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Our solar system passed through a vast wave of gas and dust around 14 million years ago, dimming Earth’s view of the night sky. The wave may even have left traces in our planet’s geological record.

Astronomers have previously discovered large ocean-like waves of stars, gas and dust in the Milky Way that undulate up and down over millions of years. One of the closest and best-studied of these is the Radcliffe wave, which is nearly 9000 light years in width and sits only about 400 light years from our solar system.

Now, at the University of Vienna and his colleagues have found that the Radcliffe wave used to be much closer to us, crossing our solar system between 11 million and 18 million years ago.

Maconi and his team used data from the Gaia space telescope, which has tracked billions of stars in the Milky Way, to identify recently formed groups of stars within the Radcliffe wave, along with the dust and gas clouds from which they formed.

Using these stars to indicate how the wave as a whole is moving, they tracked the orbits of the clouds back in time to reveal their historic location. They also calculated the past path of the solar system, winding the clock back 30 million years, and found that the wave and our sun made a close approach between around 15 million and 12 million years ago. Estimating exactly when the crossing began and ended is difficult, but the team thinks the solar system was within the wave around 14 million years ago.

This would have made Earth’s galactic environment darker than it appears today, as we currently live in a relatively empty region of space. “If we are in a denser region of the interstellar medium, that would mean that the light coming from the stars to you would be dimmed,” says Maconi. “It’s like being in a foggy day.”

The encounter may also have left evidence in Earth’s geological record, depositing radioactive isotopes in the crust, though this would be hard to measure given how long ago it happened, he says. Explaining Earth’s geological record is an ongoing problem, so finding galactic encounters like these is useful, says at University College London.

More speculatively, the crossing appears to have happened during a period when Earth was cooling called the Middle Miocene. It is possible the two are linked, says Maconi, although this would be difficult to prove. Schoenrich thinks it is unlikely. “A rule of thumb is that geology trumps any cosmic influence,” he says. “If you shift continents or interrupt ocean currents, you get climate shifts from that, so I’m very sceptical you need anything in addition.”

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