K2-239 b
K2-239 b is a terrestrial planet orbiting the M3.0 V star K2-239 in the constellation Sextans. It lies about 101 light-years from Earth and was discovered in 2018 using the transit method.
How Big Is K2-239 b?
K2-239 b has a radius of 1.10 times that of Earth.
Is K2-239 b in the Habitable Zone?
K2-239 b orbits inside the inner edge of the habitable zone of K2-239. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of K2-239: 0.104–0.270 AU (conservative: 0.131–0.256 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 K2-239 b
The equilibrium temperature of K2-239 b is about 502 K (229 °C) — hotter than anywhere on Earth. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C.
Orbit and Year Length
A year on K2-239 b — one full orbit around K2-239 — lasts 5.24 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.044 AU — closer to its star than Mercury is to the Sun.
How Was K2-239 b Discovered?
K2-239 b was discovered in 2018 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 K2-239 b?
K2-239 b is 101.4 light-years (31.1 parsecs) from Earth. Light arriving here tonight left the planet around the year 1925. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 1,784,640 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. K2-239 b scores 0.45, ranking #348 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-239
K2-239
- Spectral type
- M3.0 V
- Surface temperature
- 3,420 K (Sun: 5,772 K)
- Mass
- 0.40 M☉
- Radius
- 0.36 R☉
- Luminosity
- 0.0160 L☉
The K2-239 Planetary System
K2-239 b is one of 3 known planets in the K2-239 system. Its siblings:
K2-239 b — Complete Data
| Radius | 1.100 Earth radii (0.098 Jupiter radii) |
|---|---|
| Orbital period | 5.24 days |
| Orbital distance | 0.044 AU |
| Equilibrium temperature | 502 K (229 °C) |
| Earth Similarity Index | 0.45 |
| Distance from Earth | 101.4 light-years (31.1 parsecs) |
| Constellation | Sextans |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2018 |
Data: NASA Exoplanet Archive, last updated 2018-06-26. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-239 b
Is K2-239 b habitable?
No — K2-239 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is K2-239 b?
K2-239 b is about 101 light-years from Earth in the constellation Sextans. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 1,784,640 years to get there.
How big is K2-239 b compared to Earth?
K2-239 b has 1.10 times the radius of Earth.
How long is a year on K2-239 b?
One orbit around K2-239 takes 5.2 Earth days — short enough that 70 of its years would fit into one Earth year.
Explore K2-239 b in the app
Browse, filter and compare 6,000+ exoplanets on iPhone, iPad and Apple Watch — with habitable-zone views, widgets and offline data.
