It’s raining vaporized rock on WASP-178b, and KELT-20b has its upper atmosphere getting hotter rather than cooler because it is being sunburned by intense ultraviolet radiation from its host star, according to an analysis of data from the NASA/ESA Hubble Space Telescope. The results appear in two papers in the journal Nature and the Astrophysical Journal Letters.
An artist’s impression of the ultrahot Jupiter KELT-20b. Image credit: NASA / ESA / Leah Hustak, STScI.
“We still don’t have a good understanding of weather in different planetary environments,” said Dr. David Sing, an astronomer at the Johns Hopkins University.
“When you look at Earth, all our weather predictions are still finely tuned to what we can measure.”
“But when you go to a distant exoplanet, you have limited predictive powers because you haven’t built a general theory about how everything in an atmosphere goes together and responds to extreme conditions.”
“Even though you know the basic chemistry and physics, you don’t know how it’s going to manifest in complex ways.”
In a Nature paper, Dr. Sing and his colleagues described Hubble observations of WASP-178b, an ultrahot Jupiter exoplanet located about 1,300 light-years away in the constellation of Lupus.
On the daytime side the atmosphere of WASP-178b is cloudless, and is enriched in silicon monoxide gas.
Because one side of the planet permanently faces its star, the torrid atmosphere whips around to the nighttime side at super-hurricane speeds exceeding 3,219 km per hour (2,000 mph).
On the dark side of WASP-178b, the silicon monoxide may cool enough to condense into rock that rains out of clouds, but even at dawn and dusk, the planet is hot enough to vaporize rock.
“We knew we had seen something really interesting with this silicon monoxide feature,” said Dr. Josh Lothringer, an astronomer at the Utah Valley University.
In a paper in the Astrophysical Journal Letters, the astronomers reported on the ultrahot Jupiter KELT-20b, which is located about 400 light-years away in the constellation of Cygnus.
On this planet a blast of ultraviolet light from its parent star is creating a thermal layer in the atmosphere, much like Earth’s stratosphere.
“Until now we never knew how the host star affected a planet’s atmosphere directly,” said Dr. Guangwei Fu, an astronomer at the University of Maryland, College Park.
“There have been lots of theories, but now we have the first observational data.”
By comparison, on Earth, ozone in the atmosphere absorbs ultraviolet light and raises temperatures in a layer between 11 and 50 km (7-31 miles) above Earth’s surface.
On KELT-20b, the ultraviolet radiation from the star is heating metals in the atmosphere which makes for a very strong thermal inversion layer.
“The emission spectrum for KELT-20b is quite different from other hot-Jupiters,” Dr. Fu said.
“This is compelling evidence that planets don’t live in isolation but are affected by their host star.”
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J.D. Lothringer et al. 2022. UV absorption by silicate cloud precursors in ultra-hot Jupiter WASP-178b. Nature 604, 49-52; doi: 10.1038/s41586-022-04453-2
Guangwei Fu et al. 2022. Strong H2O and CO Emission Features in the Spectrum of KELT-20b Driven by Stellar UV Irradiation. ApJL 925, L3; doi: 10.3847/2041-8213/ac4968
