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

The future of exoplanetscience is bright, as TESS once again demonstrates with the discovery of its longest-period confirmed planetto date. We hereby present HD 21749b (TOI 186.01), a sub- Neptunein a 36-day orbit around a bright (V = 8.1) nearby (16 pc) K4.5 dwarf. TESS measures HD21749b to be 2.61$^{+0.17}_{-0.16}$ $R_{\oplus}$, and combined archival and follow-up precision radial velocitydata put the mass of the planetat $22.7^{+2.2}_{-1.9}$ $M_{\oplus}$. HD 21749b contributes to the TESS Level 1 Science Requirement of providing 50 transiting planetssmaller than 4 $R_{\oplus}$ with measured masses. Furthermore, we report the discovery of HD 21749c (TOI 186.02), the first Earth-sized ($R_p = 0.892^{+0.064}_{-0.058} R_{\oplus}$) planetfrom TESS. The HD21749 system is a prime target for comparative studies of planetary composition and architecture in multi- planetsystems.