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) R-J, a mass of 0.214(-0.038)(+0.040)M(J), an orbital period of 9.5739(-0.0001)(+0.0002) days, and an extremely low density (rho = 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.