Space pebbles and rocks play pivotal role in giant planet's formation

Space pebbles and rocks play pivotal role in giant planet's formation
by Staff and Agencies
London, UK (SPX) Jun 04, 2025

Scientists analysing an ultra-hot giant planet believe it was formed by absorbing lightweight gases like methane evaporating from tiny space pebbles, whilst being bombarded with large rocky objects.

Using the James Webb Space Telescope (JWST) to explore the atmosphere of WASP-121b, researchers successfully detected water (H2O), carbon monoxide (CO), and silicon monoxide (SiO) in the side facing its star or 'dayside'. They also found methane (CH4) in the planet's 'nightside' atmosphere.

Publishing its findings June 2nd in Nature Astronomy, the international research team's discoveries mark the first conclusive identification of SiO in any planetary atmosphere, either within or beyond our solar system.

WASP-121b orbits its host star at a distance only about twice the star's diameter, meaning its eternal dayside has temperatures locally exceeding 3000 degrees Celsius, while the nightside drops to 1,500 degrees.

Co-author Dr Anjali Piette, from the University of Birmingham, commented: "Detecting SiO in WASP-121b's atmosphere is groundbreaking - the first conclusive identification of this molecule in any planetary atmosphere.

"The 'nightside' atmospheric composition of WASP-121b also suggests 'vertical mixing' - the transport of gas from deeper atmospheric layers to the infrared photosphere. Given how hot this planet is, we weren't expecting to see methane on its nightside."

The measured atmospheric carbon-to-hydrogen (C/H), oxygen-to-hydrogen (O/H), silicon-to-hydrogen (Si/H), and carbon-to-oxygen (C/O) ratios suggest that, during its formation, WASP-121b's atmosphere was enriched by inward-drifting pebbles supplemented by a bombardment of refractory material.

"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," lead-author Dr Thomas Evans-Soma, from the University of Newcastle (Australia), explained.

Scientists analysing the atmosphere of WASP-121b used a technique called 'phase curve observation', which involves watching the planet as it orbits its star to see how its brightness changes. These observations provide a view of both the dayside and nightside hemispheres, and their chemical makeup.

"The successful use of JWST to detect these elements and characterize the atmosphere of WASP-121b demonstrates the telescope's capabilities and sets a precedent for future exoplanet studies," added Dr Piette.

Research Report:SiO and a super-stellar C/O ratio in the atmosphere of the giant exoplanet WASP-121b