K2-238 b
K2-238 b is a hot Jupiter orbiting the G2 V star K2-238 in the constellation Aquarius. It lies about 1,755 light-years from Earth and was discovered in 2018 using the transit method.
How Big Is K2-238 b?
K2-238 b has a radius of 14.57 times that of Earth, or 1.30 times the radius of Jupiter. Its mass is 273 times that of Earth, giving it a density of 0.56 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is K2-238 b in the Habitable Zone?
K2-238 b orbits inside the inner edge of the habitable zone of K2-238. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of K2-238: 1.143–2.704 AU (conservative: 1.448–2.563 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on K2-238 b
The equilibrium temperature of K2-238 b is about 1,587 K (1,314 °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 K2-238 b — one full orbit around K2-238 — lasts 3.20 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.046 AU — closer to its star than Mercury is to the Sun.
How Was K2-238 b Discovered?
K2-238 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 K2-238 b?
K2-238 b is 1,754.6 light-years (538.0 parsecs) from Earth. Light arriving here tonight left the planet about 1,755 years ago. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 30,880,960 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. K2-238 b scores 0.06, ranking #5,099 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-238
K2-238
- Spectral type
- G2 V
- Surface temperature
- 5,630 K (Sun: 5,772 K)
- Mass
- 1.19 M☉
- Radius
- 1.59 R☉
- Age
- 5.6 billion years (Sun: 4.6)
Planetary System
K2-238 b is the only planet known to orbit K2-238 so far.
K2-238 b — Complete Data
| Radius | 14.572 Earth radii (1.300 Jupiter radii) |
|---|---|
| Mass | 273.33 Earth masses (0.860 Jupiter masses) |
| Density | 0.56 g/cm³ (Earth: 5.51) |
| Orbital period | 3.20 days |
| Orbital distance | 0.046 AU |
| Equilibrium temperature | 1,587 K (1,314 °C) |
| Earth Similarity Index | 0.06 |
| Distance from Earth | 1,754.6 light-years (538.0 parsecs) |
| Constellation | Aquarius |
| 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 K2-238 b
Is K2-238 b habitable?
No — K2-238 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is K2-238 b?
K2-238 b is about 1,755 light-years from Earth in the constellation Aquarius. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 30,880,960 years to get there.
How big is K2-238 b compared to Earth?
K2-238 b has 14.57 times the radius of Earth and about 273 times its mass.
How long is a year on K2-238 b?
One orbit around K2-238 takes 3.2 Earth days — short enough that 114 of its years would fit into one Earth year.
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