New Delhi: Astronomers using observations by the James Webb Space Telescope (JWST) have determined that the exoplanet designated as 29 Cygni b, containing about 15 times the mass of Jupiter, formed through bottom-up accretion process, typical of objects such as brown dwarfs and stars that form through disk fragmentation, or from dense knots in molecular clouds. The exoplanet orbits its host star at a distance of about 2.4 billion kilometres, and sits on the dividing line between the two formation mechanisms for celestial spheres. Computer models indicate that fragmentation in a disk can easily produce objects containing much higher masses than 29 Cygni b, making it both the lowest mass plausible from fragmentation, and just about the highest mass possible from accretion.
The astronomers captured direct images of 29 Cygni b using the coronagraphic mode on the NIRCam instrument, which blocks out the light from the host star. The young, hot world has strong evidence of enrichment in heavy chemical elements such as carbon and oxygen, relative to the chemical abundances of its host star, which has a composition similar to the Sun. The amount of these metals is equivalent to about 150 Earths, indicating accretion of a large number of metal-rich solids from a protoplanetary disk, the disk of leftover material from the birth of a star.
29 Cygni b formed like a planet despite its mass
Observations with the ground-based CHARA array confirmed that the orbital plane of the planet is well-aligned with the spin of axis of its star, consistent with formation in a protoplanetary disk, similar to planets in our own Solar System. Put together, the evidence strongly suggests that 29 Cygni b formed within a protoplanetary disk through rapid accretion of metal-rich material, rather than through gas fragmentation. The research is based on an observation programme targeting young, massive worlds. A paper describing the research has been published in The Astrophysical Journal Letters.