AB Aur b: Protoplanet in a Transitional Disk

• AB Aur b Protoplanet is a candidate giant forming within the AB Aurigae disk, identified through multiwavelength imaging and spectroscopic diagnostics.
• The object shows definitive accretion signatures including an inverse P Cygni Hα profile, significant Hα flux variability, and compact emission matching theoretical models.
• The disk’s complex structure, chemical gradients, and dust-trapping features suggest gravitational instability as the likely formation mechanism.

AB Aur b is a candidate protoplanet detected at large separation (~93–100 au) within the transitional disk surrounding the Herbig Ae star AB Aurigae. Its identification as a bona fide accreting giant planet embedded in its natal disk is supported by extensive multiwavelength imaging, spectroscopic, and chemical analyses, though alternative explanations such as scattered-light disk features persist in some datasets. The AB Aur system is one of the cornerstones for studying planetary growth mechanisms, circumplanetary disk evolution, dust trapping, and accretion processes in the early phases of star and planet formation.

1. Physical Properties and Location

AB Aur b resides in the cavity of the AB Aurigae disk, with direct imaging resolving a compact source at a projected separation of $600 \pm 22$ mas ($\sim$93 au) and P.A. $182.5^\circ \pm 1.4^\circ$ (Zhou et al., 2022). Its emission properties (deconvolved spatial size of $\sim$7–14 au, aspect ratio $\sigma_x/\sigma_y \sim 1.5$) and photometry (H$\alpha$: $1.5 \pm 0.4$ mJy; NIR effective temperature $\sim$2200 K; planet mass $\sim$9 $M_{\rm Jup}$; radius $\sim$2.75 $R_{\rm Jup}$; accretion rate $\sim$1.1%%%%13$\sim$14%%%% $M_{\rm Jup}\,$yr$^{-1}$ (Currie et al., 2022)) match theoretical expectations for an accreting Jovian planet. The multi-epoch astrometry confirms orbital motion and stability (Boccaletti et al., 2020). However, UV–optical imaging reveals spatial extensions up to 14×9 au and spectral energy distributions (SEDs) consistent with scattered stellar light rather than a self-luminous planet (Zhou et al., 2023). The distinction between intrinsic planetary emission and disk-related scattered light remains a major interpretive challenge.

2. Disk Structure and Hydrodynamics

The AB Aurigae disk exhibits a complex architecture with a wide central cavity ($r\sim120$ au), a horseshoe-shaped dust ring, and marked spiral arms detected in both CO gas and dust continuum (Tang et al., 2017, Boccaletti et al., 2020). ALMA and NOEMA imaging delineate bright, trailing gaseous spirals and dust asymmetries suggestive of tidal disturbances, likely driven by massive companions at $\sim$30 and 60–80 au (Tang et al., 2017). Hydrodynamical simulations demonstrate that a putative $\sim$2–13 $M_{\rm Jup}$ companion can excite pressure bumps, drive Rossby vortices, and concentrate solids, facilitating dust trapping and growth of rocky planetesimals in the outer ring (Fuente et al., 2017). Kinematic mapping finds gas bridges connecting the ring and the central disk, with velocity profiles consistent with free-fall accretion through the cavity (Rivière-Marichalar et al., 2019). Misalignments and twisted velocity fields support two Keplerian disk components with differing inclinations ($\sim$23–43$^\circ$) and a dynamic interplay between accretion, jet launching, and disk evolution.

3. Evidence for Accretion and Circumplanetary Disk

High-contrast NIR and optical imaging (SCExAO/CHARIS, HST/WFC3, and VLT/MUSE) reveals AB Aur b in both total intensity and, in some epochs, accretion-sensitive bands (H$\alpha$, Pa$\beta$). H$\alpha$ imaging at two HST epochs shows a moderate excess ($F_\nu\sim1.5$ mJy, ratio $\sim$3.2 relative to continuum (Zhou et al., 2022)), lower than strongly accreting planets like PDS 70 b/c. Pa$\beta$ emissions are not detected, placing upper limits on accretion luminosity (Biddle et al., 19 Feb 2024). Counterarguments based on image quality and realistic source models suggest stronger Pa$\beta$ constraints are possible with improved AO data (Currie, 31 May 2024). Spectropolarimetric NIR imaging establishes that AB Aur b is not a point source of polarized light; its lack of strong polarization above $1.3\,\mu$m further supports a self-luminous origin distinct from general dust-scattered emission (Dykes et al., 15 Oct 2024). Deep variability analysis reveals H$\alpha$ flux variations of up to 330%, over 20$\times$ higher than the host star and uncorrelated, ruling out simple starlight-scattering and favoring accretion-driven emission (Bowler et al., 20 Feb 2025). Most definitively, VLT/MUSE spectroscopy resolves an inverse P Cygni profile (blueshifted emission at –100 km$\,s^{-1}$, redshifted absorption at +75 km$\,s^{-1}$), characteristic of infalling cold gas and magnetospheric accretion (Currie et al., 25 Aug 2025). This uniquely sets AB Aur b apart as the only directly imaged protoplanet with a clear accretion infall signature in H$\alpha$.

4. Disk Chemistry, Dust Trapping, and Planet Formation Conditions

Comprehensive NOEMA spectro-imaging of the disk reveals strong chemical segregation: HCO$^+$ traces accreting bridges, HCN maps cold, dense rings, and SO/H$_2$CO mark temperature gradients in the disk (Rivière-Marichalar et al., 2019, Marichalar et al., 2020). Gas-to-dust ratios vary from 10–40 in dust traps to near-interstellar values outside, signifying efficient dust concentration amenable to planetesimal assembly (Marichalar et al., 2020). Sulfur chemistry is distinguished by severe depletion ([S/H]%%%%38$\sim$39%%%%) and by H$_2$S being mainly sequestered in ices up to a transition height of $\sim$12 au, affecting the volatile inventory accreted onto any forming planet (Rivière-Marichalar et al., 2022). Dust traps in decaying vortices (with inferred total solid mass $\sim$30 $M_{\rm Earth}$) enable efficient rocky core formation, while micron-sized grains in the outer disk signal advanced coagulation and lofting (Fuente et al., 2017, Li et al., 2016). Mid-IR polarimetry confirms ordered, tilted poloidal magnetic fields, providing conditions for magneto-rotational instability and sustained accretion (Li et al., 2016). Low gas-to-dust environments, elevated C/O ratios, and sulfur-poor chemistry mark the composition of solids and gas available to AB Aur b during its assembly.

5. Formation Mechanisms: Gravitational Instability Versus Core Accretion

The physical characteristics and circumstantial environment of AB Aur b—mass ($\sim$4–13 $M_{\rm Jup}$), wide orbit ($\sim$30–100 au), rapid formation (system age $1$–$4$ Myr), and high initial disk mass ($M_{\rm d,crit}\sim0.3\,M_\odot$)—are most naturally explained by gravitational instability (GI) models (Cadman et al., 2021, Currie et al., 2022). SPH and analytical simulations demonstrate that disks exceeding $q=M_d/M_*\gtrsim0.13$ fragment into clumps matching AB Aur b’s mass domain and that inward migration on $10^4$–$10^6$ yr timescales enables their final location (Cadman et al., 2021). Core accretion is strongly disfavored due to insufficient time and low local surface densities at these wide separations. Hydrodynamic modeling confirms local Toomre $Q<1$ between 50 and 150 au, consistent with disk fragmentation (Currie et al., 2022). The lack of (sub)millimeter dust continuum emission from a circumplanetary disk around AB Aur b—a non-detection at ALMA’s $\sim$99 $\mu$Jy threshold—is reconcilable with low dust-to-gas mass supply, or with extinction-suppressed accretion rates (Shibaike et al., 5 Dec 2024). Blue optical/UV color and a Balmer jump, however, do not fit planetary photosphere or accretion shock models but rather match disk-scattered starlight (Zhou et al., 2023), arguing for caution in unifying the formation scenario.

6. Controversies, Interpretation Challenges, and Diagnostics

Multi-band imaging yields ambiguities: while NIR/total intensity supports an accreting planet origin, UV–optical and polarized-light emission resemble disk-scattered light (Zhou et al., 2023, Dykes et al., 15 Oct 2024). Accretion diagnostics in Pa$\beta$ and H$\alpha$ can be confounded by dust extinction, variability, or underlying disk structure (Biddle et al., 19 Feb 2024). Single-band Pa$\beta$ imaging does not firmly discriminate accretion sources (Currie, 31 May 2024). The accretion light echo method, leveraging temporally resolved imaging of host and candidate, finds AB Aur b’s strong H$\alpha$ variability is uncorrelated with the host and inconsistent with unobstructed scattered starlight (Bowler et al., 20 Feb 2025). VLT/MUSE spectroscopic detection of an inverse P Cygni H$\alpha$ line profile at the AB Aur b location constitutes strong evidence for infalling gas and active accretion, distinct from both the host and disk (Currie et al., 25 Aug 2025). This unique spectroscopic signature is not present in PDS 70 b/c or other known protoplanets.

7. Implications and Perspectives for Planet Formation

The AB Aur system provides an active template for understanding early planet formation via disk instability in massive, structured disks. The complex interplay between disk morphology, gas flows, magnetic fields, and dust chemistry shapes the mass assembly and accretion history of embedded protoplanets such as AB Aur b. While its identification as a truly accreting planet is supported by direct imaging, resolved orbital motion, NIR SED, accretion line profiles, and light echo variability, alternative interpretations invoking disk light scattering remain plausible in specific wavebands. The case of AB Aur b underscores the necessity of multi-epoch, multiwavelength diagnostics—including high-resolution line spectroscopy, variability studies, and polarimetry—to confirm planetary nature and follow accretion events in real time. Ongoing and future spectroscopic campaigns (notably at high spectral resolution for H$\alpha$ and related tracers), improved continuum sensitivity at (sub)mm wavelengths (ALMA Band 7), and advanced radiative transfer models will further clarify the physical and chemical evolution of AB Aur b and systems analogous to it.

Table: Summary of AB Aur b Key Observational Metrics

Metric	Value/Range	Reference
Separation from AB Aur	600±22 mas (~93 au)	(Zhou et al., 2022)
Planet Mass	~4–13 $M_{\rm Jup}$	(Cadman et al., 2021)
Accretion Rate	~1.1×10$^{-6}$ $M_{\rm Jup}$/yr	(Currie et al., 2022)
H$\alpha$ Flux Density	1.5±0.4 mJy	(Zhou et al., 2022)
NIR Effective Temperature	~2200 K	(Currie et al., 2022)
Pa$\beta$ 2$\sigma$ UL	9.44×10$^{-15}$ W m$^{-2}$ μm$^{-1}$	(Currie, 31 May 2024)
Dust Trap Mass	~30 $M_{\rm Earth}$	(Fuente et al., 2017)

The AB Aur b protoplanet remains one of the most comprehensively studied directly imaged planet candidates, with its accretion processes, disk interactions, and chemical environment providing a benchmark for understanding the complex assembly of giant planets in actively evolving transitional disks.