K2-55 b
K2-55 b is a Neptune-like planet orbiting K2-55 in the constellation Aquarius. It lies about 520 light-years from Earth and was discovered in 2016 using the transit method.
How Big Is K2-55 b?
K2-55 b has a radius of 4.43 times that of Earth. Its mass is 44.0 times that of Earth, giving it a density of 2.80 g/cm³ — between that of rocky and gaseous planets.
Is K2-55 b in the Habitable Zone?
The position of K2-55 b relative to the habitable zone of K2-55 cannot be precisely determined from the available orbital data.
Habitable zone of K2-55: 0.319–0.795 AU (conservative: 0.404–0.754 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on K2-55 b
The equilibrium temperature of K2-55 b is about 900 K (627 °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. It receives 141 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on K2-55 b — one full orbit around K2-55 — lasts 2.85 Earth days, shorter than Mercury's 88-day year.
How Was K2-55 b Discovered?
K2-55 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 K2-55 b?
K2-55 b is 520.3 light-years (159.5 parsecs) from Earth. Light arriving here tonight left the planet around the year 1506. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 9,157,280 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. K2-55 b scores 0.19, ranking #3,977 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-55
K2-55
- Surface temperature
- 4,300 K (Sun: 5,772 K)
- Mass
- 0.69 M☉
- Radius
- 0.72 R☉
Planetary System
K2-55 b is the only planet known to orbit K2-55 so far.
K2-55 b — Complete Data
| Radius | 4.430 Earth radii (0.395 Jupiter radii) |
|---|---|
| Mass | 44.00 Earth masses (0.138 Jupiter masses) |
| Density | 2.80 g/cm³ (Earth: 5.51) |
| Orbital period | 2.85 days |
| Equilibrium temperature | 900 K (627 °C) |
| Stellar irradiation | 141.30× Earth |
| Earth Similarity Index | 0.19 |
| Distance from Earth | 520.3 light-years (159.5 parsecs) |
| Constellation | Aquarius |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2016 |
Data: NASA Exoplanet Archive, last updated 2025-09-17. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-55 b
Is K2-55 b habitable?
K2-55 b is not known to orbit within the habitable zone of K2-55, and as a Neptune-like planet it is an unlikely candidate for life as we know it.
How far away is K2-55 b?
K2-55 b is about 520 light-years from Earth in the constellation Aquarius. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 9,157,280 years to get there.
How big is K2-55 b compared to Earth?
K2-55 b has 4.43 times the radius of Earth and about 44.0 times its mass.
How long is a year on K2-55 b?
One orbit around K2-55 takes 2.8 Earth days — short enough that 128 of its years would fit into one Earth year.
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