Meteorite paleomagnetism establishes that the solar nebula's magnetic field in the terrestrial-planet region (~1–3 astronomical units) collapsed from tens of microtesla at ~1–3 million years (Myr) to below ~0.6 microtesla by ~3.8 Myr after the formation of the first solids, implying that the magnetized gas disk had dispersed by that epoch and that the young Sun's accretion rate had fallen below roughly 10−910^{-9}–10−1010^{-10} solar masses per year. Which physical process drove this terminal decline remains unidentified: the relative contributions of magnetically driven disk winds, photoevaporation by high-energy stellar radiation, and viscous or purely hydrodynamic transport are undetermined, and the conversion of the paleomagnetic field limits into accretion-rate bounds itself depends on which magnetic mechanism previously dominated angular-momentum transport. It also remains unknown whether purely hydrodynamic torques, such as those of the vertical shear instability, could have sustained residual accretion in the weakly ionized late-time inner nebula after magnetic transport ceased. The partition among these dispersal mechanisms and the role of purely hydrodynamic transport in the late-time inner nebula remain open questions in solar-system and protoplanetary-disk research.

Background

The Sun formed ~4.567 billion years ago inside a rotating disk of gas and dust — the solar nebula — within which the planets assembled. Because giant planets must complete their gaseous growth before the disk disperses, and because disk gas drives the orbital migration of growing planets, the nebula's lifetime is a controlling parameter of planet formation. The nebula cannot be observed directly; two indirect probes exist. First, statistics of young stellar objects (YSOs) show that protoplanetary disks around Sun-like stars lose their infrared excess — the signature of warm dust — on timescales of a few million years (median lifetimes of roughly 2–6 Myr across star-forming regions), and that final gas clearing is rapid (within ~1 Myr). Second, meteorite paleomagnetism: magnetic minerals that cool through their Curie temperature in an ambient field acquire a thermoremanent magnetization (TRM) whose strength (the paleointensity) can be recovered in the laboratory. Sustaining a large-scale magnetic field in a disk requires ionized gas, so the collapse of the nebular field tracks the loss of the ionized gaseous disk. Absolute chronology is anchored by radiometric ages of calcium-aluminum-rich inclusions (CAIs, the oldest solar-system solids, 4567.30±0.16 million years ago) and of meteorites.

The key measurement is due to Wang et al. (2017). Volcanic angrites (basaltic meteorites from a differentiated planetesimal) — D'Orbigny, Sahara 99555 and Asuka 881371 — cooled at 4563.5±0.1 Ma, ~3.8 Myr after CAI formation, in a near-zero ambient field (<0.6 μT), more than two orders of magnitude weaker than the tens-of-microtesla fields recorded ~1–3 Myr earlier by chondrules (millimeter-sized silicate droplets) in the Semarkona meteorite (Fu et al. 2014). This implies the nebular field, and likely the gas, had dispersed in the 1–3 AU region by ~3.8 Myr, bounding the young Sun's terminal accretion rate at ~10−910^{-9}–10−1010^{-10} solar masses per year. The same study found that the plutonic angrite Angra dos Reis, which cooled ~11 Myr after CAIs, recorded ~17 μT from an internal field, bracketing the onset of the angrite parent body's core dynamo (magnetic field generation by convection in a molten metallic core) to the interval ~4–11 Myr.

The theoretical background is the problem of angular momentum transport in accretion disks. Gas orbits faster inside than outside, so without outward angular-momentum transport it cannot accrete. Candidate mechanisms include: the magnetorotational instability (MRI), turbulence driven by weak magnetic fields in sufficiently ionized gas (suppressed in cold, dense, weakly ionized 'dead zones'); magnetized (magnetohydrodynamic, MHD) disk winds, centrifugal outflows launched along large-scale magnetic field lines that carry away angular momentum; photoevaporation, in which extreme-ultraviolet, X-ray, or far-ultraviolet stellar radiation heats the disk surface until gas thermally escapes; and purely hydrodynamic instabilities, notably the vertical shear instability (VSI), which feeds on the vertical gradient of orbital velocity and requires gas cooling within ~1% of the local orbital period — a condition probably met only at large distances (≳10 AU). Disk-dispersal theory predicts a two-timescale history (slow depletion followed by rapid inside-out clearing), consistent with the observed population of 'transitional disks' (disks whose inner dust has cleared while outer dust persists, seen as infrared cavities). Which mechanism dominates dispersal has remained one of the central open questions of planet formation, and it directly controls how a measured nebular field-strength limit is converted into an accretion-rate limit, because the field–accretion-rate relation differs between MRI-turbulent and wind-driven disks by orders of magnitude.

Problem Statement

Identify the physical mechanism or combination of mechanisms that governed the final decline and dispersal of the solar nebula — in particular, the relative contributions of (i) magnetically driven disk winds, (ii) photoevaporation by high-energy stellar radiation, and (iii) viscous or purely hydrodynamic processes — such that the nebular magnetic field in the terrestrial-planet region (~1–3 AU) collapsed from tens of microtesla at ~1–3 Myr to below ~0.6 μT by ~3.8 Myr after CAI formation, the terminal accretion rate onto the young Sun fell to at most ~10−910^{-9}–10−1010^{-10} M⊙M_\odot yr−1^{-1} (later synthesis of the same angrite data refines this conversion to between <10−12<10^{-12} and <10−10<10^{-10} M⊙M_\odot yr−1^{-1} depending on the assumed field geometry and transport regime), and the gas disk cleared on an inside-out, two-timescale history (the outer 3–7 AU nebula dissipating within ~1.5 Myr after the inner 1–3 AU region). The question therefore has two parts: (a) how the terminal dispersal partitions quantitatively among these mechanisms, in a way consistent with the meteorite paleomagnetic timeline and with observed protoplanetary-disk demographics and accretion-rate distributions — thereby fixing how the late-time field limits convert into accretion-rate bounds — and (b) whether purely hydrodynamic angular-momentum transport (e.g., the vertical shear instability) could have sustained residual accretion in the weakly ionized, weakly magnetized late-time inner nebula after magnetic transport ceased, or whether the residual accretion bounds require magnetic transport.

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