Super-hot exoplanets could have such severe atmospheres that they don’t just boil water – they destroy it. This could explain why the exceptionally hot planets seem devoid of water, according to a new study.


Ultra-hot Jupiters are gas giants that are so close to their star that they are tidally locked, where the same side always faces the star and can superheat to between 2,000 and 3,000ºC.

“The daysides of these worlds are furnaces that look more like a stellar atmosphere than a planetary atmosphere,” says Viven Parmentier, from Aix Marseille University in France who led the study.

“In this way ultra-hot Jupiters stretch out what we think planets should look like.”

For many years these extreme exoplanets have puzzled astronomers as they appear to be completely devoid of water on their daysides, while on similar, but slightly cooler planets, water was abundant.

However, this study considered the dayside atmosphere to be more like a star than a hot planet.

They found that the dayside temperatures are high enough that they can split apart water, turning it into hydrogen and oxygen.

Strong winds could then blow these gases around to the nightside of the planet where the temperature is 1,000ºC cooler, and the water might recombine.

While it is difficult to observe the chemical composition of the nightside, water has been seen along the daylight boundary, hinting that water might be hiding in the dark.

Astronomers will continue to study these exoplanets to aid in understanding both planetary atmospheres and stellar ones.

“We now know that ultra-hot Jupiters exhibit chemical behaviour that is different and more complex than their cooler cousins, the hot Jupiters,” says Parmentier.


“The studies of exoplanet atmospheres is still really in its infancy and we have so much to learn.”


Elizabeth Pearson
Ezzy PearsonScience journalist

Ezzy Pearson is the Features Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.