Webb redefines dividing line between planets and stars Composition and orbit of 29 Cygni b point to accretion within a protoplanetary disc
Where is the dividing line between stars and the most massive planets? Scientists think it may depend on how they formed. Was it from a bottom-up approach, gradually growing larger over time, or a top-down approach in which a large collection of gas and dust fragments into smaller, planet-sized bits? Astronomers used the NASA/ESA/CSA James Webb Space Telescope to study an object weighing about 15 times as much as Jupiter, which puts it right on the dividing line between the two processes. They found that the object, called 29 Cygni b, likely formed from the bottom up rather than the top down. In other words, it formed like a planet, not a star.
Planets, like those in our Solar System, form in a bottom-up process where small bits of rock and ice clump together and grow larger over time. But the heftier the planet, the harder it is to explain its formation that way.
Astronomers used the James Webb Space Telescope to examine 29 Cygni b, an object about 15 times as massive as Jupiter orbiting a nearby star. They found multiple lines of evidence that 29 Cygni b indeed formed from this bottom-up process, bringing new insights into how the heftiest planets come to be. A paper describing these findings has been published in the Astrophysical Journal Letters.
The planet formation process is broadly understood to occur within gigantic discs of gas and dust around stars through a process called accretion. Dust gloms together into pebbles, which collide and grow larger and larger, forming protoplanets and eventually planets. The largest then collect gas to become giants like Jupiter. Since it takes more time for gas giants to form, and the disc 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 and each piece collapses under its own gravity, growing smaller and denser. A similar fragmentation process could theoretically occur within protoplanetary discs as well. That could explain why some very massive objects are found billions of kilometres from their host stars, in regions where the protoplanetary disc should have been too tenuous 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 as much as Jupiter and orbits its star at an average distance of 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.
The science team’s observing programme 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 programme, all of which are known to weigh between 1 and 15 times as much as Jupiter. The team also required their targets to orbit within about 15 billion kilometers of their stars.
The planets were all young and still hot from their formation, ranging in temperature from about 530 to 1,000 degrees Celsius. This would ensure their atmospheric chemistry was similar to the planets of HR 8799, whose system the team studied previously.
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 those heavier chemical elements, which astronomers collectively call metals.
They found strong evidence that 29 Cygni b is enriched in metals relative to its host star, which is similar to our Sun in its composition. Given the planet’s mass, the amount of heavy elements it contains is equivalent to about 150 Earths. This suggests that it accreted large amounts of metal-enriched solids from a protoplanetary disc.
The team also used a ground-based optical telescope array called CHARA (Center for High Angular Resolution Astronomy) to determine if the planet’s orbit is aligned with the spin of the star. They confirmed that alignment, which would be expected for an object that formed from a protoplanetary disc.
Collectively, this evidence strongly suggests that 29 Cygni b formed within a protoplanetary disc through rapid accretion of metal-rich material. As the team gathers data on the other three targets within their program, they plan to look for evidence of compositional differences between the lower-mass and higher-mass planets. This should provide additional insights into their formation mechanisms.
Exoplanet 29 Cygni b (Artist's Concept)
Exoplanet 29 Cygni b (NIRCam image)
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