spaceSpace and Physics

The Hottest Exoplanet We Know Of Can’t Form Molecules On Its Day Side


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockJan 27 2020, 15:16 UTC

Artist's rendering of a "hot Jupiter" called KELT-9b, the hottest known exoplanet. NASA/JPL-Caltech

KELT-9b is an incredible exoplanet. A gas giant very close to its star, it's not just classified as a regular hot-Jupiter, it's an ultra-hot Jupiter. KELT-9 is so close that it is tidally locked, with one side permanently facing the star. It's also the hottest known planet, with a temperature of 4,300°C (7,800°F), much hotter than many stars.

The incredible temperature has a very important consequence. As reported in the Astrophysical Journal Letters, researchers reveal molecules break apart on the planet's dayside, hydrogen in particular, but get re-formed when they get to its nightside. This work provides some interesting insights into what other hot planets might be like.


"This kind of planet is so extreme in temperature, it is a bit separate from a lot of other exoplanets," lead author Megan Mansfield, a graduate student at the University of Chicago, said in a statement. "There are some other hot Jupiters and ultra-hot Jupiters that are not quite as hot but still warm enough that this effect should be taking place."

Artist's rendering of a "hot Jupiter" called KELT-9b, the hottest known exoplanet - so hot, a new paper finds, that even molecules in its atmosphere are torn to shreds. Credit: NASA/JPL-Caltech

The observations were possible thanks to NASA's infrared telescope Spitzer, an observatory that looks at the heat emitted by celestial bodies, which will be retired this week. Over several hours of observations of this particular world, Spitzer saw that the heat was just too much for the molecules to bear, so hydrogen dissociates. But on the nightside, while still being very hot, it is cool enough for hydrogen molecules to recombine.

It also saw that the planet's “hot spot” on the dayside is not directly under the star, adding to its mysteries.


KELT-9b orbits an extremely hot and bright star that has a temperature of 9,900 °C (17,850 °F). The planet is so close that it orbits the star in just a day and a half. It has almost three times the mass of Jupiter but it is much bigger in size due to its incredible temperature. But because it's overblown and so close to its star, it's losing its atmosphere. 

The researchers used computer models to show that this planet’s atmosphere can only work if the molecules are being ripped apart.

"If you don't account for hydrogen dissociation, you get really fast winds of [37 miles or] 60 kilometers per second," Mansfield said. "That's probably not likely."


Understanding how gas and heat flow on this planet not only helps us understand its atmosphere, but it may help us understand how other exoplanets’ atmospheres work as well.

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