HD 5278 b
HD 5278 b is a mini-Neptune orbiting the F V star HD 5278 in the constellation Octans. It lies about 188 light-years from Earth and was discovered in 2021 using the transit method.
How Big Is HD 5278 b?
HD 5278 b has a radius of 2.45 times that of Earth. Its mass is 7.8 times that of Earth, giving it a density of 2.90 g/cm³ — between that of rocky and gaseous planets.
Is HD 5278 b in the Habitable Zone?
HD 5278 b orbits inside the inner edge of the habitable zone of HD 5278. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of HD 5278: 1.010–2.356 AU (conservative: 1.279–2.234 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on HD 5278 b
The equilibrium temperature of HD 5278 b is about 943 K (670 °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 132 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on HD 5278 b — one full orbit around HD 5278 — lasts 14.3 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.120 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.08).
How Was HD 5278 b Discovered?
HD 5278 b was discovered in 2021 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 5278 b?
HD 5278 b is 187.8 light-years (57.6 parsecs) from Earth. Light arriving here tonight left the planet around the year 1839. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 3,305,280 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. HD 5278 b scores 0.24, ranking #3,257 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: HD 5278
HD 5278
- Spectral type
- F V
- Surface temperature
- 6,203 K (Sun: 5,772 K)
- Mass
- 1.13 M☉
- Radius
- 1.19 R☉
- Luminosity
- 1.9070 L☉
- Age
- 3.0 billion years (Sun: 4.6)
The HD 5278 Planetary System
HD 5278 b is one of 2 known planets in the HD 5278 system. Its siblings:
- HD 5278 c (Neptune-like)
HD 5278 b — Complete Data
| Radius | 2.450 Earth radii (0.219 Jupiter radii) |
|---|---|
| Mass | 7.80 Earth masses (0.025 Jupiter masses) |
| Density | 2.90 g/cm³ (Earth: 5.51) |
| Orbital period | 14.34 days |
| Orbital distance | 0.120 AU |
| Eccentricity | 0.080 |
| Equilibrium temperature | 943 K (670 °C) |
| Stellar irradiation | 132.00× Earth |
| Earth Similarity Index | 0.24 |
| Distance from Earth | 187.8 light-years (57.6 parsecs) |
| Constellation | Octans |
| Discovery method | Transit |
| Discovery facility | Transiting Exoplanet Survey Satellite (TESS) |
| Discovery year | 2021 |
Data: NASA Exoplanet Archive, last updated 2021-04-23. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HD 5278 b
Is HD 5278 b habitable?
No — HD 5278 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is HD 5278 b?
HD 5278 b is about 188 light-years from Earth in the constellation Octans. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 3,305,280 years to get there.
How big is HD 5278 b compared to Earth?
HD 5278 b has 2.45 times the radius of Earth and about 7.8 times its mass.
How long is a year on HD 5278 b?
One orbit around HD 5278 takes 14.3 Earth days — short enough that 25 of its years would fit into one Earth year.
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