A new study shows planet formation might be more complicated than previously thought. This arises after researchers compared gases in a still-forming planet’s atmosphere to its natal disk. This led to the discovery that the planet surprisingly was less carbon-rich than the disk.
Many scientists have long thought that developing planets should resemble the swirling disk of gas and dust that births them. This may not be the case, according to a Northwestern University study.
By examining a still-forming exoplanet and its surrounding natal disk, the researchers uncovered a mismatched composition of gases in the planet’s atmosphere compared to gases within the disk. This suggests scientists’ current model of planet formation is too simplified.
According to lead researcher Chih-Chun “Dino” Hsu: “For observational astrophysicists, one widely accepted picture of planet formation was likely too simplified.”
He continues: “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.”
All planets are born from a natal disk. This is a rotating disk of gas and dust that surrounds a new star. Over millions of years, gravity pulls gas and dust together to form clumps, which eventually grow into planets. Until recently, it was impossible to obtain a direct view of a natal disk in order to track a planet’s birth.
An exception to this is PDS 70, a natal disk that envelopes two fledgling gas-giant exoplanets — similar to Jupiter — called PDS 70b and PDS 70c. These are located just 366 million lightyears from Earth within the constellation Centaurus. These planets are just 5 million years old.
To measure the materials, the researchers examined the light emitted from PDS 70b. This revealed each planet’s composition, motion, temperature and other characteristics. Each molecule or element produces its own spectrum. So, by studying these spectra, researchers can pinpoint the specific molecules or elements within an object.
With the spectra, the researchers obtained information about carbon monoxide and water from PDS 70b. From that, they calculated the inferred ratio of carbon and oxygen within the planet’s atmosphere. Then, they compared that ratio to previously reported measurements of gases in the disk.
The researchers expected the carbon-to-oxygen ratio in the planet might be similar to the disk. However, they found the carbon, relative to oxygen, in the planet was much lower than the ratio in the disk.
To explain this mismatch, the researchers speculate that two different scenarios might be at play. One explanation is the planet might have formed before its disk became enriched in carbon. Another explanation is the planet might have grown mostly by absorbing large amounts of solid materials in addition to gases. While the spectra show only gases, some of the carbon and oxygen initially could be accreted from solid — trapped in ice and dust.
The research appears in Astrophysical Journal Letters and it is titled “PDS 70b shows stellar-like carbon-to-oxygen ratio.”
