It’s like something out of a Hollywood film or a science fiction novel: a dark star sneaks up on Earth from just outside the Solar System, discovered too late to do anything about it (and really, what could we do?) and plows through the cloud of comets that surrounds the Sun like a haze of icy gnats, sending them flying everywhere… including on collision courses with Earth. Mass hysteria ensues.
Except that this isn’t just a story concept – scientists think this is actually something that happened 70,000 years ago! Minus the mass hysteria, of course… our ancestors were just beginning to settle down in the fertile lands of the Middle East after wandering out of Africa and would have had no idea what was happening at the edges of the Solar System (besides maybe a bright star occasionally flaring up in the night sky.)
An international team of astronomers, led by professor Eric Mamajek from the University of Rochester in upstate New York, measured the motion of a recently-discovered low-mass binary system nicknamed “Scholz’s star” (after its discoverer Ralf-Dieter Scholz) currently located about 20 light-years away. What they found was that the system, comprising a small M-dwarf and an even smaller brown dwarf companion, appears to be moving away from the Sun.
“Most stars this nearby show much larger tangential motion,” said Mamajek. “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.”
When the astronomers calculated the path of Scholz’s star back through time, they saw that the stars must have come very close to our Solar System around 70,000 years ago – within 5 trillion miles (8 trillion kilometers), or about 0.8 light-years away. This would have brought them just within the Oort cloud, the outermost known zone containing trillions of icy relics from the birth of the Solar System (and perhaps a few ‘stolen’ relics from other systems too.)
5 trillion miles is much farther from the Sun than Earth or any of the planets of course, but in terms of stars it’s really close.
As the stars passed through the cloud it’s only expected that they would have knocked more than a few comets inwards. But even so, the time it would take for any incoming comets to make the journey to the inner Solar System would be at least a couple million years, so we won’t need to be concerned about a comet deluge anytime soon.
Still, these findings show that the chances for unnervingly close passes of our Solar System by small, hard-to-spot stars (Scholz’s star, aka WISE J072003.20-084651.2, was only just discovered in 2013) are certainly not zero… especially since red dwarfs are the most abundant stars* in our area of the galaxy. Data from extra-sensitive observatories (like NASA’s WISE and ESA’s Gaia) will be crucial in the search for previously unseen stars and plotting the local stellar traffic patterns.