Recent observations by NASA’s space telescopes of ultrahot Jupiter-like exoplanets have surprised astronomers. The spectra of these planets have suggested they have exotic compositions. However, a new study proposes an explanation — that these gas-rich planets have compositions that are basically normal; what’s different is that the atmospheres on their daysides look more like the atmosphere of a star than a planet.
An artist’s impression of an ultrahot Jupiter. Image credit: Sci-News.com.
Among the growing catalogue of exoplanets, ultrahot Jupiters have stood out as a distinct class for about a decade.
These giant planets are very hot because they are extremely close to their stars, with one side of the planet permanently facing the star. Dayside temperatures reach 5,400 degrees Fahrenheit (3,000 degrees Celsius), ranking ultrahot Jupiters among the hottest exoplanets known.
“Interpreting the spectra of these planets has posed a thorny puzzle for researchers for years,” said Dr. Michael Line, an astrophysicist at Arizona State University.
The biggest puzzle is why water vapor appears to be missing from these worlds’ atmospheres, when it is abundant in similar but slightly cooler planets.
According to the new study, ultrahot Jupiters do in fact possess the ingredients for water — atoms of hydrogen and oxygen.
But due to the strong radiation on the planet’s daysides, temperatures there go high enough that water molecules are completely torn apart.
“The daysides of these worlds are furnaces that look more like a stellar atmosphere than a planetary atmosphere. In this way, ultrahot Jupiters stretch out what we think planets should look like,” said lead author Dr. Vivien Parmentier, an astrophysicist at Aix Marseille University, France.
Dr. Line, Dr. Parmentier and colleagues propose a model for what might be happening on both the illuminated and dark sides of these planets.
The team’s model is based largely on observations and analysis from three recent studies that focus on ultrahot Jupiters WASP-103b, WASP-18b, and HAT-P-7b.
The study suggests that fierce winds driven by heating may blow the torn-apart water molecules into the planets’ cooler nightside hemispheres.
There the atoms can recombine into molecules and condense into clouds, all before drifting back into the dayside to be ripped apart again.
“With these studies, we are bringing some of the century-old knowledge gained from studying the astrophysics of stars, to the new field of investigating exoplanetary atmospheres,” Dr. Parmentier said.
The findings appear in the journal Astronomy & Astrophysics (arXiv.org preprint).
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V. Parmentier et al. From thermal dissociation to condensation in the atmospheres of ultra hot Jupiters: WASP-121b in context. A&A, in press; doi: 10.1051/0004-6361/201833059
