Planets such as those in our solar system develop through a bottom-up process, where tiny fragments of rock and ice gradually stick together and grow. However, the more massive a planet becomes, the more difficult it is to account for its formation through this mechanism.
Astronomers analyzed 29 Cygni b using NASA’s James Webb Space Telescope, a massive object roughly 15 times the size of Jupiter that orbits a nearby star. Their observations revealed several lines of evidence showing that 29 Cygni b formed through a bottom-up process, offering fresh insight into the origins of the largest planets. The findings were detailed in a study published Tuesday in The Astrophysical Journal Letters.
The Formation of Planets and Stars
Scientists generally understand planet formation as taking place within vast disks of gas and dust surrounding stars through a process known as accretion. Tiny dust particles stick together to form pebbles, which collide and grow into larger bodies, eventually becoming protoplanets and then full-fledged planets. The largest of these can gather thick envelopes of gas, turning into giants like Jupiter. Because gas giants take longer to develop—and the surrounding disk of material eventually dissipates—most planetary systems end up with far more small planets than large ones.
By contrast, stars form when enormous clouds of gas break apart, and each fragment collapses under its own gravity, becoming increasingly compact and dense. In theory, a similar kind of fragmentation could also take place within protoplanetary disks, potentially explaining why some extremely massive objects are located billions of miles from their host stars—regions where the disk would have been too thin for accretion to occur.
Using NASA’s James Webb Space Telescope, astronomers directly imaged 29 Cygni b, an object about 15 times the mass of Jupiter. They detected signs of heavy elements such as carbon and oxygen, strongly indicating that it formed like a planet through accretion within a protoplanetary disk.
A Giant at the Edge of Formation Boundaries
29 Cygni b lies right at the boundary between these two formation pathways. With a mass about 15 times that of Jupiter, it orbits its star at an average distance of 1.5 billion miles (2.4 billion kilometers)—roughly comparable to Uranus in our solar system. Researchers focused on it because it could plausibly have formed through either mechanism.
“In computer simulations, fragmentation within a disk can easily produce objects far more massive than 29 Cygni b—making this close to the lowest mass expected from that process. At the same time, it’s near the upper limit of what accretion can produce,” explained William Balmer of Johns Hopkins University and the Space Telescope Science Institute in Baltimore.
The Webb Observation Program and Its Targets
William Balmer’s observing program used the James Webb Space Telescope’s NIRCam (Near-Infrared Camera) in coronagraphic mode to directly image 29 Cygni b. This planet was the first of four targets in the program, all with masses ranging from 1 to 15 times that of Jupiter. The team also selected objects orbiting within roughly 9 billion miles (15 billion kilometers) of their host stars.
All of the planets were relatively young and still retained heat from their formation, with temperatures between about 1,000 and 1,900 degrees Fahrenheit (530 to 1,000 degrees Celsius). This ensured their atmospheric chemistry would resemble that of planets in the HR 8799 system, which Balmer had previously studied.
Decoding the Planet’s Chemical Signature
By selecting specific observational filters, the team searched for light absorption features linked to carbon dioxide (CO₂) and carbon monoxide (CO). This enabled them to estimate the abundance of these heavier chemical elements—collectively referred to by astronomers as metals.
Their results show that 29 Cygni b is significantly enriched in metals compared with its host star, which has a composition similar to our Sun. Based on its mass, the planet’s heavy-element content is roughly equivalent to about 150 Earths, indicating that it likely accumulated large amounts of metal-rich material from a protoplanetary disk during its formation.
Hints from the Planet’s Orbital Orientation
The researchers also relied on the CHARA (Center for High Angular Resolution Astronomy) ground-based optical telescope array to assess whether the planet’s orbit matches the rotation of its host star. They found that the two are indeed aligned, a result consistent with formation within a protoplanetary disk.
“We refined the planet’s orbit and measured the star’s orientation relative to it,” explained Ash Messier, a co-author and graduate student at Johns Hopkins University. “The planet’s orbital inclination is well aligned with the star’s spin axis, similar to what we observe in our own solar system.”
Implications for the Origins of Giant Planets
“Taken together, these findings provide strong evidence that 29 Cygni b formed in a protoplanetary disk by rapidly accreting metal-rich material, rather than originating from gas fragmentation,” said William Balmer. “In short, it formed like a planet, not like a star.”
As the researchers continue analyzing the other three targets in their program, they aim to identify compositional differences between lower- and higher-mass planets. This could offer further clues about how different types of giant planets come into existence.
Read the original article on: Phys.Org
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