Role of Space Pebbles in the Formation of Giant Exoplanet WASP-121b

In a groundbreaking study published in *Nature Astronomy* on June 2, 2025, an international team of researchers has revealed the critical role that space pebbles and rocks played in the formation of the ultra-hot giant exoplanet WASP-121b. Utilizing data from the James Webb Space Telescope (JWST), the scientists successfully identified key atmospheric components, including water (H₂O), carbon monoxide (CO), and silicon monoxide (SiO), marking the first conclusive detection of SiO in any planetary atmosphere, whether within or beyond our solar system.

WASP-121b, which orbits its host star at a proximity approximately twice the star's diameter, exhibits extreme temperature variations, with its dayside reaching over 3000 degrees Celsius while its nightside cools to around 1500 degrees. This unique thermal gradient allows for significant atmospheric dynamics, including 'vertical mixing'—the process by which gases from the lower atmosphere rise to the upper layers. According to Dr. Anjali Piette, a co-author from the University of Birmingham, the detection of SiO is a noteworthy milestone in exoplanetary studies, underscoring the JWST's capabilities in detecting elemental compositions in hostile environments.

The research team, led by Dr. Thomas Evans-Soma from the University of Newcastle (Australia), posited that the atmospheric composition of WASP-121b is enriched by inward-drifting pebbles that were bombarded by refractory materials during its formation. This indicates a complex genesis involving not only gaseous elements but also solid compounds that have been altered by extreme heat. The team employed a technique known as 'phase curve observation', which monitors the planet's brightness variations as it orbits its star, to gain insights into both its dayside and nightside chemical makeup.

The findings have significant implications for our understanding of planetary formation processes. According to Dr. Evans-Soma, the high temperatures on the dayside allow for refractory materials to exist in gaseous states, influencing the planet's atmospheric structure. This research not only enhances the understanding of WASP-121b but also sets a precedent for future investigations into exoplanets, particularly those with extreme environments.

The implications of these findings extend beyond the study of WASP-121b itself. They challenge existing theories regarding the formation of gas giants and provide a framework for understanding similar exoplanets. The international collaboration involved experts from various institutions, including the University of Birmingham, the University of Newcastle, and additional research teams, highlighting a growing commitment to uncovering the mysteries of planetary atmospheres.

In summary, the study of WASP-121b emphasizes the significance of both gaseous and solid materials in the formation of planetary atmospheres, a realization that could reshape our understanding of planetary science. As research continues, further advancements in telescope technology and observational methods promise to unveil more secrets of the universe’s planetary bodies, offering new insights into their origins and compositions.