HD 191939 g
HD 191939 g is a Neptune-like planet orbiting the G9 V star HD 191939 in the constellation Draco. It lies about 175 light-years from Earth and was discovered in 2022 using the radial velocity method. It orbits within the habitable zone of its star — the region where liquid water could exist on a planet's surface.
Is HD 191939 g in the Habitable Zone?
HD 191939 g orbits inside the conservative habitable zone of HD 191939 — the region where a rocky planet could sustain liquid water on its surface. This makes it one of the most interesting known exoplanets in the search for life.
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.
See the full interactive habitable-zone view in the Exoplanet Explorer app ›
Temperature on HD 191939 g
The equilibrium temperature of HD 191939 g is about 278 K (5 °C) — in a range broadly comparable to Earth, whose equilibrium temperature is 255 K. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C. It receives 0.99 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on HD 191939 g — one full orbit around HD 191939 — lasts 284.0 Earth days, between the years of Mercury (88 days) and Earth (365 days). It orbits at an average distance of 0.812 AU — comparable to the inner Solar System. Its orbit is nearly circular (eccentricity 0.030).
How Was HD 191939 g Discovered?
HD 191939 g was discovered in 2022 using the radial velocity method, with observations from Multiple Observatories.
The radial velocity method measures the subtle wobble of a star caused by the gravitational pull of an orbiting planet, visible as periodic shifts in the star's light spectrum. The size of the wobble reveals the planet's minimum mass.
How Far Away Is HD 191939 g?
HD 191939 g 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 g scores 0.59, ranking #145 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 g is one of 6 known planets in the HD 191939 system. Its siblings:
- HD 191939 b (Puffy Planet)
- HD 191939 c (Puffy Planet)
- HD 191939 d (Puffy Planet)
- HD 191939 e (Cold Gas Giant)
- HD 191939 f (Cold Gas Giant)
HD 191939 g — Complete Data
| Mass (best estimate) | 13.50 Earth masses |
|---|---|
| Orbital period | 284.00 days |
| Orbital distance | 0.812 AU |
| Eccentricity | 0.030 |
| Equilibrium temperature | 278 K (5 °C) |
| Stellar irradiation | 0.99× Earth |
| Earth Similarity Index | 0.59 |
| Distance from Earth | 174.8 light-years (53.6 parsecs) |
| Constellation | Draco |
| Discovery method | Radial Velocity |
| Discovery facility | Multiple Observatories |
| Discovery year | 2022 |
Data: NASA Exoplanet Archive, last updated 2022-11-14. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HD 191939 g
Is HD 191939 g habitable?
HD 191939 g orbits within the habitable zone of HD 191939, the region where liquid water could exist on a planet's surface. It sits in the conservative habitable zone — the most promising region for habitability. Whether it is actually habitable depends on its atmosphere and composition, which remain unknown.
How far away is HD 191939 g?
HD 191939 g 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 long is a year on HD 191939 g?
One orbit around HD 191939 takes 284.0 Earth days.
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