A newly discovered binary star system was found to have passed exceedingly close to our solar system within the last 70,000 years. Associate Professor of Physics and Astronomy Eric Mamajek recently authored a paper, published in The Astrophysical Journal, detailing the trajectory of the star system WISE J072003.20-084651.2, or “Scholz’s Star.”
The star, named after its discoverer, Ralf-Dieter Scholz, was shown to have passed within 0.25 parsecs (0.8 light years, or 52,000 astronomical units) of the Sun 70,000 years ago. For comparison, one astronomical unit is defined as the average distance between the Earth and the Sun. Currently, the closest star to the Sun is Proxima Centauri at 1.3 parsecs (4.2 light years)—five times as far from the Sun as Scholz’s Star’s closest approach.
Scholz’s Star has a number of interesting characteristics that drew the attention of astronomers. It is one of the closest stars to the sun, located only 20 light years away, and it has a very low tangential velocity—the speed at which the star is moving across the sky. Objects as close to the Sun as Scholz’s Star are generally expected to have a much higher tangential velocity—they should appear to be moving across the sky much faster than objects further away due to the parallax effect. This suggested to the astronomers that most of the star’s velocity was radial velocity—the speed of an object moving either toward the sun or away from it. Independent studies from multiple researchers confirmed this; the radial velocity was measured to be 83 kilometers per second, compared to the the tangential velocity of only three kilometers per second.
No one in the astronomy community had noticed this star until quite recently—it was first mentioned in a paper published in November 2013. This is partly because Scholz’s Star is a very faint star, located in a very crowded region of the night sky and surrounded by many other, similar-looking stars. The low tangential velocity made it more difficult to spot, as it is easier to search for nearby stars that appear to be moving quickly. Scholz’s Star only garnered attention when its distance to the Sun was measured.
The star came to Mamajek’s attention in a conversation with one of his colleagues in January 2014. After learning of its properties, he suspected that it had passed close to the Solar System. A quick calculation showed that he was right. He then spent his spare time working on the paper and performing further calculations while his colleagues around the world obtained more information on the star. Former UR undergrad and Caltech graduate student Scott Barrenfield helped with these calculations by deriving a more accurate trajectory. In addition, Mamajek had to intensively research the Oort Cloud, a branch of astronomy with which he was relatively unfamiliar. The article was submitted to The Astrophysical Journal in January and was published on Feb. 12.
Scholz’s Star represents the closest known flyby of a star to the solar system. Calculations have shown that its passing had even perturbed the outer layer of the Oort Cloud, a massive, spherical cloud of debris surrounding the outer edges of the Solar System. Perturbations in the Oort Cloud can cause comets to come loose and travel to the inner parts of the solar system. If an object were to pass through the inner part of the cloud, starting at 20,000 astronomical units from the Sun, more comets would be affected and it would be a more tumultuous event. Such events are incredibly rare. Mamajek cited the geological record and stated that such occurrences probably happen only once in about one billion years.
Simulations of Scholz’s Star’s orbit showed a 98 percent probability that it passed through the outer Oort Cloud but only a one in 10,000 chance that it passed through the inner Oort Cloud. The small number of comets it did impact will take close to one million years to travel to the interior of the Solar System, moving at very slow velocities.
Mamajek mentioned the questions he received regarding the interactions between the gravitational fields of Scholz’s Star and the Earth or the Sun. Calculations showed negligible effects of the Sun’s gravitational pull on the star’s velocity. In addition, Scholz’s Star produced negligible effects on the Earth and its tides.
Mamajek’s paper also highlights an error when calculating the future trajectory of the HIP 85605 star, the previous closest flyby. The 2014 Bailer-Jones survey predicted, with trepidation, that the star’s trajectory would bring it within 0.1 parsecs (21,000 astronomical units) 300,000 years in the future. Mamajek notes that this would force the star to have a brightness entirely inconsistent with the type of star it is, a typical orange dwarf star. A more reasonable distance estimation would be ten times further than the initial prediction.
The discovery of Scholz’s Star and its trajectory suggests the existence of other undiscovered stars that may have performed similarly close flybys. According to Mamajek, such flybys are actually not uncommon. Around ten stars pass through the Oort Cloud every ten million years, few being large or slow enough to cause a significant impact. There is a lack of statistical data on unassuming stars like Scholz’s star. The GAIA project, launched in 2013 by the European Space Agency, will help collect more measurements on these kinds of stars.
Going forward, Mamajek will return to his study of the ring system of J1407b but will be open to helping anyone who picks up the research on Scholz’s star.
Davrenov is a member of
the class of 2017.
