HD 278555 b
HD 278555 b is a cold gas giant orbiting the F8 star HD 278555 in the constellation Perseus. It lies about 945 light-years from Earth and was discovered in 2026 using the transit method.
How Big Is HD 278555 b?
HD 278555 b has a radius of 11.03 times that of Earth, or 0.98 times the radius of Jupiter. Its mass is 267 times that of Earth, giving it a density of 1.09 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is HD 278555 b in the Habitable Zone?
HD 278555 b orbits inside the inner edge of the habitable zone of HD 278555. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of HD 278555: 1.306–3.057 AU (conservative: 1.654–2.899 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on HD 278555 b
The equilibrium temperature of HD 278555 b is about 853 K (580 °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 278555 b — one full orbit around HD 278555 — lasts 28.8 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.199 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.09).
How Was HD 278555 b Discovered?
HD 278555 b was discovered in 2026 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 278555 b?
HD 278555 b is 945.3 light-years (289.8 parsecs) from Earth. Light arriving here tonight left the planet around the year 1081. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 16,637,280 years to make the journey.
The Host Star: HD 278555
HD 278555
- Spectral type
- F8
- Surface temperature
- 6,047 K (Sun: 5,772 K)
- Mass
- 1.25 M☉
- Radius
- 1.62 R☉
- Luminosity
- 3.1300 L☉
- Age
- 3.8 billion years (Sun: 4.6)
Planetary System
HD 278555 b is the only planet known to orbit HD 278555 so far.
HD 278555 b — Complete Data
| Radius | 11.030 Earth radii (0.984 Jupiter radii) |
|---|---|
| Mass | 266.98 Earth masses (0.840 Jupiter masses) |
| Density | 1.09 g/cm³ (Earth: 5.51) |
| Orbital period | 28.84 days |
| Orbital distance | 0.199 AU |
| Eccentricity | 0.085 |
| Equilibrium temperature | 853 K (580 °C) |
| Distance from Earth | 945.3 light-years (289.8 parsecs) |
| Constellation | Perseus |
| Discovery method | Transit |
| Discovery facility | Transiting Exoplanet Survey Satellite (TESS) |
| Discovery year | 2026 |
Data: NASA Exoplanet Archive, last updated 2026-09-11. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HD 278555 b
Is HD 278555 b habitable?
No — HD 278555 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is HD 278555 b?
HD 278555 b is about 945 light-years from Earth in the constellation Perseus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 16,637,280 years to get there.
How big is HD 278555 b compared to Earth?
HD 278555 b has 11.03 times the radius of Earth and about 267 times its mass.
How long is a year on HD 278555 b?
One orbit around HD 278555 takes 28.8 Earth days — short enough that 13 of its years would fit into one Earth year.
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