Free Astronomy Magazine July-August 2025

N ASA’s James Webb Space Telescope’s observations of what is thought to be the first ever recorded planetary engulfment event revealed a hot ac- cretion disk surrounding the star, with an expanding cloud of cooler dust enveloping the scene. Webb also revealed that the star did not swell to swallow the planet, but the planet’s orbit actually slowly decayed over time. This illustration depicts the sequence of events that took place over mil- lions of years, based on observations from Webb’s MIRI (Mid-Infrared Instrument) and NIRSpec (NIR-Infrared Spectrograph). Panel 1: The planet was about Jupiter-sized, and orbited very close to the star – even closer that Mercury’s orbit around our Sun. Panel 2: The planet’s orbit slowly shrank, or decayed, over time, and the planet approached the star. It eventually started to graze the star’s atmosphere. As the planet was falling in, it smeared around the star. Panel 3: The planet was engulfed by the star completely, and blasted gas away from the outer layers of the star. Panel 4: As that gas expanded and cooled off, the heavy elements in this gas condensed into cold dust over the next year. There is a hot circumstellar disk of molecular gas closer to the star. [NASA, ESA, CSA, Ralf Crawford (STScI)] JULY-AUGUST 2025 member Morgan MacLeod of the Harvard-Smithsonian Center for As- trophysics and the Massachusetts In- stitute of Technology in Cambridge, Massachusetts. “The planet, as it’s falling in, started to sort of smear around the star.” In its final splashdown, the planet would have blasted gas away from the outer layers of the star. As it ex- panded and cooled off, the heavy el- ements in this gas condensed into cold dust over the next year. While the researchers did expect an expanding cloud of cooler dust around the star, a look with the powerful NIRSpec revealed a hot cir- cumstellar disk of molecular gas closer in. Furthermore, Webb’s high spectral resolution was able to de- tect certain molecules in this accre- tion disk, including carbon monox- ide. “With such a transformative tel- escope like Webb, it was hard for me to have any expectations of what we’d find in the immediate sur- roundings of the star,” said Colette Salyk of Vassar College in Pough- keepsie, New York, an exoplanet re- searcher and co-author on the new paper. “I will say, I could not have ex- pected seeing what has the charac- teristics of a planet-forming region, even though planets are not form- ing here, in the aftermath of an en- gulfment.” The ability to charac- terize this gas opens more questions for researchers about what actually happened once the planet was fully swallowed by the star. “This is truly the precipice of study- ing these events. This is the only one we’ve observed in action, and this is the best detection of the aftermath after things have settled back down,” Lau said. “We hope this is just the start of our sample.” These observations, taken under Guaranteed Time Observation pro- gram 1240, which was specifically designed to investigate a family of mysterious, sudden, infrared bright- ening events, were among the first Target of Opportunity programs per- formed by Webb. These types of study are reserved for events, like su- pernova explosions, that are ex- pected to occur, but researchers don’t exactly know when or where. NASA’s space telescopes are part of a growing, international network that stands ready to witness these fleeting changes, to help us under- stand how the universe works. Researchers expect to add to their sample and identify future events like this using the upcoming Vera C. Rubin Observatory and NASA’s Nancy Grace Roman Space Tele- scope, which will survey large areas of the sky repeatedly to look for changes over time. !