The Atmosphere of an Ultra Hot Jupiter Exoplanet
An artist's impression of the planet KELT-9b, an Ultra Hot Jupiter that orbits its host star in 1.48 days. Astronomers have used TESS satellite measurements and Spitzer/IRAC data to determine more precisely the day and night-side atmospheric temperatures, about 4600 and 3040 kelvin, respectively, and to model the chemical behavior of its primary constituent, hydrogen.
A hot Jupiter is a gas giant exoplanet roughly similar to Jupiter in our solar system but orbiting so close to its host star (with a period less than about ten days) that its atmosphere is heated to temperatures above a thousand degrees kelvin. An Ultra Hot Jupiter exoplanet (UHJ) is an extreme hot Jupiter whose dayside temperature exceeds two thousand degrees. An example of an UHJ is KELT-9b, discovered in 2016 by the ground-based Kilodegree Extremely Little Telescope. The planet has a mass of 2.9 Jupiter-masses and orbits its host star (a 2.3 solar-mass A0/B9 star) every 1.48 days, resulting in a dayside temperature of a whopping 4600K, more than 1500K higher than the atmosphere of the next hottest known planet and close to the surface temperature of a modest mass star.
UHJs lie in an extreme regime of temperature where molecules are dissociated and the dayside atmosphere is dominated by atomic gases. Previous observations of KELT-9b have concluded that it has an extended atmosphere dominated by atomic hydrogen and day-night temperatures that are roughly comparable, the result of the efficient transport of hydrogen around the planet. CfA astronomers Joseph Rodriguez, Dave Latham, and Daniel Yahalomi were on a team that used the TESS satellite, combined with observations by the IRAC infrared camera on Spitzer, to study KELT-9b to refine the temperature distribution and heat transport models for this exceptional planet. They fix the daytime temperature at 4600 kelvin and the nighttime temperature at 3040 kelvin (both with an uncertainty of only 100 kelvin), and consistent with this UHJ having a much smaller day-night contrast than other planets. Their results confirm previous suggestions that the night-time atmosphere is being heated as hydrogen atoms recombine to form molecular hydrogen, releasing the heat that is primarily responsible for the night side being so warm. The resultant atmospheric composition has roughly equal parts of atomic and molecular hydrogen. However, contrary to earlier speculation that the nighttime atmosphere has no spectral features, the scientists find that it should be expected to show significant spectral signatures of molecular absorption lines. Not least, the scientists conclude that the high temperatures -- even at night -- preclude the formation of condensate clouds.
Reference(s):
"Exploring the Atmospheric Dynamics of the Extreme Ultrahot Jupiter KELT-9b Using TESS Photometry," Ian Wong et al., The Astronomical Journal 160, 88, 2020.