In 2003, Hubble provided evidence of a massive exoplanet around a very old star. Such stars possess only small amounts of heavier elements that are the building blocks of planets. This implied that some planet formation happened when our Universe was very young, and those planets had time to form and grow big inside their primordial disks, even bigger than Jupiter. But how? To answer this question, astronomers used the NASA/ESA/CSA James Webb Space Telescope to study stars in the nearby Small Magellanic Cloud that, much like the early Universe, lacks large amounts of heavy elements. They found that not only do some stars there have planet-forming disks, but that those disks are longer-lived than those seen around young stars in our Milky Way Galaxy.
This Webb image shows NGC 346, a massive star cluster in the Small Magellanic Cloud. Yellow circles overlaid on the image indicate the positions of the ten stars surveyed in the study. Image credit: NASA / ESA / CSA / STScI / Olivia C. Jones, UK ATC / Guido De Marchi, ESTEC / Margaret Meixner, USRA.
“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,” said Dr. Guido De Marchi, a researcher at the European Space Research and Technology Centre.
“In the early Universe, stars formed from mostly hydrogen and helium, and very few heavier elements such as carbon and iron, which came 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,” said Dr. Elena Sabbi, chief scientist for Gemini Observatory at NSF’s NOIRLab.
“But Hubble did see those planets, so what if the models were not correct and disks could live longer?”
To test this idea, the astronomers trained Webb on the Small Magellanic Cloud, a dwarf galaxy that is one of the Milky Way’s nearest neighbors.
In particular, they examined the massive, star-forming cluster NGC 346, which also has a relative lack of heavier elements.
The cluster served 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 disks around them.
This went against the conventional belief that such disks 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,” Dr. De Marchi said.
“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.”
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 disks and are still in the process of gobbling material, even at the relatively old age of 20 or 30 million years,” said De Marchi.
“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.”
This finding refutes previous theoretical predictions that when there are very few heavier elements in the gas around the disk, the star would very quickly blow away the disk.
So the disk’s life would 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 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, for planet-forming disks to persist in environments scarce in heavier elements.
First, to be able to blow away the disk, the star applies radiation pressure.
For this pressure to be effective, elements heavier than hydrogen and helium would have to reside in the gas.
But the massive star cluster NGC 346 only has about ten percent of the heavier elements that are present in the chemical composition 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 cloud of gas.
A bigger gas cloud will produce a bigger disk. So there is more mass in the disk and therefore it would take longer to blow the disk away, even if the radiation pressure were working in the same way.
“With more matter around the stars, the accretion lasts for a longer time,” Dr. 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.”
The study was published today in the Astrophysical Journal.
_____
Guido De Marchi et al. 2024. Protoplanetary Disks around Sun-like Stars Appear to Live Longer When the Metallicity is Low. ApJ 977, 214; doi: 10.3847/1538-4357/ad7a63
This article was adapted from an original release by the Webb Mission Team, NASA’s Goddard Space Flight Center.
