Ultrahot Jupiters destroy water
“Ultrahot Jupiters” are a class of incredibly hot giant exoplanets that orbit their star more closely than Mercury orbits the Sun. These worlds, such as Wasp-121b, are tidally locked, meaning one side of the planet permanently faces its star. But what separates this world from others?
One of the mysteries about these exoplanets is the absence of water vapor in their atmospheres, when it has been found to be abundant in planets that are slightly cooler, but otherwise similar. A new study, based on data from NASA’s Spitzer and Hubble space telescopes, along with computer simulations, may provide an answer to this mystery.
According to the authors of the new study, ultrahot Jupiters possess the hydrogen and oxygen atoms necessary to form water. Because of the intense irradiation on the planet’s day sides, temperatures there get so high that water molecules are torn apart.
“The day sides 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 scientists can use space telescopes like Hubble and Spitzer to observe the daysides of ultrahot Jupiters, the night sides are difficult to study with current instruments.
Researchers have proposed a model of what may be happening on both sides of these worlds based mostly on observations of WASP-121b, along with three previous studies coauthored by Parmentier. These focus on the ultrahot Jupiters WASP-103b, WASP-18b and HAT-P-7b. The new study indicates that strong winds may blow broken-up water molecules over to the planets’s night side hemispheres. Atoms can recombine on the planet’s cooler, dark side and condense into clouds, eventually drifting back to the day side to be split again.
Temperatures on the day side of Ultrahot Jupiters range from 3,600 to 5,400 degrees Fahrenheit (2,000 to 3,000 degrees Celsius), making them some of the hottest exoplanets on record. Temperatures on the night side are about 1,800 degrees Fahrenheit cooler (about 1,000 degrees Celsius). This would be cool enough for water to re-form and coalesce into clouds with other molecules.
Hot Jupiters, exoplanets with day side temperatures below 3,600 degrees Fahrenheit (2,000 Celsius), were the first commonly discovered type of exoplanet. These were found starting in the mid-1990s. Water was commonly found in their atmospheres. One hypothesis for the apparent lack of water in ultra hot Jupiters is that these planets may have formed with very high levels of carbon instead of oxygen. The authors of the new study contend that this hypothesis cannot account for traces of water which are sometimes detected at the day side-night side boundary.
For the new study, Parmentier and colleagues applied well-established physical models of the atmospheres of stars, as well as ‘failed stars,” called brown dwarfs, whose properties somewhat overlap. The researchers adapted a brown dwarf model developed by Mark Marley, a research scientist at NASA’s Ames Research Center and one of the study’s co-authors.
“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,” said Parmentier.
Researchers studying ultrahot Jupiter are looking forward to using NASA’s James Webb Space Telescope, currently scheduled to launch in 2021 to learn more about these distant worlds. Parmentier and colleagues expect that the new space telescope will be powerful enough to discern new details about the day sides of ultrahot Jupiters, as well as confirming that water and other molecules of interest missing from the day sides have gone to the planets’ night sides.
“We now know that ultrahot Jupiters exhibit chemical behavior that is different and more complex than their cooler cousins, the hot Jupiters,” said Parmentier. “The studies of exoplanet atmospheres is still really in its infancy and we have so much to learn.”
Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.