TOI-257 b
TOI-257 b is a hot Jupiter orbiting TOI-257 in the constellation Horologium. It lies about 251 light-years from Earth and was discovered in 2021 using the transit method.
How Big Is TOI-257 b?
TOI-257 b has a radius of 7.16 times that of Earth, or 0.64 times the radius of Jupiter. Its mass is 43.9 times that of Earth, giving it a density of 0.65 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is TOI-257 b in the Habitable Zone?
TOI-257 b orbits inside the inner edge of the habitable zone of TOI-257. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of TOI-257: 1.532–3.582 AU (conservative: 1.940–3.396 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on TOI-257 b
The equilibrium temperature of TOI-257 b is about 1,027 K (754 °C) — hot enough to melt many metals. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C. It receives 186 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on TOI-257 b — one full orbit around TOI-257 — lasts 18.4 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.153 AU — closer to its star than Mercury is to the Sun.
How Was TOI-257 b Discovered?
TOI-257 b was discovered in 2021 using the transit method, with observations from Transiting Exoplanet Survey Satellite (TESS).
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 TOI-257 b?
TOI-257 b is 250.8 light-years (76.9 parsecs) from Earth. Light arriving here tonight left the planet around the year 1776. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 4,414,080 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. TOI-257 b scores 0.12, ranking #4,332 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: TOI-257
TOI-257
- Surface temperature
- 6,095 K (Sun: 5,772 K)
- Mass
- 1.41 M☉
- Radius
- 1.87 R☉
- Luminosity
- 4.3300 L☉
Planetary System
TOI-257 b is the only planet known to orbit TOI-257 so far.
TOI-257 b — Complete Data
| Radius | 7.163 Earth radii (0.639 Jupiter radii) |
|---|---|
| Mass | 43.86 Earth masses (0.138 Jupiter masses) |
| Density | 0.65 g/cm³ (Earth: 5.51) |
| Orbital period | 18.39 days |
| Orbital distance | 0.153 AU |
| Equilibrium temperature | 1,027 K (754 °C) |
| Stellar irradiation | 186.00× Earth |
| Earth Similarity Index | 0.12 |
| Distance from Earth | 250.8 light-years (76.9 parsecs) |
| Constellation | Horologium |
| Discovery method | Transit |
| Discovery facility | Transiting Exoplanet Survey Satellite (TESS) |
| Discovery year | 2021 |
Data: NASA Exoplanet Archive, last updated 2021-04-23. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About TOI-257 b
Is TOI-257 b habitable?
No — TOI-257 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is TOI-257 b?
TOI-257 b is about 251 light-years from Earth in the constellation Horologium. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 4,414,080 years to get there.
How big is TOI-257 b compared to Earth?
TOI-257 b has 7.16 times the radius of Earth and about 43.9 times its mass.
How long is a year on TOI-257 b?
One orbit around TOI-257 takes 18.4 Earth days — short enough that 20 of its years would fit into one Earth year.
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