K2-9 b
K2-9 b is a mini-Neptune orbiting the M2.5 V star K2-9 in the constellation Virgo. It lies about 271 light-years from Earth and was discovered in 2015 using the transit method. It orbits within the habitable zone of its star — the region where liquid water could exist on a planet's surface.
How Big Is K2-9 b?
K2-9 b has a radius of 2.25 times that of Earth.
Is K2-9 b in the Habitable Zone?
K2-9 b orbits within the optimistic habitable zone of K2-9 — the broader region where liquid water might be possible under favorable atmospheric conditions.
Habitable zone of K2-9: 0.090–0.234 AU (conservative: 0.114–0.222 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on K2-9 b
The equilibrium temperature of K2-9 b is about 314 K (41 °C) — in a range broadly comparable to Earth, whose equilibrium temperature is 255 K. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C. It receives 1.36 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on K2-9 b — one full orbit around K2-9 — lasts 18.4 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.091 AU — closer to its star than Mercury is to the Sun.
How Was K2-9 b Discovered?
K2-9 b was discovered in 2015 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 K2-9 b?
K2-9 b is 270.6 light-years (83.0 parsecs) from Earth. Light arriving here tonight left the planet around the year 1756. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 4,762,560 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. K2-9 b scores 0.70, ranking #61 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-9
K2-9
- Spectral type
- M2.5 V
- Surface temperature
- 3,390 K (Sun: 5,772 K)
- Mass
- 0.30 M☉
- Radius
- 0.31 R☉
- Luminosity
- 0.0120 L☉
- Age
- 1.0 billion years (Sun: 4.6)
Planetary System
K2-9 b is the only planet known to orbit K2-9 so far.
K2-9 b — Complete Data
| Radius | 2.250 Earth radii (0.201 Jupiter radii) |
|---|---|
| Orbital period | 18.45 days |
| Orbital distance | 0.091 AU |
| Equilibrium temperature | 314 K (41 °C) |
| Stellar irradiation | 1.36× Earth |
| Earth Similarity Index | 0.70 |
| Distance from Earth | 270.6 light-years (83.0 parsecs) |
| Constellation | Virgo |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2015 |
Data: NASA Exoplanet Archive, last updated 2016-01-20. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-9 b
Is K2-9 b habitable?
K2-9 b orbits within the habitable zone of K2-9, the region where liquid water could exist on a planet's surface. Whether it is actually habitable depends on its atmosphere and composition, which remain unknown.
How far away is K2-9 b?
K2-9 b is about 271 light-years from Earth in the constellation Virgo. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 4,762,560 years to get there.
How big is K2-9 b compared to Earth?
K2-9 b has 2.25 times the radius of Earth.
How long is a year on K2-9 b?
One orbit around K2-9 takes 18.4 Earth days — short enough that 20 of its years would fit into one Earth year.
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