KELT-21 b
KELT-21 b is a hot Jupiter orbiting the A8 V star KELT-21 in the constellation Cygnus. It lies about 1,536 light-years from Earth and was discovered in 2018 using the transit method.
How Big Is KELT-21 b?
KELT-21 b has a radius of 17.78 times that of Earth, or 1.59 times the radius of Jupiter. Its mass is 1,243 times that of Earth, giving it a density of 1.24 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is KELT-21 b in the Habitable Zone?
The position of KELT-21 b relative to the habitable zone of KELT-21 cannot be precisely determined from the available orbital data.
Temperature on KELT-21 b
The equilibrium temperature of KELT-21 b is about 2,051 K (1,778 °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. It receives 2,950 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on KELT-21 b — one full orbit around KELT-21 — lasts 3.61 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 KELT-21 b Discovered?
KELT-21 b was discovered in 2018 using the transit method, with observations from KELT-North.
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 KELT-21 b?
KELT-21 b is 1,535.8 light-years (470.9 parsecs) from Earth. Light arriving here tonight left the planet about 1,536 years ago. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 27,030,080 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. KELT-21 b scores 0.05, ranking #5,261 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: KELT-21
KELT-21
- Spectral type
- A8 V
- Surface temperature
- 7,598 K (Sun: 5,772 K)
- Mass
- 1.46 M☉
- Radius
- 1.64 R☉
- Luminosity
- 8.0301 L☉
- Age
- 1.6 billion years (Sun: 4.6)
Planetary System
KELT-21 b is the only planet known to orbit KELT-21 so far.
KELT-21 b — Complete Data
| Radius | 17.777 Earth radii (1.586 Jupiter radii) |
|---|---|
| Mass | 1,242.72 Earth masses (3.910 Jupiter masses) |
| Density | 1.24 g/cm³ (Earth: 5.51) |
| Orbital period | 3.61 days |
| Orbital distance | 0.052 AU |
| Equilibrium temperature | 2,051 K (1,778 °C) |
| Stellar irradiation | 2,950.00× Earth |
| Earth Similarity Index | 0.05 |
| Distance from Earth | 1,535.8 light-years (470.9 parsecs) |
| Constellation | Cygnus |
| Discovery method | Transit |
| Discovery facility | KELT-North |
| Discovery year | 2018 |
Data: NASA Exoplanet Archive, last updated 2018-02-01. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About KELT-21 b
Is KELT-21 b habitable?
KELT-21 b is not known to orbit within the habitable zone of KELT-21, and as a hot Jupiter it is an unlikely candidate for life as we know it.
How far away is KELT-21 b?
KELT-21 b is about 1,536 light-years from Earth in the constellation Cygnus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 27,030,080 years to get there.
How big is KELT-21 b compared to Earth?
KELT-21 b has 17.78 times the radius of Earth and about 1,243 times its mass.
How long is a year on KELT-21 b?
One orbit around KELT-21 takes 3.6 Earth days — short enough that 101 of its years would fit into one Earth year.
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