The Grand Tack Hypothesis

Core Idea

The Grand Tack hypothesis proposes that Jupiter migrated inward toward the Sun (the inbound tack) and then outward again (the outbound tack) early in solar system history. This inward-outward migration would explain the solar system's unusual architecture—the scarcity of terrestrial planets in the inner system and the asymmetric asteroid belt. The hypothesis elegantly reconciles observed planetary spacing with formation models.

Explainer

From your study of planetary migration mechanisms and multi-planet system architecture, you know that giant planets do not necessarily stay where they form — gravitational interactions with the protoplanetary gas disk can cause them to migrate inward or outward over millions of years. The Grand Tack hypothesis applies this understanding to our own solar system, proposing a specific migration history for Jupiter that solves several longstanding puzzles about why the inner solar system looks the way it does.

The scenario begins about 3–5 million years after the Sun formed, when Jupiter had already accreted its massive gas envelope and was embedded in the remnant gas disk. Gravitational torques between Jupiter and the disk caused it to migrate inward — a well-understood process called Type II migration that has been observed in simulations of many planetary systems. In the Grand Tack model, Jupiter migrated inward to approximately 1.5 AU (roughly where Mars is today). This inward sweep was catastrophic for the inner disk: Jupiter's gravity scattered planetesimals and disrupted the solid material available to form terrestrial planets, effectively truncating the inner disk's mass supply.

The "tack" — the reversal — happened when Saturn, which formed more slowly, caught up to Jupiter and became locked in a mean-motion resonance (specifically a 2:3 resonance, where Saturn orbits twice for every three Jupiter orbits). Hydrodynamic simulations show that when two giant planets share a gap in the gas disk in this resonance configuration, the torques reverse: instead of migrating inward, the pair migrates outward together. Jupiter reversed course and retreated to approximately its current orbital distance of 5.2 AU, with Saturn following to about 7 AU (later evolving to 9.5 AU through subsequent dynamical interactions).

This inward-then-outward journey explains several otherwise puzzling features of the solar system. First, it accounts for Mars's small mass: Jupiter's passage through the Mars-forming region depleted the available building material, leaving Mars with only about one-tenth of Earth's mass — a result that standard formation models without migration consistently fail to reproduce. Second, it explains the structure of the asteroid belt, which contains two distinct populations (dry S-type asteroids in the inner belt and water-rich C-type asteroids in the outer belt). Jupiter's outward migration would have scattered C-type material inward from beyond the snow line while mixing it with S-type material left behind, naturally producing the observed compositional gradient. Third, the Grand Tack helps explain why the inner solar system has relatively little total mass compared to the tightly packed planetary systems discovered around other stars — Jupiter's early incursion cleared out material that might otherwise have built super-Earths. The hypothesis remains debated, with alternative models (like the "empty primordial belt" scenario) offering competing explanations, but it stands as one of the most influential frameworks for understanding our solar system's architecture as a product of dynamic history rather than static initial conditions.