A Warm Planet Twice Earth's Size Is Confirmed Around a Small Red Star

Astronomers have confirmed a planet about twice the width of Earth orbiting TOI-210, a small, cool red dwarf, once every 9.01 days. The result comes from a preprint posted to arXiv on Sept. 28 by a 66-author team led by C. Cadieux, submitted to the journal Astronomy & Astrophysics and under minor revision, so it has not completed peer review.

The preprint says super-Earths and sub-Neptunes dominate the known planet population while their makeup, how they form and whether they can keep an atmosphere all remain poorly understood. Small red stars are good places to settle such questions: a planet of a given size blocks more of their light and pulls them around harder than it would a larger star.

The team combined transits from 40 sectors of NASA's TESS survey, ground-based follow-up with the LCOGT and ExTrA telescopes, and radial velocities from the NIRPS infrared spectrograph. It reports a mass of 6.75 ± 1.25 Earth masses and a radius of 2.234 ± 0.074 Earth radii. That bulk density is what the authors call volatile-rich, meaning light materials rather than rock, and it fits an extended hydrogen-helium atmosphere holding about 1% of the planet's mass, a water reservoir of at least 29% of it, or a mixture of both.

The paper calls TOI-210 b temperate on the strength of an equilibrium temperature of 368 ± 9 K, about 95 °C. That figure is calculated from the starlight the planet receives rather than measured at the planet, and it describes no surface: on the compositions the authors allow, a sub-Neptune this size is wrapped in gas or water. The paper makes no claim about habitability, and this is not an Earth analogue.

The NIRPS velocities also show what the team describes as moderate evidence for at least one further planet orbiting closer to the star than TOI-210 b, and a targeted search turned up two candidate signals, at 2.15 and 3.76 days. The authors present neither as a confirmed planet; neither produces a transit in the TESS data.