The James Webb Space Telescope has allowed scientists to get a good look at several objects beyond the Solar System. As a result, scientists now have a better understanding of where the boundary between planets and stars lies.
How planets and stars form
Planets similar to those in our Solar System form through a “bottom-up” process, in which small fragments of rock and ice clump together and gradually grow larger. However, the more massive a planet is, the more difficult it is to explain its formation in this way.
Using NASA’s James Webb Space Telescope, astronomers have studied 29 Cygni b—an object with a mass about 15 times that of Jupiter that orbits a nearby star. They found ample evidence that 29 Cygni really formed through this “bottom-up” process, which has shed new light on how the most massive planets came into being.
It is generally accepted that planets form within the massive disks of gas and dust surrounding stars through a process known as accretion. Dust clumps together into rocks, which collide and grow larger, forming protoplanets and, eventually, planets. The largest of these then accumulate gas and become giants, such as Jupiter. Since gas giants take longer to form, and the disk of material from which planets form eventually dissipates and disappears, planetary systems ultimately end up with far more small planets than large ones.
In contrast, stars form when a massive cloud of gas breaks apart into fragments, and each of these fragments collapses under its own gravity, becoming smaller and denser. In theory, such a disintegration process could occur within protoplanetary disks. This could explain why some very massive objects are found billions of kilometers away from their parent stars—in regions where the protoplanetary disk should be too sparse for material to accumulate.
Astronomers used NASA’s James Webb Space Telescope to capture a direct image of 29 Cygni b, a star 15 times more massive than Jupiter. They found traces of heavy chemical elements, such as carbon and oxygen, which strongly suggest that it formed, like the planet, through accretion within the protoplanetary disk.
A giant at the crossroads of two mechanisms
29 Cygni b lies on the boundary between what can be explained by two different mechanisms. Its mass is 15 times that of Jupiter, and its orbit around the star lies at an average distance of 1.5 billion miles (2.4 billion kilometers)—roughly the same distance as Uranus in our Solar System. The research team chose this planet as the subject of their study because it could have formed as a result of any of these processes.
“In computer models, it’s very easy for fragmentation in a disk to run away to much higher masses than 29 Cygni b. This is the lowest mass you could plausibly get. But at the same time, it’s about the highest mass you could get from accretion,” said lead author William Balmer of Johns Hopkins University and the Space Telescope Science Institute in Baltimore.
Infrared observation of planets and their chemical composition
As part of the Balmer observation program, the Webb telescope’s NIRCam (Near-Infrared Camera) was used in coronagraph mode to obtain a direct image of 29 Cygni b. This planet was the first of four targets of the program, all of which are known to have masses ranging from 1 to 15 times that of Jupiter. The team also specified that the observed objects should orbit at a distance of about 9 billion miles (15 billion kilometers) from their stars.
All the planets were young and hot shortly after their formation, with temperatures ranging from approximately 530 to 1,000 degrees Celsius. This indicated that the chemical composition of their atmospheres was similar to that of the atmospheres of the planets in the HR 8799 system, which Balmer had studied previously.
By selecting the appropriate filters, the team was able to detect signs of light absorption by carbon dioxide (CO₂) and carbon monoxide (CO), which allowed them to determine the abundance of these heavier chemical elements, which astronomers collectively refer to as metals.
They found compelling evidence that 29 Cygni b is metal-rich compared to its parent star, which is similar in composition to our Sun. Given the planet’s mass, the amount of heavy elements it contains is equivalent to approximately 150 Earths. This indicates that it has accumulated a large amount of metal-rich solid material from the protoplanetary disk.
Refine the orbit of 29 Cygni b
The team also used a ground-based array of optical telescopes called CHARA (Center for High Angular Resolution Astronomy) to determine whether the planet’s orbit aligns with the star’s axis of rotation. They confirmed this match, which is entirely expected for an object that formed from a protoplanetary disk.
“We were able to update the planet’s orbit, and also observed the host star to determine its orientation with respect to that orbit,” said Ash Messier, a co-author of the study and a graduate student at Johns Hopkins University. “We showed that the inclination of the planet is well-aligned with the spin axis of the star, which is similar to what we see for the planets of our solar system.”
Further research on giant planets
Taken together, these data provide strong evidence that 29 Cygni b formed in a protoplanetary disk through the rapid accretion of metal-rich material, rather than through the fragmentation of gas. In other words, it formed as a planet, not as a star.
While collecting data on the other three objects as part of its program, the team plans to look for evidence of differences in the composition of planets with lower and higher masses. This should provide additional insight into the mechanisms underlying their formation.
According to phys.org
