HD 181720 b
HD 181720 b is a cold gas giant orbiting HD 181720 in the constellation Sagittarius. It lies about 196 light-years from Earth and was discovered in 2009 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 181720 b in the Habitable Zone?
HD 181720 b orbits inside the conservative habitable zone of HD 181720 — 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 181720: 1.112–2.623 AU (conservative: 1.409–2.487 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 ›
Orbit and Year Length
A year on HD 181720 b — one full orbit around HD 181720 — lasts 956.0 Earth days, longer than an Earth year. It orbits at an average distance of 1.850 AU. Its orbit is mildly elliptical (eccentricity 0.26).
How Was HD 181720 b Discovered?
HD 181720 b was discovered in 2009 using the radial velocity method, with observations from La Silla Observatory.
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 181720 b?
HD 181720 b is 196.4 light-years (60.2 parsecs) from Earth. Light arriving here tonight left the planet around the year 1830. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 3,456,640 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 181720 b scores 0.36, ranking #699 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: HD 181720
HD 181720
- Surface temperature
- 5,736 K (Sun: 5,772 K)
- Mass
- 1.03 M☉
- Radius
- 1.50 R☉
Planetary System
HD 181720 b is the only planet known to orbit HD 181720 so far.
HD 181720 b — Complete Data
| Mass (best estimate) | 127.13 Earth masses |
|---|---|
| Orbital period | 956.00 days |
| Orbital distance | 1.850 AU |
| Eccentricity | 0.260 |
| Earth Similarity Index | 0.36 |
| Distance from Earth | 196.4 light-years (60.2 parsecs) |
| Constellation | Sagittarius |
| Discovery method | Radial Velocity |
| Discovery facility | La Silla Observatory |
| Discovery year | 2009 |
Data: NASA Exoplanet Archive, last updated 2018-09-04. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HD 181720 b
Is HD 181720 b habitable?
HD 181720 b orbits within the habitable zone of HD 181720, 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 181720 b?
HD 181720 b is about 196 light-years from Earth in the constellation Sagittarius. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 3,456,640 years to get there.
How long is a year on HD 181720 b?
One orbit around HD 181720 takes 956.0 Earth days.
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