A new study of the exoplanet WASP-121b, an ultra-hot Jupiter, has challenged traditional theories about planet formation. The research, conducted by a team from Arizona State University's School of Earth and Space Exploration (ASU-SESE), suggests that this gas giant may have formed close to its star, contrary to the prevailing belief that such planets form in the outer reaches of star systems.
The current exoplanet census contains 5,832 confirmed candidates, with more than 7,500 still awaiting confirmation. Most of these confirmed exoplanets are gas giants ranging from Neptune-like bodies to those similar to or many times the size and mass of Jupiter and Saturn. Astronomers have traditionally theorized that these gas giants form in the outer regions of their star systems, where temperatures are cold enough for gases like hydrogen and helium and volatile compounds to condense or freeze solid.
However, the discovery of numerous gas giants orbiting close to their stars, known as "Hot Jupiters," has raised questions about whether these planets migrate after formation to their final orbits. The ASU-SESE team, part of the Roasting Marshmallows Program, focused on studying the atmospheres of hot and ultra-hot Jupiters using the Immersion GRating INfrared Spectrograph (IGRINS) instrument, which is part of the Gemini South telescope in Chile.
The team's analysis of WASP-121b revealed a high rock-to-ice ratio in its atmosphere, indicating that it accreted an excess of rocky material during formation. This finding suggests that the planet formed closer to its star than previously thought, challenging the traditional model of gas giant formation. The reason for this unexpected finding lies in the extreme temperatures on the dayside of WASP-121b, which are so hot that rocky material and metals are vaporized into the atmosphere. Powerful winds then carry these vaporized materials to the night side, where they condense, leading to a phenomenon known as "metal rain."
This research highlights the importance of studying the atmospheres of exoplanets to gain insights into their formation and evolution. The team's findings demonstrate that our understanding of planet formation is still evolving, and that new discoveries continue to challenge our existing models.