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Top Comments: How Did Planets Almost as Old as the Universe Form? [1]
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Date: 2025-01-12
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When was it in the history of the universe that planets started to form?
We know that stars began to form about 100 million years after the Big Bang, after the universe cooled enough for hydrogen atoms to form. Hydrogen is the fuel that stars burn, using nuclear fusion, the product of which are heavier elements (such as carbon, oxygen, iron, etc). The early universe was made up of 75 % hydrogen, 25 % helium, and very little else. Essentially all of the heavier elements were produced in the stars of the early universe.
Since planets, particularly rocky ones, are generally made out of elements heavier than hydrogen, it would seem a safe assumption that the first generation of stars would have to distribute the heavier elements, via supernovae or some other process, before planets could form. That could take a few billion years. However, back in 2003, researchers using NASA’s Hubble Space Telescope discovered a 13 billion year old planet in our own galaxy. The planet formed far earlier than anyone could have suspected based on available elements from which to form a planet, and caused researchers to go back to the drawing board on how planets and solar systems form.
Prevailing theories hold that, after star formation, the star is circled by a disk of debris, and planets form by accretion of the various bits of debris. However, debris disks that lack much in the way of elements heavier than helium are expected to dissipate too quickly for planets to form, lasting perhaps 2 or 3 million years, whereas 20 or 30 million years are required for planet formation. So unless planets formed much more quickly in the early universe—which is not likely—the light-element debris disks must be persisting for much longer than the theories suggest.
Recently, the James Web Space Telescope was turned toward the star cluster NGC 356, which resides in the Small Magellanic Cloud, a dwarf galaxy that orbits our Milky Way Galaxy. This star cluster has far less in the way of heavier elements than is typical for interstellar gas clouds, and thus can be substituted for the environment of the early universe. Researchers observed that stars in this cluster with ages on the range of 20-30 million years still had observable debris disks.
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.
This work then specifies the environment in which planets could form in the early universe, where much more material than typical must be present in the star’s debris disk.
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