K2-28 b
K2-28 b is a mini-Neptune orbiting the M4 star K2-28 in the constellation Aquarius. It lies about 206 light-years from Earth and was discovered in 2016 using the transit method.
How Big Is K2-28 b?
K2-28 b has a radius of 2.32 times that of Earth.
Is K2-28 b in the Habitable Zone?
K2-28 b orbits inside the inner edge of the habitable zone of K2-28. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of K2-28: 0.073–0.192 AU (conservative: 0.093–0.182 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on K2-28 b
The equilibrium temperature of K2-28 b is about 568 K (295 °C) — hotter than anywhere on Earth. 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 K2-28 b — one full orbit around K2-28 — lasts 2.26 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.021 AU — closer to its star than Mercury is to the Sun.
How Was K2-28 b Discovered?
K2-28 b was discovered in 2016 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 K2-28 b?
K2-28 b is 205.5 light-years (63.0 parsecs) from Earth. Light arriving here tonight left the planet around the year 1821. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 3,616,800 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. K2-28 b scores 0.29, ranking #1,885 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-28
K2-28
- Spectral type
- M4
- Surface temperature
- 3,214 K (Sun: 5,772 K)
- Mass
- 0.26 M☉
- Radius
- 0.29 R☉
Planetary System
K2-28 b is the only planet known to orbit K2-28 so far.
K2-28 b — Complete Data
| Radius | 2.320 Earth radii (0.207 Jupiter radii) |
|---|---|
| Orbital period | 2.26 days |
| Orbital distance | 0.021 AU |
| Equilibrium temperature | 568 K (295 °C) |
| Earth Similarity Index | 0.29 |
| Distance from Earth | 205.5 light-years (63.0 parsecs) |
| Constellation | Aquarius |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2016 |
Data: NASA Exoplanet Archive, last updated 2016-02-24. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-28 b
Is K2-28 b habitable?
No — K2-28 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is K2-28 b?
K2-28 b is about 206 light-years from Earth in the constellation Aquarius. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 3,616,800 years to get there.
How big is K2-28 b compared to Earth?
K2-28 b has 2.32 times the radius of Earth.
How long is a year on K2-28 b?
One orbit around K2-28 takes 2.3 Earth days — short enough that 162 of its years would fit into one Earth year.
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