K2-21 c
K2-21 c is a mini-Neptune orbiting K2-21 in the constellation Aquarius. It lies about 273 light-years from Earth and was discovered in 2015 using the transit method.
How Big Is K2-21 c?
K2-21 c has a radius of 2.49 times that of Earth.
Is K2-21 c in the Habitable Zone?
K2-21 c orbits inside the inner edge of the habitable zone of K2-21. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of K2-21: 0.279–0.697 AU (conservative: 0.353–0.661 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
See the full interactive habitable-zone view in the Exoplanet Explorer app ›
Temperature on K2-21 c
K2-21 c receives 5.10 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on K2-21 c — one full orbit around K2-21 — lasts 15.5 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.107 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.21).
How Was K2-21 c Discovered?
K2-21 c was discovered in 2015 using the transit method, with observations from K2.
The transit method watches a star for the tiny, regular dip in brightness that occurs when a planet crosses in front of it. The depth and timing of these dips reveal the planet's size and orbital period.
How Far Away Is K2-21 c?
K2-21 c is 272.8 light-years (83.6 parsecs) from Earth. Light arriving here tonight left the planet around the year 1754. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 4,801,280 years to make the journey.
Earth Similarity Index
The Earth Similarity Index (ESI) scores how physically similar a planet is to Earth, from 0 to 1, based on radius, density, escape velocity and surface temperature. K2-21 c scores 0.36, ranking #728 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-21
K2-21
- Surface temperature
- 4,222 K (Sun: 5,772 K)
- Mass
- 0.68 M☉
- Radius
- 0.65 R☉
The K2-21 Planetary System
K2-21 c is one of 2 known planets in the K2-21 system. Its siblings:
- K2-21 b (Super Earth)
K2-21 c — Complete Data
| Radius | 2.490 Earth radii (0.222 Jupiter radii) |
|---|---|
| Orbital period | 15.50 days |
| Orbital distance | 0.107 AU |
| Eccentricity | 0.210 |
| Stellar irradiation | 5.10× Earth |
| Earth Similarity Index | 0.36 |
| Distance from Earth | 272.8 light-years (83.6 parsecs) |
| Constellation | Aquarius |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2015 |
Data: NASA Exoplanet Archive, last updated 2017-12-12. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-21 c
Is K2-21 c habitable?
No — K2-21 c orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is K2-21 c?
K2-21 c is about 273 light-years from Earth in the constellation Aquarius. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 4,801,280 years to get there.
How big is K2-21 c compared to Earth?
K2-21 c has 2.49 times the radius of Earth.
How long is a year on K2-21 c?
One orbit around K2-21 takes 15.5 Earth days — short enough that 24 of its years would fit into one Earth year.
Explore K2-21 c in the app
Browse, filter and compare 6,000+ exoplanets on iPhone, iPad and Apple Watch — with habitable-zone views, widgets and offline data.
