WASP-121 b
WASP-121 b is a hot Jupiter orbiting WASP-121 in the constellation Puppis. It lies about 880 light-years from Earth and was discovered in 2016 using the transit method.
How Big Is WASP-121 b?
WASP-121 b has a radius of 19.53 times that of Earth, or 1.74 times the radius of Jupiter. Its mass is 372 times that of Earth, giving it a density of 0.28 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is WASP-121 b in the Habitable Zone?
WASP-121 b orbits inside the inner edge of the habitable zone of WASP-121. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of WASP-121: 1.371–3.177 AU (conservative: 1.737–3.012 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on WASP-121 b
The equilibrium temperature of WASP-121 b is about 2,409 K (2,136 °C) — hot enough to melt many metals. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C.
Orbit and Year Length
A year on WASP-121 b — one full orbit around WASP-121 — lasts just 30.6 hours, shorter than Mercury's 88-day year. It orbits at an average distance of 0.026 AU — closer to its star than Mercury is to the Sun.
How Was WASP-121 b Discovered?
WASP-121 b was discovered in 2016 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-121 b?
WASP-121 b is 880.3 light-years (269.9 parsecs) from Earth. Light arriving here tonight left the planet around the year 1146. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 15,493,280 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-121 b scores 0.05, ranking #5,301 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: WASP-121
WASP-121
- Surface temperature
- 6,628 K (Sun: 5,772 K)
- Mass
- 1.33 M☉
- Radius
- 1.46 R☉
- Age
- 1.1 billion years (Sun: 4.6)
Planetary System
WASP-121 b is the only planet known to orbit WASP-121 so far.
WASP-121 b — Complete Data
| Radius | 19.526 Earth radii (1.742 Jupiter radii) |
|---|---|
| Mass | 371.86 Earth masses (1.170 Jupiter masses) |
| Density | 0.28 g/cm³ (Earth: 5.51) |
| Orbital period | 1.27 days |
| Orbital distance | 0.026 AU |
| Equilibrium temperature | 2,409 K (2,136 °C) |
| Earth Similarity Index | 0.05 |
| Distance from Earth | 880.3 light-years (269.9 parsecs) |
| Constellation | Puppis |
| Discovery method | Transit |
| Discovery facility | SuperWASP |
| Discovery year | 2016 |
Data: NASA Exoplanet Archive, last updated 2025-11-20. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About WASP-121 b
Is WASP-121 b habitable?
No — WASP-121 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is WASP-121 b?
WASP-121 b is about 880 light-years from Earth in the constellation Puppis. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 15,493,280 years to get there.
How big is WASP-121 b compared to Earth?
WASP-121 b has 19.53 times the radius of Earth and about 372 times its mass.
How long is a year on WASP-121 b?
One orbit around WASP-121 takes 1.3 Earth days — short enough that 286 of its years would fit into one Earth year.
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