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

We present Spitzer 4.5μm 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 1.220^(+0.073)_(−0.063) R_⊕ and orbits within its host star's conservative habitable zone with a period of 37.42 days (T_(eq) ∼269 K). TOI-700 also hosts two small inner planets (R_b = 1.044^(+0.065)_(−0.063) R_⊕ & R_c = 2.64^(+0.16)_(−0.14) R_⊕) 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. With an expected RV semi-amplitude of ∼80 cm/s, it may be possible to measure the mass of TOI-700 d using state-of-the-art radial velocity instruments.