JWST reveals surprising origin of fiery giant planet reaching 3000°C

WASP-121b is unlike anything in our Solar System. This giant planet orbits its star so closely that it completes a full orbit in just 30.5 hours. Its dayside reaches a scorching 3000°C, while the eternal nightside cools down to 1500°C. You would think this fiery world has no secrets left. But recent observations from the James Webb Space Telescope (JWST) have uncovered surprising clues about its origins and atmosphere.

Using JWST’s Near-Infrared Spectrograph, scientists detected water vapor, carbon monoxide, silicon monoxide, and methane in WASP-121b’s atmosphere. These molecules reveal a story about where this giant was born and how it formed. “Dayside temperatures are high enough for refractory materials to exist as gaseous components,” explained Thomas Evans-Soma from the Max Planck Institute for Astronomy and the University of Newcastle, Australia.

When planets form, they start as small icy dust particles in a disc of gas and dust surrounding a young star. These particles stick together, growing into pebbles, and eventually, larger rocky planetesimals. In the case of WASP-121b, it formed far from its star, where temperatures were cold enough for water to freeze but warm enough for methane to remain a gas. In our Solar System, such conditions exist between Jupiter and Uranus.

This means WASP-121b likely began its life far away and migrated inward over time. As it moved closer to its star, it accumulated gas rich in carbon while water-rich pebbles remained frozen in the outer regions. This created a higher carbon-to-oxygen ratio in its atmosphere compared to its star. “The relative abundances of carbon, oxygen, and silicon offer insights into how this planet formed,” said Evans-Soma.

Methane’s presence on the nightside was a surprise. Scientists believed that because WASP-121b’s dayside is too hot for methane to survive, the nightside would also lack it. Strong winds were expected to mix gases between the two hemispheres, preventing methane from building up. But the detection showed otherwise.

To explain this unexpected discovery, the research team proposed that powerful vertical currents lift methane-rich gas from deeper, cooler layers to the upper atmosphere on the nightside. “This challenges exoplanet dynamical models,” Evans-Soma said. Current models will need updates to explain this strong vertical mixing.

The JWST observations also revealed super-stellar values for carbon, oxygen, and silicon in the atmosphere. This means the planet has more of these elements than its star. Silicon was detected as silicon monoxide gas, which originally came from rocky materials like quartz stored in planetesimals. These rocky bodies formed later in WASP-121b’s youth, suggesting the planet first gathered its gas envelope and then pulled in solid materials.

Using JWST, scientists detected water vapor with a confidence of up to 13.5 sigma, carbon monoxide up to 12.8 sigma, silicon monoxide up to 6.2 sigma, and methane up to 5.1 sigma. In astronomy, these sigma levels indicate extremely strong evidence. The team’s observations lasted nearly 38 hours, covering two secondary eclipses and a primary transit. By watching WASP-121b throughout its orbit, they studied its dayside and nightside separately.

On the dayside, water vapor, silicon monoxide, and carbon monoxide appeared in emission. However, due to the high temperatures, water and silicon monoxide features were muted as molecules broke apart. On the nightside, methane was detected in absorption, showing the gas absorbed light from the star as it passed behind the planet.

The scientists used phase-curve fitting to analyze brightness changes as the planet orbited. They found the dayside temperature ranged from 2600K to 3200K, with a thermal inversion where upper layers were hotter than lower ones. The nightside cooled from 1400K to 1100K.

They compared the planet’s elemental abundances to its host star’s. Carbon, oxygen, and silicon levels were higher than in the star, suggesting WASP-121b gathered gas enriched by inward-drifting methane-rich pebbles. If it had formed between the water and carbon monoxide ice lines, the planet’s composition would be richer in oxygen. Instead, the higher carbon levels point to its formation beyond the water ice line but within the methane ice line.

The super-stellar silicon-to-hydrogen ratio suggests WASP-121b also accumulated rocky materials after gathering gas. Scientists estimate the silicon abundance matches what would result from adding rocky materials equal to a fraction of Earth’s mass to its atmosphere. Pebble accretion alone could not provide this much silicon, supporting the idea that planetesimals played a role in delivering rocky material.

This study provides fresh evidence for how giant planets form. In the past, detecting both refractory elements like silicon and volatile gases like water vapor in a single observation was difficult because their signals occur at different wavelengths. The sensitivity of JWST allowed scientists to see all these elements together, offering a more complete picture.

The findings also show that a planet’s composition can be complex. Many factors affect it, including where it formed, how far it migrated, the types of materials available, and the timing of gas and solid accretion. Erosion of a planet’s core can also change its atmospheric makeup. Future models will need to account for these complexities.

Researchers involved in this study included scientists from MPIA, University of Newcastle, Heidelberg University, University of St Andrews, Johns Hopkins University, The Open University, University of Birmingham, University of Oxford, Space Telescope Science Institute, Utah Valley University, and NISER India.

The JWST’s NIRSpec instrument was built by a European consortium led by Airbus Defence and Space. NASA’s Goddard Space Flight Center provided detectors and micro-shutters, while MPIA procured electrical components for its grating wheels.

The detection of silicon monoxide was particularly significant. Previously, evidence for SiO came from near-ultraviolet data, which could also be explained by magnesium and iron. JWST’s infrared observations confirmed SiO’s presence and revealed its role in driving the dayside thermal inversion.

Silicon is important for understanding planet formation because it is a primary cloud constituent in hot atmospheres. Its detection alongside volatiles allows scientists to measure both refractory and volatile contents, shedding light on where and how planets formed. “Our SiO detection stands out from previous detections of atomic silicon and silicate clouds,” Evans-Soma noted.

The study also challenges previous assumptions about methane in hot giant planets. Traditional models assumed solar-like carbon-to-oxygen ratios. When this ratio increases, methane abundance rises by 2–4 orders of magnitude while water vapor decreases. This explains why methane dominated the nightside opacity and why water vapor was not detected there.

Atmospheric dynamics add another layer of complexity. On WASP-121b, horizontal winds travel at speeds up to 10 km/s, moving gas between the dayside and nightside faster than chemical reactions can adjust. This should result in a methane-poor composition on the nightside, similar to the dayside. But the detection of methane suggests that vertical mixing, not horizontal winds, controls nightside chemistry.

Scientists ran model calculations to test this idea. They found that strong vertical mixing can lift methane-rich gas from deeper layers to the upper atmosphere, matching observed methane levels. This process is more effective than horizontal winds at maintaining methane abundance on the nightside.

These findings show that exoplanet atmospheres are shaped by complex interactions between chemistry and atmospheric dynamics. Models will need to include both vertical mixing and different carbon-to-oxygen ratios to accurately predict atmospheric compositions.

JWST continues to revolutionize our understanding of planets beyond our Solar System. Its ability to detect multiple elements in a single observation gives scientists new tools to study how planets form and evolve. As more exoplanets are studied with this telescope, researchers will refine their models and uncover the diverse ways that planets like WASP-121b come to be.

This study reminds us that the universe still holds many secrets. WASP-121b’s journey from the icy outskirts to a scorching orbit, combined with powerful vertical winds and a unique atmospheric composition, reveals how dynamic and surprising planetary systems can be.

Research findings are available online in the journal Nature Astronomy.

Note: The article above provided above by The Brighter Side of News.

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