In 2003, Hubble observed a massive planet orbiting an ancient star nearly as old as the Universe itself. This raised questions about how such planets could form in an era where heavier elements - the critical building blocks of planets - were scarce. Scientists have now resolved this mystery using Webb to study nearby environments that mirror the conditions of the early Universe.
Researchers targeted the Small Magellanic Cloud, a dwarf galaxy neighboring the Milky Way, focusing on NGC 346, a massive, star-forming cluster known to be poor in heavy elements. "With Webb, we have a strong confirmation of what we saw with Hubble, and we must rethink how we create computer models for planet formation and early evolution in the young Universe," said Guido De Marchi of the European Space Research and Technology Centre in Noordwijk, the Netherlands.
Previous models predicted that planet-forming discs around stars would dissipate quickly, especially in environments low in heavier elements. However, Webb's observations revealed that stars in NGC 346 retain their discs for far longer periods - up to 20 or 30 million years. This finding contrasts sharply with the conventional understanding that such discs disappear after just 2 or 3 million years.
"Current theoretical models predict that with so few heavier elements, the discs around stars have a short lifetime, so short in fact that planets cannot grow big," said the Webb study's co-investigator Elena Sabbi, chief scientist for Gemini Observatory at the National Science Foundation's NOIRLab in Tucson, Arizona, USA. "But Hubble did see one of those planets, so what if the models were not correct and discs could live longer?"
Thanks to Webb's unparalleled resolution and sensitivity, scientists captured the first-ever spectra of Sun-like stars and their surrounding discs in the Small Magellanic Cloud. The data confirmed that these stars are actively accreting material at an age when discs in the Milky Way would have long dissipated. "We see that these stars are indeed surrounded by discs and are still in the process of gobbling material, even at the relatively old age of 20 or 30 million years," added Guido.
The persistence of these discs implies that planets in such environments have significantly more time to form and grow. This discovery also challenges assumptions about how stellar radiation influences disc dissipation in regions with minimal heavy elements.
The research team proposed two mechanisms that might explain why planet-forming discs survive longer under such conditions. The first suggests that stars in metal-poor environments are less efficient at dispersing discs because heavier elements amplify the pressure applied by stellar light to gas in the disc. In NGC 346, the abundance of heavy elements is only ten percent of what exists in the Sun.
The second possibility points to larger gas clouds being necessary to form Sun-like stars when heavier elements are scarce. Larger clouds naturally produce larger discs, which, in turn, take longer to dissipate.
"With more matter around the stars, the accretion lasts for a longer time," said Elena. "The discs may take ten times longer to disappear. This has implications for how you form a planet, and the type of planetary systems that you can have in these different environments. This is so exciting."
The findings not only validate Hubble's controversial discovery but also demand a reevaluation of planet formation theories. The extended lifespan of planet-forming discs under metal-poor conditions reshapes how scientists envision planetary system development in the Universe's early epochs.
The team's research appears in the December 16, 2024, issue of The Astrophysical Journal.
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