Chemical clues reveal the origins of a hot exoplanet

Observations from the James Webb Space Telescope (JWST) have unlocked new clues about the formation of the exoplanet WASP-121b.

By detecting key molecules like water vapor, carbon monoxide, silicon monoxide, and methane, astronomers have pieced together a clearer picture of where this planet may have started its life and how it evolved. These findings have also revealed surprising behavior in its atmosphere that challenges current models.

The research was conducted by a team of scientists from the Max Planck Institute for Astronomy and the University of Newcastle.

A planet of extremes

WASP-121b is no ordinary planet. It’s an ultra-hot gas giant that orbits extremely close to its host star – just about twice the star’s diameter away.

One complete orbit takes only 30.5 hours. Its proximity leads to two dramatically different hemispheres: a dayside with temperatures soaring above 3,000 degrees Celsius and a permanent nightside that cools down to 1,500 degrees.

“Dayside temperatures are high enough for refractory materials – typically solid compounds resistant to strong heat – to exist as gaseous components of the planet’s atmosphere,” explained Thomas Evans-Soma, lead author of the study.

Chemical signatures of the past

The research team focused on detecting compounds that evaporate at different temperatures. These compounds act as fingerprints, providing vital information about the planet’s history.

“Gaseous materials are easier to identify than liquids and solids,” said MPIA student Cyril Gapp. “Since many chemical compounds are present in gaseous form, astronomers use WASP-121b as a natural laboratory to probe the properties of planetary atmospheres.”

The data suggests WASP-121b formed where water could freeze but methane remained a gas. This region would have been at a distance from its star similar to the area between Jupiter and Uranus in our solar system.

Yet, the planet is now perilously close to its star. This points to a long migration from the outer, colder regions inward.

The turbulent life of WASP-121b

The chemical makeup of WASP-121b tells a story of its turbulent youth. Silicon, found as silicon monoxide gas today, likely arrived via rocky materials like quartz embedded in planetesimals – small building blocks similar to asteroids.

“The relative abundances of carbon, oxygen, and silicon offer insights into how this planet formed and acquired its material,” said Evans-Soma.

Planets begin as clumps of icy dust, which stick together and grow into pebbles. These pebbles attract gas and smaller particles, growing larger over time. However, the surrounding gas causes the pebbles to spiral inward toward the star. As they move closer, the ice embedded in them begins to evaporate.

At certain stages, young planets become massive enough to carve gaps in the disk of gas and dust around their star. This blocks the inward drift of pebbles but leaves gas flowing, allowing planets to build thick atmospheres.

For WASP-121b, this process likely took place in a region where methane could evaporate but water remained frozen. Consequently, the planet captured carbon-rich, oxygen-poor gas, accounting for its higher carbon-to-oxygen ratio than its host star.

Unexpected winds on the nightside

Methane shouldn’t survive at the high temperatures of WASP-121b’s dayside. It breaks down quickly under such heat.

Scientists expected the same for the nightside, where, despite cooler temperatures, gas circulation from the dayside would typically prevent methane from accumulating.

However, they found abundant methane on the nightside, which came as a surprise. To explain this, the researchers propose that strong vertical currents are rapidly lifting methane from deeper, cooler atmospheric layers.

“This challenges exoplanet dynamical models, which will likely need to be adapted to reproduce the strong vertical mixing we’ve uncovered on the nightside of WASP-121b,” said Evans-Soma.

This discovery highlights the need to rethink how scientists model atmospheric behavior on extreme planets.

Observing the planet’s entire orbit

Astronomers made the observations using JWST’s Near-Infrared Spectrograph (NIRSpec). The team monitored WASP-121b over its entire orbit, capturing how the heat signature of the planet changed as different parts of its atmosphere rotated into view.

The experts also gathered data during the planet’s transit in front of its star. During this transit, some starlight passed through the planet’s atmosphere, leaving distinct spectral signatures.

“The emerging transmission spectrum confirmed the detections of silicon monoxide, carbon monoxide, and water that were made with the emission data,” Gapp explained. “However, we could not find methane in the transition zone between the day and nightside.”

The absence of methane in the transition zone further supports the idea that vertical mixing is confined to the nightside, a detail that current models fail to predict.

The full study was published in the journal Nature Astronomy.

Image Credit: T. Müller (MPIA/HdA)

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