Key takeaways

  • Isaac Newton’s theories of motion and universal gravity laid the foundation for understanding gravitational forces and planetary orbits. Stellar flybys, where stars pass close to the Solar System, can cause significant gravitational disturbances.
  • Recent research by Canadian astrophysicists Garett Brown and Hanno Rein indicates that a star is unlikely to pass close enough to destabilize the Solar System for another 100 billion years.
  • Stellar flybys significantly impact planetary systems, particularly during their early formation in star clusters. These events can truncate protoplanetary disks and destabilize or remove planets on wide orbits.
  • The Solar System likely formed from a collapsing nebula, influenced by stellar flybys which may have played a role in planet formation. The Sun may have formed in a star cluster, experiencing frequent flybys in its early history.
  • Predicting the long-term stability of the Solar System involves solving the N-body problem, which requires complex numerical simulations. These simulations reveal that significant perturbations from stellar flybys are rare and require close encounters.

Sir Isaac Newton’s greatest opus, Philosophiae Naturalis Principia Mathematica, was published in 1687, and it effectively synthesized his theories of motion, velocity, and universal gravity. In terms of the latter, Newton proposed a method for calculating gravitational force and forecasting planet orbits. Since then, astronomers have determined that the Solar System is only a little point of light around the Milky Way Galaxy’s core. Other stars occasionally pass close to the Solar System, causing a violent shakeup that can knock things out of their orbits.

These “stellar flybys” occur often and play a significant role in the long-term evolution of planetary systems. As a result, for centuries, scientists have been investigating the Solar System’s long-term stability. A recent study by a team of Canadian astrophysicists suggests that dwellers of the Solar System can rest comfortable. After running a number of simulations, scientists concluded that a star will not pass by and disrupt our Solar System for another 100 billion years. Beyond that, the possibilities are quite frightening!

Garett Brown, a graduate student in computational physics at the University of Toronto Scarborough‘s Department of Physical and Environmental Sciences (PES), led the study. He was accompanied by Hanno Rein, an associate professor of astrophysics and Brown’s mentor at UT Scarborough’s PES. The report summarizing their findings was recently published in the Monthly Notices of the Royal Astronomical Journal. As they stated in their report, the study of stellar flybys could disclose a lot about the history and evolution of solar systems.

As Brown explained to Universe Today via email, this is particularly true of stars like the Solar System during its early history:

“The full extent that stellar flybys play in the evolution of planetary systems is still an active area of research. For planetary systems that form in a star cluster, the consensus is that stellar flybys play an important role while the planetary system remains within the star cluster. This is typically the first 100 million years of planetary evolution. After the star cluster dissipates the occurrence rate of stellar flybys dramatically decreases, reducing their role in the evolution of planetary systems.”

The Nebula Hypothesis is the most widely accepted theory for the creation of the Solar System. It argues that the Sun formed from a vast cloud of dust and gas (also known as a nebula) that collapsed gravitationally at its center. The remaining dust and gas create a disk around the Sun, which gradually accretes to form a system of planets. According to one theory, the Sun evolved as a result of nebula disturbances, probably caused by a close encounter with another star (or a supernova). However, as Brown explained, stellar flybys are also likely to have influenced planet formation.

“During planet development, when there is a disk of dust and gas around a star, stellar flybys are expected to be responsible for disk truncation, which would prevent the formation of planets on wider, more distant orbits,” the scientist stated. “For planets which have already formed on wide orbits, stellar flybys are thought to be responsible for removing or destabilizing the outermost planets.”

Another commonly recognized theory holds that our Sun formed approximately 4.5 billion years ago as part of a star cluster that it has since departed. With these notions in mind, Brown and Rein studied how being part of a cluster (and so exposed to star flybys) would have changed the Solar System once its planets formed and were part of an established system. They discovered that the role of stellar flybys is determined by the degree to which the passing star disturbs the system. They discovered that a star flyby can dynamically destabilize a system, causing planets to collide or be expelled.

Artist’s impression of a solar system in the process of formation. Credit: NASA/JPL-Caltech

This provided a huge hurdle due to an issue that has plagued astronomers since Newton proposed his Theory of Universal Gravitation. To put it simply, it all boils down to the N-body issue, which outlines the difficulties of predicting the individual trajectories of a group of astronomical objects interacting gravitationally. Because exact solutions are mathematically impossible, astronomers are obliged to use numerical approximations. However, as Brown mentioned, there are still two main flaws with these calculations:

“One, the motion of the planets are chaotic, meaning small differences in the initial conditions of the system will result in dramatically different outcomes (even differences as small as one part in a trillion). And two, the timescales involved are dramatically different. We can get a sense for the statistical outcome of a chaotic system using an ensemble of numerical solutions. For the long-term stability of the Solar System this can give us a ratio of simulations that end up destabilizing compared to the number of simulations that remain stable to the end of the integration time.”

“However, solving the timescales issue is much more difficult. Sophisticated numerical methods have been developed over the past 50 years which make this more tractable, but we essentially need to simulate the motion of the planets one day at a time for billions of years. This requires an incredible amount of computational resources. We typically want to know if the Solar System will remain stable for the remaining lifetime of the Sun (about 5 billion years). Even with modern computers (as fast as they are) it can easily take 3-4 weeks to run just one 5 billion year simulation of the Solar System.”

Brown stated that in order to obtain meaningful statistics, researchers must run thousands of simulations. This can be accomplished in two ways: by running the simulations on a single computer for up to 70 years or more, or by employing thousands of different computers at the same time for one month. This complicates statistical analysis and increases its cost. Brown and Rein conducted their analytics using the Niagara supercomputer at the University of Toronto’s SciNet center, which is part of the Digital Research Alliance of Canada network.

The Niagara supercomputer at the SciNet center. Credit: University of Toronto

As Brown explained, he and Rein employed two main methods to calculate the potential perturbations caused by steller flybys.

“The first was an analytical approximation developed in 1975 by Douglas Heggie and refined over the years with his collaborators. It’s an approximation that assumes the relative velocity between the two stars is small compared to the orbital velocity of the planets. This analytical estimate allows us to very quickly compute order of magnitude estimates for how a stellar flyby will change the semi-major axis of a planet.”

The second way involved numerical integrations with REBOUND, an open-source multi-purpose N-body code for collisional dynamics created by Hanno Rein and associates. Brown and Rein used these two methods to numerically simulate a star flyby and then measure the system’s status before and afterward. Finally, their findings revealed that disturbances to the Solar System would necessitate a very close flyby, and that such an encounter was unlikely to occur for a long period. Said Brown:

“We found that critical changes to the orbit of Neptune needed to be on the order of 0.03 AU or 4.5 billion meters in order to have any impact on the long-term stability of the Solar System. These critical changes could increase the likelihood of instability over the lifetime of the Solar System by 10 times. Additionally, we estimated that a critical stellar flyby like this could occur once every 100 billion years in the region the Solar System is currently in.

“[W]e estimated that we would need to wait about 100 billion years before a stellar flyby past the Solar System would simply increase the odds of dismantling its current architecture by 10 times (and that’s still not a guarantee of destruction).”

Given the Solar System’s chaotic past, it’s reasonable that the prospect of star flybys (and the resulting perturbations) might raise concern for some. After all, astronomers believe that “planetary shakeups” are a common aspect of a system’s history, and that flybys frequently eject huge objects from a system’s outer reaches. Neptune’s largest moon, Triton, is assumed to have formed in the Kuiper Belt and was propelled into the inner Solar System, where Neptune caught it (resulting in the loss of Neptune’s initial satellites).

This artwork shows a rocky planet being bombarded by comets. Credit: NASA/JPL-Caltech

Furthermore, gravitational interactions with other star systems are responsible for the existence of long-period comets, which travel through the inner Solar System on a regular basis after being expelled out of the Oort Cloud. The thought that a close flyby could send multiple comets our way (or larger things such as a planetoid) sounds like a disaster scenario! But, as Douglas Adams famously advised, “Don’t Panic!” Not only do stellar flybys occur on a regular basis, but they are often light years away and have little effect on the Solar System.

Furthermore, new observations by missions such as the ESA’s Gaia Observatory have supplied the most precise information on the correct movements and velocities of stars in the Milky Way Galaxy. As Brown highlighted, this includes data on upcoming flybys and how close they will pass by our system:

“Two notable stars are HD 7977, which may have passed within 3,000 AU (0.0457 light-years) of the Sun some 2.5 million years ago, and Gliese 710 (or HIP 89825), which is expected to pass within about 10,000 AU (0.1696 light-years) of the Sun in about 1.3 million years from now. Doing some rough calculations, both of these stars will have no appreciable effect to the evolution of the Solar System.”

Furthermore, a lot will happen between now and then, and it is quite unlikely that humanity will be around to witness such an occurrence. Assuming we have not driven ourselves to extinction or left Earth to explore other parts of the galaxy, planet Earth will cease to be livable long before then. “Considering the Sun will expand and engulf the Earth in about 5 billion years, physically distancing from other stars is not an issue we need to worry about,” Brown informed us.

Further ReadingarXiv

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