Astronomers have reported evidence for a rare super-Jupiter candidate orbiting a distant star, inferred from a prolonged dimming event that blocked the host star’s light for months. The discovery, based on ground-based and space-based survey data and follow-up analysis, points to a likely ringed companion hiding in the outer reaches of its stellar system. The finding adds to a small but growing catalog of giant exoplanet candidates at wide orbital separations, a region where planet formation theories remain contested and where each new object provides a valuable test of competing models.
An Eight-Month Blackout Reveals a Hidden Giant
The star designated ASASSN-24fw drew attention when it began fading dramatically, according to a preprint detailing the event’s observational profile. The dimming, which reached a depth of 4.12 ± 0.02 magnitudes in the g-band, started in September 2024 and lasted roughly eight months. The fade was near-achromatic in optical wavelengths, meaning the star dimmed almost equally across visible colors, a signature that rules out many common causes such as stellar pulsations, flares, or simple foreground dust clouds with strong color dependence. Polarization measurements climbed to about 4%, indicating that a significant fraction of the light was scattered by aligned or structured material, likely large dust grains or ring particles, rather than merely absorbed by a smooth veil of gas.
This combination of depth, duration, color neutrality, and polarization pointed researchers away from intrinsic stellar behavior and toward something blocking the star from the outside. A separate account in Monthly Notices describes the occultation as flat-bottomed and lasting approximately 200 days, consistent with a large, opaque structure passing in front of the stellar disk. The two timelines differ slightly: the peer-reviewed study frames the event as running from late 2024 to mid-2025, while the preprint pins the start to September 2024 and estimates about eight months total. Both sources, however, converge on the same physical picture of a deep, sustained eclipse caused by an external companion rather than a transient change in the star’s own output.
Rings Around a Substellar Companion
The leading explanation treats the dimming as an occultation by optically thick rings encircling a substellar companion on a wide orbit. Based on the modeling presented in the Monthly Notices study, the companion has a minimum mass of around 3.42 Jupiter masses, placing it squarely in the super-Jupiter regime where it is too light to ignite sustained fusion like a star but far more massive than typical gas giants. The flat-bottomed shape of the light curve, together with the long ingress and egress times, fits a configuration in which an extended ring system, rather than the compact planetary body alone, sweeps across the star and blocks its light. In this scenario, the planet itself would only account for a tiny fraction of the total eclipse duration, with the rings dominating the observed blackout.
Pre-dimming infrared observations of the host star revealed excess emission that researchers modeled with two-component blackbody dust temperatures, implying that both warm and cool dust populations were already present in the system before the transit began. That infrared signature strengthens the case that the dimming is caused by an external body embedded in a dusty environment, rather than an intrinsic change in the star’s atmosphere. Ringed exoplanets are notoriously difficult to confirm, because even Saturn’s spectacular rings would be nearly invisible at interstellar distances. What makes the ASASSN-24fw companion unusual is the sheer scale implied by the occultation geometry: a ring system large and dense enough to block more than four magnitudes of starlight for months. If the ring model holds, this object joins a very short list of candidates where circumplanetary material has been inferred from transit-like data, offering a rare window into how moons and rings might assemble around giant planets beyond our Solar System.
How Super-Jupiters Compare Across Detection Methods
The ASASSN-24fw companion was found indirectly, through the shadow it cast during its slow passage across the star. By contrast, the temperate super-Jupiter Epsilon Indi Ab was directly imaged using JWST’s MIRI instrument in mid-infrared light, allowing astronomers to measure its thermal glow rather than rely on transits or radial-velocity wobbles. In the Nature report, that planet is estimated at about 6.3 Jupiter masses and is modeled to orbit at a semimajor axis near 28 astronomical units, based on orbit fitting and long-term astrometric monitoring. Direct imaging and occultation-based inference probe different corners of the exoplanet population: imaging favors young, relatively hot, self-luminous worlds at wide separations, while long-duration dimming events can uncover older, cooler companions whose own light is too faint to detect directly. The fact that both techniques now reveal super-Jupiters on distant orbits suggests these massive planets could be more common than earlier surveys implied.
A separate discovery reported on Phys.org describes astronomers using NASA’s Transiting Exoplanet Survey Satellite, or TESS, to identify another rare super-Jupiter orbiting a distant star with an effective temperature of 6,310 K. That temperature refers to the host star, placing it somewhat hotter than the Sun and in a regime where both transit photometry and radial-velocity methods work efficiently. TESS was designed primarily to find smaller planets around nearby bright stars on relatively short orbits, so its sensitivity to massive companions at wider separations is limited by cadence and mission design. Unearthing a super-Jupiter in TESS data underscores how these giant worlds can appear as by-products in surveys optimized for Earth-sized planets, and it suggests the census of wide-orbit giants may still be incomplete, even in well-studied regions of the sky.
Formation Pathways Still Under Debate
The planets in our own Solar System are thought to have grown from a disk of gas and dust that swirled around the young Sun, a process well established for rocky worlds and gas giants alike. But super-Jupiters at large orbital distances challenge the standard core accretion model, which struggles to assemble solid cores quickly enough at tens of astronomical units before the protoplanetary gas dissipates. An alternative mechanism, disk instability, allows a massive clump of gas in the disk to collapse directly under its own gravity into a giant planet, bypassing the need for a large solid core. Work highlighted by Caltech researchers emphasizes that super-Jupiters on wide orbits may owe their existence to this rapid collapse pathway, especially when they are found far from their stars in regions where core accretion is inefficient.
Systems like ASASSN-24fw provide a crucial testing ground for these ideas because the companion’s mass, orbit, and circumplanetary material can all be probed through follow-up observations. If the ringed object formed via disk instability, it might retain a substantial envelope of gas and a rich system of debris that could be sculpted into moons over time. Conversely, if future measurements reveal a more compact orbit or evidence of migration, core accretion followed by dynamical scattering could still be viable. Either way, the combination of a deep, months-long occultation, infrared excess from surrounding dust, and parallels with other super-Jupiter discoveries across different detection methods is pushing astronomers to refine their models of how the most massive planets come to be. As survey coverage expands and more long-duration dimming events are cataloged, the once-rare class of ringed super-Jupiters may evolve from curiosity to key constraint on planet formation theories.
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*This article was researched with the help of AI, with human editors creating the final content.
