70,000 years ago a Star passed through our Solar System



An alien star passed through our Solar System just 70,000 years ago, astronomers have discovered.





Eric E. Mamajek1, Scott A. Barenfeld2, Valentin D. Ivanov3,4, Alexei Y. Kniazev5,6,7, Petri Väisänen5,6,Yuri Beletsky8, and Henri M. J. Boffin3

Published 2015 February 12© 2015. The American Astronomical Society. All rights reserved.
The Astrophysical Journal Letters, Volume 800, Number 1


Passing stars can perturb the Oort Cloud, triggering comet showers and potentially extinction events on Earth. We combine velocity measurements for the recently discovered, nearby, low-mass binary system WISE J072003.20-084651.2 (“Scholz’s star”) to calculate its past trajectory. Integrating the Galactic orbits of this ~0.15 M binary system and the Sun, we find that the binary passed within only 52+23−14 kAU (0.25+0.11−0.07 pc) of the Sun 70+15−10 kya (1σuncertainties), i.e., within the outer Oort Cloud. This is the closest known encounter of a star to our solar system with a well-constrained distance and velocity. Previous work suggests that flybys within 0.25 pc occur infrequently (~0.1 Myr−1). We show that given the low mass and high velocity of the binary system, the encounter was dynamically weak. Using the best available astrometry, our simulations suggest that the probability that the star penetrated the outer Oort Cloud is ~98%, but the probability of penetrating the dynamically active inner Oort Cloud (<20 kAU) is ~10−4. While the flyby of this system likely caused negligible impact on the flux of long-period comets, the recent discovery of this binary highlights that dynamically important Oort Cloud perturbers may be lurking among nearby stars.


Perturbations by passing stars on Oort cloud comets have previously been proposed as the source of long-period comets visiting the planetary region of the solar system (Oort 1950; Biermann et al. 1983; Weissman 1996; Rickman 2014), and possibly for generating Earth-crossing comets that produce biological extinction events (Davis et al. 1984). Approximately 30%, of craters with diameters <10 km on the Earth and Moon are likely due to long-period comets from the Oort Cloud (Weissman 1996). Periodic increases in the flux of Oort cloud comets due to a hypothetical substellar companion have been proposed (Whitmire & Jackson 1984); however, recent time series analysis of terrestrial impact craters are inconsistent with periodic variations (Bailer-Jones 2011), and sensitive infrared sky surveys have yielded no evidence for any wide-separation substellar companion (Luhman 2014). A survey of nearby field stars with Hipparcosastrometric data (Perryman et al. 1997) by García-Sánchez et al. (1999) identified only a single candidate with a pass of within 0.9 pc of the Sun (Gl 710; 1.4 Myr in the future at ~0.34 pc); however, it is predicted that ~12 stars pass within 1 pc of the Sun every Myr (García-Sánchez et al. 2001). A recent analysis by Bailer-Jones (2014) of the orbits of ~50,000 stars using the revisedHipparcos astrometry from van Leeuwen (2007), identified four Hipparcos stars whose future flybys may bring them within 0.5 pc of the Sun (however, the closest candidate HIP 85605 has large astrometric uncertainties; see discussion in Section 3).

A low-mass star in the solar vicinity in Monoceros, WISE J072003.20-084651.2 (hereafter W0720 or “Scholz’s star”) was recently discovered with a photometric distance of ~7 pc and initial spectral classification of M9 ± 1 (Scholz 2014). This nearby star likely remained undiscovered for so long due to its combination of proximity to the Galactic plane (b = +2fdg3), optical dimness (V = 18.3 mag), and low proper motion (~0farcs1 yr−1). The combination of proximity and low tangential velocity for W0720 (Vtan sime 3 km s−1) initially drew our attention to this system. If most of the star’s motion was radial, it was possible that the star may have a past or future close pass to the Sun. Indeed, Burgasser et al. (2014) and Ivanov et al. (2014) have recently reported a high positive radial velocity. Burgasser et al. (2014) resolved W0720 as a M9.5+T5 binary and provided a trigonometric parllax distance of 6.0+1.2−0.9 pc. Here we investigate the trajectory of the W0720 system with respect to the solar system, and demonstrate that the star recently (~70,000 years ago) passed through the Oort Cloud.

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