A new study of the ultra-hot Jupiter WASP-121b has revealed a surprising discovery about its formation, challenging existing theories about how gas giants form. The research, led by Peter Smith of Arizona State University, used the Immersion GRating INfrared Spectrograph (IGRINS) on the Gemini South telescope in Chile to analyze the planet's atmosphere.
WASP-121b is a gas giant that orbits its star so closely that its dayside temperature reaches over 2,500 degrees Celsius. This extreme heat vaporizes elements like iron, magnesium, and silicon, making them detectable via spectroscopy. The observations showed that WASP-121b has a high rock-to-ice ratio, indicating that it accreted an excess of rocky material while it was forming.
This finding is surprising because it's generally believed that gas giants need solid ices to form in the outer regions of protoplanetary disks. The high rock-to-ice ratio suggests that WASP-121b formed in a region of the disk where it was too hot for ices to condense. This challenges the current understanding of how gas giants form and may require a reassessment of planet formation models.
The study also revealed remarkable characteristics of WASP-121b's atmosphere. Strong winds blow the vaporized metals to the planet's nightside, where it is cool enough for them to condense and rain out, creating a phenomenon of calcium rain.
This research highlights the capabilities of IGRINS, a high-resolution infrared spectrograph that allows scientists to measure the chemical composition of exoplanet atmospheres with unprecedented detail. The instrument's success has led to the development of IGRINS-2, which is now in operation on the Gemini North telescope in Hawaii.