K2-181 b
K2-181 b is a mini-Neptune orbiting K2-181 in the constellation Cancer. It lies about 1,177 light-years from Earth and was discovered in 2018 using the transit method.
How Big Is K2-181 b?
K2-181 b has a radius of 2.69 times that of Earth.
Is K2-181 b in the Habitable Zone?
The position of K2-181 b relative to the habitable zone of K2-181 cannot be precisely determined from the available orbital data.
Habitable zone of K2-181: 0.739–1.754 AU (conservative: 0.936–1.662 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 K2-181 b
The equilibrium temperature of K2-181 b is about 949 K (676 °C) — hot enough to melt many metals. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C. It receives 192 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on K2-181 b — one full orbit around K2-181 — lasts 6.89 Earth days, shorter than Mercury's 88-day year.
How Was K2-181 b Discovered?
K2-181 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 K2-181 b?
K2-181 b is 1,177.1 light-years (360.9 parsecs) from Earth. Light arriving here tonight left the planet about 1,177 years ago. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 20,716,960 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. K2-181 b scores 0.22, ranking #3,618 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-181
K2-181
- Surface temperature
- 5,528 K (Sun: 5,772 K)
- Mass
- 0.96 M☉
- Radius
- 1.06 R☉
Planetary System
K2-181 b is the only planet known to orbit K2-181 so far.
K2-181 b — Complete Data
| Radius | 2.690 Earth radii (0.240 Jupiter radii) |
|---|---|
| Orbital period | 6.89 days |
| Equilibrium temperature | 949 K (676 °C) |
| Stellar irradiation | 192.00× Earth |
| Earth Similarity Index | 0.22 |
| Distance from Earth | 1,177.1 light-years (360.9 parsecs) |
| Constellation | Cancer |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2018 |
Data: NASA Exoplanet Archive, last updated 2025-09-17. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-181 b
Is K2-181 b habitable?
K2-181 b is not known to orbit within the habitable zone of K2-181, and as a mini-Neptune it is an unlikely candidate for life as we know it.
How far away is K2-181 b?
K2-181 b is about 1,177 light-years from Earth in the constellation Cancer. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 20,716,960 years to get there.
How big is K2-181 b compared to Earth?
K2-181 b has 2.69 times the radius of Earth.
How long is a year on K2-181 b?
One orbit around K2-181 takes 6.9 Earth days — short enough that 53 of its years would fit into one Earth year.
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