Researchers at Northwestern University have discovered that the composition of gases in a young exoplanet's atmosphere does not match the gases in its natal disk, challenging conventional theories of planet formation. The study, led by Chih-Chun “Dino” Hsu and to be published in the Astrophysical Journal Letters, marks the first time scientists have compared an exoplanet's atmospheric data with its natal disk and host star.
“For observational astrophysicists, one widely accepted picture of planet formation was likely too simplified,” said Hsu. “According to that simplified picture, the ratio of carbon and oxygen gases in a planet’s atmosphere should match the ratio of carbon and oxygen gases in its natal disk — assuming the planet accretes materials through gases in its disk. Instead, we found a planet with a carbon and oxygen ratio that is much lower compared to its disk. Now, we can confirm suspicions that the picture of planet formation was too simplified.”
The research focused on PDS 70b, a gas-giant exoplanet located 366 million lightyears away within the constellation Centaurus. This system includes two young planets still forming within their natal disk. Jason Wang, an assistant professor at Northwestern University and member of CIERA (Center for Interdisciplinary Exploration and Research in Astrophysics), noted: “This is a system where we see both planets still forming as well as the materials from which they formed.”
To measure PDS 70b's atmospheric composition, researchers examined light spectra emitted from the planet. This method allowed them to identify specific molecules or elements present in PDS 70b's atmosphere. The team used advanced photonics technologies co-developed by Wang to capture these faint spectral features despite interference from nearby bright stars.
“We initially expected the carbon-to-oxygen ratio in the planet might be similar to the disk,” Hsu explained. “But instead, we found the carbon relative to oxygen in the planet was much lower than the ratio in the disk.”
Hsu and Wang propose two possible explanations for this discrepancy: either PDS 70b formed before its surrounding disk became enriched with carbon or it grew by absorbing solid materials like ice and dust along with gases.
“If the planet preferentially absorbed ice and dust, then that ice and dust would have evaporated before going into the planet,” Wang said. “So it might be telling us that we can’t just compare gas versus gas. The solid components might be making a big difference in the carbon-to-oxygen ratio.”
Future studies will focus on analyzing spectra from another exoplanet within this system to gain further insights into planetary formation processes.
“By studying these two planets together, we can understand the system’s formation history even better,” Hsu stated.
The research received support from several foundations including Heising-Simons Foundation, Simons Foundation as well as National Science Foundation.