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Circumplanetary disk
Circumplanetary disk
Circumplanetary disk
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Circumplanetary disk around exoplanet PDS 70c (point-like source and surrounding cloud in the center of the right image; brightness indicates the hot hydrogen circumplanetary disk),[1] within a circumstellar disk (left image)

A circumplanetary disk (or circumplanetary disc, short CPD) is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a planet. They are reservoirs of material out of which moons (or exomoons or subsatellites) may form.[2] Such a disk can manifest itself in various ways.

In August 2018, astronomers reported the probable detection of a circumplanetary disk around CS Cha B.[3] The authors state that "The CS Cha system is the only system in which a circumplanetary disc is likely present as well as a resolved circumstellar disc."[4] In 2020 though, the parameters of CS Cha B were revised, making it an accreting red dwarf star, and making the disk circumstellar.[5]

Theory

[edit]
Hydrodynamic simulation of the inner part of a circumplanetary disk after exomoons were added to the disk. Simulation by Sun et al.[6]

A giant planet will mainly form via core accretion. In this scenario a core forms via the accretion of small solids. Once the core is massive enough it might carve a gap onto the circumstellar disk around the host star. Material will flow from the edges of the circumstellar disk towards the planet in streams and around the planet it will form a circumplanetary disk. A circumplanetary disk does therefore form during the late stage of giant planet formation.[7][8] The size of the disk is limited by the Hill radius. A circumplanetary disk will have a maximal disk size of 0.4 times the Hill radius.[9][10] The disk also has a "dead zone" at the mid-plane that is non-turbulent and a turbulent disk surface. The dead zone is a favourable region for satellites (exomoons) to form.[11] The circumplanetary disk will go through different stages of evolution. A classification similar to young stellar objects was proposed. In the early stage the circumplanetary disk will be full. Newly forming satellites will carve a gap close to the planet, turning the disk into a "transitional" disk. In the last stage the disk is full, but has a low density and can be classified as "evolved".[6] Additional to a circumplanetary disk, a protoplanet can also drive an outflow.[12][13] One such outflow is identified via shocked SiS for HD 169142b.[14]

Circumplanetary disks are consistent with the formation of the Galilean satellites. The older models at the time were not consistent with the icy composition of the moons and the incomplete differentiation of Callisto. A circumplanetary disk with an inflow of 2*10−7 MJ/year of gas and solids was consistent with the conditions needed to form the moons, including the low temperature during the late stage of the formation of Jupiter.[15] But later simulations found the circumplanetary disk too hot for the satellites to form and survive.[16][10] This was later solved by introducing the dead zone within circumplanetary disks which is a favourable region for satellite formation and explains the compact orbit of Galilean satellites.[11]

Candidates around other exoplanets

[edit]

Possible circumplanetary disks have also been detected around exoplanets, HD 100546 b,[17] AS 209 b[18] and HD 169142 b[19] or planetary-mass companions (PMC; 10-20 MJ, separation ≥100 AU), such as GSC 06214-00210 b[20] and DH Tauri b.[21]

A disk was detected in sub-mm with ALMA around SR 12 c, a planetary-mass companion. SR 12 c might not have formed from the circumstellar disk material of the host star SR 12, so it might not be considered a true circumplanetary disk. PMC disks are relative common around young objects and are easier to study when compared to circumplanetary disks.[22] The protoplanet Delorme 1 (AB)b shows strong evidence of accretion from a circumplanetary disk, but the disk is as of now (September 2024) not detected in the infrared.[23] A disk was detected around the planet YSES 1b with the James Webb Space Telescope. The disk shows emission by small hot olivine grains. This is seen as evidence for collisions between satellites forming inside the disk.[24]

Several disks were detected around nearby isolated planetary-mass objects. Disks around such objects within 300 parsecs were found in Rho Ophiuchi Complex,[25] Taurus Complex (e.g. KPNO-Tau 12),[25][26] Lupus I Cloud[27] and the Chamaeleon Complex (e.g. the well studied OTS 44 and Cha 110913−773444[28]). One remarkable close free-floating disk-bearing object is 2MASS J11151597+1937266, which is only 45 parsec distant. It could be a planetary-mass object or a low-mass brown dwarf.[29] These objects with disks are free-floating and are most of the time called circumstellar disks, despite likely being similar to circumplanetary disks.

2M1207b was suspected to have a circumplanetary disk in the past.[30] New observations from JWST/NIRSpec were able to confirm accretion from an unseen disk by detecting emission from hydrogen and helium. The classification of a circumplanetary disk is however being disputed because 2M1207b (or 2M1207B) might be classified as a binary together with 2M1207A and not an exoplanet. This would make the disk around 2M1207b a circumstellar disk, despite not being around a star, but around a 5-6 MJup planetary-mass object.[31]

PDS 70

[edit]
Main article: PDS 70

The disk around the planet c of the PDS 70 system is the best evidence for a circumplanetary disk at the time of its discovery. The exoplanet is part of the multiplanetary PDS 70 star system, about 370 light-years (110 parsecs) from Earth.[32]

PDS 70b

[edit]

In June 2019 astronomers reported the detection of evidence of a circumplanetary disk around PDS 70b[33] using spectroscopy and accretion signatures. Both types of these signatures had previously been detected for other planetary candidates. A later infrared characterization could not confirm the spectroscopic evidence for the disk around PDS 70b and reports weak evidence that the current data favors a model with a single blackbody component.[34] Interferometric observations with the James Webb Space Telescope's Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph and archived data found tentative evidence that PDS 70b has a circumplanetary disk.[35]

PDS 70c

[edit]

In July 2019 astronomers reported the first-ever detection using the Atacama Large Millimeter/submillimeter Array (ALMA)[36][37][38] of a circumplanetary disk.[36][37][39] ALMA studies, using millimetre and submillimetre wavelengths, are better at observing dust concentrated in interplanetary regions, since stars emit comparatively little light at these wavelengths, and since optical observations are often obscured by overwhelming glare from the bright host star. The circumplanetary disk was detected around a young massive, Jupiter-like exoplanet, PDS 70c.[36][37][39]

According to Andrea Isella, lead researcher from the Rice University in Houston, Texas, "For the first time, we can conclusively see the tell-tale signs of a circumplanetary disk, which helps to support many of the current theories of planet formation ... By comparing our observations to the high-resolution infrared and optical images, we can clearly see that an otherwise enigmatic concentration of tiny dust particles is actually a planet-girding disk of dust, the first such feature ever conclusively observed."[38] Jason Wang from Caltech, lead researcher of another publication, describes, "if a planet appears to sit on top of the disk, which is the case with PDS 70c"[40] then the signal around PDS 70c needs to be spatially separated from the outer ring, not the case in 2019. However, in July 2021 higher resolution, conclusively resolved data were presented.[41]

The planet PDS 70c is detected in H-alpha, which is seen as evidence that it accretes material from the circumplanetary disk at a rate of 10−8±0.4 MJ per year.[42] From ALMA observations it was shown that this disk has a radius smaller than 1.2 astronomical units (AU) or a third of the Hill radius. The dust mass was estimated around 0.007 or 0.031 M🜨 (0.57 to 2.5 Moon masses), depending on the grain size used for the modelling.[41] Later modelling showed that the disk around PDS 70c is optically thick and has an estimated dust mass of 0.07 to 0.7 M🜨 (5.7 to 57 Moon masses). The total (dust+gas) mass of the disk should be higher. The planet's luminosity is the dominant heating mechanism within 0.6 AU of the CPD. Beyond that the photons from the star heat the disk.[43] Observations with JWST NIRCam showed a large spiral-like feature near PDS 70c. This feature is only seen after the disk around PDS 70 was removed. Part of this spiral-like feature was interpreted as an accretion stream that feeds the circumplanetary disk around PDS 70c.[44]

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Circumplanetary disk

View on Grokipedia
from Grokipedia
A circumplanetary disk (CPD) is a rotating disk of gas and encircling a , serving as a site for the formation of moons through the accretion of material, much like s around facilitate formation. These disks typically form around young giant s during the late stages of formation, when the accretes gas from the surrounding , creating a subdisk with sufficient to become rotationally supported. CPDs can arise through two primary mechanisms: core accretion, where a solid core grows by capturing gas and forms a hot, optically with masses around 0.1–1% of the 's mass, or gravitational instability, which produces cooler disks (temperatures below 100 K) that may be up to 8 times more massive relative to the . The properties of CPDs vary with the planet's mass and the circumstellar disk's aspect ratio; higher planet masses (e.g., several masses) and lower disk scale heights promote well-defined, thin disks, while lower masses lead to more envelope-like structures. In our solar , CPDs are hypothesized to have formed the and Saturn, with models suggesting they were fed by gas from the primordial solar nebula. Observationally, CPDs are rare and challenging to detect due to their small size and the prevalence of at large orbital radii where disk formation is less likely, but the first clear came in 2021 from ALMA observations of a disk around the PDS 70c, a -like world about 5.1 billion kilometers from its , with a disk roughly equal to 1 AU and mass sufficient to form up to three Moon-sized moons. Subsequent studies, including a 2025 analysis, confirm their scarcity at wide orbits but highlight potential detections around systems like AB Aur b, underscoring CPDs' role in testing theories of satellite formation and evolution.

Definition and Properties

Physical Characteristics

A circumplanetary disk is a -, -, or ring-shaped structure of gas, , planetesimals, and rocky material that orbits a forming , distinct from the larger circumstellar disk surrounding the central star. These disks arise from material accreted onto the planet, forming a rotationally supported subdisk embedded within the planet's . Unlike circumstellar disks, which span tens to hundreds of astronomical units, circumplanetary disks are confined to scales much smaller than the planet's orbital radius around the star. The radial extent of a circumplanetary disk typically begins at an inner edge just beyond the planet's radius, around 2 planetary radii, and extends outward to approximately 0.4–0.5 times the planet's Hill radius, where tidal forces from the star truncate the disk. For a Jupiter-mass planet at 5 au, the Hill radius is about 50 Jupiter radii, so the disk spans roughly 2 to 20–25 Jupiter radii. Mass estimates for these disks range from 10−510^{-5} to 10−310^{-3} times the planet's mass, though simulations show values up to 4×10−2Mp4 \times 10^{-2} M_p for more massive planets (5–10 MJupM_\mathrm{Jup}), with the disk primarily composed of hydrogen and helium gas containing embedded dust grains from micron to centimeter sizes. The temperature profile decreases radially from around 1000 K near the inner edge, driven by viscous heating and stellar irradiation, to about 100 K at the outer edge, ensuring the entire disk remains above the water freezing point in many models but allowing for volatile condensation farther out. Recent JWST observations of systems like Delorme 1 AB b (2025) reveal carbon-rich disks with blackbody temperatures around 295 K, effective radii of approximately 19 R_Jup, and low gas masses on the order of 10^{-6} M_Jup, aligning with theoretical expectations for young, accreting CPDs. Key structural parameters include an aspect ratio H/rH/r of approximately 0.1–0.3, reflecting the disk's moderate flaring due to thermal support, and a turbulent viscosity parameterized by α∼10−3\alpha \sim 10^{-3} to 10−210^{-2}, which governs angular momentum transport and accretion rates. Disk stability is maintained via the Toomre parameter Q>1Q > 1, typically Q≫1Q \gg 1 in the inner regions and approaching 1 at the outer edge to prevent gravitational fragmentation. Compositionally, the disks are gas-dominated with a dust-to-gas mass ratio of about 0.01, though midplane depletion to 10−310^{-3}–10−410^{-4} enhances ice formation; ice lines for water and other volatiles occur at 5–10 planetary radii (or 0.05–0.1 Hill radii), where temperatures drop below 150–180 K, enabling condensation of ices that influence moon compositions.

Role in Satellite Formation

Circumplanetary disks primarily function as nurseries for the formation of regular satellites, where planetesimals and particles coalesce through processes such as or core accretion to produce . In these disks, solid materials aggregate into larger bodies, with the disk's gaseous environment facilitating the capture and growth of these precursors to . A key mechanism within circumplanetary disks is pebble accretion, wherein centimeter- to meter-sized pebbles drift inward and accrete onto moon embryos, enabling rapid growth. This process allows protosatellites to form moon embryos on timescales of 10410^4 to 10510^5 years for large , outpacing gas dissipation in the disk. Additionally, remnants of the disk or material from disrupted satellites can coalesce into ring systems, preserving circumplanetary debris as observed in planetary ring structures. The disks influence satellite architecture through dynamical interactions, including inward migration driven by type I and type II torques between satellites and the disk gas, which can lead to resonant chains in satellite systems. In the gas-starved disk paradigm, the limited and time-variable supply of gas from the parent restricts satellite growth primarily to rocky or icy compositions, preventing the formation of moons. This paradigm favors the accretion of solids over extended periods, aligning with the compositions of known regular satellites. Moons formed in water-rich regions of circumplanetary disks, influenced by temperature gradients that position lines, hold implications for through the potential development of subsurface oceans. These oceans may arise from incorporated volatiles and subsequent , creating environments conducive to liquid persistence.

Theoretical Models

Formation Mechanisms

The primary mechanism for the formation of circumplanetary disks (CPDs) involves the transfer of from infalling circumstellar material during the runaway accretion phase of a , leading to rotational instability that sheds material into a disk. This process partitions the incoming gas between direct accretion onto the planet and disk formation, with the disk emerging as excess cannot be fully absorbed by the planet's spin. In the core accretion model, a planetary core captures gas from the protoplanetary disk (PPD), and the shedding of excess angular momentum results in CPD formation. As the planet contracts during this phase, the transition from a spherical envelope to a rotationally supported disk occurs, driven by the inflow's specific angular momentum bias relative to the Hill sphere. Theoretical models predict distinct properties for CPDs formed via different mechanisms. In the core accretion scenario, CPDs are hot and optically thick, with masses typically 0.1–1% of the planet's mass. In contrast, the gravitational instability model involves rapid collapse of gas clumps in the PPD to form the planet, potentially producing cooler disks (temperatures below 100 K) that may be up to 8 times more massive relative to the planet. Bondi-Hoyle-Lyttleton accretion contributes by capturing ambient gas onto the moving planet, with the accretion rate given by M˙∝(GMp)2ρv3\dot{M} \propto \frac{(G M_p)^2 \rho}{v^3}, where ρ\rho is the and vv is the . This influences the inflow pattern into the Hill sphere, enhancing disk buildup through Bondi radius effects. CPD formation typically occurs within 10310^3 to 10410^4 years after the planet reaches approximately 10 masses, aligning with the viscous spreading timescale during the initial contraction. The depends strongly on PPD properties, such that higher disk aspect ratios (H/r>0.05H/r > 0.05) promote denser CPDs by facilitating more efficient mass delivery and transport into the Hill sphere.

Evolutionary Dynamics

Circumplanetary disks undergo viscous spreading after their initial formation, during which transport causes the disk to expand radially outward while facilitating inward accretion of gas onto the central . This is governed by the standard viscous for accretion disks, ∂Σ∂t=3r∂∂r[r1/2∂∂r(νΣr1/2)],\frac{\partial \Sigma}{\partial t} = \frac{3}{r} \frac{\partial}{\partial r} \left[ r^{1/2} \frac{\partial}{\partial r} (\nu \Sigma r^{1/2}) \right], where Σ(r,t)\Sigma(r,t) is the surface density profile and ν\nu is the kinematic viscosity, typically parameterized in the α\alpha-prescription as ν=αcsH\nu = \alpha c_s H, with α∼10−3\alpha \sim 10^{-3}--10−210^{-2}, sound speed csc_s, and scale height HH. In circumplanetary disks, this spreading limits the disk extent to roughly 0.3--0.5 times the planet's Hill radius, preventing excessive mass buildup and enabling efficient satellite formation within the inner regions. Simulations show that higher viscosity accelerates outward expansion, reducing the disk's inner density and altering the torque balance on embedded bodies. External from the host drives photoevaporation in circumplanetary disks, eroding the outer disk edge through heating and hydrodynamic that remove gas at rates scaling with the stellar . This significantly shortens the disk lifetime, typically to 10^4--10^5 years for Jupiter-mass planets in solar-type systems, though the overall evolutionary phase aligns with the dispersal over 1--10 Myr. Photoevaporation truncates the disk at radii where the ionization front balances viscous inflow, with mass-loss rates of M˙∼10−10\dot{M} \sim 10^{-10}--10−8M⊙10^{-8} M_\odot yr−1^{-1} for far- of 10^3--10^4 G_0. In clustered environments, intracluster can enhance dispersal, explaining architectural differences in satellite systems like those of and Saturn. Embedded moons in circumplanetary disks experience Type I migration driven by gravitational torques from Lindblad resonances, where density waves excited at orbital commensurabilities with the disk lead to net inward or outward drift depending on the torque imbalance. These torques can truncate the disk at approximately 0.5 times the Hill radius by clearing material through resonant interactions, limiting further outward spreading. Migration timescales for moonlets of 10^{-4}--10^{-2} masses range from 10^3--10^5 years, slower than viscous evolution in low-α\alpha disks, allowing moons to accrete before falling onto the planet. The disk mass is constrained by tidal truncation within the Hill sphere, typically limited to about 0.01 times the planet's mass to prevent overflow and , as excess material would escape or accrete rapidly. This limit arises from the balance between viscous supply and tidal removal, with simulations showing steady-state masses of Mdisk∼10−3M_\mathrm{disk} \sim 10^{-3}--$10^{-2} M_p) for α∼10−2\alpha \sim 10^{-2}. Beyond this threshold, the disk destabilizes, ejecting gas and altering moon formation efficiency. Following gas dissipation via and photoevaporation, circumplanetary disks transition to debris disks composed of planetesimals, , and remnant rings on timescales of around 10 Myr, mirroring the dispersal of the parent . This phase features collisional evolution of solid material, forming structures like narrow rings or belts analogous to Saturn's system, with replenished by impacts over 10^6--10^7 years. The gas-poor remnants provide long-term reservoirs for irregular satellites or ring maintenance.

Hypothetical Evidence in the Solar System

Jupiter's Early Disk

A circumplanetary disk around is hypothesized to have formed approximately 4.5 billion years ago, during the planet's rapid accretion phase within the solar nebula, when grew to its current mass of about 1 M_Jup. This disk arose from material shed via transport as contracted under its own gravity, with an estimated initial mass on the order of 10^{-3} M_Jup, sufficient to supply the building blocks for its satellite system. The disk's formation occurred over a short timescale of roughly 10^4 years during the planet's Kelvin-Helmholtz contraction, transitioning from a rotationally supported structure to a viscously evolving gaseous disk fed by inflow from the surrounding solar nebula. Model-based evidence for this early disk draws heavily from the characteristics of Jupiter's Galilean moons—Io, Europa, Ganymede, and Callisto—which exhibit a clear compositional gradient: the inner moons (Io and Europa) are predominantly rocky with low ice fractions, while the outer ones (Ganymede and Callisto) are increasingly icy. This gradient is explained by the presence of a snow line in the disk at approximately 15 R_Jup (Jupiter radii), beyond which water ice could condense and incorporate into accreting satellites, while inner regions remained warmer and drier. Simulations indicate the disk extended outward to about 50 R_Jup, limited by tidal truncation near Jupiter's Hill sphere, with moon formation occurring via pebble accretion processes that assembled the satellites in as little as 10^5 years. Seminal models by Canup and Ward (2002, updated in 2010) demonstrate how angular momentum shedding during Jupiter's spin-down phase populated the disk, enabling efficient satellite growth through the capture and coagulation of solid pebbles drifting inward. Although no direct observational evidence exists for this ancient disk, its influence is inferred from the current orbital architecture of the , particularly the Laplace resonance among Io, Europa, and Ganymede (with mean motion ratios of 4:2:1), which models suggest was shaped by differential torques from the dissipating disk on migrating satellites. Possible remnants include Jupiter's faint , potentially originating from disrupted disk material or collisions involving small moons, as dust particles ejected by impacts on inner satellites like Amalthea could trace back to primordial debris. Compositional analyses further support a shared disk origin, with the moons' bulk densities and ice fractions aligning with accretion from a common reservoir where volatiles sublimated in the inner disk but preserved outward, consistent with isotopic and elemental similarities indicative of nebular processing.

Saturn's Early Disk

During Saturn's formation approximately 4.5 billion years ago, a circumplanetary disk of gas and solids is inferred to have surrounded the planet, with an estimated mass of about 10^{-3} M_{Sat} and extending outward to roughly 100 R_{Sat}. This disk provided the material reservoir for the accretion of Saturn's regular satellites, with its structure influenced by the planet's rapid growth phase within the solar nebula. Evidence for this early disk is drawn from the formation histories of Saturn's moons, particularly Titan, which likely accreted via pebble accretion in the cooler outer regions beyond approximately 20 R_{Sat}. Smaller moons such as are thought to have originated from remnants of this disk material, accreting as the disk viscously evolved and spread. The high content in both the rings and moons, exceeding 90% in many cases, points to a disk located around 20 R_{Sat}, where temperatures allowed to condense and dominate the composition. The current is interpreted as a dilute remnant of this massive primordial disk, which underwent disruption through formation processes or external impacts, leaving behind the observed icy structure. Models by Charnoz et al. (2011) propose that the initial ring mass was approximately 10^{-3} M_{Sat}, sufficient to accrete the mid-sized moons like , , Tethys, Dione, and Rhea as the disk spread viscously outward. Dynamical simulations indicate that interactions between the evolving disk and drove orbital migrations, leading to the current alignment of Tethys, Dione, and through capture into mean-motion resonances such as the 2:1 -Dione configuration. Data from the Cassini mission, including measurements of satellite orbital parameters, provide constraints on past disk interactions, with observed tidal migration rates implying prior viscous torques from a denser disk that influenced early and resonance locking. These interactions highlight the disk's role in shaping the system's before its left the enduring ring-moon configuration.

Observed Exoplanet Candidates

PDS 70 System

The PDS 70 system is a young centered on a of spectral type K7, with an age of approximately 5–6 million years and a distance of about 370 light-years from . The star hosts a featuring prominent gaps cleared by two forming giant s, PDS 70 b and PDS 70 c, which were directly imaged in the near-infrared using high-contrast instruments such as on the (VLT) and GPI. These planets, embedded within the disk's cavities, provide a unique laboratory for studying ongoing planet and formation processes. PDS 70 b, a Jupiter-mass (approximately 2–8 M_Jup) orbiting at about 22 AU from the star, was the site of the first submillimeter detection of potential circumplanetary disk material in using ALMA observations at 855 μm. The emission source, offset by 0.074 arcsec from the planet, exhibited a flux of 73–100 μJy beam⁻¹, corresponding to a mass of roughly 1.8–3.2 × 10⁻³ M_⊕ for optically thin 1 mm grains at 20 K, or up to four times higher if marginally optically thick. Assuming a standard gas-to-dust ratio of 100, the total disk mass could reach ~0.001 M_Jup, with a compact size limited to ≲4 AU (about 0.2 R_Hill, where R_Hill is the planet's Hill radius). This dust emission is brighter than that around PDS 70 c, suggesting a dust-rich environment potentially conducive to moon formation, though its exact association with a circumplanetary disk remains tentative due to the offset. PDS 70 c, a more massive (1–10 M_Jup) at ~34 AU, hosts the clearest evidence of a circumplanetary disk, confirmed through both continuum and molecular gas observations. In 2019 ALMA data at 855 μm, a spatially unresolved source was detected with a flux of 106 ± 19 μJy beam⁻¹, yielding a of 2–4.2 × 10⁻³ M_⊕ and a size ≲4 AU (~0.1–0.2 R_Hill). Higher-resolution ALMA in refined this to a compact disk with radius <1.2 AU, ~0.007–0.031 M_⊕ (depending on grain size), and temperature ~26 K, consistent with viscous heating and irradiation models. The first gaseous detection came in 2022 via ALMA Band 6 observations of ¹²CO and ¹³CO J=2–1 lines, revealing a point-source emission in ¹³CO with integrated intensity 2.54 mJy beam⁻¹ km s⁻¹, gas temperature ≥35 K, and ≥0.095 M_Jup—confirming an active, gaseous circumplanetary disk as a moon-forming site. This gas emission, warmer and more localized than the surrounding material, indicates ongoing accretion. These circumplanetary disks are embedded within the parental of , with no spatial overlap between their substructures; b's emission is dust-dominated and fainter in gas, while c's shows stronger gas signatures. Millimeter with ALMA has been crucial for resolving these features at ~0.1 arcsec scales, complementing near-infrared high-contrast imaging that confirms the planets' positions and accretion activity. The detections align with theoretical expectations for circumplanetary disk formation via gravitational capture from the circumstellar disk, providing direct evidence for satellite formation mechanisms in exoplanetary systems.

Recent Discoveries

In September 2025, NASA's (JWST) detected a moon-forming circumplanetary disk around the young CT Cha b using its (MIRI) for . CT Cha b, with an estimated mass of 10-15 masses, orbits its star at an angular separation of approximately 1.3 arcseconds, located about 625 light-years away. The disk, rich in carbon-bearing molecules, extends to roughly 0.3 times the planet's Hill radius and shows potential for forming icy moons through ongoing dust and gas processes. This observation marks one of the clearest views of a cooler, outer disk region, highlighting JWST's sensitivity to such structures. In June 2025, JWST observations of the YSES-1 system revealed clouds in the atmosphere of the inner YSES-1 c and signatures of a circumplanetary disk around the outer YSES-1 b, orbiting a young solar-type star about 300 light-years distant. These findings, based on mid-infrared , indicate active disk accretion feeding material into the 's envelope, with the disk potentially enabling formation through silicate-rich debris. The dual- setup provides a snapshot of differing evolutionary stages, where the inner disk shows atmospheric interactions akin to haze formation. A November 2025 report detailed a candidate circumplanetary disk transit in the ASASSN-24fw system, identified through a transient 4-magnitude dip in the light curve of a main-sequence approximately 1,000 parsecs away. The , lasting about 8 months from late 2024, suggests an by a gas-rich disk with an estimated of around 0.01 times that of its host substellar companion, possibly resulting from a planetary collision or debris accumulation. Follow-up photometry and polarization data support the disk's optically thick nature, distinguishing it from stellar variability. By late 2025, JWST's infrared capabilities have enabled the detection of cooler, carbon- and silicate-rich disks, with follow-up observations from TESS and ALMA confirming around five circumplanetary disk candidates overall. These advancements build on earlier foundational observations like those of , shifting focus to diverse chemical compositions and accretion dynamics. Key challenges in identifying these disks include differentiating circumplanetary emission from extended planetary atmospheres and their rarity at large orbital radii beyond 50 AU, where disk stability diminishes due to tidal influences. Future prospects involve (ELT) observations targeting transitional systems like , aiming to resolve finer disk structures and gas kinematics with mid-infrared spectrographs such as METIS.

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