K2-188 b
K2-188 b is a lava world orbiting K2-188 in the constellation Cancer. It lies about 1,457 light-years from Earth and was discovered in 2018 using the transit method.
How Big Is K2-188 b?
K2-188 b has a radius of 1.35 times that of Earth.
Is K2-188 b in the Habitable Zone?
K2-188 b orbits inside the inner edge of the habitable zone of K2-188. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of K2-188: 0.944–2.213 AU (conservative: 1.196–2.099 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on K2-188 b
The equilibrium temperature of K2-188 b is about 1,692 K (1,419 °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 K2-188 b — one full orbit around K2-188 — lasts just 41.8 hours, shorter than Mercury's 88-day year. It orbits at an average distance of 0.029 AU — closer to its star than Mercury is to the Sun.
How Was K2-188 b Discovered?
K2-188 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 K2-188 b?
K2-188 b is 1,457.3 light-years (446.8 parsecs) from Earth. Light arriving here tonight left the planet about 1,457 years ago. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 25,648,480 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. K2-188 b scores 0.29, ranking #2,045 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-188
K2-188
- Surface temperature
- 5,977 K (Sun: 5,772 K)
- Mass
- 1.06 M☉
- Radius
- 1.19 R☉
The K2-188 Planetary System
K2-188 b is one of 2 known planets in the K2-188 system. Its siblings:
- K2-188 c (Mini Neptune)
K2-188 b — Complete Data
| Radius | 1.350 Earth radii (0.120 Jupiter radii) |
|---|---|
| Orbital period | 1.74 days |
| Orbital distance | 0.029 AU |
| Equilibrium temperature | 1,692 K (1,419 °C) |
| Earth Similarity Index | 0.29 |
| Distance from Earth | 1,457.3 light-years (446.8 parsecs) |
| Constellation | Cancer |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2018 |
Data: NASA Exoplanet Archive, last updated 2020-03-30. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-188 b
Is K2-188 b habitable?
No — K2-188 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is K2-188 b?
K2-188 b is about 1,457 light-years from Earth in the constellation Cancer. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 25,648,480 years to get there.
How big is K2-188 b compared to Earth?
K2-188 b has 1.35 times the radius of Earth.
How long is a year on K2-188 b?
One orbit around K2-188 takes 1.7 Earth days — short enough that 210 of its years would fit into one Earth year.
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