HR 858 b
HR 858 b is a mini-Neptune orbiting HR 858 in the constellation Fornax. It lies about 104 light-years from Earth and was discovered in 2019 using the transit method.
How Big Is HR 858 b?
HR 858 b has a radius of 2.00 times that of Earth. Its mass is 3.6 times that of Earth, giving it a density of 2.44 g/cm³ — between that of rocky and gaseous planets.
Is HR 858 b in the Habitable Zone?
HR 858 b orbits inside the inner edge of the habitable zone of HR 858. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of HR 858: 1.101–2.563 AU (conservative: 1.395–2.430 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on HR 858 b
The equilibrium temperature of HR 858 b is about 1,552 K (1,279 °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 HR 858 b — one full orbit around HR 858 — lasts 3.59 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.049 AU — closer to its star than Mercury is to the Sun.
How Was HR 858 b Discovered?
HR 858 b was discovered in 2019 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 HR 858 b?
HR 858 b is 104.3 light-years (32.0 parsecs) from Earth. Light arriving here tonight left the planet around the year 1922. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 1,835,680 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. HR 858 b scores 0.25, ranking #3,060 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: HR 858
HR 858 b belongs to a system of 2 stars; it orbits HR 858.
HR 858
- Surface temperature
- 6,354 K (Sun: 5,772 K)
- Mass
- 1.20 M☉
- Radius
- 1.26 R☉
- Luminosity
- 2.3100 L☉
- Age
- 2.0 billion years (Sun: 4.6)
The HR 858 Planetary System
HR 858 b is one of 3 known planets in the HR 858 system. Its siblings:
HR 858 b — Complete Data
| Radius | 2.004 Earth radii (0.179 Jupiter radii) |
|---|---|
| Mass | 3.55 Earth masses (0.011 Jupiter masses) |
| Density | 2.44 g/cm³ (Earth: 5.51) |
| Orbital period | 3.59 days |
| Orbital distance | 0.049 AU |
| Equilibrium temperature | 1,552 K (1,279 °C) |
| Earth Similarity Index | 0.25 |
| Distance from Earth | 104.3 light-years (32.0 parsecs) |
| Constellation | Fornax |
| Discovery method | Transit |
| Discovery facility | Transiting Exoplanet Survey Satellite (TESS) |
| Discovery year | 2019 |
Data: NASA Exoplanet Archive, last updated 2025-03-03. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HR 858 b
Is HR 858 b habitable?
No — HR 858 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is HR 858 b?
HR 858 b is about 104 light-years from Earth in the constellation Fornax. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 1,835,680 years to get there.
How big is HR 858 b compared to Earth?
HR 858 b has 2.00 times the radius of Earth and about 3.6 times its mass.
How long is a year on HR 858 b?
One orbit around HR 858 takes 3.6 Earth days — short enough that 102 of its years would fit into one Earth year.
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