Seventy thousand years ago, when modern humans were on the verge of migrating from Africa and before Neanderthals died out, an alien star flew through the outer reaches of the solar system.
Passing less than a light-year from Earth, the flyby was the closest stellar near-miss identified so far, scientists reported Tuesday in Astrophysical Journal Letters.
Ordinarily too dim for human eyes to perceive, the small red dwarf may have flared up during its extremely close brush with Earth, making it visible to early humans if they chanced to look skyward at the right moment.
When scientists reconstructed the past orbit of the star, known as Scholz's star, they found that it once came within 0.8 light-years (roughly eight trillion kilometers) of Earth's solar system. That distance puts it within the outer reaches of the Oort cloud, a vast, faraway realm populated by trillions of comets.
Why It Matters
Though it's in a cosmic blind spot and has never been seen directly, the Oort cloud is thought to host an enormous number of comets, some of which already visit the inner solar system with regularity. But many more of these comets could come flying inward if something massive-say, a passing star-were to approach and perturb the cloud.
A hailstorm of comets could have catastrophic consequences for life on Earth, so astronomers hope to figure out how common these close encounters are. So far, it doesn't look like we have to worry: The next closest stellar approach is forecast to miss the Oort cloud and occur more than a million years from now.
The Big Picture
The recently discovered star, which is part of a binary system with a brown dwarf, stood out because of its peculiar motion in the sky. With scant sideways movement, it appeared to be zooming either directly away from-or directly toward-Earth.
The closest star to our sun today is Proxima Centauri, which is about 4.2 light-years away, or about five times farther away than Scholz's star passed.
The European Space Agency's Gaia satellite, launched in 2013, will map the locations of roughly one billion stars in the galaxy. Among the data the mission aims to gather are estimates of which stars are likely to have swung by the solar system-and which stars still could.
As for the red dwarf, it has been speeding away ever since its brush with Earth and is now located about 20 light-years away in the equatorial constellation Monoceros.
Editor's note: This article has been updated to reflect new research on the next stellar near-miss. An earlier version referred to an event forecast to occur between 240,000 and 470,000 years from now.