Kepler-138 b
Kepler-138 b is a terrestrial planet orbiting Kepler-138 in the constellation Lyra. It lies about 218 light-years from Earth and was discovered in 2014 using the transit method.
How Big Is Kepler-138 b?
Kepler-138 b has a radius of 0.64 times that of Earth. Its mass is 0.07 times that of Earth, giving it a density of 1.70 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is Kepler-138 b in the Habitable Zone?
Kepler-138 b orbits inside the inner edge of the habitable zone of Kepler-138. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of Kepler-138: 0.193–0.491 AU (conservative: 0.244–0.466 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on Kepler-138 b
The equilibrium temperature of Kepler-138 b is about 452 K (179 °C) — hotter than anywhere on Earth. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C. It receives 9.90 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on Kepler-138 b — one full orbit around Kepler-138 — lasts 10.3 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.075 AU — closer to its star than Mercury is to the Sun. Its orbit is nearly circular (eccentricity 0.020).
How Was Kepler-138 b Discovered?
Kepler-138 b was discovered in 2014 using the transit method, with observations from Kepler.
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 Kepler-138 b?
Kepler-138 b is 218.1 light-years (66.9 parsecs) from Earth. Light arriving here tonight left the planet around the year 1808. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 3,838,560 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. Kepler-138 b scores 0.40, ranking #462 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: Kepler-138
Kepler-138
- Surface temperature
- 3,841 K (Sun: 5,772 K)
- Mass
- 0.54 M☉
- Radius
- 0.54 R☉
- Luminosity
- 0.0560 L☉
The Kepler-138 Planetary System
Kepler-138 b is one of 4 known planets in the Kepler-138 system. Its siblings:
- Kepler-138 c (Super Earth)
- Kepler-138 d (Super Earth)
- Kepler-138 e (Terrestrial)
Kepler-138 b — Complete Data
| Radius | 0.640 Earth radii (0.057 Jupiter radii) |
|---|---|
| Mass | 0.07 Earth masses (0.000 Jupiter masses) |
| Density | 1.70 g/cm³ (Earth: 5.51) |
| Orbital period | 10.31 days |
| Orbital distance | 0.075 AU |
| Eccentricity | 0.020 |
| Equilibrium temperature | 452 K (179 °C) |
| Stellar irradiation | 9.90× Earth |
| Earth Similarity Index | 0.40 |
| Distance from Earth | 218.1 light-years (66.9 parsecs) |
| Constellation | Lyra |
| Discovery method | Transit |
| Discovery facility | Kepler |
| Discovery year | 2014 |
Data: NASA Exoplanet Archive, last updated 2023-01-04. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About Kepler-138 b
Is Kepler-138 b habitable?
No — Kepler-138 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is Kepler-138 b?
Kepler-138 b is about 218 light-years from Earth in the constellation Lyra. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 3,838,560 years to get there.
How big is Kepler-138 b compared to Earth?
Kepler-138 b has 0.64 times the radius of Earth and about 0.07 times its mass.
How long is a year on Kepler-138 b?
One orbit around Kepler-138 takes 10.3 Earth days — short enough that 35 of its years would fit into one Earth year.
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