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

Abstract

We present $Spitzer$ 4.5$\hat{I}¼$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 $1.144_{-0.061}^{+0.062}{R}_{\oplus }$ 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${}_b$=$1.037_{-0.064}^{+0.065}{R}_{\oplus }$ & R${}_c$=$2.65_{-0.15}^{+0.16}{R}_{\oplus }$) 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).

14 Pages, 5 Figures, 2 Tables, Accepted to AJ