A mystery object is dimming a distant star. Could it be a massive exoplanet, or a 'failed star'?

(Main) illustration of a dimming event caused by a mystery object (Inset) A 'failed star' brown dwarf, one of the candidates for this object (Image credit: S. Shah et al./Robert Lea created with Canva)

Scientists investigating the puzzling, long-lasting, and extreme dimming of a distant star have narrowed the suspects down to either a so-called "failed star" brown dwarf or a truly massive super-Jupiter exoplanet.

The dramatic fading of this star, located around 3,200 light-years away in the Monoceros constellation, was first spotted at the end of 2024. The dimming came as a surprise, as the star, designated ASASSN-24fw and measuring around twice the size of the sun, had previously been known to be stable. The stellar dimming was one of the longest ever observed, lasting for around 200 days. It was also extreme, with the brightness of ASASSN-24fw reduced by 97%. Such extreme and long-duration dimming events are incredibly rare.

Astronomers deduced that this dimming was caused by the saucer-like rings extending out from 15.8 million miles (26 million kilometers), or about half the distance between the sun and its closest planet, Mercury. The big question is, what is this ring system looped around? The leading suspects have now emerged as a brown dwarf or a massive extrasolar planet, or exoplanet, many times the size of Jupiter, the solar system's largest planet.

"Various models made by our group show that the most likely explanation for the dimming is a brown dwarf – an object heavier than a planet but lighter than a star – surrounded by a vast and dense ring system. It is orbiting the star at a farther distance with the ring," team leader Sarang Shah, of the Inter-University Centre for Astronomy and Astrophysics (IUCAA), India, said in a statement. "Long-lasting dimming events like this are exceptionally uncommon as they require very perfect line-ups. 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."

Small failed star or giant planet?

Brown dwarfs get the unfortunate nickname "failed stars" because, like stars, they form from collapsing clouds of gas and dust, but fail to gather enough material from what remains of that natal cloud to pile on the mass needed to trigger the fusion of hydrogen to helium in their cores. That is the process that defines what a main-sequence star is.

The dividing line between brown dwarfs and massive gas giant planets is a blurred one in terms of mass. The lower limit of brown dwarf mass is considered to be around 13 times the mass of Jupiter, with the upper limit dividing the heaviest brown dwarfs and the lightest stars being around 80 Jupiter masses, or around 0.08 times the mass of the sun. Currently, a ringed brown dwarf is the prime suspect for the ASASSN-24fw dimming event.

However, because the team can't yet pin down the mass of the dimming body beyond determining it to be greater than three times the mass of Jupiter, they still can't rule out the possibility that the culprit is a super-Jupiter exoplanet.

NASA's Wide-field Infrared Survey Explorer, or WISE, will uncover many failed stars, or brown dwarfs, in infrared light. This diagram shows a brown dwarf in relation to Earth, Jupiter, a low-mass star and the sun. — A diagram shows a brown dwarf in relation to Earth, Jupiter, a low-mass star and the sun. (Image credit: NASA)

What is more certain is the fact that ASASSN-24fw itself is closely circled by gas and dust fragments, which seem to be the remains of previous planetary collisions. This is something that is common around young stars that have turbulent and violent environments, but is uncommon for stars as old as ASASSN-24fw, estimated to be around 1 billion years old.

"Large ring systems are expected around massive objects, but they are very difficult to observe directly to determine their characteristics," team member Jonathan Marshall, an independent post-doctoral researcher affiliated with Academia Sinica, Taiwan, said. "This rare event allows us to study such a complex system in remarkable detail. In fact, while studying this dimming, we also serendipitously discovered that ASASSN-24fw also has a red dwarf star in its vicinity."

The team now intends to measure the temperature, age, and chemical composition of ASASSN-24fw as well as determine the evolutionary stage it is in. Gathering further data about this star from the Very Large Telescope (VLT) in the Atacama Desert region of northern Chile and the James Webb Space Telescope (JWST) should help to better reveal how planetary systems like this one evolve.

Astronomers will have to wait a little longer to get another look at the extreme dimming of ASASSN-24fw, however. The team expects the mysterious object to once again impact the brightness of the star in between 42 and 43 years.

The team's results were published on Thursday (Feb. 12) in Monthly Notices of the Royal Astronomical Society.

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

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