[Paper Review] A new view on planet formation

Sergei Nayakshin|arXiv (Cornell University)|Dec 8, 2010

Astro and Planetary Science17 references4 citations

TL;DR

This paper proposes the Tidal Downsizing (TD) hypothesis, where terrestrial planets form as remnants of massive gas clumps (giant planet embryos) that migrate inward and are stripped of their atmospheres by tidal forces and stellar irradiation. The model explains the Solar System's architecture—terrestrial planets as 'dried bones' of former gas giants—offering a solution to core accretion's sticking and drift problems while accounting for planetary compositions and orbital coherence.

ABSTRACT

The standard picture of planet formation posits that giant gas planets are over-grown rocky planets massive enough to attract enormous gas atmospheres. It has been shown recently that the opposite point of view is physically plausible: the rocky terrestrial planets are former giant planet embryos dried of their gas "to the bone" by the influences of the parent star. Here we provide a brief overview of this "Tidal Downsizing" hypothesis in the context of the Solar System structure.

Motivation & Objective

• To address the theoretical challenges of the core accretion model, particularly the difficulty of meter-sized rock sticking and radial drift in protoplanetary disks.
• To resolve the long-standing puzzle of the Solar System's prograde, coherent planetary rotation, which contradicts random-impact formation scenarios.
• To provide a physically viable alternative to core accretion by reinterpreting terrestrial planets as remnants of disrupted giant planet embryos.
• To explain the observed structure of the Solar System—terrestrial planets, asteroid belt, gas and ice giants—within a unified formation framework.

Proposed method

• Formation of massive gas clumps (giant planet embryos, GEs) at radii >50 AU via gravitational instability in a massive protoplanetary disk.
• Radial inward migration of GEs due to gravitational torques from the surrounding gas disk, as described by Goldreich & Tremaine (1980).
• Internal grain growth and sedimentation within GEs, leading to formation of massive, dense rocky cores (up to ~60 M⊕) from high-Z elements.
• Tidal disruption and/or irradiation-driven envelope stripping in the inner few AU, depending on proximity to the star and core mass.
• Use of radiation hydrodynamics simulations to model envelope unbinding, with results showing complete gas loss for cores >10 M⊕.
• Comparison of binding energies of solid cores and gas clumps to assess internal disruption feasibility, using E_bind ∝ GM²/R scaling.

Experimental results

Research questions

• RQ1 Can the formation of terrestrial planets be explained by the remnants of disrupted giant planet embryos rather than in-situ core accretion?
• RQ2 How does radial migration of massive gas clumps enable the formation of rocky cores in the inner Solar System despite initial formation in the outer disk?
• RQ3 What determines whether a disrupted embryo becomes a terrestrial planet, a gas giant, or an ice giant, based on the degree of envelope loss?
• RQ4 Why do the terrestrial planets in the Solar System exhibit coherent, prograde rotation, and can this be explained by a common formation mechanism?
• RQ5 Can the Tidal Downsizing hypothesis account for the asteroid belt's composition and location as leftover solids from disrupted embryos?

Key findings

• The Tidal Downsizing hypothesis provides a physically viable alternative to core accretion by placing giant planet embryo formation in the outer disk and enabling inward migration.
• Simulations confirm that a 20 M⊕ core formed in a 10 MJ gas clump can unbind all but 0.03 M⊕ of its gaseous envelope, consistent with terrestrial planet formation.
• The binding energy of a 10 M⊕ solid core is comparable to that of a 3 MJ gas clump at ~0.8 AU, supporting the feasibility of internal disruption.
• The model naturally explains the absence of massive atmospheres in terrestrial planets due to their location within the tidal disruption radius (rt ≈ 2–3 AU).
• The asteroid belt is interpreted as solids that grew in GEs but were not incorporated into cores, left behind after disruption at r ≈ 2–3 AU.
• The model accounts for Uranus and Neptune as partially disrupted embryos, with internal energy release from massive core formation likely evaporating most of their primordial envelopes.