TOI-210 b joins the temperate sub-Neptunes

A new preprint combines TESS transits, ground-based photometry, and NIRPS radial velocities to confirm TOI-210 b. The planet measures 2.234 plus or minus 0.074 Earth radii and 6.75 plus or minus 1.25 Earth masses, with an estimated equilibrium temperature of 368 plus or minus 9 kelvins. Those values place it in the comparatively sparse sample of temperate sub-Neptunes with both mass and radius measured.

Interior models allow more than one physical picture: a small hydrogen-helium envelope of roughly one percent by mass, a water-rich interior with a reported two-sigma lower limit near 29 percent water, or a mixture. The radial-velocity analysis also finds moderate evidence for at least one additional non-transiting inner planet, but the specific 2.15-day and 3.76-day signals remain candidates. Confirmation of TOI-210 b is not confirmation of its interior—or of the neighboring signals.

Why it matters

Temperate sub-Neptunes have no Solar System analogue. A well-measured member helps test whether these common worlds are gas-wrapped rock, water-rich bodies, or a family with multiple formation paths.

What the source establishes

Fresh submitted preprint. TOI-210 b is supported by transit and radial-velocity data; the proposed interior compositions are model-dependent and the additional planets are provisional.