August 10, 2018
Imagine a place where the weather forecast is always the same: scorching temperatures, relentlessly sunny, and with absolutely zero chance of rain. This hellish scenario exists on the permanent daysides of a type of planet found outside our solar system dubbed an “ultrahot Jupiter.” These worlds orbit extremely close to their stars, with one side of the planet permanently facing the star.
What has puzzled scientists is why water vapor appears to be missing from the toasty worlds’ atmospheres, when it is abundant in similar but slightly cooler planets. Observations of ultrahot Jupiters by NASA’s Spitzer and Hubble space telescopes, combined with computer simulations, have served as a springboard for a new theoretical study that may have solved this mystery.
According to the new study, ultrahot Jupiters do in fact possess the ingredients for water (hydrogen and oxygen atoms). But due to strong irradiation on the planet’s daysides, temperatures there get so intense 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,” said Vivien Parmentier, an astrophysicist at Aix Marseille University in France and lead author of the new study. “In this way, ultrahot Jupiters stretch out what we think planets should look like.”
While telescopes like Spitzer and Hubble can gather some information about the daysides of ultrahot Jupiters, the nightsides are difficult for current instruments to probe. The new paper proposes a model for what might be happening on both the illuminated and dark sides of these planets, based largely on observations and analysis of the ultrahot Jupiter known as WASP-121b, and from three recently published studies, coauthored by Parmentier, that focus on the ultrahot Jupiters WASP-103b, WASP-18b and HAT-P-7b, respectively. The new study suggests that fierce winds may blow the sundered water molecules into the planets’ nightside hemispheres. On the cooler, dark side of the planet, the atoms can recombine into molecules and condense into clouds, all before drifting back into the dayside to be splintered again.
image credit: NASA/JPL-Caltech/Vivien Parmentier/Aix-Marseille University (AMU)