![KELT-9 b: The Planet That Tears Its Own Air Apart](https://cdn.slatesource.com/3/6/6/36658afd-634c-4c1c-ae7d-8d610bebd467.webp) # KELT-9 b: The Planet That Tears Its Own Air Apart - [Made in Slatesource](https://slatesource.com/@steph/kelt-9-b-the-planet-that-tears-its-own-air-apart) - By [Steph](https://slatesource.com/@steph) - Created on Sep 19, 2026 ## Hotter than most of the stars in the sky KELT-9 b holds the temperature record among known planets. Its equilibrium temperature is 3921 K and the dayside runs hotter still, which puts it above the surface of a typical red dwarf. That is not a figure of speech: this planet is hotter than most stars in the galaxy. It orbits a blue A-type star of 9270 K, more than twice the Sun's mass, and it does so in 35 hours at a distance of 0.034 AU. At those temperatures chemistry stops working the way it does anywhere else. Equilibrium temperature 3921 K, measured not derived Radius 21.7 Earth radii, about 1.9 Jupiters Mass and density 915 Earth masses at 0.491 g/cm3 One year 1.481 Earth days Host star KELT-9 at 9270 K, an A-type blue star Distance about 667 light-years journey·5 Stages # What heat does to matter, step by step Severity / Stage "On a cooler gas giant, iron and titanium rain out into the interior. Here nothing ever condenses, so the metals stay in the air. "Iron and titanium appear in the spectrum as individual atoms rather than locked into molecules. The first heavy metals ever detected in a planet's air. "On the dayside hydrogen molecules are torn into separate atoms by the heat alone. "Carried to the nightside, those atoms recombine into molecules, then are destroyed again on the next pass. "Under that ultraviolet flux the atmosphere is streaming away into space. 🔭🔥⚛️01.06.2017 – 01.01.2020 01.06.2017Announced from the KELT survey, immediately the hottest planet on record and the only one known to orbit a star this hot. 01.06.2017 01.08.2018Iron and titanium found in its atmosphere as bare atoms, the first heavy metals detected in any planet's air. 01.08.2018 01.01.2020Modelling shows hydrogen molecules are torn apart on the dayside and reassemble on the nightside. 01.01.2020 The temperature here is the rare case where the archive has a measured value rather than one derived from the star, so the 3921 K on the cover carries no asterisk. The dayside figure often quoted at around 4600 K is a different quantity: equilibrium temperature averages the whole planet, while the dayside of a world that never rotates relative to its star runs hotter than the average. [A spectral survey of an ultra-hot Jupiter: metals in the transmission spectrum of KELT-9 b](https://arxiv.org/abs/1905.02096?utm_source=slatesource)