K2-257 b
K2-257 b is a terrestrial planet orbiting K2-257 in the constellation Virgo. It lies about 209 light-years from Earth and was discovered in 2018 using the transit method.
How Big Is K2-257 b?
K2-257 b has a radius of 0.83 times that of Earth.
Is K2-257 b in the Habitable Zone?
K2-257 b orbits inside the inner edge of the habitable zone of K2-257. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of K2-257: 0.169–0.435 AU (conservative: 0.215–0.412 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on K2-257 b
The equilibrium temperature of K2-257 b is about 789 K (516 °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 K2-257 b — one full orbit around K2-257 — lasts just 38.5 hours, shorter than Mercury's 88-day year. It orbits at an average distance of 0.022 AU — closer to its star than Mercury is to the Sun.
How Was K2-257 b Discovered?
K2-257 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 K2-257 b?
K2-257 b is 208.8 light-years (64.0 parsecs) from Earth. Light arriving here tonight left the planet around the year 1818. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 3,674,880 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. K2-257 b scores 0.31, ranking #1,239 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-257
K2-257
- Surface temperature
- 3,725 K (Sun: 5,772 K)
- Mass
- 0.52 M☉
- Radius
- 0.50 R☉
Planetary System
K2-257 b is the only planet known to orbit K2-257 so far.
K2-257 b — Complete Data
| Radius | 0.830 Earth radii (0.074 Jupiter radii) |
|---|---|
| Orbital period | 1.61 days |
| Orbital distance | 0.022 AU |
| Equilibrium temperature | 789 K (516 °C) |
| Earth Similarity Index | 0.31 |
| Distance from Earth | 208.8 light-years (64.0 parsecs) |
| Constellation | Virgo |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2018 |
Data: NASA Exoplanet Archive, last updated 2022-03-28. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-257 b
Is K2-257 b habitable?
No — K2-257 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is K2-257 b?
K2-257 b is about 209 light-years from Earth in the constellation Virgo. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 3,674,880 years to get there.
How big is K2-257 b compared to Earth?
K2-257 b has 0.83 times the radius of Earth.
How long is a year on K2-257 b?
One orbit around K2-257 takes 1.6 Earth days — short enough that 227 of its years would fit into one Earth year.
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