Astronomers uncover the topsy-turvy atmosphere of a distant planet

The atmosphere of a distant world has been mapped in detail for the first time, revealing a strange, topsy-turvy weather system, with the fastest winds ever seen inexplicably blowing around the planet’s stratosphere.

Astronomers have studied WASP-121b, also known as Tylos, since 2015. The planet, which is 900 light-years away, is a vast ball of gas double the size of Jupiter and orbits its star extremely closely, completing a full orbit in just 30 Earth hours. This close orbit heats the planet’s atmosphere to temperatures of 2500°C, hot enough to boil iron.

Now, Julia Seidel at the European Southern Observatory in Chile and her colleagues have looked inside Tylos’ scorchingly hot atmosphere using the Very Large Telescope, and found it has at least three distinct layers of gas moving in different directions around the planet – a structure unlike anything astronomers have ever seen. “It’s absolutely crazy, science fiction-y patterns and behaviours,” says Seidel.

The planetary atmospheres in our solar system share a broadly similar structure, where a jet stream of powerful winds blowing in the lower portion of an atmosphere is driven by internal temperature differences, while winds in the upper layers are more affected by temperature differences created by heat from the sun warming the day side of the planet but not the other.

But in Tylos’ atmosphere, the lower layer winds are driven by the heat from its star, travelling away from the warm side, while the jet stream appears to be mostly in the middle layer of the atmosphere, travelling around the planet’s equator in the direction of the planet’s rotation. An upper layer also shows jetstream-like features, but with hydrogen gas drifting outwards from the planet. This is difficult to explain using our current models, says Seidel. “What we see now is actually exactly the inverse of what comes out of theory.”

What’s more, the jet stream on Tylos is the most powerful ever seen, blasting at around 70,000 kilometres per hour across half the planet – almost double the speed of the previous record holder. Exactly what is driving this speed is unclear, but the researchers think that it may be due to the planet’s strong magnetic field or because of ultraviolet radiation from its star. “This could possibly change the flow patterns, but this is all highly speculative,” says Seidel.