In the quest to understand the birthplaces of planets, astronomers are increasingly turning their attention to the intricate structures hidden within protoplanetary disks. These disks, rotating clouds of gas and dust encircling young stars, give rise to planetary systems, yet many of their secrets remain elusive. A recent study, spearheaded by Á. Ribas and colleagues, pushes the boundaries of observational astronomy by unveiling compelling evidence of a nascent gas giant planet along with subtle substructures embedded in the MP Mus protoplanetary disk. Utilizing the unparalleled capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA), the team harnessed multi-wavelength data along with sophisticated simulations to reveal a rich tapestry of disk features, shedding light on planet formation processes at unprecedented scales.
The crux of this breakthrough lies in the meticulous analysis of ALMA Band 3 (3 mm) and Band 6 (1.3 mm) continuum observations. The Band 3 data, originating from ALMA project 2022.1.01758.S, were obtained using both compact and extended array configurations, enabling the capture of spatial scales ranging from tens of meters to over 16 kilometers in baseline. These configurations permitted high-resolution mapping of the disk’s faint millimeter-wave emission, a crucial tracer of the distribution of dust grains that compose the architectural elements within the disk. Each spectral window arrayed around frequencies between 90 and 105 GHz maximized continuum sensitivity, with hundreds of spectral channels finely resolving the emission features.
To extract meaningful insights from these complex datasets, the researchers executed a series of precision calibrations and imaging steps. Data calibration employed multiple versions of the Common Astronomy Software Applications (CASA), reflecting the evolving capabilities of this standard toolkit. Phase-only self-calibration was conducted to correct time-dependent atmospheric and instrumental effects, first segregated by configuration to optimize image fidelity. Coordinated re-centering of phase centers to a common spatial reference frame, along with flux rescaling of the extended observations, ensured consistent photometric integrity across observing epochs. The data then underwent a final combined self-calibration, resulting in exquisite images with beam sizes as fine as 0.06 by 0.04 arcseconds and remarkably low noise levels. These high dynamic range images exhibited peak signal-to-noise ratios near 100, enabling reliable identification of delicate disk structures.
Parallel to the Band 3 observations, the Band 6 data sourced from earlier ALMA projects offered complementary spatial resolutions and sensitivity to different grain populations. These observations featured spectral windows designed to capture both continuum emission and, crucially, the 12CO (2–1) molecular line, providing potential insights into gas kinematics. However, the focus remained firmly on continuum emission, with CO channels flagged to isolate dust signatures. Self-calibration protocols mirrored those applied to Band 3, including re-centering, flux matching, and joint imaging of compact and extended configurations to yield high-fidelity continuum maps.
Imaging methods incorporated the multi-term multi-frequency synthesis (mtmfs) technique within CASA’s tclean routine, balancing the competing demands of spatial resolution and sensitivity. The choice of robust weighting parameters optimized the resolution to reveal sub-beam scale features, while carefully chosen imaging scales accounted for emission across multiple spatial extents. The resulting maps unveiled a complex morphology within MP Mus’ disk, including clear evidence for a ring structure, multiple radial gaps, and, significantly, a subtle inner cavity suggestive of dynamic disk clearing.
To rigorously validate these features, the team employed a non-parametric radial profile extraction tool known as FRANK. FRANK’s use of Hankel transforms directly on visibilities allowed reconstruction of the disk’s radial brightness distribution without the biases often introduced by image synthesis techniques. By incorporating initial estimates of the disk’s inclination and position angle, and carefully tuning hyperparameters that control image smoothness and regularization strength, the team derived profiles reinforcing the presence of ringed substructures and the inner cavity. Crucially, residual analysis revealed no significant unmodeled emission, affirming the robustness of the extracted structures.
Additional sanity checks employed the GoFish and GPUVMEM imaging codes. GoFish facilitated radial profile extraction directly from synthesized images, providing a complementary assessment despite a moderate loss in spatial resolution. Meanwhile, GPUVMEM’s maximum entropy reconstruction further sharpened the images beyond conventional CLEAN-based methods. Its regularized maximum likelihood approach, which enforces positivity constraints without additional entropy terms, yielded super-resolved images that reproduced all key disk features while enhancing sensitivity and contrast. Together, these methods confirmed that the observed inner cavity and ring-gap architecture are intrinsic properties of the MP Mus disk, not processing artifacts.
Interpreting these morphological indicators requires understanding their origins — do they stem from inherent disk physics or the gravitational influence of forming planetary companions? To address this, the team conducted three-dimensional hydrodynamical simulations using the smoothed particle hydrodynamics (SPH) code PHANTOM, which is well established for modeling gas and dust dynamics in planet-forming environments. A grid of 25 simulations explored varying companion masses and orbital distances consistent with proper motion anomalies identified by Gaia astrometry. These simulations modeled the star with a fixed mass of 1.3 solar masses, enveloped by a disk of 0.01 solar masses, with gas temperature and surface density profiles reflecting both theoretical expectations and observational constraints.
The simulations treated crucial physical processes such as viscous angular momentum transport through SPH artificial viscosity, using parameters calibrated to yield moderate disk viscosity levels. Importantly, they included a dust component resolved through a one-fluid approach covering grain sizes from micron to centimeter scales, distributed following a power-law size spectrum. The disk’s inner boundary was set to a radius about 1.5 times the companion’s orbit, allowing tidal interactions to carve a cavity. Sink particles modeled the central star and companion, with carefully chosen accretion radii preventing numerical artifacts in cavity formation.
Over the course of 1,000 planetary orbits, the simulations demonstrated that companions with masses ranging from a few up to 15 Jupiter masses and orbits within a few astronomical units could reproduce cavities and density substructures aligning with ALMA observations. These models underscore how evolving giant planets dynamically sculpt their natal disks, creating pressure traps and gaps that influence dust grain distributions, potentially accelerating planet formation through localized enhancements.
To translate these hydrodynamical outputs into synthetic observables, the team leveraged the Monte Carlo radiative transfer code MCFOST. This allowed them to compute thermal structures and multi-wavelength continuum emission maps consistent with the simulated dust and gas distributions. The disk’s temperature was computed assuming passive stellar heating from a 5,000 K blackbody source with parameters mirroring MP Mus. The adoption of DIANA-standard dust compositions including silicates, amorphous carbon, and porosity informed the dust opacity calculations, ensuring realistic emission properties across the millimeter regime.
By discretizing the radiation field with an enormous number of photons, MCFOST achieved precise temperature equilibrium and produced ray-traced images matching ALMA’s observing frequencies and viewing geometry. The resulting synthetic images closely resembled the observed data in terms of disk morphology, cavity size, and substructure contrasts, confirming the viability of embedded gas giant companions as architects of the observed features.
This integrative study marks a crucial advance in our understanding of protoplanetary disks as dynamic, planet-forming systems rather than passive dusty reservoirs. The extensive observational campaigns combined with rigorous data processing, non-parametric imaging, and state-of-the-art simulations provide a holistic picture linking subtle disk morphologies to the presence of young gas giant planets. Such planets, often elusive to direct detection, leave telltale imprints on their birth environments that, as this work demonstrates, can be effectively deciphered with high-resolution millimeter observations and sophisticated analysis.
Looking ahead, the methodologies refined here promise to unlock similar insights into other nearby disks, expanding our census of infant planets and enriching theoretical models of planetary system assembly. With ongoing improvements in interferometric capabilities and computational modeling, the once opaque birthplaces of planets are now yielding their secrets, bringing us closer to comprehending the genesis of complex planetary architectures like our own solar system.
—
Subject of Research: Protoplanetary disk structures and planet formation signatures in the MP Mus system
Article Title: A young gas giant and hidden substructures in a protoplanetary disk
Article References:
Ribas, Á., Vioque, M., Zagaria, F. et al. A young gas giant and hidden substructures in a protoplanetary disk. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02576-w
Image Credits: AI Generated
