The James Webb Space Telescope (JWST) has resolved a long-standing controversy by confirming a conclusion reached by the Hubble Observatory more than 20 years ago. Its point is that protoplanetary disks in the early Universe existed longer than now.
In 2003, Hubble presented evidence for the existence of a massive exoplanet near a very old star, almost as old as the Universe. This finding stumped astronomers. The fact is that the stars that existed at the dawn of the Universe were formed mostly of hydrogen and helium, with virtually no heavy elements, being the building blocks of planets. Theoretical models showed that protoplanetary disks should dissipate very quickly under the influence of stellar radiation at such inputs, preventing large exoplanets from forming.
To solve this mystery, the scientists used JWST. The telescope was pointed at the star cluster NGC 346 in the Small Magellanic Cloud, a dwarf galaxy that is one of the Milky Way’s closest neighbors. NGC 346 was chosen as it contains few heavy elements. Thus, it can be used as a kind of natural laboratory to study stellar environments with conditions similar to those in the early Universe.
Hubble had already observed NGC 346 in the mid-noughties and identified many stars between 20 and 30 million years old, around which protoplanetary disks still existed. This contradicted the results of observations of active star-forming regions of the Milky Way, which showed that such disks typically dissipate after only 2 to 3 million years.
JWST was able to obtain spectra of stars in the NGC 346 cluster and confirm the conclusions of the “colleague”. Indeed, even at a relatively old age, they are still surrounded by disks and are still in the process of absorbing material. This means that exoplanets in such systems have much more time to form and grow than those in the star-forming regions of our galaxy, which contain more heavy elements.
According to scientists, there are two different mechanisms, or even a combination of them, that allow protoplanetary disks to persist longer in heavy element-deficient environments. First, the disks are indeed deflated by stellar radiation. But apparently this process is only effective when there are a large number of elements heavier than hydrogen and helium.
The second possibility is that in order for a Sun-like star to form, with the lack of heavier elements, it would have to start with a larger gas cloud. A larger gas cloud will produce a larger disk. So it will take longer for stellar radiation to blow away the rest of the disk.
Provided by NASA
