How planets form: It's much more complicated than we thought

Like parents passing down their distinctive nose or eye color to their children, young planets were thought to be formed after inheriting their chemical makeup directly from the swirling disks of gas and dust that gave birth to them. But nature, it seems, had other plans.

A remarkable study led by Chih-Chun “Dino” Hsu, a postdoctoral associate at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), suggests that we still have alot to learn about how planets form.

Front row seat to planet formation

Most planets that we find outside of our solar system are already fully formed, with their birth materials long since scattered to the cosmic winds.

That’s what makes the PDS 70 system so special. Located 366 million light-years away in the constellation Centaurus, this stellar nursery gives scientists a “front-row seat” to planet formation in action.

The system features two baby gas giants, similar to Jupiter, that are still in their formative years. At just 5 million years old, these planetary toddlers – named PDS 70b and PDS 70c – are still surrounded by their natal disks, the very materials from which they are forming.

Chemical surprise in a forming planet

When the research team studied PDS 70b’s atmosphere, they found something unexpected. The ratio of carbon to oxygen didn’t match what they saw in the surrounding disk of gas and dust.

“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.”

Faint planets next to bright stars

Studying a planet that is forming next to its bright parent star is no small feat. It’s like trying to spot a firefly next to a spotlight.

Jason Wang is an assistant professor at Northwestern University who co-developed the technology used in the study.

“These new tools make it possible to take really detailed spectra of faint objects next to really bright objects. Because the challenge here is there’s a really faint planet next to a really bright star. It’s hard to isolate the light of the planet in order to analyze its atmosphere,” explained Wang.

The experts used specialized photonics technologies to capture the planet’s spectrum – which is essentially its chemical fingerprint. This allowed the team to measure the amount of carbon monoxide and water in the atmosphere of PDS 70b, and calculate the ratio of carbon to oxygen.

Possible explanations for the mismatch

The mismatch between the planet’s composition and that of its natal disk poses an interesting puzzle. The research team proposes two possible explanations.

First, the planet might have formed before its disk became enriched with carbon. Think of it like baking a cake. The finished product’s ingredients depend on what was available in your kitchen when you started baking – not what you added to your pantry later.

The second possibility involves solid materials. The planet might have grown by absorbing large amounts of ice and dust, not just gas. When these solid materials evaporated, they could have altered the final ratio of carbon to oxygen in the planet’s atmosphere.

A better understanding of planet formation

This study marks the first time scientists have been able to compare information from an exoplanet, its natal disk, and host star. But it’s just the beginning. The team plans to study PDS 70c next, hoping to understand better how this entire system formed.

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

This discovery reminds us that the universe often defies our expectations. As we continue to peer deeper into space, we are likely to find more surprises that challenge our understanding of how planetary systems come into existence.

The research was supported by the Heising-Simons Foundation, the Simons Foundation, and the National Science Foundation. The findings are published in The Astrophysical Journal Letters.

Image Credit: ESO/A. Müller et al.

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