A super-Earth in a triple-star system
Gliese 667 Cc
Orbits Gliese 667 C · 23.6 light-years · discovered 2011 (radial velocity)
Gliese 667 Cc orbits a red dwarf that belongs to a triple-star system about 23.6 light-years away in the constellation Scorpius. It has a minimum mass of nearly four Earths and receives a little less energy from its star than Earth does from the Sun, placing it within the star's habitable zone.
When it was reported in 2011 and 2012, it was one of the first planets of modest mass found in a habitable zone, and it quickly became a fixture of lists of potentially habitable worlds. Its size, composition and atmosphere remain unknown, however, and even the full make-up of its planetary system has been debated.
Key facts
- Star
- Gliese 667 CM1.5V red dwarf
- Distance
- 23.6 light-years
- Radius
- Not measuredIt has not been seen to transit its star
- Mass
- ≥ 3.80 × EarthMinimum mass (radial velocity)
- Orbital period
- 28.1 days
- Equilibrium temperature
- 277 K (4 °C)Before any greenhouse warming
- Starlight received
- 0.88× Earth's
- Surface gravity
- ≈ 1.85 gRough estimate (rocky composition assumed)
- Discovered
- 2011Radial velocity
- Star
- Gliese 667 C (M1.5V red dwarf), part of a triple system
- Distance
- About 23.6 light-years
- Discovered
- 2011–2012, radial velocity
- Mass
- At least about 3.8 Earth masses (minimum mass)
- Radius
- Unknown — estimated at roughly 1.5 Earth radii if rocky
- Orbital period
- About 28.1 days
- Starlight received
- Roughly 90% of what Earth gets from the Sun
- Equilibrium temperature
- About 277 K (4 °C), a simple estimate that ignores any atmosphere
- Constellation
- Scorpius
Measured values follow the NASA Exoplanet Archive and discovery papers; many carry large uncertainties. The 3D globe and walkable surface on this page are an artist's impression generated from these numbers.
What we know
Gliese 667 Cc is known only from the periodic shift it causes in its star's spectrum. Those measurements give an orbit of about 28 days and a minimum mass of roughly 3.8 Earth masses, making it a super-Earth — larger than our planet, but smaller than Neptune.
Its radius has never been measured because it has not been seen to transit its star. A planet of this mass could be rocky, but it could also hold a large share of water or a thicker gas envelope; current data cannot tell these apart.
Its star
Gliese 667 C is a red dwarf with roughly a third of the Sun's mass. It orbits at a large distance — a few hundred times the Earth–Sun separation — around a close pair of orange dwarfs, Gliese 667 A and B. The system is poorer in heavy elements than the Sun, which made the discovery of small planets there especially interesting to astronomers.
Could it be habitable?
In terms of incoming energy alone, Gliese 667 Cc lies comfortably within its star's habitable zone. That is the main reason it is often listed among potentially habitable planets.
There are significant caveats. Its mass leaves open whether it has a solid surface at all. A 2013 study suggested that tidal forces from its star could heat its interior far more strongly than Earth's, which might drive intense volcanism. As with other red dwarf planets, stellar activity may have affected any atmosphere. None of these factors has been tested by observation.
Orbit and climate
The planet orbits about 0.125 astronomical units from its star — roughly a third of Mercury's distance from the Sun. At that range it is likely to be tidally locked, or close to it. If it has a substantial atmosphere, winds could carry heat from the dayside to the night side; if not, temperatures would differ enormously between the two hemispheres.
Discovery
The planet first appeared in an analysis of HARPS spectrograph data released by a European team led by Xavier Bonfils in 2011. In early 2012, a team led by Guillem Anglada-Escudé independently analysed the data and confirmed it, publishing the first refereed description. Credit for the discovery has sometimes been disputed as a result.
How many planets?
A 2013 analysis proposed as many as seven planets around Gliese 667 C. Later studies argued that several of those signals could be noise or stellar activity, and only planets b and c are widely regarded as secure.
Future observations
Gliese 667 Cc does not transit, so its atmosphere cannot be probed with transmission spectroscopy. Future direct-imaging efforts with extremely large telescopes may be able to detect its light, which would be a first step towards learning what it is actually made of.
About the 3D impression
Artist's impression — the real surface has never been seen. The globe and the surface you can walk on are generated from what is measured — size, the starlight it receives and the kind of star it orbits — using the same procedural engine as our imaginary worlds. Seas, ice, clouds and any plant-like colours are possibilities, not observations: around a red dwarf the daylight is redder, so the scene uses warmer skies and darker vegetation.
Earth-like exoplanets compared
| Planet | Distance | Radius | Year | Sunlight |
|---|---|---|---|---|
| Proxima Centauri b | 4.2 ly | — | 11.2 days | 0.65× Earth |
| Ross 128 b | 11.0 ly | — | 9.87 days | 1.38× Earth |
| Teegarden's Star b | 12.5 ly | — | 4.91 days | 1.08× Earth |
| Gliese 667 Cc | 23.6 ly | — | 28.1 days | 0.88× Earth |
| TRAPPIST-1d | 40.5 ly | 0.79 R⊕ | 4.05 days | 1.11× Earth |
| TRAPPIST-1e | 40.5 ly | 0.92 R⊕ | 6.10 days | 0.65× Earth |
| TRAPPIST-1f | 40.5 ly | 1.04 R⊕ | 9.21 days | 0.37× Earth |
| TRAPPIST-1g | 40.5 ly | 1.13 R⊕ | 12.4 days | 0.25× Earth |
| LHS 1140 b | 48.9 ly | 1.73 R⊕ | 24.7 days | 0.43× Earth |
| TOI-700 d | 101 ly | 1.07 R⊕ | 37.4 days | 0.87× Earth |
| Kepler-186f | 580 ly | 1.17 R⊕ | 129.9 days | 0.30× Earth |
| Kepler-22b | 635 ly | 2.10 R⊕ | 289.9 days | 1.01× Earth |
| Kepler-62f | 980 ly | 1.41 R⊕ | 267.3 days | 0.41× Earth |
| Kepler-442b | 1,194 ly | 1.34 R⊕ | 112.3 days | 0.70× Earth |
| Kepler-452b | 1,800 ly | 1.63 R⊕ | 384.8 days | 1.10× Earth |
Surprising facts about Gliese 667 Cc
- From Gliese 667 Cc, the two other stars of the system would appear as a very bright pair in the sky, though far fainter than the planet's own sun.
- Its host star is the smallest and faintest of three stars bound together in the Gliese 667 system.
- The planet's year lasts about four Earth weeks.
- Gliese 667 Cc was one of the first planets of only a few Earth masses found in a habitable zone, years before Proxima Centauri b or TRAPPIST-1.