The First Habitable Zone Earth-Sized Planet From TESS II: \(Spitzer\) Confirms TOI-700 d

We present \(Spitzer\) 4.5\(\mu\)m observations of the transit of TOI-700 d, a habitable zone Earth-sized planet in a multiplanet system transiting a nearby M-dwarf star (TIC 150428135, 2MASS J06282325-6534456). TOI-700 d has a radius of \(1.144^{+0.062}_{-0.061}R_\oplus\) and orbits within its host star's conservative habitable zone with a period of 37.42 days (\(T_\mathrm{eq} \sim 269\)K). TOI-700 also hosts two small inner planets (R\(_b\)=\(1.037^{+0.065}_{-0.064}R_\oplus\) & R\(_c\)=\(2.65^{+0.16}_{-0.15}R_\oplus\)) with periods of 9.98 and 16.05 days, respectively. Our \(Spitzer\) observations confirm the TESS detection of TOI-700 d and remove any remaining doubt that it is a genuine planet. We analyze the \(Spitzer\) light curve combined with the 11 sectors of TESS observations and a transit of TOI-700 c from the LCOGT network to determine the full system parameters. Although studying the atmosphere of TOI-700 d is not likely feasible with upcoming facilities, it may be possible to measure the mass of TOI-700 d using state-of-the-art radial velocity instruments (expected RV semi-amplitude of \(\sim\)70 cm/s).