How a Passing Star Could Oust Planets from the Solar System

Scientific American

2 days ago

There's a bit of a paradox about our galaxy: it's both jam-packed with stars and cavernously empty.
The Milky Way is crowded in the sense that it holds hundreds of billions of stars, as well as sprawling clouds of gas and dust. But even so, there is a lot of elbow room: the nearest star to the sun is more than four light-years distant, separated from us by tens of trillions of kilometers. That's an immense distance and difficult to even analogize. Saying our fastest space probes would take tens of thousands of years to reach the nearest star is still such a ponderous concept that it's hard to grasp.
Of course, there are more crowded spots, too. Some stellar clusters pack thousands of stars into a small volume of space, and the bustling galactic center swarms with stars. But out here in the galactic suburbs, stars are more spread out, providing one another plenty of room as they orbit through the Milky Way.
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Still, given enough time, some stars will encroach on our personal space. About 80,000 years ago a small red dwarf called Scholz's star passed the sun at a distance of just 0.85 light-year. Looking ahead, about 1.3 million years hence, the star Gliese 710 will give us a close shave by 0.17 light-year.
That may seem like a long time on a human scale, but that's barely a tick of the galactic clock. The sun and its retinue of planets, asteroids and comets have been around for more than 4.5 billion years. Across that yawning stretch of time, it's a near certainty the sun has had some close encounters of the stellar kind.
What sort of effect does that have on the solar system?
We know that the sun is surrounded by a vast halo of trillions of icy bodies collectively called the Oort Cloud; although each individual object is far too faint to see with modern equipment, every now and again, one will drop down into the inner solar system and grace our skies as a long-period comet. Estimates of the cloud's size vary, but it could stretch more than a light-year from the sun. A star passing through that region could gravitationally poke at those ice balls, nudging hundreds or even thousands of them toward the sun, and some of them could hit an inner planet. Some researchers have even speculated that such a close pass could provoke a mass extinction event.
Research published in the Astrophysical Journal Letters showed that the passage of Scholz's star was unlikely to trigger such an event; the star is too much of a lightweight and was moving too quickly to significantly jostle the Oort Cloud and rain death upon our world. Given enough time, however, other stars could indeed stir up trouble in the distant reaches of the outer solar system. Happily, we probably have thousands of millennia to prepare.
But such a celestial drive-by can have other unsettling consequences as well.
Many astronomers have wondered about the long-term stability of the solar system's planets, given that they interact with each other gravitationally over the eons. The early solar system was wracked with profound instabilities, but more recently such effects have been far more subtle. Oddly enough, Mercury, the innermost planet, is particularly susceptible to these. The physics behind this is complex, but in a nutshell, small changes in the orbit of Neptune—the major planet most affected by a star passing by—propagate inward. It tugs on Uranus, which tugs on Saturn, which tugs on Jupiter, and the solar system's most massive planet affects everything else. Its orbit and that of Mercury can fall into a resonance in which the orbital periods (the 'years' of both planets) are simple ratios of each other. When this happens, Mercury gets an added kick (literally, like when a child on a playground swing kicks at the right moment, pumping up their oscillations).
It's been known for decades that these effects can change the ellipticity of Mercury's orbit, sometimes stretching it out into a long oval. If the orbit were to get too elongated, Mercury could fall into the sun or get close enough to Venus to get flung out of the solar system. Mars, too, could fall prey to this; like Mercury, it has a more oval-shaped orbit than that of Earth or Venus and can find its orbit changing shape radically over a sufficiently long time frame.
In the past, most of those simulations assumed the solar system to be in isolation, with no other stars nearby sticking their noses in our business. But we know that's not the case, and such stellar interference must be accounted for to understand the solar system's evolution.
Many simulations that do include passing stars don't usually take all the effects into complete consideration; for example, they run their models for a few tens of millions of years even though it can take billions for gradually growing instabilities to have an impact. Others have used limited modeling of stellar encounters, meaning that they haven't included the entire possible range of masses, velocities and passage distances expected from stars in the galaxy.
Research published online in the planetary science journal Icarus last month attempts to address all these factors in more robust simulations of the solar system's dynamic evolution. What the authors find is that some celestial bodies are a little less stable than previously thought, given how often stars pass by the sun.
Not surprisingly, Pluto is the hardest hit. (The researchers only modeled the eight major planets plus Pluto.) Previously, Pluto was thought to have a pretty stable orbit, but the new simulations show that over the course of about five billion years, there's a 4 percent chance for Pluto to be ejected from the solar system entirely.
These passes also increase Mercury's odds of an unhappy end. Previous studies showed a roughly 1 percent chance of it dropping into the sun or being ejected from the solar system because of planetary dynamics in the next five billion years or so, but according to the new study, there's an additional 0.56 percent chance that these events could occur via stellar interactions. Mars, too, has a 0.3 percent chance of the fate of getting an extreme sunburn or starlessly wandering the galaxy.
Earth isn't immune, either. The new research finds that our own fair world has a 0.2 percent chance of being involved in a planetary collision or ejected into interstellar space. The odds are low, certainly, but higher than I'd care for given the world-shattering stakes.
At this point I think I should remind you of the timescale involved: we're talking five billion years into the future, which is roughly the same amount of time that's elapsed since the solar system was born in the first place. That's a long time, so this is not something you or I should personally worry about. Plus, we don't know of any stars that will pass terribly close to us for several million years anyway. In the shorter term, I'm more concerned about—in chronologically ascending order— global warming (at the timescale of decades), medium-sized asteroids (centuries), supervolcanoes (hundreds of millennia) and giant-sized asteroids (tens of millions of years).
Remember, too, that the solar system has been around a long time and, crucially, Earth is still here. It's been batted around a bit, but life persists. Over the very long term, the universe is a dangerous place, but for now, for us—cosmically speaking, at least—we can breathe easy.