HD 77946 b
HD 77946 b is a mini-Neptune orbiting HD 77946 in the constellation Ursa Major. It lies about 324 light-years from Earth and was discovered in 2024 using the transit method.
How Big Is HD 77946 b?
HD 77946 b has a radius of 2.90 times that of Earth. Its mass is 10.9 times that of Earth, giving it a density of 2.44 g/cm³ — between that of rocky and gaseous planets.
Is HD 77946 b in the Habitable Zone?
HD 77946 b orbits inside the inner edge of the habitable zone of HD 77946. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of HD 77946: 1.056–2.474 AU (conservative: 1.338–2.346 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on HD 77946 b
The equilibrium temperature of HD 77946 b is about 1,129 K (856 °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 77946 b — one full orbit around HD 77946 — lasts 6.53 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.073 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.21).
How Was HD 77946 b Discovered?
HD 77946 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 77946 b?
HD 77946 b is 324.4 light-years (99.5 parsecs) from Earth. Light arriving here tonight left the planet around the year 1702. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 5,709,440 years to make the journey.
The Host Star: HD 77946
HD 77946
- Surface temperature
- 6,007 K (Sun: 5,772 K)
- Mass
- 1.20 M☉
- Radius
- 1.32 R☉
- Luminosity
- 2.0360 L☉
- Age
- 3.1 billion years (Sun: 4.6)
Planetary System
HD 77946 b is the only planet known to orbit HD 77946 so far.
HD 77946 b — Complete Data
| Radius | 2.896 Earth radii (0.258 Jupiter radii) |
|---|---|
| Mass | 10.90 Earth masses (0.034 Jupiter masses) |
| Density | 2.44 g/cm³ (Earth: 5.51) |
| Orbital period | 6.53 days |
| Orbital distance | 0.073 AU |
| Eccentricity | 0.211 |
| Equilibrium temperature | 1,129 K (856 °C) |
| Distance from Earth | 324.4 light-years (99.5 parsecs) |
| Constellation | Ursa Major |
| Discovery method | Transit |
| Discovery facility | Transiting Exoplanet Survey Satellite (TESS) |
| Discovery year | 2024 |
Data: NASA Exoplanet Archive, last updated 2024-07-23. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HD 77946 b
Is HD 77946 b habitable?
No — HD 77946 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is HD 77946 b?
HD 77946 b is about 324 light-years from Earth in the constellation Ursa Major. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 5,709,440 years to get there.
How big is HD 77946 b compared to Earth?
HD 77946 b has 2.90 times the radius of Earth and about 10.9 times its mass.
How long is a year on HD 77946 b?
One orbit around HD 77946 takes 6.5 Earth days — short enough that 56 of its years would fit into one Earth year.
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