A different environment in early times

In the early Universe, stars formed from mostly hydrogen and helium, and very few heavier elements such as carbon and iron, which came through the first generation of supernova explosions.

“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?”

To test this idea, scientists trained Webb on the Small Magellanic Cloud, a dwarf galaxy that is one of the Milky Way’s nearest neighbours. In particular, they examined the massive, star-forming cluster NGC 346, which is also poor of heavier elements. The cluster serves as a nearby proxy for studying stellar environments with similar conditions in the early, distant Universe.

Hubble observations of NGC 346 from the mid 2000s revealed many stars about 20 to 30 million years old that seemed to still have planet-forming discs around them. This went against the conventional belief that such discs would dissipate after 2 or 3 million years.

“The Hubble findings were controversial, going against not only empirical evidence in our galaxy but also against the current models,” said Guido. “This was intriguing, but without a way to obtain spectra of those stars, we could not really establish whether we were witnessing the stars’ growth and the presence of discs, or just some spurious effect.”

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

“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,” said Guido. “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.”