If you thought you were hot chasing a child around Walt Disney World in the dead of a Florida summer surrounded by 56,000 people, imagine how hydrogen feels on KELT-9b. Astronomers using NASA’s Spitzer space telescope have discovered that the exoplanet is so hot molecules of hydrogen are ripped apart during the day, only reassembling when the atoms flow around to the planet’s night side.
KELT-9b orbits its star so tightly that a “year" takes only 1.5 days. That means the planet is tidally locked, presenting one face to its star all the time. On the far side of KELT-9b, nighttime lasts forever (hello Pink Floyd fans). But gases and heat flow from one side to the other. Though still extremely hot, the night side is 2010 K cooler than the day side, enough to allow hydrogen gas molecules to reform—until they flow back to the day side, where they're torn apart all over again.
Megan Mansfield, a graduate student at the University of Chicago and lead author of the new paper published in Astrophysical Journal Letters, and her team used the Spitzer space telescope to parse the temperature profiles of KELT-9b. Spitzer, which makes observations in infrared light, can measure subtle variations in heat. Repeated over many hours, these observations allow Spitzer 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. This is what gave the researchers a view of the hot day side and the just-a-tiny-bit cooler night side.
"New models of these ultra-hot planets have been developed recently that incorporate hydrogen dissociation (where hydrogen molecules are ripped apart on the day side of the planet and combine again on the night side)" Mansfield explained to Laboratory Equipment. "This process was only recognized recently as something that could impact heat transport on ultra-hot exoplanets, and my collaborators incorporated it into their models for this paper. We decided to look at a phase curve of this planet with Spitzer because it has a legacy of observing interesting phase curves of hot exoplanets, so we thought a phase curve of KELT-9b with Spitzer would be a good way to test the new models incorporating hydrogen dissociation."
Subsequent computer models based on the images and data captured showed how the atmosphere of KELT-9b is likely to behave in different temperatures, including tearing apart and reassembling hydrogen molecules.
"This same process has been suggested to be happening on a few other ultra-hot planets and should be happening, to some extent, on any planets with temperatures above about 2500 K," Mansfield said. "This ripping apart could be happening to other molecules on hot planets, too. For example, on these hot planets, water may also be ripped apart into hydrogen and oxygen atoms."
KELT-9b—an ultra-hot Jupiter, one of several varieties of exoplanets—was first detected in 2017 using the Kilodegree Extremely Little Telescope system. It weighs nearly three times the mass of the Milky Way’s Jupiter and orbits a star 670 light years away. With a surface temperature of 4566 K (7800 F, 4300 C) on the day side, it is the hottest planet found to date.
While the molecule question has a hypothetical answer, KELT-9b still holds a wealth of mysteries for astronomers. For example, according to the models, the "hot spot" on the day side should be directly under the planet's star, but that is not the case.
"We saw in our observations that the hottest point was about 20 degrees away from the point facing the star," Mansfield said. "We don’t yet fully understand what is causing this offset. One possibility is magnetic fields. The atmosphere of KELT-9b contains a lot of charged ions, so if the planet has a strong magnetic field the ions in the atmosphere may get caught up in that magnetic field, which may shape how the atmosphere circulates. This could potentially cause the hottest point to be offset from where we expect. There aren’t a lot of models that incorporate the effects of magnetic fields, though, so in order to fully understand whether this theory could be correct we will need new models including magnetic fields."
New models, like the phase curve ones Mansfield and her co-authors pioneered, are key to future planetary observations. Additionally, while space telescopes like the Spitzer Space Telescope and the Hubble Space Telescope have made enormous contributions toward understanding hot Jupiters, Mansfield said bigger and more sensitive telescopes, like the James Webb Space Telescope, are needed to give astronomers the chance to look at rocky planets in detail, including searching for signs of habitability.
Photo: Artist’s rendering of KELT-9b, the hottest known exoplanet. Credit: NASA/JPL-Caltech