TOI-5817 b
TOI-5817 b is a sub-Neptune orbiting the G2 IV-V star TOI-5817 in the constellation Pegasus. It lies about 263 light-years from Earth and was discovered in 2025 using the transit method.
How Big Is TOI-5817 b?
TOI-5817 b has a radius of 3.08 times that of Earth. Its mass is 10.3 times that of Earth, giving it a density of 1.93 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is TOI-5817 b in the Habitable Zone?
TOI-5817 b orbits inside the inner edge of the habitable zone of TOI-5817. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of TOI-5817: 1.075–2.533 AU (conservative: 1.362–2.402 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on TOI-5817 b
The equilibrium temperature of TOI-5817 b is about 950 K (677 °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.
Orbit and Year Length
A year on TOI-5817 b — one full orbit around TOI-5817 — lasts 15.6 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.122 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.20).
How Was TOI-5817 b Discovered?
TOI-5817 b was discovered in 2025 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-5817 b?
TOI-5817 b is 263.4 light-years (80.8 parsecs) from Earth. Light arriving here tonight left the planet around the year 1763. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 4,635,840 years to make the journey.
The Host Star: TOI-5817
TOI-5817 b belongs to a system of 2 stars; it orbits TOI-5817.
TOI-5817
- Spectral type
- G2 IV-V
- Surface temperature
- 5,770 K (Sun: 5,772 K)
- Mass
- 0.97 M☉
- Radius
- 1.43 R☉
- Luminosity
- 2.0500 L☉
- Age
- 9.7 billion years (Sun: 4.6)
Planetary System
TOI-5817 b is the only planet known to orbit TOI-5817 so far.
TOI-5817 b — Complete Data
| Radius | 3.080 Earth radii (0.275 Jupiter radii) |
|---|---|
| Mass | 10.30 Earth masses (0.032 Jupiter masses) |
| Density | 1.93 g/cm³ (Earth: 5.51) |
| Orbital period | 15.61 days |
| Orbital distance | 0.122 AU |
| Eccentricity | 0.200 |
| Equilibrium temperature | 950 K (677 °C) |
| Distance from Earth | 263.4 light-years (80.8 parsecs) |
| Constellation | Pegasus |
| Discovery method | Transit |
| Discovery facility | Transiting Exoplanet Survey Satellite (TESS) |
| Discovery year | 2025 |
Data: NASA Exoplanet Archive, last updated 2025-08-14. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About TOI-5817 b
Is TOI-5817 b habitable?
No — TOI-5817 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is TOI-5817 b?
TOI-5817 b is about 263 light-years from Earth in the constellation Pegasus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 4,635,840 years to get there.
How big is TOI-5817 b compared to Earth?
TOI-5817 b has 3.08 times the radius of Earth and about 10.3 times its mass.
How long is a year on TOI-5817 b?
One orbit around TOI-5817 takes 15.6 Earth days — short enough that 23 of its years would fit into one Earth year.
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