Around the time modern humans are thought to have first spread across Asia, a red dwarf star passed just 0.8 light-years from the sun, a group of astronomers have concluded.
Our wandering ancestors probably never noticed. Scholz's star, as the red dwarf star is nicknamed, is so faint that, despite being just 20 light-years away, it was only discovered in 2013. Even when 25 times closer, and therefore 600 times brighter, the star officially known as WISE J072003.20-084651.2 would have required binoculars to detect (had they existed at the time). However, magnetically active stars like Scholz's can flare and it's possible that it may have occasionally become bright enough to puzzle an observant early human.
Scholz's star almost certainly passed through the Oort cloud, where most comets dwell, but probably didn't reach the inner cloud where a gravitational disturbance can trigger a cascade of comets into the inner solar system.
In The Astrophysical Journal Letters, a team led by Dr. Eric Mamajek of the University of Rochester describes the event as “The closest known flyby of a star to the solar system.” The claim follows reports that the star HIP 85605 may come as close as 0.13 light-years away in a quarter of a million years' time, but Mamajek's team says they have found evidence to refute this.
Scholz's star is one of many red dwarfs that turned up on photographic images of the sky, but went unnoticed until recently. Once their distances were measured, some of the closer objects started to draw attention, but at 20 light-years away, Scholz's star didn't stand out. The star is also not moving very quickly across the sky.
To determine whether the star is flying towards us or away, co-author Dr. Valentin Ivanov of the European Southern Observatory measured its radial velocity via Doppler shift and found it to be moving away from the sun rather fast.
"Most stars this nearby show much larger tangential motion," says 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." The star is currently moving away from us five times as fast as the Voyager 1 spacecraft.
Calculating the path of a star involves some uncertainty, since the gravity of other nearby objects may have introduced distortions. Nevertheless, Mamajek concludes there is a 98% chance that Scholz's star passed through the Oort cloud, but not through its inner region.
The European Space Agency's Gaia satellite will map the distance and velocities of a billion stars, so cases like Scholz's star and HIP 85605 may be just a foretaste of what we are about to learn.