Giant ring system around substellar object causes rare eclipse of its star for nine months

Santa Cruz de Tenerife. An international scientific team, in which the University of La Laguna (ULL) and the Institute of Astrophysics of the Canary Islands (IAC) participate, identified the cause of an unusually long dimming observed in a distant star. The phenomenon is explained by the passage of a substellar object with a giant ring system, similar to a “cosmic saucer”, in front of the host star.

The star, ASASSN-24fw, is located in the Monoceros constellation, about 3 thousand light years from Earth. The star gradually lost brightness for more than nine months, between the end of 2024 and mid-2025, until reaching a dimming of approximately 97 percent, before recovering its usual luminosity.

The IAC reminds in a note that these stellar eclipses are extremely rare. In fact, most of them last only a few days or weeks, but this dimming continued for almost 200 days, making it one of the longest ever observed.

Thus, the celestial body is approximately 50 percent more massive than our Sun and approximately twice as large. The star itself is known to be stable and not prone to sudden changes. This rules out internal stellar activity as the cause of the strange dimming. Instead, a recently published detailed analysis of several observations supports that the phenomenon was caused by a large companion object crossing the line of sight to the star, blocking its light for an exceptionally long period of time.

“First, a deep drop was detected in the light curve (intensity as a function of time) of ASASSN-24fw obtained with the All-Sky Automated Survey for Supernovae (ASAS-SN). This event caught our attention and we observed this star with different instruments, in addition to gathering additional information. Thus we were able to carry out the stellar characterization and model the transit, which turned out to be extraordinarily long, revealing some ‘treasures’ hidden in the surroundings of this fascinating star,” explains Carlos del Burgo, Participating researcher at the ULL and the IAC.

Several models carried out by the research show that the explanation for the dimming is the crossing of a brown dwarf (an object with a mass intermediate between that of a planet and a star) surrounded by a vast and dense ring system,” says Sarang Shah, researcher at the Interuniversity Center for Astronomy and Astrophysics (Iucaa; Pune, India).

“Cases of dimming as long-lasting as this are exceptional, as they require very perfect alignments. The dimming began gradually, since the outer parts of the rings are thin, and only became evident when the densest regions passed in front of the star,” he added.