The extreme exoplanet that is hotter than most stars
KELT-9b is a giant gas exoplanet located 670 light-years away. It orbits so close to its host star that its dayside temperature reaches a blistering 7,800 degrees Fahrenheit. This makes KELT-9b hotter than the surface of most stars in the galaxy, causing molecules in its atmosphere to tear themselves apart.
A World Hotter Than Stars
In the constellation Cygnus, roughly 670 light-years from Earth, orbits one of the most extreme celestial bodies discovered to date: KELT-9b. Classified as an ultra-hot Jupiter, this gas giant is roughly 2.8 times the mass of Jupiter and has puffed up to nearly twice its radius. Its most staggering characteristic is its temperature. With a dayside surface temperature approaching 4,600 Kelvin (more than 7,800 degrees Fahrenheit or 4,300 degrees Celsius), KELT-9b is hotter than the vast majority of stars in the Milky Way galaxy.
Most stars in the universe are red dwarfs, also known as M-dwarfs, which typically burn at surface temperatures between 2,400 and 3,700 Kelvin. Even orange K-type dwarfs often have surface temperatures that hover around 4,000 to 5,200 Kelvin. This means that KELT-9b, despite being a planet lacking internal nuclear fusion, maintains an outer atmosphere that is considerably hotter than the stellar surfaces of billions of actual stars. It effectively bridges the thermal gap between planetary science and stellar astrophysics.
The Host Star and an Intimate Orbit
The immense heat of KELT-9b is not generated from within the planet itself, but is instead driven by fierce irradiation from its parent star, KELT-9 (also cataloged as HD 195689). KELT-9 is an exceptionally hot, rapidly rotating star belonging to the late B or early A spectral class. Its surface temperature sits around 10,170 Kelvin, making it more than twice as hot as our Sun. Stars of this class emit ferocious amounts of high-energy ultraviolet radiation, subjecting anything nearby to severe stellar winds and radiation pressure.
Compounding this intense stellar furnace is the planet's tight orbit. KELT-9b circles its parent star at a distance of only about 0.035 astronomical units—just a tiny fraction of the distance between Mercury and our Sun. At this proximity, the planet completes a full orbit in just under 36 hours (approximately 1.48 Earth days). Gravitational tidal forces have locked the planet into synchronous rotation, meaning one side permanently faces the stellar inferno while the other side faces perpetual night.