70,000 Years Ago A Rogue Star Passed Through the Solar System

Like piecing together a car accident, by looking at the results of a collision in space, we can use our knowledge of science to piece together the past and determine what happened to a high degree of accuracy.  The velocity of a star, along with high powered computer simulations and statistical techniques, can help us determine where it once was.  But since space is so vast, why do we care where stars have moved?

Artist’s conception of Scholz’s Star with it’s Brown Dwarf companion, shown during it’s pass close to the Sun 70,000 years ago. Credit: Michael Osadciw/University of Rochester.

The short answer is that there are so many stars out there that we don’t often care where individual stars have moved.  Some exceptions are when the stars are in unique environments, such as near a black hole, or in a globular cluster, or in this case, close to home.

A star 20 light years away was recently found to have passed through the outer solar system in the closest passing of a star ever discovered.  Known as Scholz’s star, it was discovered in 2013 as part of NASA’s Wide Field Infrared Survey (WISE).  Scholz’s star is a small and dim red dwarf star located in the constellation of Monoceros.

Astronomers looked at both the radial and tangential motion of the star and noticed that even though the star was only 20 light years away from the solar system, it has a relatively small tangential velocity, meaning it didn’t move very much against the background of more distant stars.  Most stars close to the Sun are much easier to discover due to their noticeable motion against the stellar background.

This false-color composite image shows the Scholz’s binary star (center). Image credit: V. D. Ivanov et al., 10.1051/0004-6361/201424883.

“Most stars this nearby show much larger tangential motion,” says Eric Mamajek, associate professor of physics and astronomy at the University of Rochester. “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.”

To work out the likelihood of a close pass, Mamajek and a colleague ran 10,000 simulations of the star’s trajectory, taking into account the current distance, position, radial velocity, and tangential velocity of the star, and discovered that in 98% of the simulations, the star would have passed through the outer Oort cloud 70,000 years ago, roughly 0.8 light years from the Sun.  This distance is 52,000 astronomical units (the distance from the Earth to the Sun), or about 8 Trillion Km.  This is very far to an Earthbound observer, but if you compare it to the closest star, Proxima Centauri, at a distance of 4.2 light years, this is very close.

The Oort cloud is the spherical distribution of comets that populates the outermost regions of the solar system.  Having a star pass through it is harmless unless it passes through the inner Oort cloud, where the gravitational field of the star can trigger ‘comet showers,’ sending thousands of comets hurtling toward the inner solar system.

This is a rare occurrence, but it gives us an understanding of the size of the solar system and the stellar neighbourhood.  It also shows how some stars, even though they are very close to us, can be elusive, hiding among the backdrop of distant stars.  It’s almost like a ‘Where’s Waldo’ book, except ‘Waldo’ is a star hidden among hundreds of thousands of other nearly-identical stars.

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