On its day side, KELT-9 b is hotter than most stars. When its discovery was published in the journal Nature in 2017, it was the hottest giant planet known, with a day side of about 4,600 kelvin (roughly 4,300 °C). It was found by the KELT-North telescope at Winer Observatory in Arizona, part of the KELT survey run by Ohio State, Lehigh and Vanderbilt universities. The survey used the transit method: it spotted a small, regular dip in the star's light each time the planet passed in front of it.
The heat comes from its star. KELT-9 is a young, blue A-type star, nearly twice as hot as the Sun, and the planet circles it every 1.5 days. The planet is tidally locked, meaning one side always faces the star. Its orbit is also tipped so that it passes over the star's poles rather than around its equator. KELT-9 b has nearly three times Jupiter's mass but only about half its density, because the star's radiation has puffed up its atmosphere.
At these temperatures, molecules are broken apart into single atoms. In 2018 a team led by the University of Geneva used the HARPS-North spectrograph on the Telescopio Nazionale Galileo in La Palma to split the starlight passing through the planet's atmosphere into its colours. They found the fingerprints of iron and titanium vapour, including iron and titanium atoms that had lost an electron. Iron had never been detected in an exoplanet's atmosphere before. A separate 2018 study found hydrogen gas reaching out to about 1.64 times the planet's radius, close to the Roche lobe, the boundary beyond which the star's gravity pulls gas away. That points to a planet losing its atmosphere to space.
How fast it is losing gas, and what that means for its future, is still open. The discovery team suggested it could be largely stripped of its atmosphere over the star's lifetime. The Geneva team noted that it is massive enough to resist evaporating completely. Either way, the star is expected to swell into a red giant within a few hundred million years, which could swallow the planet.
TerraForge's model, working only from its mass, orbit and star, classifies KELT-9 b as a gas giant, which matches the observations. Its modelled temperature of 3,782 K sits close to the archive's whole-planet equilibrium temperature of 3,921 K, and below the roughly 4,600 K measured on the star-facing side.