Early Planet Formation in eDisk

• Early planet formation in embedded disks (eDisk) is characterized by the emergence of dynamically mature, Keplerian disks during the deeply embedded protostellar phase.
• High-resolution ALMA observations uncover subtle substructures and brightness asymmetries that hint at rapid disk evolution or the masking effect of high dust optical depths.
• The eDisk program integrates advanced imaging of disk kinematics and chemistry to illuminate the initial conditions for planetesimal growth and subsequent planet formation.

Early planet formation in embedded disks (“eDisk”) refers to the processes, structure, and evolution of solid and gaseous material leading to the first stages of planet formation, occurring while the disk is still deeply enshrouded in its natal protostellar envelope (the Class 0/I phase). The eDisk program, based on extensive high-resolution Atacama Large Millimeter/submillimeter Array (ALMA) surveys, systematically investigates disk structure, kinematics, chemistry, and substructure in 19 nearby embedded protostars with the aim of constraining how and when planet formation is initiated, and how its earliest observable signatures differ from those in more evolved (Class II) protoplanetary disks (Ohashi et al., 2023).

1. Embedded Disk Substructure and Its Evolution

High-angular-resolution (∼0.04″, ≈7 au) 1.3 mm continuum imaging across the eDisk sample reveals spatially resolved, disk-like emission in all embedded Class 0/I sources, but the disks generally lack the prominent concentric rings, gaps, and spiral arms seen in many Class II disks (Ohashi et al., 2023). Instead:

• Substructure is rare and, where present, subtle: L1489 IRS shows hints of low-contrast ring-like features, Oph IRS63 displays weak substructure, and IRAS 04169+2702 exhibits a bean-like emission feature.
• Brightness asymmetries are frequently detected along the disk minor axis. These are interpreted as geometric effects of inclined, vertically thick (flared) and optically thick disks, not as clear indicators of axisymmetric substructures.

This distinction from Class II disks signals two possibilities: (1) that substructures develop very rapidly as envelopes dissipate and disks evolve, or (2) that high dust optical depths in embedded disks obscure fine substructures which may still be developing beneath the observable millimeter surface.

2. Disk Dynamics: Kinematic Signatures of Keplerian Rotation

A central result is the detection of Keplerian rotation in embedded disks by means of spectral-line imaging (especially CO isotopologues like C$^{18}$O 2–1). Position–velocity (PV) diagram analysis along disk major axes is modeled by the scaling

$$v(r) = v_{\rm sys} + v_{b} \left( \frac{|r|}{r_b} \right)^{-p}$$

with $p \approx 0.5$ for Keplerian rotation. Multiple systems (e.g., R CrA IRS7B, L1527 IRS, IRAS 16544–1604, and R CrA IRS5N) yield indices near 0.5, confirming rotationally supported disks.

Dynamical masses are computed using

$M_\star = \frac{rv^2}{G \sin^2 i}$

where $i$ is the disk inclination. Measured masses span $\sim$0.1–3 M$_\odot$, encompassing both very low-mass and Sun-like protostars (Hoff et al., 2023, Sharma et al., 2023, Kido et al., 2023).

Kinematic evidence demonstrates that, even in the mass accretion-dominated, envelope-embedded phase, disks attain dynamical maturity with Keplerian support—a necessary condition for subsequent planetesimal formation.

3. Comparison to More Evolved (Class II) Disks

In Class II disks, prominent, high-contrast rings, gaps, and spirals (e.g., from programs like DSHARP) are often interpreted as dust traps or planet–disk gravitational perturbations. The relative absence of such features in eDisk sources supports two non-exclusive scenarios:

• Substructures rapidly emerge after envelope dispersal and are a product of advanced dust coagulation or planet-induced dynamics.
• The high dust optical depths in embedded disks, combined with geometric flaring and incomplete dust vertical settling, result in a smooth observed morphology, masking any underlying density structure.

This contrast emphasizes that the observable fingerprint of planet formation is evolutionary-stage dependent. The lack of sharp substructure in the eDisk sample suggests planet formation seeds may be sown during the embedded phase, but their subsequent growth and disk sculpting become apparent only later (Ohashi et al., 2023).

4. Sample Selection, ALMA Observational Strategy, and Data Analysis

The eDisk sample comprises 12 Class 0 and 7 Class I protostars located in star-forming regions within 200 pc, selected to span a range of luminosities, envelope properties, and evolutionary status (Ohashi et al., 2023). Key aspects include:

• ALMA Band 6 observations at ∼0.04″ (∼7 au) resolution in the continuum and ∼0.08″ in CO isotopologue lines.
• Correlator settings optimize coverage for dust continuum plus high-fidelity imaging of key molecular transitions (C$^{18}$O, $^{13}$CO, SO, etc.).
• Data reduction combines standard ALMA pipeline processing using CASA, and extensive self-calibration and continuum subtraction for optimal sensitivity and image fidelity.

This approach enables separation of disk, envelope, and outflow components, and supports quantitative analysis of faint substructure or asymmetric features.

5. Early Disk Chemistry, Thermal Structure, and Vertical Settling

Embedded disks display high optical depths and significant vertical thickness. Modeling and observations (e.g., L1527 IRS, IRAS 04302+2247) demonstrate:

• Limited dust settling: Millimeter-sized grains are not confined to a narrow midplane layer but instead remain vertically extended, resulting in a “puffy” disk morphology (Lin et al., 2023).
• The temperature structure is complex, with warm (20–60 K) disks often too hot for CO freeze-out except at large radii; CO snowlines are mapped at hundreds of au in envelopes.
• SO, H$_2$CO, and other chemical tracers often reveal localized enhancements at disk surfaces, outflow cavity walls, or shock interfaces (e.g., at streamer landing zones), indicating a chemically active environment whose thermal and ionization profiles are critical for grain coagulation and surface chemistry (Hoff et al., 2023).

These conditions may delay vertical settling and planetesimal layer formation, but also provide locally enhanced regions (e.g., at shocks or stream landing sites) conducive to dust concentration and early coagulation.

6. The Emergence of Disk Substructure and Implications for Planet Formation

Despite the general smoothness of eDisk continuum images, individual systems show hints of early substructure:

• Faint ring-like features, low-contrast plateaus, or “shoulders” in Ced110 IRS4, L1489 IRS, and IRAS 04169+2702 are interpreted as either nascent density/pressure maxima (potentially sites of dust trapping and planet formation) or as artifacts of optical depth and temperature gradients (Sai et al., 2023, Yamato et al., 2023, Han et al., 19 Jun 2025).
• Large-scale, infalling streamers and spiral arms feeding disks are commonly detected in optically thin tracers (C$^{18}$O, SO, H$_2$CO), suggesting continuous envelope-to-disk mass supply and the presence of accretion shocks that may further concentrate solid material or trigger chemical changes.
• Non-axisymmetric features, such as spiral arms, warp-induced velocity gradients, and misalignment between disks and outflow axes, highlight the dynamic, non-steady-state conditions under which the earliest planet-forming seeds develop (Yamato et al., 2023, Han et al., 19 Jun 2025, Kido et al., 1 Apr 2025).

These observations support the view that key conditions for planetesimal formation—pressure maxima, vertical and radial dust concentration, and a continuous supply of fresh material—are established early but evolve as envelopes dissipate and disks settle.

7. Theoretical and Observational Context, and Future Directions

The eDisk program’s findings challenge the classical view that planet formation is largely a Class II (post-embedded) disk phenomenon, instead demonstrating that:

• Keplerian disks, with sizes of a few tens to hundreds of au and sufficient mass reservoirs, form early and are dynamically mature in the deep embedded stage.
• Disks may be “flared” with limited dust settling, high optical depths, and no clear substructure prior to envelope dispersal, setting distinct initial conditions for planetesimal growth.
• Dynamical features (warps, substructure, and infall) and chemical complexity are set by the interplay of disk gravity, envelope accretion, turbulence, and possibly magnetic fields.

A future research priority is to extend observations to longer wavelengths and utilize line-of-sight velocity tomography to disentangle optical depth effects, vertical disk structure, and hidden substructure. High spatial and spectral resolution imaging, combined with sophisticated radiative transfer and chemical models, will be essential to connect smooth dust disks, the emergence of substructure, and the first steps of planetary core assembly.

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The early planet formation picture emerging from eDisk is thus one of rapid disk assembly, incomplete dust settling, and dynamic infall and transport shaping the raw ingredients for planets, with the first detectible signs of planet formation processes already realized during the embedded disk phase (Ohashi et al., 2023, Hoff et al., 2023, Yamato et al., 2023, Lin et al., 2023, Kido et al., 2023, Sai et al., 2023, Sharma et al., 2023, Han et al., 19 Jun 2025, Kido et al., 1 Apr 2025).