HD 191939 c
HD 191939 c is a puffy low-density planet orbiting the G9 V star HD 191939 in the constellation Draco. It lies about 175 light-years from Earth and was discovered in 2020 using the transit method.
How Big Is HD 191939 c?
HD 191939 c has a radius of 3.20 times that of Earth. Its mass is 8.0 times that of Earth, giving it a density of 1.35 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is HD 191939 c in the Habitable Zone?
HD 191939 c orbits inside the inner edge of the habitable zone of HD 191939. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of HD 191939: 0.620–1.481 AU (conservative: 0.786–1.404 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on HD 191939 c
The equilibrium temperature of HD 191939 c is about 600 K (327 °C) — hotter than anywhere on Earth. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C. It receives 21.00 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on HD 191939 c — one full orbit around HD 191939 — lasts 28.6 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.175 AU — closer to its star than Mercury is to the Sun. Its orbit is nearly circular (eccentricity 0.034).
How Was HD 191939 c Discovered?
HD 191939 c was discovered in 2020 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 191939 c?
HD 191939 c is 174.8 light-years (53.6 parsecs) from Earth. Light arriving here tonight left the planet around the year 1852. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 3,076,480 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 191939 c scores 0.26, ranking #2,922 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: HD 191939
HD 191939
- Spectral type
- G9 V
- Surface temperature
- 5,348 K (Sun: 5,772 K)
- Mass
- 0.81 M☉
- Radius
- 0.94 R☉
- Luminosity
- 0.6500 L☉
- Age
- 8.7 billion years (Sun: 4.6)
The HD 191939 Planetary System
HD 191939 c is one of 6 known planets in the HD 191939 system. Its siblings:
- HD 191939 b (Puffy Planet)
- HD 191939 d (Puffy Planet)
- HD 191939 e (Cold Gas Giant)
- HD 191939 f (Cold Gas Giant)
- HD 191939 g (Neptune-like)
HD 191939 c — Complete Data
| Radius | 3.195 Earth radii (0.285 Jupiter radii) |
|---|---|
| Mass | 8.00 Earth masses (0.025 Jupiter masses) |
| Density | 1.35 g/cm³ (Earth: 5.51) |
| Orbital period | 28.58 days |
| Orbital distance | 0.175 AU |
| Eccentricity | 0.034 |
| Equilibrium temperature | 600 K (327 °C) |
| Stellar irradiation | 21.00× Earth |
| Earth Similarity Index | 0.26 |
| Distance from Earth | 174.8 light-years (53.6 parsecs) |
| Constellation | Draco |
| Discovery method | Transit |
| Discovery facility | Transiting Exoplanet Survey Satellite (TESS) |
| Discovery year | 2020 |
Data: NASA Exoplanet Archive, last updated 2022-11-14. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HD 191939 c
Is HD 191939 c habitable?
No — HD 191939 c orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is HD 191939 c?
HD 191939 c is about 175 light-years from Earth in the constellation Draco. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 3,076,480 years to get there.
How big is HD 191939 c compared to Earth?
HD 191939 c has 3.20 times the radius of Earth and about 8.0 times its mass.
How long is a year on HD 191939 c?
One orbit around HD 191939 takes 28.6 Earth days — short enough that 13 of its years would fit into one Earth year.
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