K2-108 b
K2-108 b is a Neptune-like planet orbiting K2-108 in the constellation Cancer. It lies about 1,557 light-years from Earth and was discovered in 2017 using the transit method.
How Big Is K2-108 b?
K2-108 b has a radius of 5.33 times that of Earth.
Is K2-108 b in the Habitable Zone?
K2-108 b orbits inside the inner edge of the habitable zone of K2-108. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of K2-108: 1.253–2.968 AU (conservative: 1.588–2.814 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on K2-108 b
The equilibrium temperature of K2-108 b is about 1,360 K (1,087 °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.
Orbit and Year Length
A year on K2-108 b — one full orbit around K2-108 — lasts 4.73 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.058 AU — closer to its star than Mercury is to the Sun.
How Was K2-108 b Discovered?
K2-108 b was discovered in 2017 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 K2-108 b?
K2-108 b is 1,556.7 light-years (477.3 parsecs) from Earth. Light arriving here tonight left the planet about 1,557 years ago. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 27,397,920 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. K2-108 b scores 0.14, ranking #4,236 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-108
K2-108
- Surface temperature
- 5,599 K (Sun: 5,772 K)
- Mass
- 1.17 M☉
- Radius
- 1.76 R☉
Planetary System
K2-108 b is the only planet known to orbit K2-108 so far.
K2-108 b — Complete Data
| Radius | 5.330 Earth radii (0.476 Jupiter radii) |
|---|---|
| Orbital period | 4.73 days |
| Orbital distance | 0.058 AU |
| Equilibrium temperature | 1,360 K (1,087 °C) |
| Earth Similarity Index | 0.14 |
| Distance from Earth | 1,556.7 light-years (477.3 parsecs) |
| Constellation | Cancer |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2017 |
Data: NASA Exoplanet Archive, last updated 2018-10-24. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-108 b
Is K2-108 b habitable?
No — K2-108 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is K2-108 b?
K2-108 b is about 1,557 light-years from Earth in the constellation Cancer. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 27,397,920 years to get there.
How big is K2-108 b compared to Earth?
K2-108 b has 5.33 times the radius of Earth.
How long is a year on K2-108 b?
One orbit around K2-108 takes 4.7 Earth days — short enough that 77 of its years would fit into one Earth year.
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