Tandem Planet Formation Theory

Updated 3 July 2026

Tandem planet formation is a theory where multiple planet-building processes operate concurrently across diverse regions of a protoplanetary disk.
Models combine disk accretion dynamics, MRI-suppressed zones, and gravitational instabilities to regulate pebble drift and embryo growth.
This framework links observational signatures, such as ALMA dust gaps and simulated N-body dynamics, to explain varied exoplanet compositions and orbital configurations.

Tandem planet formation encompasses a class of theories wherein multiple major planet-building processes operate simultaneously or at synchronously coupled sites within a protoplanetary disk, both spatially and temporally. These frameworks span scenarios of (i) concurrent accretion of stars and planets, (ii) dual-source or dual-zone planetesimal formation linked to the disk's physical and magnetic structure, (iii) interconnected growth of multiple planets via mutual regulation of solids flow, and (iv) planet–binary or planet–fragment tandem emergence in gravitationally unstable environments. Common across these models is the rejection of strictly sequential, spatially monotonic planet-forming paradigms, in favor of a coupled, multi-site, multi-scale view wherein accretion, migration, and compositional differentiation are inherently non-local and contemporaneous.

1. Simultaneous Star and Planet Formation in Young Disks

The earliest and most direct realization of tandem planet formation is observed where massive planet assembly coincides with ongoing protostellar accretion. ALMA and VLA observations of disks like [BHB2007] 1 reveal a deep, $\sim$ 70 AU-wide dust continuum gap at $a\sim 50$ AU, while the molecular gas within the gap is not fully depleted and exhibits localized free-free emission coincident with the gap's interior. This is interpreted as a young, accreting $\sim$ 4–70 $M_\mathrm{Jup}$ protoplanet formed in the outer disk while the central star ( $M_*=2.23\,M_\odot$ ) continues to gain mass from ISM infall and the disk itself. Hydrodynamical gap models indicate that such a planet's Hill radius must satisfy $R_H \gtrsim H$ and $K \gg 1$ for deep clearing, with gap width-mass scaling $\Delta/a \sim 0.78$ at $a\sim 50$ AU. Filamentary gas inflows (traced in $^{12}$ CO) feed both star and planet, coupling their growth and mass budgets. Observational and theoretical signatures—dust gaps, molecular gas, infalling filaments, and ionized jets—are all mutually consistent with this "tandem" growth regime, which fundamentally links the mass accretion histories of both protostar and planet (Alves et al., 2020).

2. MRI-Suppressed Disk Structure and Dual Front Planetesimal Formation

A major class of tandem planet formation models asserts that planetesimal and planetary building blocks are preferentially assembled at two distinct radii, defined by physical transitions in disk turbulence and ionization. In the United Theory of Planet Formation (Imaeda & Ebisuzaki and successors), a magneto-rotationally regulated disk naturally splits into three zones: an inner turbulent region (ITR), a central MRI-suppressed region (MSR), and an outer turbulent region (OTR). MRI activation is controlled by the local Elsasser number, $a\sim 50$ 0, which depends on thermal ionization at small $a\sim 50$ 1 and cosmic-ray penetration at large $a\sim 50$ 2. Planetesimal formation is concentrated at the MSR boundaries ("inner" at $a\sim 50$ 3–1 AU, "outer" at $a\sim 50$ 4–60 AU), where pressure maxima cause accumulation and subsequent gravitational instability of solids:

At the outer MRI front ( $a\sim 50$ 5 10–60 AU): Fluffy, icy aggregates drift inward and are converted by GI into icy planetesimals, seeding giant planets.
At the inner front ( $a\sim 50$ 6–1.0 AU): Rocky solids pile up at the pressure bump, halting inward drift; rapid GI produces dry, rocky planetesimals, the precursors of terrestrial planets.

This mechanistic dual-front process naturally yields a dichotomy of inner anhydrous and outer volatile-rich bodies and explains the depleted planetesimal population (and small Mars mass) at intermediate radii (Ebisuzaki et al., 2016).

3. Quantitative Dynamics in Tandem Disk Models

Disk-structure-driven tandem models, including those by Ebisuzaki & Imaeda and Nimura & Ebisuzaki, employ 1D accretion disk and particle-gas coupling equations to model mass delivery and concentration at the MRI fronts. In a representative case (Case D, $a\sim 50$ 7), porous aggregate pebbles are delivered to $a\sim 50$ 8–1.5 AU at a rate sufficient to amass a total solid mass $a\sim 50$ 9 at the inner front. Once local midplane density satisfies the GI threshold, the first planetesimal (seed) forms and accretes pebbles rapidly ( $\sim$ 0), reaching $\sim$ 1 and migrating outward via positive disk torque ( $\sim$ 2 for typical disk gradients). The next embryo forms from remaining pebbles, also reaching Earth mass, thus producing two Earth-mass worlds in the terrestrial zone—quantitatively matching the mass and relative abundance of Venus and Earth (which jointly constitute $\sim$ 3% of the system's terrestrial mass) (Nimura et al., 18 Aug 2025). The process at the outer MRI front similarly establishes initial conditions for gas and ice giants.

The system evolution is characterized by the following sequence:

Porous pebble growth and radial drift into the MSR.
Pileup and GI at pressure maxima at the MRI fronts.
Rapid embryo growth by pebble accretion.
Type I (and possibly II) migration; feedback onto pebble supply and additional embryo growth.
Outer front: icy planetesimal/embryo genesis; inner front: rocky planet formation.

All stages proceed in parallel, emphasizing the synchronized, coupled character of the formation process.

4. Tandem Reservoir and Radial Source Zone Models in Terrestrial Planet Formation

N-body, gas-drag, and migration-inclusive dynamical models of the terrestrial planet region now demonstrate that the Solar System's configuration is reproduced only when two separated, spatially distinct reservoirs of planetesimals are invoked: an "inner ring" near the silicate-sublimation line ( $\sim$ 4–0.6 AU) and an "outer reservoir" at $\sim$ 5–2.0 AU. The total solid mass is divided roughly 2:1 between the reservoirs. Mercury, Venus, and Earth accrete from the inner ring, with convergent Type-I migration (enabled by disk surface density "bulge" in the inner region) stalling Venus and Earth near their observed orbits. Mars is a survivor from the depleted outer reservoir. This geometry delivers:

Modern Venus/Earth analogs in 50% of stochastic simulations.
Mars analogs in 35%; Mercury analogs near 30%.
Earth's compositional split: 70% from reduced, inner-ring material, 30% from oxidized, outer-reservoir material, exactly matching cosmochemical models and major isotope groupings.

No single-annulus or monotonic radial disk models can produce the combined constraints on planetary masses, separations, and compositions (Nesvorny et al., 20 Jul 2025).

5. Tandem Formation in Multi-Planet and Binary Systems

Tandem and "sandwiched" planet formation scenarios extend the concept to planet–planet dynamical coupling. Simulations of two-planet systems reveal that a sufficiently massive outer planet can starve the inner planet's exterior pressure bump of pebbles, sharply suppressing the dust surface density and thus the mass available for a third, intermediate planet. The analytic and numerical result is a "Big–Small–Big" architecture, deviating from naive monotonic mass ladders with orbital distance. Observed exoplanet systems (HD 219134, Kepler-80, TRAPPIST-1, etc.) display such configurations, indicating widespread occurrence of tandem formation via mutual flow regulation (Pritchard et al., 2023).

In tight binary systems, rapid gravitational fragmentation in massive, infall-fed disks produces bound planets and secondary stars nearly simultaneously. A dominant "oligarch" fragment accretes most of the local mass (and becomes the secondary), while early-formed, high-mass fragments ( $\sim$ 6–3 $\sim$ 7) survive as close-orbit S-type planets; lower-mass or late-formed objects are dynamically ejected. This natural mass-dependent tandem survival process aligns with observed trends in binary systems and the frequency/mass function of free-floating planets as inferred from microlensing (Zhang et al., 2 Mar 2026).

6. Broader Implications and Theoretical Synthesis

Tandem planet formation frameworks challenge the classical sequential accretion paradigm in several key respects:

Planet formation begins contemporaneously with star formation, often before the disk is dispersed.
Mass budgets and evolutionary trajectories of stars and planets are linked via global disk accretion and infall.
MRI-driven disk structure, pebble drift, and feedback/migration effects lead to sharply non-monotonic radial distributions, explaining the compositional and mass contrasts across planet types, small Mars/massive Earth-Venus, and "sandwiched" configurations.
The simultaneous operation of multiple particle/planetesimal traps and dynamical feedback between forming planets are required to match observed exoplanet architectures.

This prompts a shift toward multi-zone, multi-phase, and multi-scale planet formation models, supported by both detailed simulations and direct imaging/spectroscopic observations. Tandem models robustly unify dynamical, chemical, and observational constraints on planet origins across both single and binary stars, as well as exoplanetary systems (Alves et al., 2020, Ebisuzaki et al., 2016, Nesvorny et al., 20 Jul 2025, Nimura et al., 18 Aug 2025, Pritchard et al., 2023, Zhang et al., 2 Mar 2026).