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A passing star may have unleashed a comet deluge
Some 2.5 million years ago, a passing star might have jostled a throng of icy objects at the edges of our solar system, sending them soaring in toward the sun as long-period comets. That’s according to scientists from the Planetary Science Institute in Arizona and the University of Bordeaux in France, who said on June 25, 2026, that the influx of comets from this event might be continuing to this day.
Thanks to the Gaia mission, the scientists identified a star – HD 7977 – that appeared to have passed near our sun some 2.5 million years ago. That’s relatively recent for our solar system, which is 4.6 billion years old. For context, long-period comets are those that take between 200 and tens of millions of years to orbit the sun once. So if HD 7977 passed close to our sun 2.5 million years ago, there might still be many comets tugged by the passing star that are making their way inward.
Why do comets travel to the inner solar system?
Comets are ‘dirty snowballs’, left over from our solar system’s formation. These icy remnants formed in the region of the giant planets early in the solar system’s history. Gravitational interactions with the young, giant planets flung them to the solar system’s outermost reaches. And the gravity of the surrounding Milky Way galaxy has been gently pulling on the fringes of our solar system, spreading out the pool of comets into a sphere, which we call the Oort Cloud. And these same tugs also can put a comet in motion toward the sun.
But now, the researchers said a close encounter with a passing star might also have been the trigger for many comets to begin their journey to the inner solar system.
A stellar flyby left its mark
So, some 2.5 million years ago, a sunlike star – HD 7977 – that currently lies in the direction of Cassiopeia the Queen in our sky passed close to Earth. According to Gaia data, HD 7977 passed between 4,000-25,000 astronomical units (AU) from the sun 2.5 million years ago. (1 AU equals the distance between Earth and the sun.) And the researchers have narrowed down that range to 6,000-10,000 AU from our sun.
They narrowed the range by running computer simulations of the orbits of long-period comets. Then they compared the simulations to the passage of 112 known long-period comets. These are all comets with professional observations since 1989. And they found that the star’s gravity likely did disturb the Oort Cloud, sending a wave of comets inward.
Nathan Kaib of the Planetary Science Institute said:
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.
Their results indicated that newer comets – those on their first pass through the inner solar system – could be the result of the passing star. Meanwhile, older comets on subsequent passes have orbits that are more consistent with the galactic pull.
More comet data needed to refine the theory
But the researchers admit their results aren’t perfect. The size of the long-period comets’ orbits don’t precisely match the closest approach of the star. The forces at play might not be as simplistic as the modeled simulations. But, Kaib said:
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.
The Vera Rubin Observatory’s Legacy Survey of Space and Time is set to begin any day. It should detect many comets over the coming years, allowing astronomers to better understand all the forces that start them on their journey toward the sun.
Bottom line: Scientists think that a passing star 2.5 million years ago might have sent a wave of comets in toward the sun. And some of those comets could still be arriving today.
Via Planetary Science Institute
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