70,000 Years Ago, a Passing Star Shook Up Our Solar System

The red dwarf likely came within one light-year of the sun, altering the course of some asteroids and comets

Caveman Star
José A. Peñas/SINC

About 70,000 years ago, any hominins who happened to gaze up at the night sky may have caught an incredible sight. At that time, astronomers hypothesize, a small red-dwarf star was skirting the edges of our solar system, cruising along within one light year of the sun.

The idea was first announced in a 2015 study, which focused on ​Scholz’s Star, a small red dwarf with an orbiting brown dwarf that lies some 20 light years away from Earth. The researchers measured its velocity and simulated possible past pathways of the star. Out of 10,000 potential orbits, 98 percent passed through the edges of the Oort Cloud, a shell of over a trillion icy bodies surrounding our solar system.

Now, a new analysis of the orbits of objects in the distant Oort Cloud bolsters the case for this ancient stellar flyby, reports Mike Wall at Space.com.​

Researchers from the Complutense University of Madrid and Cambridge University analyzed the orbits of nearly 340 objects in the solar system with hyperbolic orbits, which means they are closer to a V-shape than an elliptical orbit, according to a press release. What they found is that a flyby of Scholz’s Star explains some of those unusual orbits.

A statistically significant number of them, 36, have radiants—or the points from which they seem to radiate from the sky—pointing back towards the constellation Gemini. Normally, the radiants of those objects would be evenly distributed across the sky if they randomly came out of the Oort cloud.

As Carlos de la Fuente Marcos, co-author of the paper, explains in the release, this positioning "fits the close encounter with Scholz´s star." The gravity of that passing body could have nudged them out of the Oort Cloud into their unusual orbits. The study appears in The Monthly Notices of the Royal Astronomical Society.

There may be many other objects perturbed by Scholz’s Star as well. “Keep in mind that the sample detected is made of objects that passed relatively close to our planet,” de la Fuente Marcos tells George Dvorsky at Gizmodo. “The number of objects that were possibly perturbed by this stellar flyby could have been significantly higher.”

The researchers weren’t just looking for evidence of Scholz’s star. Hyperbolic orbits are also a sign that an object arrived not from the Oort Cloud, but interstellar space, like the recently discovered interstellar asteroid ‘Oumuamua. The paper also flagged eight objects that may also be interstellar travelers that need follow up study.

Not everyone is convinced by the study's claim that these objects were gravitationally bumped by Scholz’s Star. Wesley Fraser from Queen’s University-Belfast tells Dvorsky that the dataset they pull from appears to be observationally biased. As Dvorsky reports, other researchers argue that the precision of the data on these bodies limits the claims that can be made about their trajectories, since many of them were only glimpsed briefly.

Eric Mamajek, the lead author of the 2015 study, tells Dvorsky that Scholz’s star is probably not the only star to pass through the Oort Cloud, and that over the course of millions of years other stars have also likely made close passes. But those flybys, he says, don’t affect us much on Earth. Most of those stars don’t impact Oort Cloud objects at all. “I’m not losing sleep over comets perturbed by Scholz’s Star,” he says. “There are many, many more immediate concerns on Earth, and most are fixable.”

If Scholz’s Star did make a pass when humanity and our relatives were around, what would they see?

As Nola Taylor at Space.com reports, the show may have been brief. Even though the star passed close by, overall it would have been faint. It was likely a 10th magnitude star, which would make it about 50 times too faint to see. But since it’s a red dwarf, which have a tendency to flare up, it could have occasionally turned up the juice for a few minutes or hours, putting on a little show for the wondering eyes below.

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