HD 28109 b
HD 28109 b is a mini-Neptune orbiting the F8 star HD 28109 in the constellation Hydrus. It lies about 455 light-years from Earth and was discovered in 2022 using the transit method.
How Big Is HD 28109 b?
HD 28109 b has a radius of 2.69 times that of Earth. Its mass is 6.2 times that of Earth, giving it a density of 1.80 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is HD 28109 b in the Habitable Zone?
The position of HD 28109 b relative to the habitable zone of HD 28109 cannot be precisely determined from the available orbital data.
Habitable zone of HD 28109: 1.205–2.812 AU (conservative: 1.526–2.666 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on HD 28109 b
The equilibrium temperature of HD 28109 b is about 867 K (594 °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 28109 b — one full orbit around HD 28109 — lasts 22.9 Earth days, shorter than Mercury's 88-day year. Its orbit is nearly circular (eccentricity 0.007).
How Was HD 28109 b Discovered?
HD 28109 b was discovered in 2022 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 28109 b?
HD 28109 b is 455.3 light-years (139.6 parsecs) from Earth. Light arriving here tonight left the planet around the year 1571. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 8,013,280 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 28109 b scores 0.25, ranking #3,038 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: HD 28109
HD 28109
- Spectral type
- F8
- Surface temperature
- 6,189 K (Sun: 5,772 K)
- Mass
- 1.23 M☉
- Radius
- 1.43 R☉
- Luminosity
- 2.7100 L☉
- Age
- 3.2 billion years (Sun: 4.6)
The HD 28109 Planetary System
HD 28109 b is one of 3 known planets in the HD 28109 system. Its siblings:
- HD 28109 c (Neptune-like)
- HD 28109 d (Puffy Planet)
HD 28109 b — Complete Data
| Radius | 2.690 Earth radii (0.240 Jupiter radii) |
|---|---|
| Mass | 6.20 Earth masses (0.020 Jupiter masses) |
| Density | 1.80 g/cm³ (Earth: 5.51) |
| Orbital period | 22.89 days |
| Eccentricity | 0.007 |
| Equilibrium temperature | 867 K (594 °C) |
| Earth Similarity Index | 0.25 |
| Distance from Earth | 455.3 light-years (139.6 parsecs) |
| Constellation | Hydrus |
| Discovery method | Transit |
| Discovery facility | Transiting Exoplanet Survey Satellite (TESS) |
| Discovery year | 2022 |
Data: NASA Exoplanet Archive, last updated 2025-12-04. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HD 28109 b
Is HD 28109 b habitable?
HD 28109 b is not known to orbit within the habitable zone of HD 28109, and as a mini-Neptune it is an unlikely candidate for life as we know it.
How far away is HD 28109 b?
HD 28109 b is about 455 light-years from Earth in the constellation Hydrus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 8,013,280 years to get there.
How big is HD 28109 b compared to Earth?
HD 28109 b has 2.69 times the radius of Earth and about 6.2 times its mass.
How long is a year on HD 28109 b?
One orbit around HD 28109 takes 22.9 Earth days — short enough that 16 of its years would fit into one Earth year.
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