Water on ultrahot Jupiters may experience cycles of death and rebirth

Hottest gas giants in close orbits around their stars act like star-planet hybrids.
By Laurel Kornfeld | Aug 10, 2018
Ultrahot Jupiters, a class of massive, extremely hot gas giants in close orbits around their parent stars, may experience cycles in which their water is regularly destroyed on one side and re-created on the other side.

Because these planets orbit so close to their stars, they are tidally locked to them, meaning one of their sides always faces the star while the other always faces away from it.

Their star-facing or daysides can reach temperatures between 3,600 and 5,400 degrees Fahrenheit (2,000 and 3,000 degrees Celsius) while their opposite or nightsides are about 1,800 degrees Fahrenheit (1,000 degrees Celsius) cooler.

Water cannot exist on these planets' daysides because the extreme heat and high levels of radiation from the star tear water molecules 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," noted Vivien Parmentier of Aix Marseille University in France, who led a study that indicates these planets act as star-planet hybrids.

While the planets' dark nightsides are difficult to observe, scientists know their temperatures get low enough for water as well as other molecules such as titanium oxide and aluminum oxide to re-form and condense as clouds. Both water and liquid metals may fall back from these clouds to the planet's nightside surface as rain.

Parmentier's research team proposed this theory based on studies of three separate hot Jupiters, WASP-103b, WASP-18b, and HAT-P-7b, which propose winds on the planets carry the separated components from the daysides to the nightsides, where temperatures allow them to recombine.

These earlier studies were conducted through observations with the Hubble and Spitzer space telescopes.

Eventually, the clouds are carried by wind to the daysides, where the molecules are once again broken apart.

While scientists have observed water molecules in hot Jupiters, close-orbiting gas giants with temperatures below 3,600 degrees Fahrenheit (2,000 degrees Celsius), they have not observed them in ultrahot Jupiters. This difference has not been well understood.

According to the new study, which has been accepted for publication in the journal Astronomy and Astrophysics, ultrahot Jupiters do have hydrogen and oxygen, the components of water, which undergo these endless cycles.

Infrared observations with Spitzer proved key to this discovery. Scientists observed carbon monoxide, which can withstand the heat and radiation of these planets' daysides, in the planets' atmospheres, and found their atmospheric temperatures to not be uniform. Instead, upper regions of the atmospheres have higher temperatures while regions further down have lower ones.

Researchers hope to conduct more detailed observations of ultrahot Jupiters' atmospheres using the James Webb Space Telescope (JWST), now scheduled for launch in 2021.



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