A super-Earth in an orange dwarf's habitable zone
Kepler-442b
Orbits Kepler-442 · 1,194 light-years · discovered 2015 (transit)
Kepler-442b orbits an orange K-type dwarf star about 1,200 light-years away in the constellation Lyra. Roughly a third larger than Earth, it lies within its star's habitable zone and receives around 70% of the energy Earth gets from the Sun.
Its size is close to the boundary where planets tend to be rocky, and its star is calmer and longer-lived than the red dwarfs that host many habitable-zone candidates. Those factors have made it a frequent entry near the top of habitability rankings. Its mass, atmosphere and surface, however, remain unmeasured.
Key facts
- Star
- Kepler-442K-type orange dwarf
- Distance
- 1,194 light-years
- Radius
- 1.34 × Earth
- Mass
- Not measured
- Orbital period
- 112.3 days
- Equilibrium temperature
- 233 K (-40 °C)Before any greenhouse warming
- Starlight received
- 0.70× Earth's
- Surface gravity
- ≈ 1.64 gRough estimate (rocky composition assumed)
- Discovered
- 2015Transit
- Star
- Kepler-442 (K-type orange dwarf)
- Distance
- About 1,200 light-years
- Discovered
- 2015, transit (Kepler)
- Radius
- About 1.34 Earth radii
- Mass
- Not measured — estimated at roughly 2.3 Earth masses if rocky, with large uncertainty
- Orbital period
- About 112 days
- Starlight received
- Roughly 70% of what Earth gets from the Sun
- Equilibrium temperature
- About 233 K (−40 °C), a simple estimate that ignores any atmosphere
- Constellation
- Lyra
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-442b was identified from repeated transits in Kepler data, which reveal its radius of about 1.34 Earth radii and its 112-day orbit. Its mass has not been measured, so its density and make-up are inferred rather than known.
Planets of this size are more often rocky than not, according to statistical studies of the Kepler sample. A mass estimate of a little over twice Earth's assumes a rocky composition and comes with wide error bars.
Its star
Kepler-442 is an orange dwarf somewhat smaller, cooler and dimmer than the Sun. Stars of this type are often considered good hosts for habitable planets: they live for tens of billions of years, are generally less violently active than red dwarfs, and allow habitable-zone planets to orbit far enough out to avoid the strongest tidal effects.
Could it be habitable?
Kepler-442b receives somewhat less energy than Earth, so with an Earth-like atmosphere it would probably be colder than our planet. A moderately stronger greenhouse effect could bring temperatures into the range where liquid water is stable.
A 2021 study estimating the light available for photosynthesis found that Kepler-442b could, in principle, receive enough usable starlight to support a large biosphere. That is a theoretical calculation, not evidence of life, and it depends on the planet having water and a suitable atmosphere — neither of which has been observed.
Orbit and climate
The planet orbits roughly 0.4 astronomical units from its star — about the distance of Mercury from the Sun — and completes a year in about 112 days. At that distance around an orange dwarf, it may or may not be tidally locked; its rotation and seasons are unknown.
Discovery
Kepler-442b was announced in January 2015 in a study led by Guillermo Torres of the Harvard-Smithsonian Center for Astrophysics. Rather than measuring its mass, the team validated the planet statistically, showing that other explanations for the signal, such as an eclipsing pair of background stars, were extremely unlikely.
Future observations
At around 1,200 light-years, Kepler-442 is too faint for current instruments to measure the planet's mass precisely or study its atmosphere. Kepler-442b is likely to remain a statistical benchmark rather than a target for detailed study, at least until telescopes far more capable than today's are built.
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-442b
- Light from Kepler-442b left the planet more than a thousand years ago, around the early Middle Ages.
- Its orange host star is expected to shine steadily for far longer than the Sun's roughly ten-billion-year lifetime.
- Kepler-442b's year lasts about 112 days — not quite a third of an Earth year.
- It was confirmed without ever measuring its mass, using statistical validation to rule out false alarms.