HD 7977: Did a Star Trigger Our Comet Shower?

A star about the size of the Sun, catalogued as HD 7977, slipped past our Solar System roughly 2.5 million years ago, close enough to reach into the Oort Cloud and shake comets loose. Some of those comets are falling toward us now.

That is the claim behind new work from Nathan Kaib of the Planetary Science Institute and Sean Raymond of the Université de Bordeaux, and it turns a quiet piece of galactic bookkeeping into something you can walk outside and see. Are the bright comets crossing our sky today the wake of a star that left our neighborhood before our species existed?

The short answer: probably yes, at least for the freshest arrivals. Kaib and Raymond find that the newest long-period comets carry the fingerprint of a single passing star rather than the steady pull of the whole Milky Way, and they name HD 7977 as the star. It is a suggestive result presented at a conference, not a settled one in a journal, and the authors are the first to say the fit is imperfect.

What HD 7977 did to our comets

It reshuffled the outer edge of the Solar System. As the star drifted by, its gravity tugged on the icy bodies of the distant Oort Cloud and knocked a fresh batch of them inward, where they become the long-period comets we catalog today. That is the headline Kaib and Raymond presented at the American Astronomical Society’s Division on Dynamical Astronomy.

The argument rests on a contrast. Normally the gentle tide of the whole Milky Way disk is what stirs the Oort Cloud and dislodges comets, a slow and even process. A single star doing the stirring is a different kind of event, and it leaves a different mark on the comets that arrive. The team says that mark is readable in the comets logged over the past few decades.

How Gaia caught an ancient encounter

Gaia found it by running the sky backward. The European Space Agency’s Gaia mission measured the positions and motions of well over a billion stars, building a three-dimensional map precise enough to play in reverse. Wind HD 7977’s path back about 2.5 million years and it swings close to the Sun.

Picture the Sun and its neighbors as walkers on a huge looping track around the galaxy’s center, each in a slightly different lane at a slightly different pace. Now and then two lanes nearly touch. Gaia is the camera quick enough to catch who almost bumped into whom, and when.

The spacecraft stopped taking data in early 2025, and its archive is nowhere near exhausted. Sharper numbers on HD 7977 are expected from data releases still to come, which matters for a claim that lives or dies on how close the star actually passed.

The star crossed into the Oort Cloud

Close enough to trespass. Gaia’s first estimates put HD 7977’s nearest approach somewhere between 4,000 and 25,000 astronomical units, a range too wide to be useful. By modeling how comet orbits should respond to a passing mass, Kaib and Raymond narrowed it to between 6,000 and 10,000 AU. One AU is the Earth–Sun distance, so 10,000 of them come to about a sixth of a light-year, 0.16 to be exact.

That drops the star squarely inside the Oort Cloud, the vast shell of icy bodies thought to stretch from roughly 2,000 out to 200,000 AU. Named for the Dutch astronomer Jan Oort, it is the reservoir that feeds us long-period comets whenever something jostles it, and no telescope has ever imaged it directly. For the wider map of where comets are born, our explainer on the Kuiper Belt and the outer Solar System lays out the neighborhood.

For scale, Voyager 1, the most distant thing humans have ever launched, sits near 171 AU after almost half a century of flight and is closing on one light-day from the Sun, which works out to 173 AU. HD 7977 reached dozens of times deeper than that, and it did so dragging a whole star’s worth of gravity behind it.

Two comet groups, two different memories

The proof sits in two comet populations that point in different directions. Kaib and Raymond studied 112 long-period comets recorded since 1989, the year professional surveys grew good enough to catch most newcomers in both hemispheres. They sorted the sample by orbital history, and that sorting is where the story earns its keep.

New arrivals remember the star

Comets on their very first plunge inward, with orbits spanning more than a million years, match the signature of a single star sweeping through. If the galaxy’s tide were doing the stirring, these fresh comets should show a tidy bias tied to the plane of the Milky Way. That galactic bias is smeared out or missing among the newest arrivals, which is what a recent stellar shove predicts.

Repeat visitors remember the galaxy

Older comets tell the other half of the story. Their orbits, already shortened by past run-ins with the planets, line up with the steady pull of the galactic disk instead. One group carries HD 7977’s mark; the other carries the galaxy’s. Having both signatures in the same catalog is what makes the split convincing rather than a statistical fluke.

Are we still inside the comet shower?

Very likely, and we may be near the end of it. “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,” Kaib said in the Planetary Science Institute release. “This would also mean we’re living through the late stages of a pretty rare and powerful comet shower.”

You may have already seen the evidence yourself. Comet C/2023 A3 Tsuchinshan-ATLAS put on a memorable show in late 2024, and C/2024 E1 Wierzchos climbed into view soon after; both are long-period visitors of exactly the kind that fits the wave. When the next bright one appears, it is fair to wonder whether a long-gone star gave it the first push. For how much a single well-tracked comet can teach us, see our look at the interstellar comet 3I/ATLAS.

A daytime star our ancestors may have seen

From Earth, HD 7977 would have outshone everything but the Sun and the Moon. Drop a Sun-like star to 10,000 AU and the inverse-square law puts it near magnitude −7; its absolute magnitude of about +4.8 gives −6.7 when you run the numbers, bright enough to cast a faint shadow and to stay visible in daylight. Australopithecus africanus walked the African savanna at the time, and there is a decent chance some of them glanced up at a strange new point of light in the afternoon sky.

Updated July 2026: an earlier version of this article repeated a widely quoted line putting the star at forty times the brightness of Venus. Our own arithmetic makes it closer to seven times Venus at its most brilliant, since the forty-times figure only holds when you compare against Venus near its dimmest. We have corrected it here.

HD 7977 is a G-type star carrying 1.07 times the Sun’s mass. It has since drifted into the constellation Cassiopeia, dimmed by distance to a speck you would need a telescope to find, and no planets have turned up around it so far.

How it compares with other close passes

HD 7977 stands out because it was both massive and near. Other stars have grazed the Solar System, and at least one is due to visit in the far future, but the well-known cases are smaller, dimmer red dwarfs that carry far less gravitational punch.

Table 1 — Notable close stellar passes to the Sun, distances in astronomical units (Kaib & Raymond via the Planetary Science Institute; Universe Today, 2026).

The pattern is worth sitting with. A red dwarf skimming past barely ruffles the Oort Cloud. A star with the Sun’s heft, arriving at a similar distance, can dominate it for a stretch of cosmic time. Gliese 710 will pass about as close as HD 7977 did, yet it weighs far less, so its comet shower should be the milder one. That is why HD 7977 earns its own headline while heavier traffic barely registers.

Where the study could be wrong

The result is intriguing rather than airtight, and the authors say so plainly. “Like many other works that simulate long-period comet production, we find that our comets’ orbit sizes aren’t a great match to the observed distribution,” Raymond said. “It’s possible we’re missing some important physics.” When the people making a claim flag its weak point first, that is a good sign for their honesty and a caution for the rest of us.

Some of that missing physics is ordinary. Comets leak: gas jetting off their surfaces acts like a small wandering thruster that bends an orbit over time, and sunlight pressure adds another nudge. Neither looks dramatic on its own, and both blur any attempt to read orbits across millions of years. We watched one comet reverse its own spin under forces like these in our piece on how a comet’s jets can flip it around.

Two more cautions belong here, and they are ours rather than the team’s. There is no peer-reviewed paper to read yet, only a conference presentation and a press release, so the 112-comet statistics and the 6,000–10,000 AU refinement sit on the authors’ word until the full write-up appears. And we are leaving aside whether HD 7977 could have nudged anything onto a collision course with the inner planets; the odds are tiny, and it is a separate question that deserves its own piece.

The good news is that a verdict is coming. Gaia is still publishing refined stellar motions, and within roughly a year those numbers should tighten HD 7977’s closest approach and either back the idea or sink it. Sky surveys like Pan-STARRS and ATLAS keep hauling in fainter comets, and the Vera Rubin Observatory’s decade-long survey will catalog them in bulk. If the galactic fingerprint really is missing from fresh comet orbits, that flood of data will show it.

A star we can no longer see

Step back and the picture is quietly strange. A star we can no longer pick out by eye, one that passed before there were people to watch it, may still be steering the comets that decorate our nights. The Oort Cloud kept the record, Gaia handed us the means to read it, and if the reading holds, our descendants will meet fewer first-time comets than we do. That makes right now a lucky window to be looking up.

We wrote this for you at FreeAstroScience, where we take tangled science and hand it back in plain language, and where we would rather show you the seams in a result than pretend they aren’t there. Keep questioning what you read, this article included.

Signed,
Gerd Dani, President, FreeAstroScience — Science and Cultural Group.

Sources

1. Kaib, N. and Raymond, S. “Long-Period Comets’ Orbits Reflect Close Passage by Star HD 7977.” Planetary Science Institute, June 25, 2026. Presented at the American Astronomical Society Division on Dynamical Astronomy, 2026. psi.edu.
2. Dickinson, D. “An Ancient Stellar Passage Altered the Orbits of Comets We See Today.” Universe Today, July 14, 2026. universetoday.com.
3. European Space Agency. “Gaia overview.” ESA Science and Exploration, esa.int.