Kepler-257 d
Kepler-257 d is a Neptune-like planet orbiting Kepler-257 in the constellation Cygnus. It lies about 2,545 light-years from Earth and was discovered in 2014 using the transit method.
How Big Is Kepler-257 d?
Kepler-257 d has a radius of 4.95 times that of Earth.
Is Kepler-257 d in the Habitable Zone?
Kepler-257 d orbits inside the inner edge of the habitable zone of Kepler-257. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of Kepler-257: 0.647–1.554 AU (conservative: 0.820–1.473 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 ›
Orbit and Year Length
A year on Kepler-257 d — one full orbit around Kepler-257 — lasts 24.7 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.160 AU — closer to its star than Mercury is to the Sun.
How Was Kepler-257 d Discovered?
Kepler-257 d 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 Kepler-257 d?
Kepler-257 d is 2,544.9 light-years (780.3 parsecs) from Earth. Light arriving here tonight left the planet about 2,545 years ago. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 44,790,240 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. Kepler-257 d scores 0.22, ranking #3,683 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: Kepler-257
Kepler-257
- Surface temperature
- 5,180 K (Sun: 5,772 K)
- Radius
- 1.04 R☉
The Kepler-257 Planetary System
Kepler-257 d is one of 3 known planets in the Kepler-257 system. Its siblings:
- Kepler-257 b (Mini Neptune)
- Kepler-257 c (Neptune-like)
Kepler-257 d — Complete Data
| Radius | 4.950 Earth radii (0.442 Jupiter radii) |
|---|---|
| Orbital period | 24.66 days |
| Orbital distance | 0.160 AU |
| Earth Similarity Index | 0.22 |
| Distance from Earth | 2,544.9 light-years (780.3 parsecs) |
| Constellation | Cygnus |
| Discovery method | Transit |
| Discovery facility | Kepler |
| Discovery year | 2014 |
Data: NASA Exoplanet Archive, last updated 2014-05-14. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About Kepler-257 d
Is Kepler-257 d habitable?
No — Kepler-257 d orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is Kepler-257 d?
Kepler-257 d is about 2,545 light-years from Earth in the constellation Cygnus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 44,790,240 years to get there.
How big is Kepler-257 d compared to Earth?
Kepler-257 d has 4.95 times the radius of Earth.
How long is a year on Kepler-257 d?
One orbit around Kepler-257 takes 24.7 Earth days — short enough that 15 of its years would fit into one Earth year.
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