NGTS-21 b
NGTS-21 b is a hot Jupiter orbiting the K3 V star NGTS-21 in the constellation Microscopium. It lies about 1,934 light-years from Earth and was discovered in 2022 using the transit method.
How Big Is NGTS-21 b?
NGTS-21 b has a radius of 14.91 times that of Earth, or 1.33 times the radius of Jupiter. Its mass is 750 times that of Earth, giving it a density of 1.25 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is NGTS-21 b in the Habitable Zone?
NGTS-21 b orbits inside the inner edge of the habitable zone of NGTS-21. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of NGTS-21: 0.445–1.091 AU (conservative: 0.563–1.034 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on NGTS-21 b
The equilibrium temperature of NGTS-21 b is about 1,357 K (1,084 °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 NGTS-21 b — one full orbit around NGTS-21 — lasts just 37.0 hours, shorter than Mercury's 88-day year. It orbits at an average distance of 0.024 AU — closer to its star than Mercury is to the Sun.
How Was NGTS-21 b Discovered?
NGTS-21 b was discovered in 2022 using the transit method, with observations from Next-Generation Transit Survey (NGTS).
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 NGTS-21 b?
NGTS-21 b is 1,933.7 light-years (592.9 parsecs) from Earth. Light arriving here tonight left the planet about 1,934 years ago. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 34,033,120 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. NGTS-21 b scores 0.06, ranking #5,104 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: NGTS-21
NGTS-21
- Spectral type
- K3 V
- Surface temperature
- 4,660 K (Sun: 5,772 K)
- Mass
- 0.72 M☉
- Radius
- 0.86 R☉
- Age
- 10.0 billion years (Sun: 4.6)
Planetary System
NGTS-21 b is the only planet known to orbit NGTS-21 so far.
NGTS-21 b — Complete Data
| Radius | 14.908 Earth radii (1.330 Jupiter radii) |
|---|---|
| Mass | 750.08 Earth masses (2.360 Jupiter masses) |
| Density | 1.25 g/cm³ (Earth: 5.51) |
| Orbital period | 1.54 days |
| Orbital distance | 0.024 AU |
| Equilibrium temperature | 1,357 K (1,084 °C) |
| Earth Similarity Index | 0.06 |
| Distance from Earth | 1,933.7 light-years (592.9 parsecs) |
| Constellation | Microscopium |
| Discovery method | Transit |
| Discovery facility | Next-Generation Transit Survey (NGTS) |
| Discovery year | 2022 |
Data: NASA Exoplanet Archive, last updated 2022-10-18. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About NGTS-21 b
Is NGTS-21 b habitable?
No — NGTS-21 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is NGTS-21 b?
NGTS-21 b is about 1,934 light-years from Earth in the constellation Microscopium. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 34,033,120 years to get there.
How big is NGTS-21 b compared to Earth?
NGTS-21 b has 14.91 times the radius of Earth and about 750 times its mass.
How long is a year on NGTS-21 b?
One orbit around NGTS-21 takes 1.5 Earth days — short enough that 237 of its years would fit into one Earth year.
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