K2-9 b — 2.3 times Earth's width, a 'sub-Neptune' — bigger than Earth, smaller than Neptune; its year lasts 18 days; about 41 °C, between freezing and boiling; found in 2015 by transit.
NASA Exoplanet Archive, Planetary Systems Composite Parameters (fetched 2026-11-02). Composite values: where a planet's size or mass was not measured, the archive estimates it from the other.
This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with NASA under the Exoplanet Exploration Program.