K2-321 b
K2-321 b is a mini-Neptune orbiting K2-321 in the constellation Sextans. It lies about 253 light-years from Earth and was discovered in 2020 using the transit method.
How Big Is K2-321 b?
K2-321 b has a radius of 2.05 times that of Earth.
Is K2-321 b in the Habitable Zone?
K2-321 b orbits inside the inner edge of the habitable zone of K2-321. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of K2-321: 0.205–0.524 AU (conservative: 0.259–0.497 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on K2-321 b
The equilibrium temperature of K2-321 b is about 830 K (557 °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 K2-321 b — one full orbit around K2-321 — lasts 2.30 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.028 AU — closer to its star than Mercury is to the Sun. Its orbit is highly elliptical (eccentricity 0.44), swinging dramatically closer to and farther from its star.
How Was K2-321 b Discovered?
K2-321 b was discovered in 2020 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 K2-321 b?
K2-321 b is 253.4 light-years (77.7 parsecs) from Earth. Light arriving here tonight left the planet around the year 1773. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 4,459,840 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. K2-321 b scores 0.30, ranking #1,490 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-321
K2-321
- Surface temperature
- 3,750 K (Sun: 5,772 K)
- Mass
- 0.58 M☉
- Radius
- 0.60 R☉
- Luminosity
- 0.0630 L☉
Planetary System
K2-321 b is the only planet known to orbit K2-321 so far.
K2-321 b — Complete Data
| Radius | 2.051 Earth radii (0.183 Jupiter radii) |
|---|---|
| Orbital period | 2.30 days |
| Orbital distance | 0.028 AU |
| Eccentricity | 0.440 |
| Equilibrium temperature | 830 K (557 °C) |
| Earth Similarity Index | 0.30 |
| Distance from Earth | 253.4 light-years (77.7 parsecs) |
| Constellation | Sextans |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2020 |
Data: NASA Exoplanet Archive, last updated 2024-11-20. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-321 b
Is K2-321 b habitable?
No — K2-321 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is K2-321 b?
K2-321 b is about 253 light-years from Earth in the constellation Sextans. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 4,459,840 years to get there.
How big is K2-321 b compared to Earth?
K2-321 b has 2.05 times the radius of Earth.
How long is a year on K2-321 b?
One orbit around K2-321 takes 2.3 Earth days — short enough that 159 of its years would fit into one Earth year.
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