An international team of scientists led by Dr Eric Mamajek of the University of Rochester has determined that 70,000 years ago the recently discovered, nearby, low-mass binary system WISE J072003.20-084651.2 passed within only 0.8 light-years of the Sun, i.e. within the Solar System’s distant cloud of comets – the outer Oort Cloud. No other star is known to have ever approached the Solar System this close – about 5 times closer than the current closest star, Proxima Centauri.
This false-color composite image shows the Scholz’s binary star (center). Image credit: V. D. Ivanov et al., 10.1051/0004-6361/201424883.
WISE J072003.20-084651.2 is a binary star system, containing a red dwarf star (with mass about 8 percent that of the Sun) and a brown dwarf (with mass about 6 percent that of the Sun). It is located in the constellation of Monoceros, about 20 light years away.
The system has been nicknamed Scholz’s star to honor its discoverer – Dr Ralf-Dieter Scholz of the Leibniz-Institut für Astrophysik Potsdam in Germany, who first reported the discovery of the dim nearby star in 2014.
In a new study published in Astrophysical Journal Letters (arXiv.org preprint), Dr Eric Mamajek and his colleagues analyzed the velocity and trajectory of this binary.
The star’s trajectory suggests that 70,000 years ago it passed about 52,000 astronomical units away (or about 0.8 light years). This is astronomically close – our closest neighbor star Proxima Centauri is 4.2 light-years distant.
According to the team, Scholz’s star has an unusual mix of characteristics: despite being fairly close, it shows very slow tangential motion.
“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,” Dr Mamajek said.
“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.”
Artist’s conception of Scholz’s binary star during its flyby of the Solar System 70,000 years ago; the Sun (left, background) would have appeared as a brilliant star. Image credit: Michael Osadciw / University of Rochester.
The scientists simulated 10,000 orbits for Scholz’s star, taking into account its position, distance, and velocity, the Milky Way Galaxy’s gravitational field, and the statistical uncertainties in all of these measurements.
Of those 10,000 simulations, 98 percent showed the star passing through the outer Oort cloud, but fortunately only one of the simulations brought the star within the inner Oort cloud, which could trigger so-called comet showers.
At the closest point in its flyby of the Solar System, the star would have been a 10th magnitude star – about 50 times fainter than can normally be seen with the naked eye at night. It is magnetically active, however, which can cause stars to flare and briefly become thousands of times brighter.
So it is possible that the star may have been visible to the naked eye by our ancestors 70,000 years ago for minutes or hours at a time during rare flaring events.
Until now, the top candidate for the closest known flyby of a star to the Sun was HIP 85605, which was predicted to come close to the Solar System in 240,000 to 470,000 years from now.
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Eric E. Mamajek et al. 2015. The Closest Known Flyby of a Star to the Solar System. ApJ 800, L17; doi: 10.1088/2041-8205/800/1/L17
