K2-388 b
K2-388 b is a sub-Neptune orbiting K2-388 in the constellation Aquarius. It lies about 948 light-years from Earth and was discovered in 2022 using the transit method.
How Big Is K2-388 b?
K2-388 b has a radius of 3.90 times that of Earth.
Is K2-388 b in the Habitable Zone?
The position of K2-388 b relative to the habitable zone of K2-388 cannot be precisely determined from the available orbital data.
Habitable zone of K2-388: 0.593–1.416 AU (conservative: 0.751–1.343 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 K2-388 b
The equilibrium temperature of K2-388 b is about 874 K (601 °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 138 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on K2-388 b — one full orbit around K2-388 — lasts 6.34 Earth days, shorter than Mercury's 88-day year.
How Was K2-388 b Discovered?
K2-388 b was discovered in 2022 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 K2-388 b?
K2-388 b is 947.5 light-years (290.5 parsecs) from Earth. Light arriving here tonight left the planet around the year 1079. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 16,676,000 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. K2-388 b scores 0.19, ranking #3,941 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-388
K2-388
- Surface temperature
- 5,310 K (Sun: 5,772 K)
- Mass
- 0.93 M☉
- Radius
- 0.91 R☉
Planetary System
K2-388 b is the only planet known to orbit K2-388 so far.
K2-388 b — Complete Data
| Radius | 3.900 Earth radii (0.348 Jupiter radii) |
|---|---|
| Orbital period | 6.34 days |
| Equilibrium temperature | 874 K (601 °C) |
| Stellar irradiation | 138.00× Earth |
| Earth Similarity Index | 0.19 |
| Distance from Earth | 947.5 light-years (290.5 parsecs) |
| Constellation | Aquarius |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2022 |
Data: NASA Exoplanet Archive, last updated 2025-09-17. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-388 b
Is K2-388 b habitable?
K2-388 b is not known to orbit within the habitable zone of K2-388, and as a sub-Neptune it is an unlikely candidate for life as we know it.
How far away is K2-388 b?
K2-388 b is about 948 light-years from Earth in the constellation Aquarius. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 16,676,000 years to get there.
How big is K2-388 b compared to Earth?
K2-388 b has 3.90 times the radius of Earth.
How long is a year on K2-388 b?
One orbit around K2-388 takes 6.3 Earth days — short enough that 58 of its years would fit into one Earth year.
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