The temperature on the nightsides of short-period gas giants (hot Jupiters) is surprisingly uniform, suggesting the dark sides of these exoplanets have clouds made of minerals, a study published in the journal Nature Astronomy has found.
An artist’s impression of a hot-Jupiter exoplanet. Image credit: C. Carreau / ESA.
Hot Jupiters circle very close to their host star — so close that it typically takes fewer than three days to complete an orbit.
As a result, these massive planets have daysides that permanently face their host stars and nightsides that always face the darkness of space.
The tight orbit also means these worlds receive more light from their star, which is what makes them extremely hot on the dayside.
But astronomers had previously measured significant amounts of heat on the nightside of hot Jupiters, as well, suggesting some kind of energy transfer from one side to the other.
“Atmospheric circulation models predicted that the nightside temperatures should vary much more than they do,” said Dylan Keating, a PhD student at McGill University.
“This is surprising because the planets we studied all receive different amounts of irradiation from their host stars and the dayside temperatures among them vary by almost 1,700 degrees Celsius.”
Using data from NASA’s Spitzer Space and the NASA/ESA Hubble Space telescopes, Keating and colleagues found that the nightside temperature of 12 hot Jupiters they studied was about 800 degrees Celsius.
“The nightside temperatures are probably the result of condensation of vaporized rock in these very hot atmospheres,” Keating said.
“The uniformity of the nightside temperatures suggests that clouds on this side of the planets are likely similar to one another in composition.”
The team’s analysis suggests that these clouds are likely made of minerals such as manganese sulfide or silicates — in other words, rocks.
“Because the basic physics of cloud formation is universal, the study of the nightside clouds on hot Jupiters could give insight into cloud formation elsewhere in the Universe, including on Earth,” said McGill University’s Professor Nicolas Cowan.
“Future space telescope missions — such as the James Webb Space Telescope and ESA’s ARIEL mission — could be used to further characterize the dominant cloud composition on hot Jupiter nightsides, as well as to improve models of atmospheric circulation and cloud formation of these planets,” Keating said.
“Observing hot Jupiters at both shorter and longer wavelengths will help us determine what types of clouds are on the nightsides of these planets.”
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Dylan Keating et al. Uniformly hot nightside temperatures on short-period gas giants. Nature Astronomy, published online August 26, 2019; doi: 10.1038/s41550-019-0859-z
