Kepler-18 c
Kepler-18 c is a Neptune-like planet orbiting Kepler-18 in the constellation Cygnus. It lies about 1,412 light-years from Earth and was discovered in 2011 using the transit method.
How Big Is Kepler-18 c?
Kepler-18 c has a radius of 5.49 times that of Earth. Its mass is 17.3 times that of Earth, giving it a density of 0.59 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is Kepler-18 c in the Habitable Zone?
Kepler-18 c orbits inside the inner edge of the habitable zone of Kepler-18. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of Kepler-18: 0.743–1.772 AU (conservative: 0.940–1.680 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 ›
Orbit and Year Length
A year on Kepler-18 c — one full orbit around Kepler-18 — lasts 7.64 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.075 AU — closer to its star than Mercury is to the Sun.
How Was Kepler-18 c Discovered?
Kepler-18 c was discovered in 2011 using the transit method, with observations from Kepler.
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 Kepler-18 c?
Kepler-18 c is 1,412.4 light-years (433.0 parsecs) from Earth. Light arriving here tonight left the planet about 1,412 years ago. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 24,858,240 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. Kepler-18 c scores 0.15, ranking #4,223 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: Kepler-18
Kepler-18
- Surface temperature
- 5,345 K (Sun: 5,772 K)
- Mass
- 0.97 M☉
- Radius
- 1.11 R☉
- Luminosity
- 0.9311 L☉
- Age
- 10.0 billion years (Sun: 4.6)
The Kepler-18 Planetary System
Kepler-18 c is one of 3 known planets in the Kepler-18 system. Its siblings:
- Kepler-18 b (Super Earth)
- Kepler-18 d (Cold Gas Giant)
Kepler-18 c — Complete Data
| Radius | 5.490 Earth radii (0.490 Jupiter radii) |
|---|---|
| Mass | 17.30 Earth masses (0.054 Jupiter masses) |
| Density | 0.59 g/cm³ (Earth: 5.51) |
| Orbital period | 7.64 days |
| Orbital distance | 0.075 AU |
| Earth Similarity Index | 0.15 |
| Distance from Earth | 1,412.4 light-years (433.0 parsecs) |
| Constellation | Cygnus |
| Discovery method | Transit |
| Discovery facility | Kepler |
| Discovery year | 2011 |
Data: NASA Exoplanet Archive, last updated 2019-03-01. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About Kepler-18 c
Is Kepler-18 c habitable?
No — Kepler-18 c orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is Kepler-18 c?
Kepler-18 c is about 1,412 light-years from Earth in the constellation Cygnus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 24,858,240 years to get there.
How big is Kepler-18 c compared to Earth?
Kepler-18 c has 5.49 times the radius of Earth and about 17.3 times its mass.
How long is a year on Kepler-18 c?
One orbit around Kepler-18 takes 7.6 Earth days — short enough that 48 of its years would fit into one Earth year.
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