Seventy thousand years ago, when the Homo erectus had just disappeared from the face of our planet and our ancestors were on the verge of migrating from Africa, an alien star grazed the outer reaches of the solar system at less than a light year away from Earth, a flyby that scientists describe as the closest stellar near-miss.

In a study published in the Astrophysical Journal Letters on Tuesday, a team of astronomers reported that 70,000 years ago, a dim star likely passed through the outer Oort Cloud, a region at the outskirts of the solar system that is filled with trillions of comets.

The approach of the star, officially named WISE J072003.20-084651.2 and nicknamed Scholz's Star, after Ralf-Dieter Scholz, from Germany's Leibniz-Institut fur Astrophysik Potsdam (AIP) who discovered it in 2013, is the closest to the solar system and is in fact five times closer than the Proxima Centauri, which is currently the Solar System's closest star.

By analyzing the velocity and trajectory of the Scholz's star, Eric Mamajek, from the University of Rochester, and colleagues found that the star passed at proximity of about 52,000 astronomical units, which is about 0.8 light years or 5 trillion miles, a distance astronomically close given that Proxima Centauri is 4.2 light years away.

"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," Mamajek wrote. "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."

The Scholz's star, a red dwarf with mass equivalent to 8 percent of that of the sun, was also not alone in its visit. It was accompanied by a brown dwarf, which has about 6 percent of the sun's mass.

"We show that given the low mass and high velocity of the binary system, the encounter was dynamically weak. Using the best available astrometry, our simulations suggest that the probability that the star penetrated the outer Oort Cloud is ~98%, but the probability of penetrating the dynamically active inner Oort Cloud (<20 kAU) is ~10-4," Mamajek and colleagues wrote.

After its close encounter with Earth, the Scholz's star has sped away and is now about 20 light-years away.

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