%ACañas, Caleb%AMahadevan, Suvrath%ACochran, William%ABender, Chad%AFeigelson, Eric%AHarman, C.%AKopparapu, Ravi%ACaceres, Gabriel%ADiddams, Scott%AEndl, Michael%AFord, Eric%AHalverson, Samuel%AHearty, Fred%AJones, Sinclaire%AKanodia, Shubham%ALin, Andrea%AMetcalf, Andrew%AMonson, Andrew%ANinan, Joe%ARamsey, Lawrence%ARobertson, Paul%ARoy, Arpita%ASchwab, Christian%AStefánsson, Guđmundur%BJournal Name: The Astronomical Journal; Journal Volume: 163; Journal Issue: 1 %D2021%I %JJournal Name: The Astronomical Journal; Journal Volume: 163; Journal Issue: 1 %K %MOSTI ID: 10344426 %PMedium: X %TA Hot Mars-sized Exoplanet Transiting an M Dwarf %XAbstract We validate the planetary nature of an ultra-short-period planet orbiting the M dwarf KOI-4777. We use a combination of space-based photometry from Kepler, high-precision, near-infrared Doppler spectroscopy from the Habitable-zone Planet Finder, and adaptive optics imaging to characterize this system. KOI-4777.01 is a Mars-sized exoplanet ( R p = 0.51 ± 0.03 R ⊕ ) orbiting the host star every 0.412 days (∼9.9 hr). This is the smallest validated ultra-short period planet known and we see no evidence for additional massive companions using our HPF RVs. We constrain the upper 3 σ mass to M p < 0.34 M ⊕ by assuming the planet is less dense than iron. Obtaining a mass measurement for KOI-4777.01 is beyond current instrumental capabilities. %0Journal Article