TESS Photometry and Radial Velocity Analysis of the sub-Neptune Exoplanet pi Mensae c and the Wider pi Mensae Planetary System

Exoplanet characterization relies on precise measurements of planetary orbital and physical parameters. This is particularly important for planetary dynamics and atmospheric evolution, as orbital parameters help constrain system evolution, resolve ambiguities, and gauge atmospheric retention. The first exoplanet discovered by the Transiting Exoplanet Survey Satellite (TESS), pi Men c, is a warm sub-Neptune orbiting a bright Sun-like star in a system containing (at least) one other planet with a wildly different period and size. Lying near the 1.5-2.0 R radius gap, pi Men c is expected to have lost its primordial hydrogen and helium, but kept heavier compounds like H 2 O and CO 2. The pi Men system is well observed with decades of radial velocity measurements, and TESS has continued to observe pi Men c, yielding six years and 21 sectors of photometry. We present a comprehensive analysis of these TESS data and 22 years of radial velocity measurements to provide updated orbital ephemerides for pi Men b, c, and the proposed third planet, pi Men d. Our newly derived pi Men c period error margins are an order of magnitude improved from previous estimates, and we estimate the mass range of pi Men d to be 13.4 at most M d < 20 M. We find that pi Men c is a uniquely interesting target for future transmission spectroscopy studies with JWST, and that existing radial velocity data are consistent with the existence of a third planet.