About 20 years ago, scientists used the powerful Hubble telescope to look at the oldest-known exoplanet in space.
How this gas giant, about 2.5 times the mass of Jupiter, could have come to be less than 1 billion years after the Big Bang befuddled them. This world, about 5,600 light-years away in the summer constellation Scorpius, is more than twice the age of Earth. Its mere existence conflicts with the commonly accepted ideas about how the universe evolved.
But a new study using observations by the infrared-sensing James Webb Space Telescope, a collaboration of NASA and its European and Canadian counterparts, is revealing insights into how planet formation was possible long, long ago, even around the primitive stars of the early universe.
“Current models predict that with so few heavier elements, the disks (of planet-building material) around stars have a short lifetime, so short in fact that planets cannot grow big,” said Elena Sabbi, one of the Webb researchers based at the National Science Foundation’s NOIRLab in Arizona, in a statement. “But Hubble did see those planets, so what if the models were not correct and disks could live longer?”
Credit: NASA / ESA / CSA / STScI / O. C. Jones / G. De Marchi / M. Meixner
The Webb team set out to study early planetary disks by homing in on the Small Magellanic Cloud, a dwarf galaxy near the Milky Way. Planetary disks are the clouds of gas and dust surrounding young stars that can eventually coalesce to form baby worlds.
Within that galaxy is a hectic star-forming cluster, dubbed NGC 346. Because the cluster lacks a lot of heavier elements — it only has about 10 percent of the heavier elements that make up the sun — scientists used it as a stand-in for the early universe’s conditions.
Researchers surveyed 10 stars in the cluster and discovered that, even at old ages, they still maintained substantial disks. Previous thinking was that these primitive stars would have lost their lightweight disks pretty quickly, after just two or three million years. The team’s findings were published Monday in The Astrophysical Journal.
“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 Guido De Marchi, the study leader based at the European Space Research and Technology Centre in the Netherlands. “This also implies that planets have more time to form and grow around these stars.”
The cores of stars are considered element factories: They make carbon, for instance, the same chemical on which humans and much of life on Earth are based. Then, through supernova explosions, they spread these heavier elements, like calcium found in bones and iron in blood, across interstellar space. This dispersal seeds new generations of stars and planets.
Given that most of the chemicals in the universe are thought to have come from exploded stars, scientists have rationalized that the firstborn must have been composed almost entirely of hydrogen and helium, the primitive material that emerged from the Big Bang. Over time, as stars died and scattered heavier elements, subsequent generations of stars formed with more diverse ingredients.
The team studying early planetary disks suggests there could be different ways for planet-forming disks to remain intact in the early universe’s environment. One explanation is that because the stars lack heavier elements in their disks, it takes longer for the stars to blow them away with radiation pressure. Another possibility is the disks might have started out much larger, requiring more time for stars to scatter them, even if the radiation pressure is working in the expected way.
In the latter scenario, the disks could take 10 times longer to disappear, Sabbi said.
“This has implications for how you form a planet, and the type of system architecture that you can have in these different environments,” she said.
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