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Webb Discovers Persistent Planet-Forming Disks in the Early Universe

According to a study published in The Astrophysical Journal, NASA’s James Webb Space Telescope has solved a conundrum by confirming a controversial discovery made with the agency’s Hubble Space Telescope more than 20 years ago.

NGC 346, a massive star cluster in the Small Magellanic Cloud
This is a James Webb Space Telescope image of NGC 346, a massive star cluster in the Small Magellanic Cloud, a dwarf galaxy that is one of the Milky Way's nearest neighbors. With its relative lack of elements heavier than hydrogen and helium, the NGC 346 cluster serves as a nearby proxy for studying stellar environments with similar conditions in the early, distant universe. Ten, small, yellow circles overlaid on the image indicate the positions of the ten stars surveyed in this study. Image Credit: NASA, ESA, CSA, STScI, Olivia C. Jones (UK ATC), Guido De Marchi (ESTEC), Margaret Meixner (USRA)

In 2003, Hubble discovered a large planet orbiting a very old star, nearly as old as the cosmos. Such stars contain only trace amounts of heavier elements, which are the building blocks of planets. This meant that certain planets formed when our universe was extremely young and that such planets had enough time to originate and grow large within their primordial disks, larger than Jupiter. But how?

To resolve this question, researchers used Webb to investigate stars in a nearby galaxy that, like the early cosmos, lacks a high concentration of heavy metals. They discovered that some stars not only have planet-forming disks, but such disks last longer than those seen surrounding young stars in the Milky Way galaxy.

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.

Guido De Marchi, Study Leader, European Space Research and Technology Centre

A Different Environment in Early Times

In the early cosmos, stars were largely made of hydrogen and helium, with just a few heavier elements like carbon and iron appearing later through supernova explosions.

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. But Hubble did see those planets, so what if the models were not correct and disks could live longer?

Elena Sabbi, Study Co-Investigator, NASA

Sabbi is also a Chief Scientist for Gemini Observatory at the National Science Foundation’s NOIRLab in Tucson.

To test this idea, scientists directed Webb at the Small Magellanic Cloud, a dwarf galaxy that is one of the Milky Way’s closest neighbors. They focused specifically on the huge, star-forming cluster NGC 346, which similarly has a low concentration of heavy elements. The cluster was used as a close proxy to explore star settings with similar circumstances in the early, distant universe.

Hubble views of NGC 346 in the mid-2000s found numerous stars 20 to 30 million years old that appeared to still have planet-forming disks surrounding them. This contradicted the common idea that such disks would disintegrate within 2 or 3 million years.

De Marchi added, “The Hubble findings were controversial, going against not only empirical evidence in our galaxy but also against the current models. 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.”

Thanks to Webb's sensitivity and resolution, astronomers now have the first spectra of developing, Sun-like stars and their immediate surroundings in a nearby galaxy.

De Marchi further added, “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. 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.”

A New Way of Thinking

This discovery contradicts prior theoretical predictions that when there are very few heavy elements in the gas around the disk, the star will soon blast away the disk. As a result, the disk's lifetime would be extremely short, possibly less than a million years. But how can planets form if a disk does not remain around the star long enough for dust grains to bind together and pebbles to form, eventually becoming the core of a planet?

The researchers indicated that planet-forming disks might endure in environments with a scarcity of heavier elements through two independent methods, or perhaps a combination of them.

First, the star uses radiation pressure to blow away the disk. For this pressure to be effective, the gas must contain elements heavier than hydrogen and helium. However, the large star cluster NGC 346 contains just around 10 percent of the heavier elements found in the sun’s chemical composition. Perhaps a star in this cluster takes longer to disseminate its disk.

The star first applies radiation pressure to blow away the disk. Elements heavier than hydrogen and helium would need to be in the gas for this pressure to work. However, only around 10% of the heavier elements found in the Sun's chemical makeup are found in the massive star cluster NGC 346. Maybe a star in this cluster just takes longer to spread its disk.

Second, a Sun-like star would have to originate from a larger cloud of gas to form when there are few heavy elements. Disk size will increase with the size of the gas cloud. Since the disk has greater mass, blowing it away would take longer, even if the radiation pressure were acting in the same manner..

Sabbi stated, “With more matter around the stars, the accretion lasts for a longer time. 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.”

Journal Reference:

De Marchi, G., et al. (2024) Protoplanetary Disks around Sun-like Stars Appear to Live Longer When the Metallicity is Low. The Astrophysical Journal. doi.org/10.3847/1538-4357/ad7a63.

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