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The grand tack hypothesis is a 2011 model regarding the development of the solar system that theorizes that Jupiter and Saturn underwent planetary migration inward after their formation, then migrated outward to their present positions. (The word tack is borrowed from sailing: it means a substantial change in the direction of a sailing vessel.) The migration would be aided by the protoplanetary disk. The theory is that Jupiter formed at about 3.5 AU from the Sun (about at the snow line), migrated inward to 1.5 AU, then returned to its present position at 5.2 AU. Saturn would have formed beyond Jupiter and followed it, eventually the two falling into orbital resonance, and their proximity would have changed the nature of the migration, causing a net outward migration. The hypothesis offers explanation for a number of characteristics of the solar system: why Mars is not larger, why there are no super-Earths, and characteristics of the asteroid belt.