The James Webb Space Telescope (JWST) has just solved a 20-year-old mystery of how ancient stars could host giant planets.
In the early 2000s, the Hubble Space Telescope spotted the oldest planet ever discovered, 2.5 times larger than Jupiter that formed in the Milky Way 13 billion years ago, less than a billion years after the birth of the universe. Other older planets were discovered soon after. This puzzled scientists, because stars in the early universe should have been composed mostly of light elements like hydrogen and helium, with almost no heavier elements like carbon and iron, which make up planets.
Astronomers believed that the disks of dust and gas around these light-element stars should have been blown away by the star’s own radiation, causing the disk to disintegrate within a few million years and leaving nothing to form planets. Scientists believed that the heavy elements needed to form a long-lasting planetary disk around a star weren’t available until later supernova explosions created them.
Now, however, the JWST has taken a closer look at a modern-day proxy for these old stars and found that Hubble wasn’t wrong. In the new research, published Dec. 16 in The Astrophysical Journal, researchers found that when heavy, metallic elements are scarce, planetary disks can last much longer than previously thought.
“We see that these stars are actually surrounded by disks and are still in the process of swallowing material, even at the relatively old age of 20 [million] or 30 million years,” study lead author Guido De Marchi, an astronomer at the European Space Research and Technology Centre in Noordwijk, the Netherlands, said in a statement. “It also means that planets get more time to form and grow around these stars than in nearby star-forming regions in our own galaxy.” James Webb’s observations
The JWST observed the spectra (measurements of different wavelengths of light) of stars in a star-forming cluster called NGC 346. Conditions in this cluster are similar to those in the early universe, with lots of lighter elements like hydrogen and helium and a relative lack of metals and other heavier elements. The cluster is in the Small Magellanic Cloud, a galaxy about 199,000 light-years from Earth.
The light and electromagnetic waves coming from these stars and their surroundings revealed that they host long-lived planetary disks. According to Marchi and his colleagues, this could work in two ways.
The first is that stars made of lighter elements don’t have a lot of elements that undergo radioactive decay — all those radioactive elements are heavier ones. This lack of radiation means the star has less power to push the planetary disk away, so it could last far longer than a disk around a star with more heavy elements.
Another possibility is that a star made of only lighter elements would form a much larger cloud of dust and gas. This extra-large dust cloud would also leave a giant disk around the newborn star, and that giant disk could take a very long time to blow away, even though lighter-element stars emit just as much radiation as heavier-element stars.
“This has implications for how you form planets, and what kinds of system architectures you can have in these different environments,” study co-author Elena Sabbi, chief scientist for the Gemini Observatory at the National Science Foundation’s NOIRLab in Tucson, said in the statement.