For centuries, astronomers have sought to understand the formation and evolution of the solar system and the dynamics that govern it. In particular, there is the long-standing question of whether or not the planets’ orbits will remain stable over time. However, these studies have generally treated the solar system as an isolated system, focusing solely on the gravitational interactions between the planets. This is in spite of the fact that astronomers have known for some time that stars in the Milky Way make close passes to each other every so often.

In short, dynamical studies of the solar system have generally failed to consider the influence that passing field stars will have on the solar planets over time. According to a recent study by a team of astronomers, stars passing close to the solar system could significantly influence the orbits of the solar planets.

According to their simulations, the passages of field stars could result in the loss of planets over the next 5 billion years. Their results also indicate that the orbits of the gas/ice giants and Pluto are not as stable as previously thought.

The study was conducted by Nathan A. Kaib, an Associate Professor of Astrophysics and Cosmology at the Planetary Science Institute (PSI), and Sean N. Raymond, an astronomer with the Laboratoire d’Astrophysique de Bordeaux (LAB). The paper that describes their findings, “The influence of passing field stars on the solar system’s dynamical future,” was recently published in the journal Icarus.

As Raymond indicates on his website, Sir Isaac Newton (who formulated the Theory of Universal Gravitation) first predicted that gravitational interactions between the planets would eventually cause their orbits to become unstable, though he couldn’t prove it.

By the 18th century, mathematicians Pierre-Simon Laplace and Joseph-Louis Lagrange concluded that planetary orbits oscillate in a wave-like pattern that keeps them stable forever. But by the 19th century, Carl Friedrich Gauss and Urbain le Verrier found that these wave-like patterns eventually decay.

Then came Henri Poincaré, who showed that the gravitational interactions between 3 or more bodies (aka the Three-Body Problem) cannot be solved analytically. Moreover, astronomers discovered multiple planets, satellites, and assorted bodies in the 19th and 20th centuries that Newton and his contemporaries did not know about, including Uranus, Neptune, Pluto, and the Kuiper Belt. Around the turn of the century, astronomers also detected several planetoids in the Trans-Neptunian region (Eris, Haumea, Makemake, etc.).

Recent studies have also raised the issue of “rogue planets,” bodies that are kicked from their star systems and become wanderers in interstellar (and even intergalactic) space. Alas, as Raymond and Kaib told Universe Today via email, the role of stellar flybys remains absent from research into solar dynamics.

“It simply hasn’t been included in the vast majority of studies. And this neglect can be justified by a simple calculation showing that the typical flyby has little to no effect on the planets’ orbits. Of course, this isn’t true of all flybys, or on the long-term, as we showed,” said Raymond. “It often takes hundreds of Myrs or even Gyrs for the effects of a stellar encounter to manifest themselves, so shorter simulations can give you the impression that they don’t matter as much.”

For their research, Kaib and Raymond performed 2,000 simulations based on the modern-day solar system derived from the Horizons System provided by the Solar System Dynamics Group at NASA JPL. It provides access to key solar system data and can predict the positions of all its objects with extreme accuracy.

They then injected stellar flybys into the mix, adjusted for various distances that would result in different perturbation levels for planetary orbits. As Kaib noted, each simulation had slightly different starting conditions for the planets based on the degree of uncertainty with their speeds and orbits:

“For half of those simulations (1,000), each system is subjected to a unique set of stellar passages while it is evolved for 5 billion years. We don’t know what types of stellar encounters the solar system will experience. But the stellar masses, passage velocities, and passage frequency are all consistent with what we’d expect for our neighborhood of the Milky Way galaxy. We then compare our 1,000 systems subjected to stellar passages with our 1,000 that weren’t to discern the effects of stellar passages.”

In addition, they also simulated a set of 1,000 systems where stellar passages would only have a 5% chance (or less) of occurring over the next 5 billion years. This gave them more examples of planetary instabilities triggered by passing stars, which provided a clearer picture of their possible outcomes.

“The punchline is that the solar system is about 50% less stable when flybys are taken into account,” Raymond added. “Internal chaos tends to take a few billion years to stir up the system, and has about a 1% chance of destabilizing the orbits of the rocky planets, preferentially in about 4–5 billion years.”

Their simulations also showed a ~5% chance that passing stars will destabilize Pluto over the next 5 billion years. This was a surprise since most prior studies assumed that Pluto has a 100% chance of remaining stable over this period. Ultimately, their simulations showed that there is about a 1 in 200 chance that the solar system will be destabilized by a stellar passage over the next several billion years. Their results also showed that these passages are more likely to happen sooner rather than later.

This makes one or more stellar passages the most likely cause of instability in the orbits of the solar planets for the next 4–4.5 Gyrs. Says Kaib:

“[T]he approximation that the solar system is isolated from the rest of our galaxy has its limits. Even in the (many) cases where passing stars did not trigger a planetary instability, they still gave slight nudges to the orbits of the giant planets, particularly Uranus and Jupiter. In addition, the past evolution of the solar system likely had a similar chance of being affected by stars as we find in our future evolution simulations.

“So, although we are confident that we did not go through a stellar-triggered planetary instability, it is quite plausible that the most powerful passages we have experienced have left their imprint on the giant planets’ orbits.”

In addition, these findings could have significant implications for more than just the field of solar dynamics. As Raymond added, they could also inform future studies about the dynamics governing other star systems.

“We know of thousands of exoplanet systems. Statistically speaking, flybys likely played a major role in destabilizing some of them. So, even though this is not a majority outcome, it is still highly relevant for interpreting exoplanet systems and our own future.”