James Webb traces birth and journey of scorching exoplanet WASP-121b

This artist’s impression shows the phase when WASP-121b gathered most of its gas, based on recent findings. The image suggests the young planet had already cleared its distant orbit of icy pebbles containing water. This gap likely stopped more solid material from reaching it. WASP-121b then appears to have drifted from the chilly outer regions to its current close orbit around its star. (Image: T. Müller/MPIA/HdA)

After decades of cosmic curiosity, a faraway giant has weighed in. Astronomers looking through the James Webb Space Telescope (JWST) have cracked the chemical narrative of WASP-121b, a hot gas planet with two opposing extremes. The world orbits its star with such proximity that it takes 30.5 hours to finish a cycle, scorching one side and freezing the other.

JWST's high-resolution image discovered water vapour, methane, carbon monoxide and silicon monoxide churning around in its atmosphere. These gases provided scientists with hints about the planet's early days. Scientists created a map of carbon, oxygen and silicon levels in WASP-121b's atmosphere based on this data. This was accomplished by Thomas Evans-Soma and Cyril Gapp, whose results are published in Nature Astronomy and The Astronomical Journal.

A World Conceived in Ice, Now Soaked in Fire

WASP-121b is infamous for its blistering dayside with temperatures of 3000°C. The nightside, on the other hand, broils at a lower temperature of 1500°C. Its proximity to the star is untypical and hints at a migration inwards from a colder point of origin.

Evans-Soma clarified that the dayside is warm enough to vaporize even heat-resistant material. Such rocky components as quartz, which initially came from planetesimals – early building blocks of planets – are such. The planet eventually acquired gases as it orbited in the cold region. In our Solar System, this would be between Jupiter and Uranus.

Chemical signatures indicate WASP-121b to have been created in an area cold enough for water to freeze but warm enough for methane to be present in gaseous form. This accounts for the higher ratio of carbon to oxygen in the planet relative to its star. The water remained frozen in pebbles as methane vented to gas. This provided WASP-121b with a carbon-dense atmosphere.

Planetary formation begins with icy dust sticking together to form pebbles. These migrate towards the star, venting gas as they warm up. When a forming planet grows large enough, it shuts off the pebbles but continues to accrete gas. That is how WASP-121b could have built a dense, carbon-rich atmosphere.

Methane on the Dark Side Raises New Questions

One surprise discovery was the presence of methane on the cold nightside. Researchers had thought gas would flow rapidly from day to night sides and be in short supply everywhere. But not so.

Astronomers think intense vertical winds would blow methane from lower, colder layers upward. Such winds defy current models of exoplanetary atmosphere behaviour. Evans-Soma theorised that existing theories could be revised to explain this mixing process.

To achieve these findings, the team employed JWST's Near-Infrared Spectrograph (NIRSpec) to monitor WASP-121b's complete orbit. This revealed temperature and chemistry changes on its surface. When it transited in front of its star, bits of starlight passed through its atmosphere. This assisted in confirming silicon monoxide, water and carbon monoxide.

But methane was absent in the area between the day and night sides. This lends credence to the notion that vertical winds are required to ensure methane is maintained only in cooler regions.

A Collaboration Across Continents and Institutions

The research involved researchers from Germany, the UK, the US, India and Australia. Evans-Soma is associated with the Max Planck Institute for Astronomy and the University of Newcastle. Gapp is also associated with Heidelberg University.

Other contributions were made by the experts from Johns Hopkins University, The Open University, University of Birmingham, the Carnegie Institution, the University of Oxford, and Indian and US institutions.

NIRSpec was built by the European Space Agency in collaboration with Airbus Defence and Space. Detectors and micro-shutters were provided by NASA. The Max Planck Institute provided the electrical components for the NIRSpec grating wheels.

JWST is still a joint NASA, ESA, and Canadian Space Agency mission. Its unparalleled perspective continues to uncover secrets of worlds far beyond our own.