exoALMA Large Program

• exoALMA Large Program is a homogeneous survey of 15 bright protoplanetary disks designed to trace gas kinematics and disk substructures.
• It utilizes ALMA Band 7 to map key tracers like 12CO, 13CO, and CS along with high-res continuum imaging for detailed dynamical analysis.
• The program applies uniform calibration and advanced modeling techniques to differentiate between planet-induced features, instabilities, and turbulence.

The exoALMA Large Program is an ALMA Cycle 8 Large Program, project 2021.1.01123.L, built to study planet formation through gas kinematics in a homogeneous sample of 15 bright, extended protoplanetary disks. It targeted disks ranging between ${\sim}1\arcsec$ and ${\sim}7\arcsec$ in radius, and mapped $^{12}$CO $J=3\!-\!2$, $^{13}$CO $J=3\!-\!2$, CS $J=7\!-\!6$, and the 330 GHz continuum at high angular and spectral resolution, with the explicit goal of detecting departures from smooth Keplerian rotation, characterizing disk structure in three dimensions, and relating dust and gas substructures to embedded planets, turbulence, winds, and self-gravity (Teague et al., 25 Apr 2025). Across the series, exoALMA is presented not as a population census of all disks, but as a biased yet homogeneous and well-characterized sample optimized for high-fidelity dynamical inference (Ruzza et al., 13 Mar 2026).

1. Program design, sample, and scientific scope

The sample was selected to maximize kinematic leverage. The main criteria were a gas radius of at least $1\arcsec$, moderate inclination $5^\circ \le i \le 60^\circ$, bright and extended $^{12}$CO emission, minimal contamination by large-scale cloud emission or remnant envelopes, and a right-ascension distribution compatible with efficient Band 7 scheduling (Teague et al., 25 Apr 2025). Distances span roughly ${\sim}7\arcsec$0–${\sim}7\arcsec$1 pc, and the targets include both T Tauri and Herbig Ae/Fe systems such as DM Tau, LkCa 15, HD 135344B, HD 143006, HD 34282, CQ Tau, MWC 758, J1604, J1615, PDS 66, SY Cha, and V4046 Sgr (Curone et al., 25 Apr 2025).

The scientific remit is broader than planet detection alone. exoALMA was designed to measure or constrain turbulent motions, characterize pressure gradients and disk self-gravity, reconstruct CO emitting surfaces and temperature structures, search for kinematic signatures of embedded planets, and calibrate gas-mass tracers against rotation-curve measurements (Teague et al., 25 Apr 2025). Later papers extend this scope to vertical flows, gravitational stability, line-broadening tomography, nonaxisymmetric dust morphology, and simulation-based inference of putative planet and disk parameters (Benisty et al., 13 Mar 2026).

A central programmatic feature is uniformity. The same survey products support source-specific studies and population-level comparisons. This is why the series repeatedly emphasizes homogeneous calibration, homogeneous imaging, and homogeneous geometric and kinematic modeling across all 15 targets (Loomis et al., 28 Apr 2025).

2. Observing strategy and principal observables

exoALMA uses ALMA Band 7 because the high observing frequency improves velocity resolution for a fixed frequency spacing and enables very deep line and continuum imaging. The core line tracers are ${\sim}7\arcsec$2CO ${\sim}7\arcsec$3, ${\sim}7\arcsec$4CO ${\sim}7\arcsec$5, and CS ${\sim}7\arcsec$6, observed with native spectral sampling of 15.25 kHz and an effective velocity resolution of about ${\sim}7\arcsec$7–${\sim}7\arcsec$8, then commonly re-imaged into fiducial cubes at ${\sim}7\arcsec$9 for CO and $^{12}$0 for CS (Teague et al., 25 Apr 2025). The delivered fiducial line products are circularized to $^{12}$1, while the continuum is imaged at about 90 mas with a point-source sensitivity of $^{12}$2 (Teague et al., 25 Apr 2025).

Seven sources required ACA observations because their $^{12}$3CO disks are so extended that 12 m-array data alone would not recover all relevant spatial scales (Loomis et al., 28 Apr 2025). exoALMA II describes three principal image sets for the lines: a fiducial $^{12}$4 product, a higher-resolution product used where the signal supports finer spatial analysis, and a $^{12}$5 high-surface-brightness product used to trace low-level outer-disk structure (Loomis et al., 28 Apr 2025).

Observable	Role in exoALMA	Representative property
$^{12}$6CO $^{12}$7	Highest-S/N tracer of the warm surface and primary kinematic line	Fiducial $^{12}$8, $^{12}$9
$J=3\!-\!2$0CO $J=3\!-\!2$1	Traces deeper layers; in some cases approaches optically thin behavior	Matched Band 7 line cube
CS $J=3\!-\!2$2	Denser/deeper gas; less affected by thermal broadening than CO	Fiducial $J=3\!-\!2$3
330 GHz continuum	Dust rings, gaps, cavities, asymmetries, faint outer disk	$J=3\!-\!2$4 mas, $J=3\!-\!2$5

The program’s observational design was explicitly tuned so that channel-by-channel perturbations at a few $J=3\!-\!2$6 are, in principle, detectable, and so that large disks are sampled by many independent beams across their CO extent (Barraza-Alfaro et al., 28 Apr 2025). A plausible implication is that exoALMA was configured not merely to detect disks, but to resolve their dynamical substructure at the scale where planet-driven and instability-driven signatures diverge.

3. Calibration, imaging, and modeling infrastructure

exoALMA II is the technical backbone of the survey. Because the program is designed to detect subtle non-Keplerian features, the collaboration treated phase decoherence, astrometric misalignment between executions, flux mismatches, and PSF non-Gaussianity as first-order systematics rather than as routine calibration nuisances (Loomis et al., 28 Apr 2025). The adopted workflow used iterative self-calibration, visibility-space alignment, explicit flux scaling when required, and morphology-driven masking during CLEAN to avoid imprinting Keplerian assumptions onto the data (Loomis et al., 28 Apr 2025).

For gas-line modeling, exoALMA III introduced a Discminer-based workflow to extract system properties and line-formation structure from $J=3\!-\!2$7CO, $J=3\!-\!2$8CO, and CS. A composite line-profile kernel accounts for projected overlap between front- and back-side emission, and an improved iterative two-component fitting method is used for inclined sources with $J=3\!-\!2$9 to mitigate contamination from the disk backside (Izquierdo et al., 28 Apr 2025). This framework yields Keplerian stellar masses, inclinations, position angles, systemic velocities, rotation directions, and emission surfaces for the full sample (Izquierdo et al., 28 Apr 2025).

The continuum analysis is likewise visibility-based. exoALMA IV used GALARIO for parametric geometry fitting and frank for nonparametric reconstruction of axisymmetric radial brightness profiles, then subtracted the axisymmetric model in the visibility domain to isolate nonaxisymmetric residuals (Curone et al., 25 Apr 2025). This provides a common geometric reference frame for comparing continuum rings and gaps with gas velocity residuals, emission surfaces, and pressure perturbations.

Imaging robustness was tested directly. exoALMA IX applied regularized maximum likelihood imaging with MPoL to seven disks with apparent non-Keplerian features and found that RML reconstructions reproduce the same features seen in the fiducial CLEAN cubes, implying that the relevant structures are not artifacts of a single deconvolution algorithm (Zawadzki et al., 27 Apr 2025). exoALMA XIX further showed that full 3D radiative-transfer fitting with MCFOST and Bayesian inference can be run directly on exoALMA-quality cubes; in DM Tau, this required more than five million models and produced a background disk structure that could be reused for other tracers such as CS (Hardiman et al., 2 Feb 2026).

4. Dust and gas structures revealed by the survey

The continuum data show that substructure is the rule rather than the exception. exoALMA IV found rings, gaps, or cavities in 14 of the 15 disks, and introduced a NonAxisymmetry Index to quantify the residual flux left after subtraction of an axisymmetric continuum model (Curone et al., 25 Apr 2025). The most asymmetric disks predominantly show an inner cavity and consistently present higher values of mass accretion rate and near-infrared excess, suggesting a connection between outer-disk dust substructures and inner-disk properties (Curone et al., 25 Apr 2025). The same paper showed that larger disks, in both dust and gas, tend to have gradually tapered outer continuum profiles, whereas compact disks show sharper outer edges (Curone et al., 25 Apr 2025).

The line data reveal even broader dynamical diversity. exoALMA X analyzed the $^{13}$0CO $^{13}$1 cubes channel by channel and found that 13 of the 15 disks display large-scale arcs or spiral arms, localized velocity kinks, and/or multiple faint arcs that appear like filamentary structures on the disk surface (Pinte et al., 25 Apr 2025). Six disks—AA Tau, SY Cha, J1842, J1615, LkCa 15, and HD 143006—show kinematic signatures consistent with planet wakes, with comparison to hydrodynamical and radiative-transfer simulations suggesting planets at orbital radii between 80 and 310 au and masses between 1 and $^{13}$2 (Pinte et al., 25 Apr 2025).

The same channel-map study also established a vertical-chemistry result that became one of the signature exoALMA findings: in the seven disks with favorable inclination for separating upper and lower emission surfaces, the vertical CO snowline is detected, the $^{13}$3CO midplane abundance is only partially depleted, with a depletion factor of approximately $^{13}$4–$^{13}$5 relative to the warm molecular layer, and there is systematic evidence for CO desorption in the outer disk (Pinte et al., 25 Apr 2025).

exoALMA XXII generalized this to the entire sample by constructing uniform 2D maps of centroid velocity, line width, and peak intensity from the $^{13}$6CO data. All 15 targets exhibit large-scale deviations from smooth Keplerian disks, with morphologies that include spiral-like structures, arc- or ring-like features, and signatures of variations in emitting-surface height (Fukagawa et al., 13 Mar 2026). Non-axisymmetric spiral-arm features are detected or suggested in five disks—CQ Tau, MWC 758, HD 135344B, HD 34282, and SY Cha—and are preferentially found in Herbig Ae/Fe systems, while J1852, PDS 66, and V4046 Sgr appear dynamically quieter despite still showing measurable deviations (Fukagawa et al., 13 Mar 2026).

Vertical flows are likewise common. exoALMA XXI analyzed 14 disks and found vertical motions in most of them, generally with amplitudes of a few tens of $^{13}$7, organized either as oscillatory up/down flows or as downward-to-upward transitions interpreted as the base of a disk wind (Benisty et al., 13 Mar 2026). MWC 758 and CQ Tau show much larger red- and blue-shifted spiral features, interpreted as vertical velocities up to $^{13}$8 ($^{13}$9), while MWC 758 also shows fast upward motions up to $J=3\!-\!2$0 ($J=3\!-\!2$1) in its outer disk (Benisty et al., 13 Mar 2026).

5. Embedded planets, turbulence, and disk masses

A major exoALMA theme is that localized non-Keplerian structure can be interpreted only in combination with broader disk physics. exoALMA XX showed, using synthetic observations of planet–disk interactions and disk instabilities, that tomographic line analysis can identify the key signatures of planets more massive than $J=3\!-\!2$2 of the stellar mass with only a few hours of ALMA integration at $J=3\!-\!2$3–$J=3\!-\!2$4 resolution (Izquierdo et al., 13 Mar 2026). In that framework, planets produce not only centroid-velocity perturbations but also localized line broadening, and line skewness becomes a diagnostic for distinguishing planetary signatures from instability-driven signatures because the underlying velocity coherence differs (Izquierdo et al., 13 Mar 2026).

When applied to exoALMA data, this tomographic method suggested three massive planets in HD 135344B—at $J=3\!-\!2$5 au, $J=3\!-\!2$6 au, and $J=3\!-\!2$7 au—and interpreted the dominant vertical-velocity spirals in MWC 758 as more consistent with moderate disk eccentricity or warps, potentially induced by a substellar companion in the inner disk, than with a purely local planet signal (Izquierdo et al., 13 Mar 2026). This is consistent with the broader exoALMA pattern that not every localized disturbance is best described by the same mechanism.

Turbulence and instability diagnostics form a parallel thread. exoALMA XVI used 3D simulations plus radiative transfer to predict how the vertical shear instability, the magneto-rotational instability, and gravitational instability would appear at exoALMA’s angular and spectral resolution (Barraza-Alfaro et al., 28 Apr 2025). The predicted morphologies differ—rings and arcs for VSI, spiral-like residuals and zonal structures for MRI, large-scale spirals for GI—and a qualitative comparison to the actual exoALMA sample suggests the presence of two laminar disks and a scarcity of ring- and arc-like VSI signatures, whereas MRI- or GI-like spiral features remain plausible in several disks (Barraza-Alfaro et al., 28 Apr 2025).

The DM Tau case shows how this plays out in practice. exoALMA XIX fit high-resolution $J=3\!-\!2$8CO $J=3\!-\!2$9 data at $J=7\!-\!6$0 and $J=7\!-\!6$1 with a Bayesian MCFOST framework and found a significant nonthermal contribution to the line width of about $J=7\!-\!6$2, inconsistent with purely thermal motions (Hardiman et al., 2 Feb 2026). The same CO-based structure reproduces CS $J=7\!-\!6$3, which is more sensitive to nonthermal broadening, and residual moment maps reveal localized perturbations that may trace forming planets (Hardiman et al., 2 Feb 2026).

Disk masses and radial scales provide the program’s dynamical calibration. Rotation-curve modeling of $J=7\!-\!6$4CO and $J=7\!-\!6$5CO for ten exoALMA disks yielded dynamical stellar masses, disk masses, and density scale radii, showed that all modeled disks are gravitationally stable, and implied an average gas-to-dust ratio of approximately 400 under the assumption of optically thin dust (Longarini et al., 25 Apr 2025). The same analysis inferred effective $J=7\!-\!6$6 values between $J=7\!-\!6$7 and $J=7\!-\!6$8 (Longarini et al., 25 Apr 2025). Complementary line-flux modeling with C$J=7\!-\!6$9O and N$1\arcsec$0H$1\arcsec$1 found that CO+N$1\arcsec$2H$1\arcsec$3-based gas masses typically agree with kinematic masses within a factor of 3, although they are on average lower by a factor $1\arcsec$4, and also established a chemical dichotomy in which Herbig disks tend to retain ISM-level CO abundances while T Tauri disks are typically depleted by factors of 3–30 (Trapman et al., 27 Apr 2025).

Dust morphology has been turned into an additional inference channel. exoALMA XXIII used DBNets2.0 to interpret 19 substructures in 13 disks under the assumption that they are produced by embedded planets at fixed locations, inferring putative planet masses, disk $1\arcsec$5-viscosities, scale heights, and dust Stokes numbers (Ruzza et al., 13 Mar 2026). The resulting planet population is predominantly Jovian to super-Jovian, the inferred migration is mostly inward, and for the Herbig stars the implied viscous accretion timescales are too long to explain the observed stellar accretion rates (Ruzza et al., 13 Mar 2026).

6. Interpretation, caveats, and legacy

Several recurring misconceptions are addressed directly by the exoALMA series. First, the sample is not a representative disk population: it is intentionally biased toward bright, large, structured systems where gas kinematics can be measured at high precision (Teague et al., 25 Apr 2025). Second, not every non-Keplerian feature is a planet signature. exoALMA XVII examined four disks with strong dust crescents—HD 135344B, HD 143006, HD 34282, and MWC 758—and found that none shows a distinctive vortex signature around the dust asymmetry in the $1\arcsec$6CO or $1\arcsec$7CO kinematics (Wölfer et al., 28 Apr 2025). exoALMA XVI and exoALMA XXI further show that MRI, GI, VSI-like flows, wind bases, and eccentric-disk vertical spirals can all generate residual structures that overlap phenomenologically with planet-driven perturbations (Barraza-Alfaro et al., 28 Apr 2025).

This is why exoALMA progressively moved from single-diagnostic interpretations to multi-diagnostic ones. The program combines continuum morphology, emission-surface reconstruction, velocity centroids, line widths, rotation-curve fitting, nonthermal broadening, and multi-line tomography, and it validates imaging products with independent deconvolution methods (Zawadzki et al., 27 Apr 2025). A plausible implication is that exoALMA’s main legacy lies not in any single planet claim, but in establishing a workflow for distinguishing between planets, instabilities, self-gravity, and winds in disks where all of those processes can operate simultaneously.

As a survey, exoALMA has shown that when large disks are observed with sufficient sensitivity at $1\arcsec$8 au scales and $1\arcsec$9 velocity resolution, physical and dynamical substructure is nearly ubiquitous (Fukagawa et al., 13 Mar 2026). The program’s public data products, uniform analysis layers, and cross-calibrated dynamical, chemical, and tomographic results therefore constitute a benchmark dataset for high-resolution studies of planet-forming disks (Teague et al., 25 Apr 2025).