The Oort Cloud is a spherical cloud of icy planetesimals that surrounds the Sun out to a distance of about 50,000 AU. For comparison, Pluto’s orbit never brings it further than 49 AU from the Sun. Passing stars can perturb these icy planetesimals in the Oort Cloud and send them on trajectories towards the Sun, resulting in the flux of long-period comets that visit the planetary region of the Solar System. Some of these comets might have even impacted Earth and produced extinction events. A study by Mamajek et al. (2015) reports on the identification of the closest known flyby of a star to the Solar System. The star belongs to a nearby, low-mass binary system known as WISE J0720.
A star’s velocity through space consists of two components - radial and tangential. Radial motion is how fast the star appears to be approaching or receding along the line of sight, while tangential motion is the motion of the star across the plane of the sky, perpendicular to the line of sight. WISE J0720 seems peculiar with its low tangential velocity of only ~3 km/s because this suggests that most of its velocity could be in the radial component. Indeed, WISE J0720 was found to have a large radial velocity of 83 km/s, implying it once might have made a close pass to the Sun. By tracing its motion through space, WISE J0720 was determined to have passed within only ~50,000 AU of the Sun ~70,000 years ago.
Currently, WISE J0720 is about 20 light years away. This binary system weighs in at only ~0.15 times the Sun’s mass. It consists of an M9 red dwarf star and a T5 brown dwarf companion. Nevertheless, the low mass of WISE J0720 and the relatively high velocity of its close passage meant that the encounter probably has a negligible statistical impact on the flux of long-period comets into the planetary region of the Solar System. The discovery of WISE J0720 suggests more of such objects may be lurking in the Sun’s stellar neighbourhood.
WISE J0720 is extremely faint. Even at closest approach, it would have been invisible to the naked eyes. Nevertheless, the stellar component of WISE J0720 is an active red dwarf star that is perhaps capable of generating powerful flares. Such flares can cause the star to brighten by orders of magnitude. Although WISE J0720 is invisible in its quiescent state, an exceptionally powerful flare event might have allowed our Pleistocene ancestors to observe a short-lived transient in the night sky. From the vantage point of WISE J0720, the Sun would be by far the brightest star in the sky.
Mamajek et al. (2015), “The Closest Known Flyby of a Star to the Solar System”, arXiv:1502.04655 [astro-ph.SR]