Webb nudges planet–star line
NASA says new James Webb observations are reshaping how scientists draw the line between planets and stars by improving understanding of formation and atmospheres. (NASA Science published a report describing Webb’s role in redefining that dividing line.) (science.nasa.gov)
Astronomers used the James Webb Space Telescope to show that 29 Cygni b, an object about 15 times Jupiter’s mass, likely formed like a planet, not like a star. (science.nasa.gov) The basic split is this: planets usually grow inside a disk of gas and dust around a young star, while stars form when a larger cloud of gas breaks apart and collapses under its own gravity. NASA and the European Space Agency said 29 Cygni b sits near the mass range where either route could have worked. (science.nasa.gov) (esawebb.org) Webb directly imaged 29 Cygni b with its Near-Infrared Camera in coronagraph mode, which blocks the star’s glare so fainter nearby objects can be studied. The observations were taken on September 1, 2025, and the science release and image were published on April 14, 2026. (science.nasa.gov 1) (science.nasa.gov 2) The object orbits 29 Cygni at an average distance of 1.5 billion miles, or 2.4 billion kilometers, about as far out as Uranus is from the Sun. That wide orbit made it a useful test case because standard planet growth gets harder to explain farther from the star. (science.nasa.gov) (esawebb.org) The key clue was chemistry. Webb found evidence for heavy elements including carbon and oxygen in 29 Cygni b’s atmosphere, which points to accretion inside a protoplanetary disk rather than direct collapse from a gas cloud. (science.nasa.gov) (stsci.edu) That pushes on an old rule of thumb that uses mass to separate planets from brown dwarfs, the dim objects between planets and stars. At around 13 Jupiter masses, objects can briefly fuse deuterium, a heavy form of hydrogen, but Webb’s result points to formation history as another way to classify them. (esawebb.org) (science.nasa.gov) NASA said the finding could help explain why some very massive worlds turn up in parts of planetary systems where the disk should have been thin and short-lived. The paper describing the result was published April 14, 2026, in The Astrophysical Journal Letters. (science.nasa.gov) (stsci.edu) The 29 Cygni b result does not erase the boundary between planets and stars, but it makes that border look less like a single number and more like a question about origin. Webb’s program targeted four objects between 1 and 15 Jupiter masses, so astronomers now have a template for testing the others the same way. (esawebb.org)
