Webb maps two very different twilights on a scorching alien world

Astronomers using the James Webb Space Telescope have mapped striking differences between the morning and evening skies of WASP-121 b, an ultra-hot gas giant orbiting a distant star. The findings were published in Nature Astronomy.

The planet is an extreme place. Tidally locked to its star, one hemisphere is permanently scorched while the other faces away in perpetual night. Daytime temperatures reach around 2,770 kelvin, while the nightside sits closer to 1,000 kelvin, creating a vast thermal gulf across the world.

That temperature contrast is so severe that the physics of the planet's atmosphere bears little resemblance to anything in our own Solar System. At these temperatures, materials that are solid rock on Earth can exist as vapour, and the boundary between the two faces of the planet becomes a region of violent change.

Winds that warp the weather

By studying the narrow boundary regions where day meets night, known as terminators, the team found the evening side to be hotter and more puffed-up than the morning side. The data indicate the evening terminator absorbs more light, consistent with powerful winds dragging heat away from the permanent dayside and around the planet.

This asymmetry is the kind of detail that was simply out of reach for previous instruments. Distinguishing between the morning and evening edges of a planet hundreds of light-years away requires extraordinary sensitivity, because both are seen at once as the planet passes in front of its star. Teasing the two apart is a testament to how far the technique has advanced.

The picture that emerges is of a world whirling with super-fast winds that redistribute heat, smearing the energy from the blistering dayside towards the cooler regions. Understanding these wind patterns is central to building accurate models of how such planets behave.

Because the planet is tidally locked, with one face permanently turned towards its star, those winds are the principal mechanism by which heat is moved around at all. The way they carry energy from day to night, and the asymmetry that leaves the evening side hotter than the morning, gives astronomers a rare chance to test their theories of atmospheric circulation against an environment far more extreme than any in our own neighbourhood.

Chemistry written in starlight

The observations also turned up chemical clues, including signs that water is being torn apart by the intense heat and hints that mineral clouds may be shaping the cooler side's atmosphere. Such detail was only possible thanks to Webb's sensitivity, which lets astronomers tease apart conditions in different parts of a single exoplanet.

The idea that water molecules are being ripped apart underscores just how hostile the dayside is. On the cooler regions, by contrast, the suggestion of clouds made not of water but of minerals points to an atmosphere where the very concept of weather takes on an alien form. The findings together build a richer portrait of the planet than any single measurement could provide:

• A hotter, more expanded evening terminator than the morning side
• Evidence of powerful winds carrying heat around the planet
• Signs that water is being broken apart by extreme dayside temperatures
• Hints of mineral clouds shaping the cooler regions
• A daytime temperature of about 2,770 kelvin against roughly 1,000 kelvin at night

“Fierce winds appear to carry heat from the planet's permanent dayside, making the evening side hotter and more expanded.”

— Nature Astronomy study summary

Background: a new era of exoplanet weather

When the first exoplanets were confirmed in the 1990s, astronomers could do little more than infer their existence and rough size. The field has since advanced at remarkable speed, moving from simply detecting distant worlds to characterising their atmospheres. Ultra-hot gas giants like WASP-121 b have become favoured targets precisely because their extreme conditions make their atmospheres easier to study, even if no one would want to visit.

The James Webb Space Telescope, launched at the end of 2021, has been central to this leap. Its infrared instruments are exquisitely suited to dissecting the faint light that filters through a planet's atmosphere, allowing chemists and physicists to read off the ingredients and conditions of worlds far beyond reach of any spacecraft.

“We are moving from cataloguing planets to studying their climates, and that shift would have seemed fantastical a generation ago.”

— Exoplanet astronomer

What it means

Mapping the weather of worlds dozens of light-years away was unthinkable not long ago. Each detailed study like this one feeds into models that researchers will eventually apply to smaller, more temperate and potentially more habitable planets. As Webb continues to probe exotic atmospheres, planets like WASP-121 b are helping researchers refine their understanding of how heat, wind and chemistry interact far beyond our Solar System, laying groundwork for the harder searches still to come.