TOI-521 b
TOI-521 b is a super-Earth orbiting the M3 star TOI-521 in the constellation Cancer. It lies about 199 light-years from Earth and was discovered in 2025 using the transit method.
How Big Is TOI-521 b?
TOI-521 b has a radius of 1.98 times that of Earth. Its mass is 5.3 times that of Earth, giving it a density of 3.80 g/cm³ — between that of rocky and gaseous planets.
Is TOI-521 b in the Habitable Zone?
TOI-521 b orbits inside the inner edge of the habitable zone of TOI-521. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of TOI-521: 0.130–0.336 AU (conservative: 0.164–0.319 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on TOI-521 b
The equilibrium temperature of TOI-521 b is about 794 K (521 °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 66.00 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on TOI-521 b — one full orbit around TOI-521 — lasts just 37.0 hours, shorter than Mercury's 88-day year. It orbits at an average distance of 0.020 AU — closer to its star than Mercury is to the Sun.
How Was TOI-521 b Discovered?
TOI-521 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-521 b?
TOI-521 b is 198.8 light-years (60.9 parsecs) from Earth. Light arriving here tonight left the planet around the year 1828. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 3,498,880 years to make the journey.
The Host Star: TOI-521
TOI-521
- Spectral type
- M3
- Surface temperature
- 3,544 K (Sun: 5,772 K)
- Mass
- 0.42 M☉
- Radius
- 0.42 R☉
- Luminosity
- 0.0252 L☉
- Age
- 0.8 billion years (Sun: 4.6)
Planetary System
TOI-521 b is the only planet known to orbit TOI-521 so far.
TOI-521 b — Complete Data
| Radius | 1.980 Earth radii (0.177 Jupiter radii) |
|---|---|
| Mass | 5.30 Earth masses (0.017 Jupiter masses) |
| Density | 3.80 g/cm³ (Earth: 5.51) |
| Orbital period | 1.54 days |
| Orbital distance | 0.020 AU |
| Equilibrium temperature | 794 K (521 °C) |
| Stellar irradiation | 66.00× Earth |
| Distance from Earth | 198.8 light-years (60.9 parsecs) |
| Constellation | Cancer |
| Discovery method | Transit |
| Discovery facility | Transiting Exoplanet Survey Satellite (TESS) |
| Discovery year | 2025 |
Data: NASA Exoplanet Archive, last updated 2025-12-11. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About TOI-521 b
Is TOI-521 b habitable?
No — TOI-521 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is TOI-521 b?
TOI-521 b is about 199 light-years from Earth in the constellation Cancer. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 3,498,880 years to get there.
How big is TOI-521 b compared to Earth?
TOI-521 b has 1.98 times the radius of Earth and about 5.3 times its mass.
How long is a year on TOI-521 b?
One orbit around TOI-521 takes 1.5 Earth days — short enough that 237 of its years would fit into one Earth year.
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