Precise Masses Reveal that TOI-700 c is Low Density and TOI-700 d is Rocky

The search for life in the Universe has a significant focus on M dwarf stars, as they are the most common stellar type, and their small sizes and low masses make it easier to detect and characterize small, temperate planets. However, M dwarfs, especially those with the lowest masses, emit frequent flares for long portions of their lifetimes that may erode planetary atmospheres. Demographic studies have shown that planets the size of Earth in the Habitable Zone — the region where liquid water could likely exist on planetary surfaces — are common. But only a handful of such worlds have been measured to be rocky, and all of them orbit active, lower-mass M-dwarfs, so the survival of their atmospheres is questionable. Here, we report new exoplanet mass measurements from radial velocity observations with the VLT/ESPRESSO spectrograph of TOI-700, a relatively quiet, early M dwarf star that hosts 4 small planets, including 2 in the star's Habitable Zone. We find that TOI-700 c (2.65 R_Earth) is a low-mass (2.5 M_Earth) planet, which must have a hydrogen/helium dominated atmosphere given its low bulk density (0.73 g/cc). Furthermore, the small (1.16 R_Earth), Habitable Zone planet, TOI-700 d, has a mass of 2.4 M_Earth, implying a rocky composition. Given that TOI-700 c and d have similar masses, the presence of TOI-700 c, a puffy sub-Neptune, orbiting interior to the Habitable Zone planets, implies that d might be able to maintain a heavier, secondary atmosphere as these planets were subjected to the same stellar history.