Ultra-hot Jupiters are half boiling hellscape, half cloudy oasis. Molecules in the atmosphere on the planets鈥� day sides may be broken into their atomic building blocks by the blistering heat, only to recombine into clouds that rain down liquid iron on the cooler night sides.
The hottest planets in the universe are gas giants a bit larger than Jupiter that orbit close to their stars. For worlds this close in, one side is always facing the star, with temperatures over 4300掳C, while the other side never sees sunlight and can be a thousand degrees cooler.
Most planets, including hot Jupiters, contain water in some form because it is common in the universe and in the discs of debris surrounding planet-forming stars. 鈥淲e鈥檝e been looking for water on planets since we鈥檝e been able to look for things on planets,鈥� says Vivien Parmentier at Aix Marseille University in France. But when astronomers found extremely hot planets, some of their day sides showed no signs of water at all.
Advertisement
Parmentier and his colleagues hypothesised that these worlds face so much radiation from their stars that water and other molecules in their dayside atmospheres get broken into their constituent atomic parts. It鈥檚 so hot that some electrons even break free of their atoms.
Two-faced worlds
The difference in heat between the day and night side also causes extreme winds that whip around the planet, carrying the atmospheric atoms with them. When those atoms reach the limb 鈥� the boundary between day and night 鈥� they start to cool down and link back up. As they blow to the night side, some of the molecules condense into clouds and rain.
That means there鈥檚 no water on the day side, only hydrogen and oxygen. But on the night side, the atmosphere has plenty of water. It might also have clouds of calcium and titanium which could rain down aluminium oxide, iron, or liquid rubies. 鈥淲e call them planets, but they鈥檙e like an intermediate object: star-like on the dayside and planet-like on the night side,鈥� Parmentier says.
He and his colleagues created a computer model of this process and tested it on data about a world called WASP-121b, as well as 13 other similar planets. Parmentier says that it was a good match for most of them, but some strange outliers remain.
The dark sides of ultra-hot Jupiters are too dim for us to observe them and check this hypothesis, but it鈥檚 clear that these Janus-faced worlds are more complicated than we thought.
Reference:
Read more: Fiery exoplanet may see a trillion lightning flashes in an hour
