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KELT-9 b: The Planet That Tears Its Own Air Apart

KELT-9 b: The Planet That Tears Its Own Air Apart

@Steph12

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

Clouds cannot formMetals go bareMolecules come apartAnd reassembleThe planet leaks12345
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.