K2-129 b
K2-129 b is a terrestrial planet orbiting K2-129 in the constellation Sagittarius. It lies about 91 light-years from Earth and was discovered in 2017 using the transit method.
How Big Is K2-129 b?
K2-129 b has a radius of 1.04 times that of Earth.
Is K2-129 b in the Habitable Zone?
K2-129 b orbits inside the inner edge of the habitable zone of K2-129. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of K2-129: 0.106–0.275 AU (conservative: 0.134–0.260 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on K2-129 b
K2-129 b receives 4.50 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on K2-129 b — one full orbit around K2-129 — lasts 8.24 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.057 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.13).
How Was K2-129 b Discovered?
K2-129 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 K2-129 b?
K2-129 b is 90.7 light-years (27.8 parsecs) from Earth. Light arriving here tonight left the planet around the year 1936. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 1,596,320 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. K2-129 b scores 0.54, ranking #198 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-129
K2-129
- Surface temperature
- 3,459 K (Sun: 5,772 K)
- Mass
- 0.36 M☉
- Radius
- 0.36 R☉
Planetary System
K2-129 b is the only planet known to orbit K2-129 so far.
K2-129 b — Complete Data
| Radius | 1.040 Earth radii (0.093 Jupiter radii) |
|---|---|
| Orbital period | 8.24 days |
| Orbital distance | 0.057 AU |
| Eccentricity | 0.130 |
| Stellar irradiation | 4.50× Earth |
| Earth Similarity Index | 0.54 |
| Distance from Earth | 90.7 light-years (27.8 parsecs) |
| Constellation | Sagittarius |
| 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 K2-129 b
Is K2-129 b habitable?
No — K2-129 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is K2-129 b?
K2-129 b is about 91 light-years from Earth in the constellation Sagittarius. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 1,596,320 years to get there.
How big is K2-129 b compared to Earth?
K2-129 b has 1.04 times the radius of Earth.
How long is a year on K2-129 b?
One orbit around K2-129 takes 8.2 Earth days — short enough that 44 of its years would fit into one Earth year.
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