If our species manages to hang on for a few billion additional years, we might be in for a wild ride — stars passing in the vicinity of the sun could cause planets in our solar system to collide or even be ejected, according to a paper published last month in the journal Icarus.
The findings even suggest a scenario in which our world ends not consumed by the sun, but in a carom prompted by the powers of gravity.
The Milky Way is home to hundreds of billions of stars. Each one is in motion, zinging in its own orbit around the galactic center. Consider a long enough span of time — something astronomers are wont to do — and it’s inevitable that another star will pass closer to the sun than Proxima Centauri, currently our nearest stellar neighbor. In fact, calculations based on orbits of stars cataloged by the Gaia spacecraft suggest that, every million years, 33 stars, give or take a few, do just that.
But for another star’s gravitational effects to have a sizable impact on our solar system, you need a much closer shave than that, according to Nate Kaib, an astronomer at the Planetary Science Institute. “Once you get a couple hundred times the distance from the Earth to the Sun, you can really start to destabilize stuff,” he said.
Dr. Kaib and Sean Raymond, an astronomer at the Bordeaux Astrophysical Laboratory in France, set about determining the likelihood and effects of such cosmic near misses. The researchers ran thousands of computer simulations, modeling the gravitational effects of passing stars on the solar system’s eight planets and Pluto. The team considered stars with masses, velocities and orbits representative of objects in our stellar neighborhood.
Each simulation modeled the passage of five billion years. Dr. Kaib said that such a long-term perspective is necessary because it often takes tens of millions of years or even longer for a planet’s orbit to be perturbed by a passing star. “You don’t see the effects for a long, long time,” he said.
The researchers found that the triple threat is a massive star that makes a close approach to the sun at a relatively low velocity, magnifying its gravitational effect. But the alignment of all these attributes is rare. “A huge fraction of stellar passages are inconsequential to our solar system,” Dr. Kaib said.
The researchers found that 0.5 percent of their simulations resulted in planets colliding or a planet being ejected from the solar system.
And the world most likely to be affected in such a cataclysm? “Mercury is the most vulnerable planet by far,” Dr. Raymond said.
That’s because Mercury is already on unsteady orbital ground. Previous work has suggested that Mercury faces a 1 percent chance or so of colliding with Venus or the sun in the future, even if no stars make a close pass. Stellar flybys are basically throwing a gravitational curveball into an already unstable situation, Dr. Raymond said.
And if Mercury were to become a loose cannon, the inner solar system wouldn’t be a place to live: One of Dr. Kaib and Dr. Raymond’s simulations showed Mercury colliding with Venus, and the resulting planet (Mernus?) colliding with our planet.
“That would be very bad for life on Earth,” Dr. Kaib said.
Roughly 4 percent of the team’s simulations showed Pluto being ejected from the solar system. It makes sense that a small dwarf planet near the solar system’s outer reaches would feel the brunt of a passing star, said Jacques Laskar, an astronomer at the Paris Observatory who was not involved in this new study. “It can be more easily perturbed,” he said.
These results build on earlier research by Dr. Laskar and others that examined the stability of the solar system in isolation, said Hanno Rein, an astrophysicist at the University of Toronto who was not involved in the research.
“Now we’re making it a more complete picture by adding other stars in,” he said.
The simulations by Dr. Kaib and Dr. Raymond model what might happen in the future. But has a massive, slow-moving, close-brushing star already wreaked havoc on the solar system?
Probably not, Dr. Kaib said.
Such an event would have significantly disrupted the solar system’s icy outer reaches more than has been observed.
But it is possible that passing stars have had an impact on other planetary systems.
Thousands of exoplanet systems have been identified in the Milky Way, and every one, just like our solar system, is exposed to a scattershot of passing stars, Dr. Raymond said. Perhaps some of the planets in those systems have already collided or been ejected, ever to wander the galaxy as rogue planets.
“Statistically speaking, at least a few of those systems have to have been sculpted by flybys,” he said.
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