HAT-P-21 b
HAT-P-21 b is a hot Jupiter orbiting HAT-P-21 in the constellation Ursa Major. It lies about 904 light-years from Earth and was discovered in 2010 using the transit method.
How Big Is HAT-P-21 b?
HAT-P-21 b has a radius of 12.44 times that of Earth, or 1.11 times the radius of Jupiter. Its mass is 1,548 times that of Earth.
Is HAT-P-21 b in the Habitable Zone?
The position of HAT-P-21 b relative to the habitable zone of HAT-P-21 cannot be precisely determined from the available orbital data.
Habitable zone of HAT-P-21: 0.859–2.035 AU (conservative: 1.088–1.929 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 HAT-P-21 b — one full orbit around HAT-P-21 — lasts 4.12 Earth days, shorter than Mercury's 88-day year. Its orbit is mildly elliptical (eccentricity 0.23).
How Was HAT-P-21 b Discovered?
HAT-P-21 b was discovered in 2010 using the transit method, with observations from HATNet.
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 HAT-P-21 b?
HAT-P-21 b is 904.4 light-years (277.3 parsecs) from Earth. Light arriving here tonight left the planet around the year 1122. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 15,917,440 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. HAT-P-21 b scores 0.07, ranking #4,825 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: HAT-P-21
HAT-P-21
- Surface temperature
- 5,588 K (Sun: 5,772 K)
- Mass
- 1.24 M☉
- Radius
- 1.21 R☉
Planetary System
HAT-P-21 b is the only planet known to orbit HAT-P-21 so far.
HAT-P-21 b — Complete Data
| Radius | 12.442 Earth radii (1.110 Jupiter radii) |
|---|---|
| Mass | 1,547.83 Earth masses (4.870 Jupiter masses) |
| Orbital period | 4.12 days |
| Eccentricity | 0.230 |
| Earth Similarity Index | 0.07 |
| Distance from Earth | 904.4 light-years (277.3 parsecs) |
| Constellation | Ursa Major |
| Discovery method | Transit |
| Discovery facility | HATNet |
| Discovery year | 2010 |
Data: NASA Exoplanet Archive, last updated 2018-04-25. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HAT-P-21 b
Is HAT-P-21 b habitable?
HAT-P-21 b is not known to orbit within the habitable zone of HAT-P-21, and as a hot Jupiter it is an unlikely candidate for life as we know it.
How far away is HAT-P-21 b?
HAT-P-21 b is about 904 light-years from Earth in the constellation Ursa Major. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 15,917,440 years to get there.
How big is HAT-P-21 b compared to Earth?
HAT-P-21 b has 12.44 times the radius of Earth and about 1,548 times its mass.
How long is a year on HAT-P-21 b?
One orbit around HAT-P-21 takes 4.1 Earth days — short enough that 89 of its years would fit into one Earth year.
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