KOI-1257 b
KOI-1257 b is a cold gas giant orbiting the G5 V star KOI-1257 in the constellation Cygnus. It 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.
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 5,568 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 |
| 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 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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