TOI-1842b: A Transiting Warm Saturn Undergoing Reinflation around an Evolving Subgiant

Abstract

The imminent launch of space telescopes designed to probe the atmospheres of exoplanets has prompted new efforts to prioritize the thousands of transiting planet candidates for follow-up characterization. We report the detection and confirmation of TOI-1842b, a warm Saturn identified by TESS and confirmed with ground-based observations from Minerva-Australis, NRES, and the Las Cumbres Observatory Global Telescope. This planet has a radius of ${1.04}_{-0.05}^{+0.06}$ RJ, a mass of ${0.214}_{-0.038}^{+0.040}$ MJ, an orbital period of ${9.5739}_{-0.0001}^{+0.0002}$ days, and an extremely low density (ρ = 0.252 ± 0.091 g cm−3). TOI-1842b has among the best known combinations of large atmospheric scale height (893 km) and host-star brightness (J = 8.747 mag), making it an attractive target for atmospheric characterization. As the host star is beginning to evolve off the main sequence, TOI-1842b presents an excellent opportunity to test models of gas giant reinflation. The primary transit duration of only 4.3 hr also makes TOI-1842b an easily-schedulable target for further ground-based atmospheric characterization.