K2-128 b
K2-128 b is a super-Earth orbiting K2-128 in the constellation Virgo. It lies about 374 light-years from Earth and was discovered in 2017 using the transit method.
How Big Is K2-128 b?
K2-128 b has a radius of 1.42 times that of Earth.
Is K2-128 b in the Habitable Zone?
K2-128 b orbits inside the inner edge of the habitable zone of K2-128. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of K2-128: 0.372–0.922 AU (conservative: 0.472–0.874 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
See the full interactive habitable-zone view in the Exoplanet Explorer app ›
Temperature on K2-128 b
K2-128 b receives 68.80 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on K2-128 b — one full orbit around K2-128 — lasts 5.68 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.056 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.23).
How Was K2-128 b Discovered?
K2-128 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 K2-128 b?
K2-128 b is 373.9 light-years (114.6 parsecs) from Earth. Light arriving here tonight left the planet around the year 1653. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 6,580,640 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. K2-128 b scores 0.31, ranking #1,150 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-128
K2-128
- Surface temperature
- 4,470 K (Sun: 5,772 K)
- Mass
- 0.71 M☉
- Radius
- 0.78 R☉
Planetary System
K2-128 b is the only planet known to orbit K2-128 so far.
K2-128 b — Complete Data
| Radius | 1.420 Earth radii (0.127 Jupiter radii) |
|---|---|
| Orbital period | 5.68 days |
| Orbital distance | 0.056 AU |
| Eccentricity | 0.230 |
| Stellar irradiation | 68.80× Earth |
| Earth Similarity Index | 0.31 |
| Distance from Earth | 373.9 light-years (114.6 parsecs) |
| Constellation | Virgo |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2017 |
Data: NASA Exoplanet Archive, last updated 2017-10-03. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-128 b
Is K2-128 b habitable?
No — K2-128 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is K2-128 b?
K2-128 b is about 374 light-years from Earth in the constellation Virgo. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 6,580,640 years to get there.
How big is K2-128 b compared to Earth?
K2-128 b has 1.42 times the radius of Earth.
How long is a year on K2-128 b?
One orbit around K2-128 takes 5.7 Earth days — short enough that 64 of its years would fit into one Earth year.
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