Using data from NASA’s Spitzer Space Telescope, astronomers have found evidence that the heat in the atmosphere of KELT-9b, the hottest known transiting exoplanet, is too much even for molecules of hydrogen to remain intact — they are likely ripped apart on the dayside of the planet, unable to reform until their disjointed atoms flow around to the planet’s nightside. The findings appear in the Astrophysical Journal Letters.
Artist’s impression of a sunset on the ultrahot Jupiter KELT-9b. Image credit: Denis Bajram.
KELT-9b belongs to a family called ultrahot Jupiters, incredibly hot giant exoplanets that orbit very close to their parent stars.
The planet is a gas giant approximately 2.8 times more massive than Jupiter but only half as dense.
It whips around its host star, KELT-9, in 36 hours at a distance only 1/30th that from Earth to the Sun.
At around 9,900 degrees Celsius (17,850 degrees Fahrenheit), the host star is at the dividing line between stars of type A and B. Also known as HD 195689, the star is about 650 light-years away in the constellation Cygnus.
KELT-9b is tidally locked to the star and its day side is perpetually bombarded by stellar radiation.
With a day-side temperature peaking at 4,327 degrees Celsius (7,820 degrees Fahrenheit), the planet is hotter than most stars.
“This kind of planet is so extreme in temperature, it is a bit separate from a lot of other exoplanets,” said Megan Mansfield, a graduate student at the University of Chicago.
“There are some other hot Jupiters and ultrahot Jupiters that are not quite as hot but still warm enough that this effect should be taking place.”
Mansfield and her colleagues used Spitzer to parse temperature profiles from KELT-9b.
Spitzer can measure subtle variations in heat. Repeated over many hours, these observations allow the telescope to capture changes in the atmosphere as the planet presents itself in phases while orbiting the star. Different halves of the planet roll into view as it orbits around its star.
That allowed the astronomers to catch a glimpse of the difference between KELT-9b’s day- and nightsides.
A big question for researchers trying to understand exoplanet atmospheres is how radiation and flow balance each other out. Computer models are major tools in such investigations, showing how these atmospheres are likely to behave in different temperatures.
The best fit for the data from KELT-9b was a model that included hydrogen molecules being torn apart and reassembled, a process known as dissociation and recombination.
“If you don’t account for hydrogen dissociation, you get really fast winds of 37 miles/sec (60 km/s). That’s probably not likely,” Mansfield said.
KELT-9b turns out not to have huge temperature differences between its day- and nightsides, suggesting heat flow from one to the other.
And the hot spot on the dayside, which is supposed to be directly under this planet’s star, was shifted away from its expected position.
Scientists don’t know why — yet another mystery to be solved on this strange, hot planet.
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Megan Mansfield et al. 2020. Evidence for H2 Dissociation and Recombination Heat Transport in the Atmosphere of KELT-9b. ApJL 888, L15; doi: 10.3847/2041-8213/ab5b09
