Astronomers discover the surprising reason for a star's disappearance

The steady beam of a star twice the size of the sun played a trick on astronomers about a year ago: It vanished. 

Then some nine months later, it reappeared in the constellation Monoceros, about 3,200 light-years away in space.

Now researchers think they've solved the mystery of one of the longest star-dimming events ever recorded. The star, called ASASSN-24fw, may have disappeared behind a giant planet with an enormous system of rings, according to new research, blocking most of its light from reaching Earth for nine months. 

The study, which appears in Monthly Notices of the Royal Astronomical Society, serves as a reminder that worlds around other stars can have ring systems far larger and more complex than anything in our own solar system.

"Long-lasting dimming events like this are exceptionally uncommon as they require very perfect line-ups," said Sarang Shah, lead author at the Inter-University Centre for Astronomy and Astrophysics in India, in a statement. "The dimming began gradually because the outer parts of the rings are thin, and only became obvious when the denser regions passed in front of the star."

Near the end of 2024, astronomers noticed the star suddenly grew faint. Rather than brighten in days or weeks like most dimming events, ASASSN-24fw faded for nearly 200 days. The star's cheeky name comes from the All Sky Automated Survey for Supernovae, a robotic telescope based in Hawaii nicknamed "Assassin." 

After trying various computer models to explain the event, the team concluded the culprit was either a brown dwarf or a super-Jupiter planet, both large objects that are smaller than stars but bigger than most worlds. Experts sometimes refer to brown dwarfs as failed stars because they're not quite massive enough to generate their own nuclear power. 

Researchers believe a brown dwarf or super-Jupiter blocking ASASSN-24fw formed a nearly opaque saucer covering the star, through which only a tiny amount of its light could escape. Credit: S. Shah et al. / DOI: 10.1093/mnras/staf2251 illustration

The team's top explanation involves a brown dwarf surrounded by humongous rings, similar in shape to Saturn's but vastly larger, eclipsing the star. In this case, the rings are estimated to stretch about 15.8 million miles from the brown dwarf, about half the distance between the sun and Mercury.

As the ring system moved in front of the star, it blocked about 97 percent of ASASSN-24fw's light. By studying changes in the star's brightness and light patterns — methods astronomers use to infer mass and motion — the team estimates the hidden object weighs more than three times as much as Jupiter.

The data also suggest the star itself has leftover material close by, possibly debris from past or ongoing planetary collisions. That is unusual for a star believed to be more than a billion years old.

Researchers think the brown dwarf or giant planet that eclipsed the star had a ring system similar to Saturn's, seen here, though vastly larger. Credit: NASA / JPL-Caltech / Space Science Institute

"Large ring systems are expected around massive objects, but they are very difficult to observe directly to determine their characteristics," said Jonathan Marshall, a coauthor affiliated with Academia Sinica in Taiwan, in a statement. 

The researchers now want to measure the star's temperature, makeup, age, and life stage. They plan to gather more data using powerful observatories, including the Very Large Telescope in Chile and the James Webb Space Telescope, NASA's leading infrared observatory.

They expect the star to dim again in roughly 42 years, when the ringed object returns, offering another chance to study this unusual system, said Marshall, whose expertise is in circumstellar material and debris disks.

"This rare event allows us to study such a complex system in remarkable detail," he said. "In fact, while studying this dimming, we also serendipitously discovered that ASASSN-24fw also has a red dwarf star in its vicinity."