Ultrahot Jupiters Are So Fiery That They Tear Water Apart

Just like their name indicates, ultrahot Jupiters are a newly-emerged class of giant planets with blistering surface temperatures, ranking among the hottest exoplanets ever discovered. These exotic planets orbit very closely to their parent star — closer than Mercury is to the sun — and are tidally locked.

This means that one side of the planet always faces the scorching heat of the star, leading to searing dayside temperatures between 3,600 and 5,400 degrees Fahrenheit (2,000 and 3,000 degrees Celsius).

But a new study argues that these exotic objects are right on the threshold between planets and brown dwarfs, also known as “failed stars,” and seem to exhibit a “starlike atmospheres” capable of tearing water molecules apart, reports Arizona State University.

According to the study, due for publication in the journal Astronomy and Astrophysics and currently available on the pre-print server arXiv, ultrahot Jupiters get so heated up on their dayside that water molecules disintegrate.

“The daysides of these worlds are furnaces that look more like a stellar atmosphere than a planetary atmosphere,” said study lead author Vivien Parmentier, an astrophysicist at Aix Marseille University in France. “In this way, ultrahot Jupiters stretch out what we think planets should look like.”

These water molecules are swept away by strong winds to the planets’ nightside, where they can recombine and be “molecularly reborn” in a process of “chemical reincarnation,” explains NASA’s Jet Propulsion Laboratory (JLP) in Pasadena, California.

“Nightside temperatures are around 1,800 degrees Fahrenheit cooler (1,000 degrees Celsius), cold enough for water to re-form and, along with other molecules, coalesce into clouds,” details a JPL news release.

Astronomers have begun wondering what happens to water on ultrahot Jupiters after telescope observations uncovered that their atmospheres are devoid of water vapor. The revelation was even more puzzling considering that these planets’ cousins, the hot Jupiters, manage to cling to their water vapor despite being only slightly cooler.

The mystery has now been unraveled by the new study, which shows that “ultrahot Jupiters do in fact possess the ingredients for water (hydrogen and oxygen atoms),” but that their daysides are pummeled with such incredibly strong radiation that water molecules get “completely torn apart,” notes JPL.

The same fate could be shared by other types of molecules as well, such as titanium oxide (known as a sunscreen) and aluminum oxide (the main ingredient in rubies), which could also get deconstructed on one side on these planets and put back together on the other side.

The nightside of ultrahot Jupiters might even see these materials “form clouds and rain down as liquid metals and fluidic rubies,” shows JPL.

The chemical reincarnation cycle of water molecules on these planets was tested in computer models of WASP-103b, WASP-18b, and HAT-P-7b, three ultrahot Jupiters previously imaged with NASA’s Spitzer and Hubble space telescopes.

The telescope observations helped the researchers run computer simulations, which were adapted from a brown dwarf model created by team member Mark Marley, a scientist at NASA’s Ames Research Center in Silicon Valley, California.

Once the team started regarding ultrahot Jupiters more like “star-planet hybrids,” too hot to be planets yet too cool to be stars, the confounding telescope observations on their water content started to make more sense.

“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.