Parts of Earth may originally be from another part of the galaxy, having crossed light years to form the ground beneath our feet. That’s the conclusion of a study suggesting that the Milky Way should be full of free-floating rocks like ‘Oumuamua, the interstellar asteroid which visited our solar system in October 2017, and they may act as seeds to form planets in nascent planetary systems.

Our traditional picture is that planets form out of discs of gas and fine dust around a star, but …