Young exoplanet’s atmosphere unexpectedly differs from its birthplace

But in a new study, a team of astrophysicists led by Northwestern University finds that the similarity may be looser than previously thought. By studying a still-forming exoplanet and its surrounding birth disk, researchers found that the composition of the gases in the planet’s atmosphere did not match the composition of the gas in the disk.

The surprising discovery may confirm long-held suspicions that scientists’ current models of planet formation are oversimplified.

The study will be published on Wednesday (December 18) in Astrophysical Journal Letters. This marks the first time physicists have compared information about an exoplanet, its birth disk and its host star.

“A widely accepted picture of planet formation may be too simplistic for observational astrophysicists,” said Chih-Chun “Dino” Hsu of Northwestern University, who led the study. “Based on this simplified picture, , the ratio of carbon to oxygen in a planet’s atmosphere should match the ratio of carbon to oxygen in its birth disk—assuming the planet accreted material through the gas in its disk. Instead, we found a planet. Now, we can confirm planet formation. The suspicion that the process is oversimplified.

Xu is a postdoc at the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). His advisor is Jason Wang, assistant professor of physics and astronomy in Northwestern University’s Weinberg College of Arts and Sciences and a member of CIERA.

Looking for visible birth material

All planets are born from a birth disk, a rotating disk of gas and dust surrounding a new star. Over millions of years, gravity held gas and dust together to form clumps that eventually grew into planets. Until recently, it was not possible to directly observe birth charts to track the birth of planets. Most observable exoplanets are so old that their birth disks have been lost.

The exception, however, is PDS 70, a birth disk enveloping two fledgling gas giant exoplanets (similar to Jupiter) called PDS 70b and PDS 70c. These planets are located in the constellation Centauri, only 366 million light-years away from Earth, and are only 5 million years old at most.

“In this system, we see two planets and the material from which they were formed that are still forming,” Wang said. “Previous studies have analyzed this gas disk to understand its composition. For the first time, we are able to measure the still forming the composition of the planet itself and understand how similar the materials in the planet are to those in the gas disk.”

Check planetary fingerprints

To measure these materials, Hsu, Wang and their team examined the light emitted by PDS 70b. This light, or spectrum, acts like a fingerprint, revealing the object’s composition, movement, temperature and other characteristics. Each molecule or element produces its own spectrum. Therefore, by studying these spectra, researchers can identify specific molecules or elements within an object.

In previous work, Wang co-developed new photonic techniques that allow astronomers to capture the spectra of faint objects near brighter stars. Researchers used this technique to zero in on the faint signatures of young planetary systems.

“These new tools make it possible to take very detailed spectra of faint objects next to really bright objects,” Wang said. “Because the challenge here is that you have a very faint planet next to a very bright star. It’s very difficult to isolate The planet’s light to analyze its atmosphere.”

Using these spectra, the researchers obtained information about carbon monoxide and water from PDS 70b. From this, they calculated the inferred proportions of carbon and oxygen in the planet’s atmosphere. They then compared this ratio with previously reported measurements of gases in magnetic disks.

“We initially expected that the carbon-to-oxygen ratio on the planet might be similar to that of a disk,” Xu said. “But, instead, we found that the ratio of carbon to oxygen in the planet was much lower than in the disk. This was somewhat surprising and suggests that our widely accepted picture of planet formation is too simplistic.”

Sturdy components could make a difference

To explain this mismatch, Hsu and Wang suggested that two different circumstances may be at play. One explanation is that the planet may have formed before its disk became rich in carbon. Another explanation is that the Earth may have grown primarily by absorbing large amounts of solid matter in addition to gases. Although the spectrum shows only gas, some of the carbon and oxygen may have been accreted from the solid initially – trapped in ice and dust.

“If the Earth preferentially absorbs ice and dust, then the ice and dust would evaporate before entering the Earth,” Wang said. “So, it may tell us that we can’t just compare gas to gas. The solid component may have an impact on the carbon-oxygen ratio A big impact.”

In this study, the team only studied PDS 70b. Next, they plan to observe the spectrum of another planet in the PDS 70 system.

“By studying these two planets together, we can better understand the system’s formation history,” Xu said. “However, this is just one system. Ideally, we need to identify more systems 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.