HD 21520 b
HD 21520 b is a puffy low-density planet orbiting HD 21520 in the constellation Eridanus. It lies about 258 light-years from Earth and was discovered in 2024 using the transit method.
How Big Is HD 21520 b?
HD 21520 b has a radius of 2.70 times that of Earth. Its mass is 17.7 times that of Earth, giving it a density of 1.35 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is HD 21520 b in the Habitable Zone?
HD 21520 b orbits inside the inner edge of the habitable zone of HD 21520. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of HD 21520: 0.805–1.892 AU (conservative: 1.020–1.794 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on HD 21520 b
The equilibrium temperature of HD 21520 b is about 637 K (364 °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 HD 21520 b — one full orbit around HD 21520 — lasts 25.1 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.173 AU — closer to its star than Mercury is to the Sun.
How Was HD 21520 b Discovered?
HD 21520 b was discovered in 2024 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 HD 21520 b?
HD 21520 b is 258.3 light-years (79.2 parsecs) from Earth. Light arriving here tonight left the planet around the year 1768. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 4,546,080 years to make the journey.
The Host Star: HD 21520
HD 21520
- Surface temperature
- 5,871 K (Sun: 5,772 K)
- Mass
- 1.09 M☉
- Radius
- 1.04 R☉
- Luminosity
- 1.1640 L☉
- Age
- 5.3 billion years (Sun: 4.6)
Planetary System
HD 21520 b is the only planet known to orbit HD 21520 so far.
HD 21520 b — Complete Data
| Radius | 2.697 Earth radii (0.241 Jupiter radii) |
|---|---|
| Mass | 17.70 Earth masses (0.056 Jupiter masses) |
| Density | 1.35 g/cm³ (Earth: 5.51) |
| Orbital period | 25.13 days |
| Orbital distance | 0.173 AU |
| Equilibrium temperature | 637 K (364 °C) |
| Distance from Earth | 258.3 light-years (79.2 parsecs) |
| Constellation | Eridanus |
| Discovery method | Transit |
| Discovery facility | Transiting Exoplanet Survey Satellite (TESS) |
| Discovery year | 2024 |
Data: NASA Exoplanet Archive, last updated 2024-09-13. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HD 21520 b
Is HD 21520 b habitable?
No — HD 21520 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is HD 21520 b?
HD 21520 b is about 258 light-years from Earth in the constellation Eridanus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 4,546,080 years to get there.
How big is HD 21520 b compared to Earth?
HD 21520 b has 2.70 times the radius of Earth and about 17.7 times its mass.
How long is a year on HD 21520 b?
One orbit around HD 21520 takes 25.1 Earth days — short enough that 15 of its years would fit into one Earth year.
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