TESS Delivers Its First Earth-sized Planet and a Warm Sub-Neptune*

Abstract

The future of exoplanet science is bright, as Transiting Exoplanet Survey Satellite ( TESS ) once again demonstrates with the discovery of its longest-period confirmed planet to date. We hereby present HD 21749b (TOI 186.01), a sub-Neptune in a 36 day orbit around a bright ( V  = 8.1) nearby (16 pc) K4.5 dwarf. TESS measures HD 21749b to be R ⊕ , and combined archival and follow-up precision radial velocity data put the mass of the planet at M ⊕ . HD 21749b contributes to the TESS Level 1 Science Requirement of providing 50 transiting planets smaller than 4 R ⊕ with measured masses. Furthermore, we report the discovery of HD 21749c (TOI 186.02), the first Earth-sized ( ) planet from TESS . The HD 21749 system is a prime target for comparative studies of planetary composition and architecture in multi-planet systems.