:INFO
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.

:STATS
[icon:FIRE] Equilibrium temperature | 3921 K, measured not derived
[icon:TARGET] Radius | 21.7 Earth radii, about 1.9 Jupiters
[icon:CHART] Mass and density | 915 Earth masses at 0.491 g/cm3
[icon:CLOCK] One year | 1.481 Earth days
[icon:STAR] Host star | KELT-9 at 9270 K, an A-type blue star
[icon:BOLT] Distance | about 667 light-years

:JOURNEY
What heat does to matter, step by step
2 Nothing condenses
3 Atoms not compounds
4 Dissociation
4 A one-way cycle
5 Escape

:TIMELINE
🔭🔥⚛️
2017-06-01 | Announced from the KELT survey, immediately the hottest planet on record and the only one known to orbit a star this hot.
2018-08-01 | Iron and titanium found in its atmosphere as bare atoms, the first heavy metals detected in any planet's air.
2020-01-01 | Modelling shows hydrogen molecules are torn apart on the dayside and reassemble on the nightside.

:NOTE
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.

:LINK
https://arxiv.org/abs/1905.02096
A spectral survey of an ultra-hot Jupiter: metals in the transmission spectrum of KELT-9 b