WASP-21 b
WASP-21 b is a hot Jupiter orbiting the G3 V star WASP-21 in the constellation Pegasus. It lies about 843 light-years from Earth and was discovered in 2010 using the transit method.
How Big Is WASP-21 b?
WASP-21 b has a radius of 11.99 times that of Earth, or 1.07 times the radius of Jupiter. Its mass is 95.3 times that of Earth, giving it a density of 0.32 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is WASP-21 b in the Habitable Zone?
The position of WASP-21 b relative to the habitable zone of WASP-21 cannot be precisely determined from the available orbital data.
Orbit and Year Length
A year on WASP-21 b — one full orbit around WASP-21 — lasts 4.32 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.052 AU — closer to its star than Mercury is to the Sun.
How Was WASP-21 b Discovered?
WASP-21 b was discovered in 2010 using the transit method, with observations from SuperWASP.
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 WASP-21 b?
WASP-21 b is 842.9 light-years (258.4 parsecs) from Earth. Light arriving here tonight left the planet around the year 1184. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 14,835,040 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. WASP-21 b scores 0.08, ranking #4,781 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: WASP-21
WASP-21
- Spectral type
- G3 V
Planetary System
WASP-21 b is the only planet known to orbit WASP-21 so far.
WASP-21 b — Complete Data
| Radius | 11.990 Earth radii (1.070 Jupiter radii) |
|---|---|
| Mass | 95.35 Earth masses (0.300 Jupiter masses) |
| Density | 0.32 g/cm³ (Earth: 5.51) |
| Orbital period | 4.32 days |
| Orbital distance | 0.052 AU |
| Earth Similarity Index | 0.08 |
| Distance from Earth | 842.9 light-years (258.4 parsecs) |
| Constellation | Pegasus |
| Discovery method | Transit |
| Discovery facility | SuperWASP |
| Discovery year | 2010 |
Data: NASA Exoplanet Archive, last updated 2014-05-14. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About WASP-21 b
Is WASP-21 b habitable?
WASP-21 b is not known to orbit within the habitable zone of WASP-21, and as a hot Jupiter it is an unlikely candidate for life as we know it.
How far away is WASP-21 b?
WASP-21 b is about 843 light-years from Earth in the constellation Pegasus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 14,835,040 years to get there.
How big is WASP-21 b compared to Earth?
WASP-21 b has 11.99 times the radius of Earth and about 95.3 times its mass.
How long is a year on WASP-21 b?
One orbit around WASP-21 takes 4.3 Earth days — short enough that 85 of its years would fit into one Earth year.
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