Kepler's first habitable-zone planet around a Sun-like star
Kepler-22b
Orbits Kepler-22 · 635 light-years · discovered 2011 (transit)
Kepler-22b orbits a star a little smaller and cooler than the Sun, about 640 light-years away in the constellation Cygnus. Announced in December 2011, it was the first planet NASA's Kepler mission confirmed in the habitable zone of a Sun-like star, and it caught the public imagination as a hint of other potentially habitable worlds.
It is not an Earth-sized planet, though. At about 2.1 times Earth's width, it is more likely to be a water-rich world or a small gas-enveloped 'mini-Neptune' than a rocky planet like ours. Its mass is only loosely constrained, and nothing is known about its atmosphere.
Key facts
- Star
- Kepler-22G5V yellow dwarf
- Distance
- 635 light-years
- Radius
- 2.10 × Earth
- Mass
- Not measured
- Orbital period
- 289.9 days
- Equilibrium temperature
- 279 K (6 °C)Before any greenhouse warming
- Starlight received
- 1.01× Earth's
- Surface gravity
- ≈ 1.61 gRough estimate (rocky composition assumed)
- Discovered
- 2011Transit
- Star
- Kepler-22 (G5V, Sun-like)
- Distance
- About 640 light-years
- Discovered
- 2011, transit (Kepler)
- Radius
- About 2.1 Earth radii
- Mass
- Not measured — upper limit of roughly 9 Earth masses
- Orbital period
- About 290 days
- Starlight received
- Roughly the same as Earth gets from the Sun
- Equilibrium temperature
- About 279 K (6 °C), a simple estimate that ignores reflection and any atmosphere
- Constellation
- Cygnus
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
Kepler-22b was found from three transits recorded by Kepler, which reveal a planet about 2.1 times Earth's radius on a 290-day orbit. Early estimates put it closer to 2.4 Earth radii; refined measurements of the star brought that down.
Attempts to measure its mass from the star's wobble have only set an upper limit, most recently around 9 Earth masses. Without a firm mass, its density and composition remain uncertain.
Its star
Kepler-22 is a G-type star like the Sun, but slightly less massive and noticeably less luminous — roughly three-quarters of the Sun's brightness. Because it is so similar to the Sun, its habitable zone lies at a comparable distance, and its planets do not face the intense flares typical of red dwarfs.
Could it be habitable?
Kepler-22b receives roughly as much energy from its star as Earth does from the Sun, so it orbits within the habitable zone. Location alone is not enough, however. Statistical studies of Kepler planets show that most planets larger than about 1.6–1.8 Earth radii have substantial envelopes of gas or water rather than bare rocky surfaces.
If Kepler-22b is a mini-Neptune, a thick hydrogen-rich atmosphere would make its deeper layers far too hot and pressurised for life as we know it. If it is a water world, it might have a deep global ocean. Neither scenario has been confirmed.
Orbit and climate
Kepler-22b orbits about 15% closer to its star than Earth is to the Sun, completing a year in about 290 days. Its orbit is wide enough that it is probably not tidally locked, so it could have days and nights, though its rotation, tilt and seasons are unknown.
Discovery
Kepler's first transit of the planet was recorded just days after the spacecraft began science operations in 2009. A third transit, needed to confirm the orbit, came in late 2010. A team led by William Borucki announced the confirmed planet in December 2011, after ground-based follow-up observations helped rule out other explanations for the signal.
Future observations
Its long orbit means Kepler-22b transits less than once a year, which makes follow-up difficult. More precise radial velocity measurements could pin down its mass and reveal whether it is rocky, watery or gas-rich, which would be the key step before any atmospheric study.
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 Kepler-22b
- Kepler-22b's year lasts about 290 days — closer to an Earth year than most known habitable-zone planets.
- Its host star is a Sun-like G-type star, unlike the red dwarfs that host most of the potentially habitable planets on this list.
- Kepler had to wait more than a year between the first and third transits to confirm the planet's orbit.
- Despite often being described as a 'super-Earth', it may have more in common with a small Neptune than with Earth.