The star
The star
K2-121
- Type
- orange dwarf
- Surface
- ≈ 4,526 K
- Size
- 0.68 × the Sun
- Mass
- 0.71 × the Sun
- Brightness
- 0.18 × the Sun
- Habitable zone
- 0.339–0.836 AU
The planets
K2-121
K2-121 b
- Year
- 5.19 days
- Distance from star
- 0.0525 AU
- Mass
- 51 Earths
- Size
- 7.2 × Earth
- Temperature
- ≈ 716 K (+443 °C)
- Found
- 2017, transit (K2)
Masses found by the wobble method are minimums, because the orbit's tilt is unknown. Temperatures are equilibrium estimates for a bare planet, before any atmosphere.
How we know
The shadow
The planet passes in front of its star and dims it by a fraction of a percent, every orbit. The dip gives the planet's size and year.
Sources
- Planet and star parameters: NASA Exoplanet Archive, Planetary Systems Composite Parameters, retrieved 1 Nov 2026. 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.
- Position: HYG Database v4.1 (CC BY-SA 4.0).
- Habitable zone: a simple estimate from the star's brightness (√L × 0.95 to 1.67 AU), shown as a guide.