Webb discovers that planet-forming disks lived longer in the early universe

The NASA/ESA/CSA James Webb Space Telescope has solved a puzzle by proving a controversial discovery made with the NASA/ESA Hubble Space Telescope more than 20 years ago.

In 2003, Hubble revealed the existence of a massive planet around a very old star, almost as old as the universe. These stars have only small amounts of the heavier elements that make up the building blocks of planets. This implied that some planet formation had occurred when our universe was very young and that these planets had had time to form and grow inside their primordial disks, even larger than Jupiter. But how? It was confusing.

To answer this question, the researchers used Webb to study stars in a nearby galaxy that, like the early universe, lacks large amounts of heavy elements. They discovered that not only do some stars have planet-forming disks, but these disks are longer-lived than those observed around young stars in our galaxy, the Milky Way. The work is published in The Astrophysics Journal.

“With Webb, we have very strong confirmation of what we saw with Hubble, and we need to rethink how we model planet formation and early evolution in the young universe,” said Guido De Marchi, head of study by ESA’s European Space Research and Technology Center. Noordwijk, Netherlands.

A different environment at the beginning

In the early universe, stars formed mainly from hydrogen and helium, and very few heavier elements such as carbon and iron, which appeared later in supernova explosions.

“Current models predict that with so few heavier elements, the disks around stars are short-lived, so short in fact that planets cannot grow,” said co-researcher Elena Sabbi. the Webb study and chief scientist of the Gemini Observatory at National Science. NOIRLab Foundation in Tucson. “But Hubble actually saw these planets, what if the models weren’t correct and the disks could live longer?”

To test this idea, scientists trained Webb on the Small Magellanic Cloud, a dwarf galaxy that is one of the Milky Way’s closest neighbors. In particular, they looked at the massive star-forming cluster NGC 346, which also has a relative lack of heavier elements. The cluster served as a nearby proxy to study stellar environments with similar conditions in the distant, early universe.

Hubble observations of NGC 346 in the mid-2000s revealed numerous stars around 20 to 30 million years old that appeared to still be surrounded by planet-forming disks. This went against the conventional belief that such disks would dissipate after 2 or 3 million years.

“Hubble’s findings were controversial, going against not only empirical evidence for our galaxy, but also current models,” De Marchi said. “It was intriguing, but without a way to obtain the spectra of these stars, we couldn’t really establish whether we were seeing true accretion and the presence of disks, or just artificial effects.”

Today, thanks to Webb’s sensitivity and resolution, scientists have the first-ever spectra of Sun-like forming stars and their immediate environments in a nearby galaxy.

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

A new way of thinking

This finding disproves previous theoretical predictions that when there were very few heavier elements in the gas around the disk, the star would explode the disk very quickly. The lifespan of the disk would therefore be very short, even less than a million years. But if a disk doesn’t stay around the star long enough for the dust grains to stick together and the pebbles to form and become the core of a planet, how can planets form?

The researchers explained that there could be two distinct mechanisms, or even a combination, allowing planet-forming disks to persist in environments poor in heavier elements.

First, to be able to blow the disk, the star applies radiation pressure. For this pressure to be effective, elements heavier than hydrogen and helium would need to reside in the gas. But the massive star cluster NGC 346 contains only about 10% of the heaviest elements found in the chemical makeup of our Sun. Perhaps it simply takes longer for a star in this cluster to disperse its disk.

The second possibility is that for a Sun-like star to form when there are few heavier elements, it would have to start from a larger gas cloud. A larger gas cloud will produce a larger disk. So there is more mass in the disk and so it would take longer to explode the disk, even if the radiation pressure worked the same.

“With more matter around stars, accretion lasts longer,” Sabbi said. “Disks take ten times longer to disappear. That has implications for how you form a planet and what kind of system architecture you can have in these different environments. It’s so exciting.”