An international group of astronomers has determined that 70,000 years ago a dim star is likely to have passed within our solar systems Oort Cloud. That is five times closer than the current closest known star Proxima Sentauri and the closest any star, other than the Sun, is known to have come to Earth.

In a paper in Astrophysical Journal Letters, the team shows that Scholz’s star’s trajectory suggests that it passed roughly 52,000 astronomical units (AU) or 0.8 light years from the Earth. That may sound like a great distance and it would be if you were going to drive it, but cosmically speaking it’s very close. Proxima Centauri is 4.2 light years away.

In the paper, the astronomers explain that they are 98% certain that the star went through the outer Oort Cloud.

The star originally caught the attention of lead author Eric Mamajek, of the University of Rochester during a conversation with co-author Valentin D. Ivanov.

The researchers noted that Scholtz’s star had unusual characteristics. Despite being only 20 light years away, it showed a slow motion across the sky. Radial velocity measurements taken by Ivanov, of the European Southern Observatory, and his collaborators showed the star moving moving directly away from our solar system at high speed.

“Most stars this nearby show much larger tangential motion. The small tangential motion and proximity initially indicated that the star was most likely either moving towards a future close encounter with the solar system, or it had ‘recently’ come close to the solar system and was moving away. Sure enough, the radial velocity measurements were consistent with it running away from the Sun’s vicinity – and we realized it must have had a close flyby in the past,” said Mamajek, in a statement.

To calculate the trajectory of the star, the astronomers needed both its tangential and radial velocities. The researchers had recently characterized the star through measuring its spectrum and radial velocity using Doppler shift. Once the researchers had put the information together they realized that the star was moving away from the solar system. Next they traced its position back in time, to the point where it was closest to our sun.

Previously, the closest flyby of a star was believed to be the rogue star HIP 85605 which will likely come close to our solar system 240,000 to 470,000 years from now.

Mamajek and his colleagues have also demonstrated, however, that the original distance to that rogue star was underestimated by a factor of 10. While it may come close, it is not likely to be as close as the Oort Cloud.

Mamajek and Scott Barenfeld, now a graduate student at Caltech, simulated 10,000 orbits for the star taking position, distance and velocity into account as well as the Milky Way’s gravitational field and statistical uncertainties for all of these measurements. 98% of the simulations showed Scholz’s star passing through the Oort cloud, only one brought the star within the inner Oort cloud which would have triggered “comet showers”.

Sholz’s star is a small, dim red dwarf. It is currently located in the constellation Monoceros. At the time of the flyby it would have been a 10th magnitude star, about 50 times fainter than what can be seen with the naked eye. However, it is magnetically active which can cause stars to flare and become much brighter than normal. That means that it’s possible that 70,000 years ago people would have been able to see it for brief periods.