Brown dwarf's giant rings likely caused star's nine-month dimming

 A star more than 3,000 light-years from Earth nearly vanished from view for nine months, and astronomers now believe they have solved the mystery: an enormous ring system orbiting an unseen brown dwarf eclipsed the distant sun.

The star, known as ASASSN-24fw, had been stable for decades before it faded dramatically in late 2024, losing about 97 percent of its brightness before returning to normal in mid-2025. A study published Wednesday in Monthly Notices of the Royal Astronomical Society concludes that a brown dwarf—an object too large to be a planet but too small to sustain nuclear fusion like a star—likely passed in front of ASASSN-24fw, its colossal Saturn-like rings blocking almost all of the star's light.

A Rare Cosmic Alignment

The approximately 200-day dimming event ranks among the longest stellar eclipses ever recorded, according to the Royal Astronomical Society. Typical stellar eclipses last only days or weeks, making this months-long event exceptionally rare.

"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," said lead author Dr. Sarang Shah, a postdoctoral researcher at the Inter-University Centre for Astronomy and Astrophysics in Pune, India.

The ring system extends roughly 0.17 astronomical units from the companion object—about half the distance between the Sun and Mercury. The companion itself has a mass at least three times that of Jupiter.

Clues from the Past and Future

While investigating the dimming, the research team discovered that ASASSN-24fw is accompanied by a nearby red dwarf star. They also found evidence that the star has a circumstellar environment, possibly remnants of past planetary collisions, which is unusual for a star likely more than a billion years old.

Historical data revealed that ASASSN-24fw previously dimmed in 1981 and 1937, suggesting the companion completes an orbit approximately every 43 years. The next eclipse is not expected until around 2068.

"Large ring systems are expected around massive objects, but they are very difficult to observe directly to determine their characteristics," said co-author Dr. Jonathan Marshall, an independent researcher affiliated with Academia Sinica in Taiwan. "This rare event allows us to study such a complex system in remarkable detail."

New Astronomy Study Reveals How Titan and Saturn’s Rings Were Born

 Saturn's largest moon, Titan, and its spectacular ring system may both owe their existence to a single cataclysmic event — a collision between two ancient moons roughly 400 million years ago, according to a study led by SETI Institute scientist Matija Ćuk. The research, posted to the preprint server arXiv on February 11 and accepted for publication in The Planetary Science Journal, presents computer simulations suggesting the crash reshaped much of the Saturnian system.

A Two-Stage Catastrophe:

The hypothesis builds on data collected by NASA's Cassini spacecraft, which during its 13-year mission revealed that Saturn's rings are far younger than the planet itself and that Titan's orbit is shifting more rapidly than expected. A 2022 study had proposed that a lost moon, dubbed "Chrysalis," was ejected from Saturn's orbit and torn apart to form the rings. But when Ćuk's team ran simulations, they found that in 42 out of 60 cases, the most likely outcome was not ejection — but a direct collision with Titan.

The team proposes that two predecessors — a large "Proto-Titan" and a smaller "Proto-Hyperion" — merged violently, producing the Titan we observe today and scattering debris that coalesced into Saturn's small, irregularly shaped moon Hyperion. "We recognized that the Titan-Hyperion lock is relatively young, only a few hundred million years old," Ćuk said in a statement from the SETI Institute. "This dates to about the same period when the extra moon disappeared. Perhaps Hyperion did not survive this upheaval but resulted from it".

The collision itself did not directly create Saturn's rings. Instead, the researchers describe a chain reaction: Titan's newly eccentric orbit gradually destabilized smaller inner moons through gravitational resonances, sending them crashing into one another. The debris from those secondary collisions eventually settled into the ring system roughly 100 million years ago — consistent with independent age estimates.

Solving Multiple Mysteries at Once:

What makes the hypothesis compelling is its ability to address several long-standing puzzles simultaneously. It accounts for Titan's surprisingly few impact craters, which would have been erased in the merger. It explains the unusual orbital tilt of Saturn's distant moon Iapetus, which the simulations show was gravitationally disturbed by Proto-Hyperion before the crash. And it offers a reason why Saturn's axial wobble fell slightly out of sync with Neptune — the loss of the extra moon's mass changed Saturn's precession rate.

"This serves as a sort of grand unified theory that addresses all major issues," Ćuk told New Scientist. "We had some understanding of each problem individually, but this might illustrate how they interconnect in a single narrative that can be validated".

​

Sarah Hörst, a planetary scientist at Johns Hopkins University who was not involved in the study, said the work could "truly place Titan at the center of our understanding of the system today". Linda Spilker, a research scientist at NASA's Jet Propulsion Laboratory, called the findings "compelling evidence that Hyperion and Saturn's rings formed well after Saturn's inception".

What Comes Next:

The hypothesis remains to be peer-reviewed in its final published form. A definitive test may come from NASA's Dragonfly mission, a nuclear-powered rotorcraft scheduled to arrive at Titan in 2034. By analyzing the moon's surface geology and chemistry, Dragonfly could reveal whether Titan bears the scars of a violent origin half a billion years ago.

Studies find "Jupiter's moons may have formed with life's building blocks"

 Jupiter's four largest moons — Europa, Ganymede, Callisto, and Io — may not have formed as chemically barren worlds. Instead, they likely accumulated complex organic molecules essential for life during their very formation billions of years ago, according to a pair of complementary studies published in The Planetary Science Journal and Monthly Notices of the Royal Astronomical Society.

An international team led by scientists from Aix-Marseille University and Southwest Research Institute demonstrated that complex organic molecules, or COMs — carbon-rich compounds containing hydrogen, oxygen, and nitrogen — could have been forged within the proto solar nebula and Jupiter's circumplanetary disk, then incorporated into the moons as they took shape.

Two Pathways to Organic Chemistry

The studies trace two primary routes by which COMs could have reached the Galilean moons. In one paper, published in Monthly Notices of the Royal Astronomical Society and led by Tom Benest Couzinou of Aix-Marseille University, the team simulated 500 individual icy particles drifting through the protosolar nebula — the vast cloud of gas and dust that surrounded the young Sun. Their models found that when particles were released at distances around seven astronomical units from the Sun, roughly 45 percent of centimeter-sized particles formed COMs through thermal processing and subsequently reached Jupiter's orbital region within 300,000 years.

The companion paper, published in The Planetary Science Journal and led by Olivier Mousis — now at SwRI — examined how COMs could form locally within Jupiter's circumplanetary disk, the swirling environment of gas and dust where the moons coalesced. Heating of ices containing ammonia and carbon dioxide emerged as the dominant pathway for COM creation in this environment.

"Our findings suggest that Jupiter's moons did not form as chemically pristine worlds," Mousis said in an SwRI press release. "Instead, they may have accreted, or accumulated, a significant inventory of COMs at birth, providing a chemical foundation that could later interact with the liquid water in their interiors."

What It Means for Habitability

Europa, Ganymede, and Callisto are believed to harbor subsurface oceans beneath their icy crusts. An early inheritance of organic molecules means these moons may possess not only water and energy sources but also the chemical precursors that could drive prebiotic processes such as the formation of amino acids and nucleotides. The research indicates that Ganymede and Callisto, which likely formed under cooler conditions farther from Jupiter, may have retained even more of their primordial organic material than Europa.

Eyes on Jupiter

The findings arrive as two spacecraft are en route to test these predictions. NASA's Europa Clipper, launched in October 2024, is expected to reach the Jupiter system in April 2030 and will conduct 49 close flybys of Europa. ESA's JUICE mission, launched in April 2023, is on track for a July 2031 arrival after completing a Venus flyby in August 2025. Both carry instruments designed to detect organic compounds on the moons' surfaces and in their thin atmospheres — data that could confirm whether the chemical inheritance modeled in these studies left a detectable signature.