A theoretical model of the first million years or so of the Solar System’s evolution in which the planet Jupiter moves inwards to a distance of about 1.5 au from the Sun before reversing course (‘tacking’) and migrating outwards to its present orbit. In this model, Jupiter would have been the first planet in the Solar System to form, at a distance of about 3.5 au. Its orbital motion was slowed down by the dense cloud of gas and dust that then existed around the infant Sun, so that it moved inwards to about the current distance of Mars, 1.5 au. Such a scenario explains the existence of hot Jupiters in close orbit around other stars. According to the theory, Saturn would have been next to form and followed Jupiter inwards. As the result of an orbital resonance, the two planets then migrated outwards to their present positions. The Grand Tack model explains many features of the present-day Solar System, including why the inner planets, particularly Mars, are so small; the reason is that much of the material in the inner region of the Solar System would have been gathered up or swept away by Jupiter when at its closest to the Sun.