HD 156668 c
HD 156668 c is a Neptune-like planet orbiting HD 156668 in the constellation Hercules. It lies about 79 light-years from Earth and was discovered in 2021 using the radial velocity method. It orbits within the habitable zone of its star — the region where liquid water could exist on a planet's surface.
Is HD 156668 c in the Habitable Zone?
HD 156668 c orbits inside the conservative habitable zone of HD 156668 — the region where a rocky planet could sustain liquid water on its surface. This makes it one of the most interesting known exoplanets in the search for life.
Habitable zone of HD 156668: 1.028–2.500 AU (conservative: 1.302–2.370 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 HD 156668 c — one full orbit around HD 156668 — lasts 811.3 Earth days, longer than an Earth year. It orbits at an average distance of 1.570 AU. Its orbit is mildly elliptical (eccentricity 0.09).
How Was HD 156668 c Discovered?
HD 156668 c was discovered in 2021 using the radial velocity method, with observations from Multiple Observatories.
The radial velocity method measures the subtle wobble of a star caused by the gravitational pull of an orbiting planet, visible as periodic shifts in the star's light spectrum. The size of the wobble reveals the planet's minimum mass.
How Far Away Is HD 156668 c?
HD 156668 c is 79.4 light-years (24.3 parsecs) from Earth. Light arriving here tonight left the planet around the year 1947. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 1,397,440 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. HD 156668 c scores 0.24, ranking #3,221 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: HD 156668
HD 156668
- Surface temperature
- 4,850 K (Sun: 5,772 K)
- Mass
- 0.77 M☉
- Radius
- 0.72 R☉
- Luminosity
- 1.6982 L☉
- Age
- 8.6 billion years (Sun: 4.6)
The HD 156668 Planetary System
HD 156668 c is one of 2 known planets in the HD 156668 system. Its siblings:
- HD 156668 b (Super Earth)
HD 156668 c — Complete Data
| Mass (best estimate) | 31.50 Earth masses |
|---|---|
| Orbital period | 811.30 days |
| Orbital distance | 1.570 AU |
| Eccentricity | 0.089 |
| Earth Similarity Index | 0.24 |
| Distance from Earth | 79.4 light-years (24.3 parsecs) |
| Constellation | Hercules |
| Discovery method | Radial Velocity |
| Discovery facility | Multiple Observatories |
| Discovery year | 2021 |
Data: NASA Exoplanet Archive, last updated 2021-09-20. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HD 156668 c
Is HD 156668 c habitable?
HD 156668 c orbits within the habitable zone of HD 156668, the region where liquid water could exist on a planet's surface. It sits in the conservative habitable zone — the most promising region for habitability. Whether it is actually habitable depends on its atmosphere and composition, which remain unknown.
How far away is HD 156668 c?
HD 156668 c is about 79 light-years from Earth in the constellation Hercules. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 1,397,440 years to get there.
How long is a year on HD 156668 c?
One orbit around HD 156668 takes 811.3 Earth days.
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