KOI-1257 b
KOI-1257 b is a cold gas giant orbiting the G5 V star KOI-1257 in the constellation Cygnus. It lies about 2,140 light-years from Earth and was discovered in 2014 using the transit method.
How Big Is KOI-1257 b?
KOI-1257 b has a radius of 10.54 times that of Earth, or 0.94 times the radius of Jupiter. Its mass is 461 times that of Earth, giving it a density of 2.10 g/cm³ — between that of rocky and gaseous planets.
Is KOI-1257 b in the Habitable Zone?
KOI-1257 b orbits inside the inner edge of the habitable zone of KOI-1257. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of KOI-1257: 0.786–1.865 AU (conservative: 0.995–1.768 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 ›
Temperature on KOI-1257 b
The equilibrium temperature of KOI-1257 b is about 511 K (238 °C) — hotter than anywhere on Earth. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C.
Orbit and Year Length
A year on KOI-1257 b — one full orbit around KOI-1257 — lasts 86.6 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.382 AU — closer to its star than Mercury is to the Sun. Its orbit is highly elliptical (eccentricity 0.77), swinging dramatically closer to and farther from its star.
How Was KOI-1257 b Discovered?
KOI-1257 b was discovered in 2014 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 KOI-1257 b?
KOI-1257 b is 2,139.6 light-years (656.0 parsecs) from Earth. Light arriving here tonight left the planet about 2,140 years ago. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 37,656,960 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. KOI-1257 b scores 0.17, ranking #4,109 of 5,565 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: KOI-1257
KOI-1257 b belongs to a system of 2 stars; it orbits KOI-1257.
KOI-1257
- Spectral type
- G5 V
- Surface temperature
- 5,520 K (Sun: 5,772 K)
- Mass
- 0.99 M☉
- Radius
- 1.13 R☉
- Age
- 9.3 billion years (Sun: 4.6)
Planetary System
KOI-1257 b is the only planet known to orbit KOI-1257 so far.
KOI-1257 b — Complete Data
| Radius | 10.540 Earth radii (0.940 Jupiter radii) |
|---|---|
| Mass | 460.83 Earth masses (1.450 Jupiter masses) |
| Density | 2.10 g/cm³ (Earth: 5.51) |
| Orbital period | 86.65 days |
| Orbital distance | 0.382 AU |
| Eccentricity | 0.772 |
| Equilibrium temperature | 511 K (238 °C) |
| Earth Similarity Index | 0.17 |
| Distance from Earth | 2,139.6 light-years (656.0 parsecs) |
| Constellation | Cygnus |
| Discovery method | Transit |
| Discovery facility | Kepler |
| Discovery year | 2014 |
Data: NASA Exoplanet Archive, last updated 2014-06-25. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About KOI-1257 b
Is KOI-1257 b habitable?
No — KOI-1257 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is KOI-1257 b?
KOI-1257 b is about 2,140 light-years from Earth in the constellation Cygnus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 37,656,960 years to get there.
How big is KOI-1257 b compared to Earth?
KOI-1257 b has 10.54 times the radius of Earth and about 461 times its mass.
How long is a year on KOI-1257 b?
One orbit around KOI-1257 takes 86.6 Earth days.
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