Kepler-21 b
Kepler-21 b is a lava world orbiting Kepler-21 in the constellation Lyra. It lies about 354 light-years from Earth and was discovered in 2012 using the transit method.
How Big Is Kepler-21 b?
Kepler-21 b has a radius of 1.64 times that of Earth. Its mass is 7.5 times that of Earth, giving it a density of 9.30 g/cm³ — comparable to rocky planets like Earth (5.51 g/cm³).
Is Kepler-21 b in the Habitable Zone?
Kepler-21 b orbits inside the inner edge of the habitable zone of Kepler-21. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of Kepler-21: 1.646–3.833 AU (conservative: 2.085–3.634 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on Kepler-21 b
The equilibrium temperature of Kepler-21 b is about 2,015 K (1,742 °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 2,749 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on Kepler-21 b — one full orbit around Kepler-21 — lasts 2.79 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.043 AU — closer to its star than Mercury is to the Sun.
How Was Kepler-21 b Discovered?
Kepler-21 b was discovered in 2012 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-21 b?
Kepler-21 b is 354.0 light-years (108.5 parsecs) from Earth. Light arriving here tonight left the planet around the year 1672. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 6,230,400 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-21 b scores 0.27, ranking #2,498 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: Kepler-21
Kepler-21 b belongs to a system of 2 stars; it orbits Kepler-21.
Kepler-21
- Surface temperature
- 6,305 K (Sun: 5,772 K)
- Mass
- 1.41 M☉
- Radius
- 1.90 R☉
- Age
- 2.8 billion years (Sun: 4.6)
Planetary System
Kepler-21 b is the only planet known to orbit Kepler-21 so far.
Kepler-21 b — Complete Data
| Radius | 1.639 Earth radii (0.146 Jupiter radii) |
|---|---|
| Mass | 7.50 Earth masses (0.024 Jupiter masses) |
| Density | 9.30 g/cm³ (Earth: 5.51) |
| Orbital period | 2.79 days |
| Orbital distance | 0.043 AU |
| Equilibrium temperature | 2,015 K (1,742 °C) |
| Stellar irradiation | 2,749.00× Earth |
| Earth Similarity Index | 0.27 |
| Distance from Earth | 354.0 light-years (108.5 parsecs) |
| Constellation | Lyra |
| Discovery method | Transit |
| Discovery facility | Kepler |
| Discovery year | 2012 |
Data: NASA Exoplanet Archive, last updated 2023-04-17. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About Kepler-21 b
Is Kepler-21 b habitable?
No — Kepler-21 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is Kepler-21 b?
Kepler-21 b is about 354 light-years from Earth in the constellation Lyra. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 6,230,400 years to get there.
How big is Kepler-21 b compared to Earth?
Kepler-21 b has 1.64 times the radius of Earth and about 7.5 times its mass.
How long is a year on Kepler-21 b?
One orbit around Kepler-21 takes 2.8 Earth days — short enough that 131 of its years would fit into one Earth year.
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