Detection of an Atmospheric Outflow from the Young Hot Saturn TOI-1268b

Abstract

Photoevaporative mass-loss rates are expected to be highest when planets are young and the host star is more active, but to date there have been relatively few measurements of mass-loss rates for young gas giant exoplanets. In this study we measure the present-day atmospheric mass-loss rate of TOI-1268b, a young (110 - 380 Myr) and low density (0.71$^{+0.17}_{-0.13}$~g~cm$^{-3}$) hot Saturn located near the upper edge of the Neptune desert. We use Palomar/WIRC to search for excess absorption in the 1083~nm helium triplet during two transits of TOI-1268b. We find that it has a larger transit depth ($0.285_{-0.050}^{+0.048}\%$ excess) in the helium bandpass than in the TESS observations, and convert this excess absorption into a mass-loss rate by modeling the outflow as a Parker wind. Our results indicate that this planet is losing mass at a rate of $\log \dot{M} = 10.2 \pm 0.3$~g~s$^{-1}$ and has a thermosphere temperature of 6900$^{+1800}_{-1200}$~K. This corresponds to a predicted atmospheric lifetime much larger than 10 Gyr. Our result suggests that photoevaporation is weak in gas giant exoplanets even at early ages.