NASA’s Webb Redefines Dividing Line Between Planets, Stars

Planets, like those in our solar system, form in a bottom-up process in which small pieces of rock and ice clump together and grow larger over time. But the heavier the planet, the more difficult it is to explain its formation in this way.

Astronomers used NASA’s James Webb Space Telescope to examine 29 Cygni b, an object about 15 times more massive than Jupiter orbiting a nearby star. They found multiple lines of evidence that 29 Cygni b did indeed form from this bottom-up process, providing new insights into how the heaviest planets form. A paper describing these findings published Tuesday in The Astrophysical Journal Letters.

The process of planet formation is generally thought to occur within gigantic disks of gas and dust around stars, through a process called accretion. The dust groups together into pebbles, which collide and grow larger and larger, forming protoplanets and eventually planets. The largest ones then collect gas to become giants like Jupiter. Because gas giants take longer to form and the disk of planet-forming material eventually evaporates and disappears, planetary systems end up with many more small planets than large planets.

In contrast, stars form when a vast cloud of gas fragments breaks up and each piece collapses under its own gravity, becoming smaller and denser. A similar fragmentation process could theoretically also occur within protoplanetary disks. This could explain why some very massive objects are found billions of kilometers from their host stars, in regions where the protoplanetary disk would have to be too thin for accretion to occur.

29 Cygni b sits on the dividing line between what can be explained by these two different mechanisms. It weighs 15 times Jupiter and orbits its star at an average distance of 1.5 billion miles (2.4 billion kilometers), about the same as Uranus in our solar system. The research team targeted it because it could potentially result from either process.

“In computer models, it’s very easy for disk fragmentation to reach masses much higher than 29 Cygni b. That’s the lowest mass you can plausibly get. But at the same time, it’s about the highest mass you can get through accretion,” said lead author William Balmer of Johns Hopkins University and the Space Telescope Science Institute, both in Baltimore.

Balmer’s observing program used Webb’s NIRCam (Near-Infrared Camera) in its coronagraphic mode to directly image 29 Cygni b. This planet was the first of four objects targeted by the program, all known to weigh between 1 and 15 times more than Jupiter. The team also required that their targets orbit about 9 billion miles (15 billion kilometers) from their stars.

The planets were all young and still hot from their formation, with temperatures ranging from about 1,000 to 1,900 degrees Fahrenheit (530 to 1,000 degrees Celsius). This would ensure that their atmospheric chemistry was similar to that of the planets of HR 8799, whose system Balmer had previously studied.

By choosing appropriate filters, the team was able to look for signs of light being absorbed by carbon dioxide (CO₂) and carbon monoxide (CO), which allowed them to determine the amount of these heavier chemical elements, which astronomers collectively call metals.

They found strong evidence that 29 Cygni b is enriched in metals compared to its host star, which is similar in composition to our Sun. Considering the mass of the planet, the amount of heavy elements it contains is equivalent to approximately 150 Earths. This suggests that it accumulated large quantities of metal-enriched solids from a protoplanetary disk.

The team also used an array of ground-based optical telescopes called CHARA (Center for High Angular Resolution Astronomy) to determine whether the planet’s orbit is aligned with the star’s rotation. They confirmed this alignment, which one would expect for an object formed from a protoplanetary disk.

“We were able to update the planet’s orbit and also observe the host star to determine its orientation relative to that orbit,” said Ash Messier, co-author and graduate student at Johns Hopkins University. “We showed that the planet’s tilt is well aligned with the star’s rotation axis, which is similar to what we observe for the planets in our solar system.”

“Together, this evidence strongly suggests that 29 Cygni b formed within a protoplanetary disk by rapid accretion of metal-rich materials, rather than by gas fragmentation,” Balmer said. “In other words, it formed as a planet and not as a star.”

As the team gathers data on the other three targets in its program, it plans to look for evidence of compositional differences between lower-mass and higher-mass planets. This should provide additional information about their formation mechanisms.

The James Webb Space Telescope is the world’s first space science observatory. Webb solves the mysteries of our solar system, looks beyond distant worlds around other stars, and probes the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

To learn more about Webb, visit:

https://science.nasa.gov/webb

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