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| NASA’s James Webb Space Telescope revealed a planetary engulfment event where a planet’s orbit decayed over time, forming a hot accretion disk and a surrounding cloud of cool dust—contrary to prior beliefs that the star had expanded to consume it. |
NASA’s James Webb Space Telescope has rewritten the narrative of what was once believed to be the first observed instance of a star engulfing a planet, offering surprising new insights into the final stages of planetary systems.
Rather than swelling into a red giant and consuming a nearby planet, as initially hypothesized, Webb’s infrared observations reveal a more gradual demise: the planet's orbit shrank over time, drawing it ever closer to its host star until it was ultimately devoured.
“Because this is such a novel event, we didn’t quite know what to expect when we decided to point this telescope in its direction,” said Ryan Lau, lead author of the study and astronomer at NSF NOIRLab in Tucson, Arizona. “With its high-resolution look in the infrared, we are learning valuable insights about the final fates of planetary systems, possibly including our own.”
Two of Webb’s instruments, the Mid-Infrared Instrument (MIRI) and the Near-Infrared Spectrograph (NIRSpec), played a crucial role in analyzing the aftermath of this cosmic event.
Rewriting the Scenario.
The star in question lies some 12,000 light-years from Earth in the Milky Way galaxy. The event, known as ZTF SLRN-2020, was first detected in optical light by the Zwicky Transient Facility at Caltech’s Palomar Observatory. Follow-up data from NASA’s NEOWISE indicated that the star had already brightened in the infrared a year earlier—suggesting the presence of dust and sparking initial theories that the star was transitioning into a red giant.
However, Webb’s observations told a different story. MIRI revealed the star wasn’t as luminous as expected for a red giant, meaning it had not expanded enough to engulf the planet in a traditional way. Instead, the data points to a scenario where the planet spiraled in slowly over time.
A Gravitational Dance to Destruction.
Researchers believe the doomed planet was roughly the size of Jupiter but orbited perilously close to its star—closer even than Mercury is to our Sun. Over millions of years, the planet’s orbit gradually decayed until it brushed the outer layers of the star, triggering a runaway plunge into the stellar atmosphere.
“The planet eventually started to graze the star's atmosphere. Then it was a runaway process of falling in faster from that moment,” explained Morgan MacLeod of the Harvard-Smithsonian Center for Astrophysics and MIT. “The planet, as it’s falling in, started to sort of smear around the star.”
The dramatic plunge released gas from the star’s outer layers, which later cooled and condensed into dust over the following year.
Unexpected Aftermath.
While researchers anticipated finding a cool dust cloud around the star, Webb’s NIRSpec instrument also revealed a hot, molecular gas disk closer in—something akin to what’s seen in planet-forming regions.
“With such a transformative telescope like Webb, it was hard for me to have any expectations of what we’d find in the immediate surroundings of the star,” said Colette Salyk, an exoplanet researcher at Vassar College and co-author of the study. “I will say, I could not have expected seeing what has the characteristics of a planet-forming region, even though planets are not forming here, in the aftermath of an engulfment.”
Notably, NIRSpec detected carbon monoxide and other molecules in the newly formed accretion disk, opening up new questions about what happens to planetary material after it’s consumed.
Looking Ahead.
“This is truly the precipice of studying these events,” Lau emphasized. “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. We hope this is just the start of our sample.”
These observations were part of Webb’s Guaranteed Time Observation program 1240, designed to investigate rare, sudden infrared-brightening events. As one of Webb’s first Target of Opportunity observations—used for unpredictable events like supernovae—this study exemplifies the telescope’s role in capturing the fleeting moments of cosmic transformation.
Future missions like the Vera C. Rubin Observatory and NASA’s Nancy Grace Roman Space Telescope are expected to dramatically increase the discovery rate of such events by surveying wide areas of the sky for changes over time. The full findings are published in The Astrophysical Journal.
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