The giant cloud of icy chunks on the outskirts of the Solar System, the Oort cloud, resembles a transparent bubble stretched to a light-year across. Usually, these chunks hang quietly in their distant corners, but sometimes something disturbs them.
Most often, it's the tidal influence of our Galaxy. The Milky Way's disk, through its gravity, gives the cloud a slight "shake," and some comets fall toward the Sun. Such comets, arriving for the first time, are called dynamically new long-period comets. Astronomers expected these newcomers to arrive not randomly but with a specific orientation of their orbits — dictated by the galactic tide.
But when scientists compared expectations with actual observations, the picture didn't match. The new comets turned out to be too evenly scattered across the sky. The puzzle: why couldn't the Galaxy's tidal "hand" align them?
The researchers tested two possible explanations. First: the Oort cloud itself is structured differently than thought: it is less dense in the center. Second: an outsider intervened. About 2.5 million years ago, the star HD 7977 passed very close — at a distance of 6000–10000 astronomical units from the Sun (one astronomical unit is the distance from Earth to the Sun). That's like a neighbor walking through your yard and brushing against the tree branches.
Modeling showed that such a stellar flyby best reproduces what we see today. HD 7977's gravity caused a real comet shower, disrupting the order imposed by the Galaxy. Interestingly, this shower hasn't ended yet: we are still observing its aftermath.
It turns out our calm harbor on the edge of the Galaxy isn't so serene. Stellar encounters like this one may occur every few million years, and each time they wake sleeping comets, sending them on a journey toward the Sun.
A distance of 6000 astronomical units is about 150 times farther than the average distance from the Sun to Pluto. For a star, that's almost a "near miss."
In Arthur C. Clarke's novel "Rendezvous with Rama," the Oort cloud serves as a staging base for an alien spacecraft: there's certainly plenty of room for maneuvering.