# TESS delivers its first Earth-sized planet and a warm sub-Neptune

## TL;DR

TESS's first Earth-sized planet and a warm sub-Neptune were discovered, showcasing its capability to find small, long-period exoplanets with measured masses, advancing exoplanet science.

## Contribution

First TESS discovery of an Earth-sized planet and a long-period sub-Neptune with measured masses, fulfilling key science requirements.

## Key findings

- HD 21749b is a 2.61 Earth-radius sub-Neptune with a 36-day orbit.
- HD 21749c is an Earth-sized planet, the first from TESS.
- The system is ideal for comparative planetary studies.

## Abstract

The future of exoplanet science is bright, as 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 HD21749b to be 2.61$^{+0.17}_{-0.16}$ $R_{\oplus}$, and combined archival and follow-up precision radial velocity data put the mass of the planet at $22.7^{+2.2}_{-1.9}$ $M_{\oplus}$. HD 21749b contributes to the TESS Level 1 Science Requirement of providing 50 transiting planets smaller than 4 $R_{\oplus}$ with measured masses. Furthermore, we report the discovery of HD 21749c (TOI 186.02), the first Earth-sized ($R_p = 0.892^{+0.064}_{-0.058} R_{\oplus}$) planet from TESS. The HD21749 system is a prime target for comparative studies of planetary composition and architecture in multi-planet systems.