WASHINGTON (WASHINGTON POST) - Some 70,000 years ago, when humans and Neanderthals shared the planet, an alien star streaked through the outer edges of our solar system and jostled its contents, astronomers say.
In a study of hundreds of solar system objects with unusual orbits, the scientists also single out eight comets that might also have interstellar origins.
This idea that a star recently sideswiped our solar system was first raised three years ago by University of Rochester astronomer Eric Mamajek. He and his colleagues had noticed something strange while studying a binary stellar system named Scholz's star, which comprises two small, dim stars orbiting each other.
Even though Scholz's star is just 20 light-years from Earth - a near neighbour by astronomical standards - it appeared to move incredibly slowly across the night sky. The best explanation was that Scholz star was cruising away from us. Which means at some point, it must have been quite close by.
Of 10,000 simulations of the star's potential orbits, 98 per cent showed it passing through the inner Oort cloud - a region of scattered tiny icy bodies encircling the edge of the solar system. Even at its closest approach, within 0.8 light-years of the sun, the dim star would have been 50 times too faint to be seen by the naked eye. It probably flew through the solar system unnoticed by anyone living in it.
There is a very, very small chance that prehistoric hominins did look up to see a new red light in the sky: Mamajek and his colleagues point out that magnetic activity may have caused Scholz's star to flare, producing a brief burst of visible light.
But Scholz's star did leave other evidence of its passing, scientists say. The new study, published in the May edition of the Monthly Notices of the Royal Astronomical Society, bolsters Mamajek's original theory by looking at the paths of more than 300 small bodies in our solar system with hyperbolic orbits.
Unlike most planets, asteroids and the like, which journey around the sun on elliptical paths, bodies with hyperbolic orbits track a V-shaped path through the solar system. The study authors found three dozen of these bodies seemed to originate in the direction of the constellation Gemini, rather than being distributed evenly across the sky. This pattern squares nicely with the trajectory of Scholz's star, said lead author Carlos de la Fuente Marcos, an astronomer at the Complutense University of Madrid, in Spain.
The researchers also point out eight comets with high radial velocities that warrant follow-up observations. These fast-moving balls of ice and dust may also have interstellar origins, the researchers say.
Mamajek, the lead author of the 2015 study, told Gizmodo the new paper was solid, adding that Scholz's star is probably just the most recent example of a stellar visitation. But other researchers questioned the study's methods and the reliability of its conclusions, given that it's based on a mishmash of comet data by biased human observers.
Scientists have seen one interstellar voyager - the cigar-shaped asteroid 'Oumuamua, named for the Hawaiian word for "scout", which passed through the solar system last year. 'Oumuamua would have been too far away to be affected by Scholz's star 70,000 years ago, De la Fuente Marcos says.
But a separate study, also published in the Monthly Notices of the Royal Astronomical Society, reports that the recent visitor was probably flung from a binary star system like Scholz's star. According to the analysis, systems of two stars orbiting one another are more likely eject rocky bodies as well as icy ones.