K2-121 b
K2-121 b is a hot Jupiter orbiting K2-121 in the constellation Cancer. It lies about 553 light-years from Earth and was discovered in 2017 using the transit method.
How Big Is K2-121 b?
K2-121 b has a radius of 7.16 times that of Earth, or 0.64 times the radius of Jupiter. Its mass is 51.0 times that of Earth, giving it a density of 0.70 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is K2-121 b in the Habitable Zone?
The position of K2-121 b relative to the habitable zone of K2-121 cannot be precisely determined from the available orbital data.
Habitable zone of K2-121: 0.331–0.817 AU (conservative: 0.419–0.774 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on K2-121 b
The equilibrium temperature of K2-121 b is about 716 K (443 °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 62.50 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on K2-121 b — one full orbit around K2-121 — lasts 5.19 Earth days, shorter than Mercury's 88-day year.
How Was K2-121 b Discovered?
K2-121 b was discovered in 2017 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 K2-121 b?
K2-121 b is 552.5 light-years (169.4 parsecs) from Earth. Light arriving here tonight left the planet around the year 1474. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 9,724,000 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. K2-121 b scores 0.13, ranking #4,273 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-121
K2-121
- Surface temperature
- 4,526 K (Sun: 5,772 K)
- Mass
- 0.71 M☉
- Radius
- 0.68 R☉
Planetary System
K2-121 b is the only planet known to orbit K2-121 so far.
K2-121 b — Complete Data
| Radius | 7.160 Earth radii (0.639 Jupiter radii) |
|---|---|
| Mass | 51.00 Earth masses (0.160 Jupiter masses) |
| Density | 0.70 g/cm³ (Earth: 5.51) |
| Orbital period | 5.19 days |
| Equilibrium temperature | 716 K (443 °C) |
| Stellar irradiation | 62.50× Earth |
| Earth Similarity Index | 0.13 |
| Distance from Earth | 552.5 light-years (169.4 parsecs) |
| Constellation | Cancer |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2017 |
Data: NASA Exoplanet Archive, last updated 2025-09-17. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-121 b
Is K2-121 b habitable?
K2-121 b is not known to orbit within the habitable zone of K2-121, and as a hot Jupiter it is an unlikely candidate for life as we know it.
How far away is K2-121 b?
K2-121 b is about 553 light-years from Earth in the constellation Cancer. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 9,724,000 years to get there.
How big is K2-121 b compared to Earth?
K2-121 b has 7.16 times the radius of Earth and about 51.0 times its mass.
How long is a year on K2-121 b?
One orbit around K2-121 takes 5.2 Earth days — short enough that 70 of its years would fit into one Earth year.
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