The Gaia mission has allowed researchers to understand the motions of stars like never before, even revealing possible interactions between our solar system and nearby stars. Planetary Science Institute Senior Scientist Nathan Kaib and collaborator Sean Raymond (Universite de Bordeaux) have found that a recent stellar passage likely triggered a huge increase in comet formation as the star’s gravity altered Oort cloud objects’ orbits, sending them cascading into the inner solar system. We may even still be feeling the effects of this passage today. This work is being presented at the American Astronomical Society Division on Dynamical Astronomy.
HD 7977 is a nearby sun-like star in the constellation Cassiopeia whose close passage was discovered by the Gaia mission. Approximately 2.5 million years ago, the orbits of the sun and HD 7977 brought the two stars close together, but exactly how close is still an open question. Gaia data suggest they passed within 4,000–25,000 astronomical units of one another. Now, Kaib and Raymond have shown that the orbits of long-period comets suggest HD 7977 came within 6,000–10,000 AU of our sun, setting off a major shower of new comets into the inner solar system.
How Gaia reshaped the picture
The Gaia mission was a European Space Agency telescope that precisely measured the changing positions and brightnesses of stars and other objects in our galaxy relative to background quasars and galaxies. Its combination of a spectrograph to measure motions toward and away from us and high-resolution cameras that measure motion across the sky has allowed the three-dimensional motions of roughly 1% of the objects in our galaxy—including objects like the star HD 7977. This allows us to understand the history of our solar system in new ways.
Normally, the gravity from our galaxy’s disk is the primary force causing icy objects in the outer solar system to change their orbits. This pull spreads out what was once a disk of material into what researchers calculate is currently a spherical shell of objects. This shell, the Oort cloud, is named after its discoverer, Jan Oort.
This galactic tug should also dominate the orbits of new comets entering our solar system for the first time (effects from the planets and passage near the sun can radically change comets’ orbits after their initial entrance), and it should leave a distinct signature in the direction that comet orbits point relative to our Milky Way’s midplane.
A stellar shove in the data
If HD 7977 passed as near as Kaib and Raymond propose, its gravitational influence would temporarily dwarf that of the galaxy, and the galactic signature should be absent from current comet orbits. This is exactly what Kaib and Raymond found when they analyzed the orbits of new comets.
“The distribution of comet orbits suggests we are living through an unusual time where HD 7977 has dominated the generation of new comets and not the larger gravitational field of the Milky Way, as it usually would. This would also mean we’re living through the late stages of a pretty rare and powerful comet shower,” says Kaib.
To test this idea, Kaib and Raymond ran a series of computer simulations to understand what comet orbits would be seen as a result of HD 7977 passing at different distances. These computer models were then compared to the passage of 112 long-period comets that have been observed since 1989, when professional surveys made it possible to detect most new comets in both hemispheres.
Their paper has been accepted by The Planetary Science Journal and is currently available on the arXiv preprint server.
Long-period comets have highly elliptical orbits that typically take thousands to millions of years to complete. New comets on their first passage through the inner solar system have orbital periods measured in millions of years, while older comets on subsequent passages have shorter periods due to their prior close interactions with the planets. The observed orbits of comets on their first passage through the solar system match HD 7977, triggering a wave of long-period comets to enter our solar system. Older comets on repeat passages are consistent with the galactic disk’s pull, creating their orbits.
Where the model falls short
This isn’t a perfectly clean result, however. That is rare in science, and in this data set, the sizes of the comets’ orbits aren’t a good match for the models of the star’s close approach.
“Like many other works that simulate long-period comet production, we find that our comets’ orbit sizes aren’t a great match for the observed distribution. It’s possible we’re missing some important physics from our simulations, and it’s conceivable that this has caused us to misinterpret comet orbit data,” said Raymond.
It is entirely possible that either our solar system’s structure is more complex than predicted, or the forces involved are more complex than the models accounted for. And it is even possible both these situations play a role. For instance, we know forces other than gravity, such as the pushes the comet experiences from its own jets and even from light, can play a role in altering orbits.
More evidence is on the way
“The nice thing about our prediction is that it will be testable pretty soon. Gaia is still publishing new data on the motions of stars, and in 6–12 months, it should be able to improve our understanding of HD 7977’s motion and tell us if we are right or wrong,” said Kaib.
With each passing year, more comets are observed and newer telescopes allow us to see a larger variety of objects—stars and comets—with more precision. In addition, the Vera Rubin Observatory’s Legacy Survey of Space and Time will detect a very large number of new comets over the next decade, and it will more confidently determine whether or not our galaxy’s gravitational signature appears in the orbits of comets.
Publication details
Nathan A. Kaib et al, A Potential Signature of HD 7977’s Passage Among Observed Long-Period Comet Orbits, arXiv (2026). DOI: 10.48550/arxiv.2606.25069
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Ancient stellar flyby may still be steering long-period comets today (2026, June 25)
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