How planets formed in the harsh early universe

NASA’s James Webb Space Telescope has resolved a long-standing mystery, confirming a controversial discovery made over 20 years ago with the Hubble Space Telescope. 

In 2003, Hubble found evidence of a massive planet orbiting a very old star – a star so ancient that it contained very few heavy elements, the building blocks of planets. 

This finding puzzled scientists. How could a planet, especially one larger than Jupiter, form under such conditions in the early universe?

To solve this mystery, researchers turned to Webb, which studied stars in a nearby galaxy with a chemical makeup similar to the early universe. 

Planet-forming disks in the early universe 

What they found changed the way scientists think about planet formation. Not only did some of these stars have planet-forming disks, but those disks also lasted far longer than what is typically seen in young stars in the Milky Way.

“With Webb, we have a really strong confirmation of what we saw with Hubble, and we must rethink how we model planet formation and early evolution in the young universe,” explained study leader Guido De Marchi, an astrophysicist at the European Space Research and Technology Center.

The early universe: A hostile place for planets?

In the early universe, stars formed mostly from hydrogen and helium, with very few heavier elements like carbon and iron. These heavier elements, which come from supernova explosions, appeared later in cosmic history. 

The current models of planet formation suggest that without a significant amount of these elements, disks of gas and dust around young stars would quickly dissipate – too quickly for planets to form.

“Current models predict that with so few heavier elements, the disks around stars have a short lifetime, so short in fact that planets cannot grow big,” said Elena Sabbi, co-investigator of the Webb study and chief scientist at the Gemini Observatory. “But Hubble did see those planets, so what if the models were not correct and disks could live longer?”

To explore this, the team focused Webb on the Small Magellanic Cloud, a dwarf galaxy close to the Milky Way. 

In particular, they studied NGC 346, a massive star-forming cluster within the galaxy. NGC 346 has a similarly low abundance of heavy elements, making it a perfect stand-in for the conditions of the early universe.

Planet-forming disks that defied expectations

Hubble’s observations of NGC 346 in the early 2000s showed something surprising: stars that were 20 to 30 million years old still appeared to have planet-forming disks. 

This contradicted the established belief that such disks should dissipate within two to three million years. At the time, the findings were controversial and lacked spectral data to confirm whether the disks were truly present.

“This was intriguing, but without a way to obtain spectra of those stars, we could not really establish whether we were witnessing genuine accretion and the presence of disks, or just some artificial effects,” De Marchi said.

Now, Webb’s incredible sensitivity and resolution have provided the first-ever spectral data for Sun-like stars in NGC 346, along with their surrounding environments. 

For the first time, scientists could see that these stars were indeed surrounded by planet-forming disks and were still actively accreting material, even at 20 to 30 million years old.

“We see that these stars are indeed surrounded by disks and are still in the process of gobbling material, even at the relatively old age of 20 or 30 million years,” De Marchi explained. “This also implies that planets have more time to form and grow around these stars than in nearby star-forming regions in our own galaxy.”

Rethinking planet formation models

The discovery challenges the theoretical prediction that stars with very few heavier elements would lose their planet-forming disks quickly. 

If radiation pressure from a young star is responsible for dispersing its disk, the process should be faster when the gas lacks heavier elements. In other words, the disk would disappear before planets could form. 

However, Webb’s findings show that disks in environments like NGC 346 are far more resilient than expected.

The team proposed two explanations for why the disks last longer. One possibility is that radiation pressure, which blows away the disk material, depends on the presence of heavier elements in the gas. 

Since NGC 346 has only about 10% of the heavy elements found in our Sun, the stars may simply take longer to disperse their disks.

The second explanation is that stars forming in these environments begin with larger clouds of gas. A larger gas cloud produces a larger disk, and the greater mass would naturally take longer to dissipate, even under the same radiation pressure.

“With more matter around the stars, the accretion lasts for a longer time,” Sabbi said. “The disks take ten times longer to disappear. This has implications for how you form a planet, and the type of system architecture that you can have in these different environments. This is so exciting.”

A universe full of surprises

The study highlights how environments with few heavy elements – once thought hostile to planet formation – can still support the growth of massive planets. 

Webb’s observations offer a glimpse into the early universe, revealing that planet-forming disks in ancient conditions not only exist but also persist far longer than expected.

The research, published in The Astrophysical Journal, opens a new chapter in our understanding of planet formation. By challenging long-standing models, Webb has shown that the universe may have been creating planets far earlier, and under far harsher conditions, than previously imagined.

As De Marchi and his team continue to study these resilient disks, the findings promise to reshape how scientists view the evolution of planets – and life – in the cosmos. 

Image Credit: NASA, ESA, CSA, STScI, Olivia C. Jones (UK ATC), Guido De Marchi (ESTEC), Margaret Meixner (USRA)

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