The event began in September 2024 when the Sun-like star J0705+0612, located about 3000 light years away, abruptly became about 40 times dimmer than normal and stayed that way until May 2025. The star's dramatic and sustained fading drew the attention of Johns Hopkins University astrophysicist Nadia Zakamska, who noted that stars like the Sun rarely show such extreme dimming without an external cause.

Zakamska and colleagues quickly organized an observing campaign using multiple ground-based facilities, including the Gemini South telescope on Cerro Pachon in Chile, the Apache Point Observatory 3.5 meter telescope, and the 6.5 meter Magellan Telescopes. By combining these new data with archival observations of J0705+0612, they concluded that the star had been occulted, or temporarily obscured, by a vast, slowly moving cloud of gas and dust.

Analysis indicates that the occulting cloud lies about two billion kilometers, or roughly 1.2 billion miles, from the star, placing it in the outer reaches of the stellar system. The cloud itself is enormous, with an estimated diameter of about 200 million kilometers, or around 120 million miles, large enough to block a substantial fraction of the star's light for many months.

The team's modeling shows that the cloud is gravitationally bound to a secondary object that orbits J0705+0612 at a large distance, rather than freely drifting through the system. This companion must be massive enough to hold the cloud together, with constraints suggesting at least several times the mass of Jupiter, although it could be more massive.

Depending on the mass of the unseen companion, the structure could be classified in different ways. If the secondary object is a low mass star in a binary system with J0705+0612, the debris would form a circumsecondary disk around the less massive stellar partner. If instead the companion is a giant planet, the material would be a circumplanetary disk orbiting that planet.

Directly catching a star being occulted by a disk surrounding a secondary body is exceptionally uncommon, and only a few such systems are known. The long duration and depth of this occultation made it an ideal laboratory for probing the physical and chemical conditions within the disk and testing ideas about how such structures form and evolve in mature planetary systems.

To investigate the composition and internal dynamics of the cloud, the researchers turned to Gemini South's Gemini High-resolution Optical Spectrograph, or GHOST. In March 2025, GHOST monitored the occultation for just over two hours, dispersing the star's light into a high resolution spectrum that encodes the fingerprints of atoms and ions in the intervening gas.

Zakamska initially hoped to identify which elements were present, since no previous measurements of this kind had been made for a similar cloud. The GHOST observations delivered that and more, revealing not only the chemical makeup of the gas but also its motion in three dimensions within the cloud.

The spectra show multiple metals, in astronomical terms elements heavier than helium, including gaseous iron and calcium within the cloud. Thanks to the precision of GHOST, the team could measure subtle shifts in the spectral lines, allowing them to track the velocities of the gas along the line of sight and to infer the structure of wind flows inside the disk.

This marks the first time astronomers have directly measured the internal gas motions of a disk orbiting a secondary object such as a massive planet, brown dwarf, or low mass star. The data portray a dynamic environment with strong winds of vaporized metals streaming through the cloud rather than a static, quiescent disk.

Zakamska notes that the sensitivity of GHOST allowed the team not only to detect the gas but also to quantify how it is moving, a capability that has not previously been available for a system of this kind. That detailed kinematic information provides a powerful new way to diagnose the physical processes at work in such disks.

Chris Davis, National Science Foundation Program Director for NOIRLab, points out that the study showcases both the capabilities of GHOST and Gemini's strength in responding quickly to transient events. The occultation of J0705+0612 was time critical, and the observatory's flexible scheduling helped capture the phenomenon while it was underway.

The measured wind speeds and directions demonstrate that the gas is moving independently of the host star's rotation. Coupled with the long duration of the occultation, these measurements reinforce the conclusion that the obscuring material is a disk bound to a secondary object orbiting J0705+0612 in the outer reaches of the system, rather than a shorter lived feature close to the star.

Infrared observations show that J0705+0612 has an excess of infrared emission, typically associated with dusty disks around young stars that are still forming planets. However, the star's estimated age of more than two billion years indicates that it is a mature system, making it unlikely that the disk is a leftover of the original planet forming disk.

To explain the presence of such a large and dusty structure in an old system, Zakamska suggests that the disk may have formed after a catastrophic collision between two planets in the outer regions of the system. Such an impact could eject vast quantities of dust, rock, and gas, which would then settle into a gravitationally bound cloud or disk around one of the surviving objects.

The newly observed cloud could therefore be the aftermath of a relatively recent giant collision, similar in concept to the impact thought to have formed Earth's Moon, but occurring far from the host star and involving different types of bodies. Over time, the debris would grind down into smaller particles and potentially feed the gaseous component seen in the metallic winds.

The results underline how modern instrumentation is opening new windows on phenomena that were previously inaccessible. By providing high resolution spectroscopy of faint, time variable targets, GHOST enables researchers to probe the internal structure and motion of disks and clouds in distant planetary systems in unprecedented detail.

This occultation shows that even in long lived planetary systems, dramatic, large scale events such as planetary collisions can still reshape the architecture and generate new disks around secondary objects. It offers a striking reminder that planetary systems continue to evolve for billions of years, with episodes of destruction and rebuilding leaving observable signatures in the light of their host stars.

The observations and analysis are presented in a paper titled "ASASSN-24fw: Candidate Gas-rich Circumsecondary Disk Occultation of a Main-sequence Star," published in The Astronomical Journal. The study uses the J0705+0612 system as a case study to explore how gas rich disks may form around secondary objects in mature stellar systems and what they reveal about the long term evolution of planetary architectures.

Research Report:ASASSN-24fw: Candidate Gas-rich Circumsecondary Disk Occultation of a Main-sequence Star

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