core-powered mass-loss models than the gas-poor formation models. However, more planets in the radius-gap regime around the low-mass stars are needed to robustly predict the atmospheric evolution in planets around low-mass stars.