Discovery and mass measurement of the hot, transiting, Earth-sized planet, GJ 3929 b

<jats:p>We report the discovery of GJ 3929 b, a hot Earth-sized planet orbiting the nearby M3.5 V dwarf star, GJ 3929 (G 180-18, TOI-2013). Joint modelling of photometric observations from TESS sectors 24 and 25 together with 73 spectroscopic observations from CARMENES and follow-up transit observations from SAINT-EX, LCOGT, and OSN yields a planet radius of <jats:italic>R</jats:italic><jats:sub>b</jats:sub> = 1.150 ± 0.040 <jats:italic>R</jats:italic><jats:sub>⊕</jats:sub>, a mass of <jats:italic>M</jats:italic><jats:sub>b</jats:sub> = 1.21 ± 0.42 <jats:italic>M</jats:italic><jats:sub>⊕</jats:sub>, and an orbital period of <jats:italic>P</jats:italic><jats:sub>b</jats:sub> = 2.6162745 ± 0.0000030 <jats:italic>d</jats:italic>. The resulting density of <jats:italic>ρ</jats:italic><jats:sub>b</jats:sub> = 4.4 ± 1.6 g cm<jats:sup>−3</jats:sup> is compatible with the Earth’s mean density of about 5.5 g cm<jats:sup>−3</jats:sup>. Due to the apparent brightness of the host star (<jats:italic>J</jats:italic> = 8.7 mag) and its small size, GJ 3929 b is a promising target for atmospheric characterisation with the JWST. Additionally, the radial velocity data show evidence for another planet candidate with <jats:italic>P</jats:italic><jats:sub>[c]</jats:sub> = 14.303 ± 0.035 d, which is likely unrelated to the stellar rotation period, <jats:italic>P</jats:italic><jats:sub>rot</jats:sub> = 122 ± 13 d, which we determined from archival HATNet and ASAS-SN photometry combined with newly obtained TJO data.</jats:p>