At least two intergalactic interlopers – stars from outside the Milky Way – are hurtling through our galaxy at more than 700 kilometres per second. These are among almost 30 runaway stars that have been spotted in the treasure trove of data from the European Space Agency’s Gaia satellite mission.

On 25 April, Gaia released its second batch of data on 1.7 billion stars. For 1.3 billion of these, Gaia measured the distances to the stars and also their motions …