Young planet's atmosphere challenges traditional formation models

Los Angeles CA (SPX) Dec 26, 2024
Scientists have long believed that planets forming within a swirling disk of gas and dust should closely resemble the composition of their birthplace. However, new research led by Northwestern University astrophysicists suggests this assumption may be oversimplified.

The study, published in Astrophysical Journal Letters, analyzed a young exoplanet and its natal disk, revealing unexpected differences in gas composition. The findings could reshape how scientists understand planetary formation processes.

"For observational astrophysicists, one widely accepted picture of planet formation was likely too simplified," said Chih-Chun "Dino" Hsu, the study's lead author and postdoctoral associate at the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). "Now, we can confirm suspicions that the picture of planet formation was too simplified."

A Unique View of Planet Formation

Planets emerge from natal disks, rotating collections of gas and dust that encircle new stars. Gravity draws these materials together over time to form planets. Most observable exoplanets are older, making it challenging to study their formative environments.

The PDS 70 system, however, offers a rare opportunity. Located 366 million light-years away in the constellation Centaurus, it hosts two forming gas-giant planets, PDS 70b and PDS 70c. Both planets are no more than 5 million years old, allowing researchers to compare their atmospheric makeup directly to their surrounding natal disk.

"This is a system where we see both planets still forming as well as the materials from which they formed," said Jason Wang, an assistant professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences and a member of CIERA.

Atmospheric Analysis

The researchers focused on PDS 70b, using advanced photonics technologies to isolate the planet's faint light from its much brighter host star. The technique allowed them to capture and analyze the planet's spectra - a unique fingerprint that reveals its chemical composition.

Examining the spectra, the team measured carbon monoxide and water in PDS 70b's atmosphere and calculated the carbon-to-oxygen ratio. Surprisingly, they found the planet's ratio to be significantly lower than that of its natal disk.

"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. That was a bit surprising."

A Closer Look at Formation Processes

Two potential scenarios could explain the discrepancy. The planet may have formed before the disk became enriched in carbon, or it may have absorbed significant amounts of solids, such as ice and dust, in addition to gases.

"If the planet preferentially absorbed ice and dust, then that ice and dust would have evaporated before going into the planet," Wang said. "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."

Expanding the Research

For now, the study focuses solely on PDS 70b, but the researchers plan to analyze PDS 70c to gain a more comprehensive understanding of the system.

"By studying these two planets together, we can understand the system's formation history even better," Hsu said. "But, also, this is just one system. Ideally, we need to identify more of them to better understand how planets form."

The study, PDS 70b shows stellar-like carbon-to-oxygen ratio, was supported by the Heising-Simons Foundation, the Simons Foundation, and the National Science Foundation.

Research Report:PDS 70b shows stellar-like carbon-to-oxygen ratio