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.
as written by the authors arXiv:2606.25069
It is generally presumed that the tidal field of the Milky Way's disk is the main perturbation that has driven observed long-period comets (LPCs) from the Oort cloud into the inner solar system. The tide's influence on the Oort cloud should produce a distinct anisotropy in the arguments of perihelion ($$\omega$$) of dynamically new LPCs with semimajor axes ($$a$$) over 10$$^4$$ au. Simulating LPC production dominated by the Galactic tide, we find that observed dynamically new LPCs are more isotropic than expected. Meanwhile, our simulation exhibits much better agreement between simulated and observed ``returning'' LPCs that have made a handful of passages through the inner solar system prior to discovery. The isotropy of new LPCs can be explained if the Oort cloud is much less centrally concentrated than the conventional Oort cloud formation model predicts. However, a second possibility also exists. Additional simulations we perform show that the observed $$\omega$$ distributions of new and returning LPCs can both be well-replicated if the star HD 7977 passed within $$\sim$$6000--10000 au of the Sun $$\sim$$2.5 Myrs ago. In such a scenario, our solar system is still undergoing the latter stages of a comet shower. These simulations imply the modern observed LPC flux is $$\sim$$twice as high as the longer-term (tide-dominated) rate. This also implies that estimates of the Oort cloud's population should be revised downward by a factor of $$\sim$$2. Our LPC analysis predicts the upcoming Gaia data release will favor an HD 7977 impact parameter of $$\sim$$6000--10000 au.
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.