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 across the sky. For a subset of 7 million stars, Gaia measured the radial velocity of these stars – how fast they are moving away from or towards Earth.
and colleagues at Leiden University in the Netherlands, studied these 7 million stars to identify those traveling at speeds greater than about 450 kilometres per second. They found 165 candidate stars.
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The team calculated that 28 of the 165 stars have a greater than 50 per cent chance of escaping the Milky Way’s gravitational pull. “They are basically flying away forever from the Milky Way,” says Marchetti.
Supernova side effect
A handful of these unbound stars have paths that are consistent with having been ejected from the galactic centre, where the gravity of the central massive black hole ripped apart a binary star system, sending one of the pair soaring away.
Yet other stars can be traced back to the stellar disc of the Milky Way. This requires positing a different mechanism for creating runaway stars, most likely a binary star system in which one star exploded as a supernova and launched the other into space.
Reconstructions of speeds and orbits suggest that about half of the 28 high velocity stars have likely originated outside our galaxy. The most intriguing are two stars with velocities in excess of 700 kilometres per second.
“They have done a really good job of identifying all the possible candidate hyper velocity stars in this subset of stars,” says at the University of Cambridge, UK. “They have one object which is traveling towards us. Its radial velocity is minus 600 kilometres per second. That is pretty much a certain discovery. I’m sure there are people racing to get more data about it from ground based telescopes.”
Fleeing dwarf stars
Boubert is a member of another team led by  at the University of California at Berkeley that has analysed the Gaia data for a particular type of runaway star: a white dwarf that was ejected from a binary system of two white dwarfs when one goes supernova. One dwarf slowly slurps matter from the other and when it reaches 1.4 times the mass of our sun, it explodes, flinging its companion into space at speeds exceeding 1000 kilometres per second.
Shen’s team found seven possible hypervelocity white dwarfs in the Gaia data. They then used ground-based telescopes to study the spectrum of light from these stars and showed that three of the seven are very likely white dwarfs that were once companions to other white dwarfs that went supernova.
Marchetti says that such findings are “showing how Gaia can change dramatically our view and understanding of high velocity stars.” His team is now using artificial intelligence to analyse the bigger dataset of 1.3 billion stars to find even more runaway stars.
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Read more: We got a good look at the interstellar asteroid and it’s weird
Article amended on 8 May 2018
We have corrected the spelling of Tommaso Marchetti’s name
