At the heart of the constellation Monoceros, the star named ASASSN-24fw saw its brightness drop by 97% for nearly two hundred days. Such an observation immediately sparked the curiosity of scientists, because dimmings this pronounced and long-lasting are extremely rare. Astronomers are therefore questioning the origin of such a drop in luminosity for this star.
The star, located about 3,200 light-years away, is about twice the size of our Sun. Normally stable, it exhibited this phenomenon at the end of 2024. The research team then attempted to determine the cause of this prolonged occultation, which surpasses in duration and intensity most known comparable events.
The analyses reveal the presence of a massive object surrounded by a vast ring system. Extending about 16 million miles (26 million kilometers), these structures blocked the star's light as they passed in front of it. The main suspects identified are a brown dwarf, sometimes called a "failed star," or a giant exoplanet, several times more massive than Jupiter. The team led by Sarang Shah estimates that this object would have a mass exceeding three times that of Jupiter.
A brown dwarf forms similarly to a star, but it does not gather the necessary mass to initiate nuclear fusion in its core. It thus occupies an intermediate zone between gaseous giant planets and low-mass stars. The models developed by the researchers show that the object, equipped with dense rings, orbits at a distance from its star, which would explain the slow progression of the dimming.
Around ASASSN-24fw, astronomers have also spotted fragments of gas and dust, probably the remnants of ancient planetary collisions. Furthermore, a red dwarf star has been identified nearby. These elements are surprising for a star about one billion years old, because such turbulent environments are more typical of young stars.
To refine their results, scientists plan to use state-of-the-art instruments such as the Very Large Telescope in Chile and the James Webb Space Telescope. These tools will allow them to measure the temperature, age, and chemical composition of the star, while observing the evolution of this planetary system. The upcoming data could provide clarification on the exact nature of the occulting object.
A new episode of comparable dimming is anticipated in 42 to 43 years. This work, published in
Monthly Notices of the Royal Astronomical Society, contributes to a better understanding of the diversity of planetary systems in our Galaxy and the mechanisms that shape them over time.
What is a brown dwarf?
Brown dwarfs are celestial objects born from the collapse of clouds of gas and dust, similar to stars. However, they do not accumulate enough mass to cross the critical threshold that triggers the fusion of hydrogen into helium in their core. This absence of nuclear fusion distinguishes them from main sequence stars, which shine thanks to this reaction.
In terms of mass, brown dwarfs fall between gas giant planets, like Jupiter, and the lightest stars. Their minimum mass reaches about 13 times that of Jupiter, while the upper limit is around 80 Jupiter masses. This intermediate position makes them objects that are difficult to classify, as they share traits with both planets and stars.
Unlike planets, brown dwarfs can emit a faint luminosity from the residual heat of their formation or from the fusion of deuterium, an isotope of hydrogen. This emission sometimes allows them to be detected in the infrared, although they remain much less luminous than stars.
In the case of ASASSN-24fw, a brown dwarf surrounded by rings could explain the recorded dimming. These rings, comparable to those of Saturn but on a much larger scale, could be composed of ice, dust, or rock.