A Stellar Flyby Jumbled Up the Outer Solar System

An ancient passerby may have visited the Sun and inadvertently helped shape the Solar System into what it is today. It happened billions of years ago when a stellar drifter came to within 110 astronomical units (AU) of our Sun. The effects were long-lasting and we can see evidence of the visitor’s fleeting encounter throughout the Solar System.

Neptune is the outermost planet in the Solar System, and by a simple definition, that can mark the edge of the Solar System. There’s an entire realm of other objects beyond Neptune called the Kuiper Belt. It’s the home of Pluto, most of the dwarf planets, and some comets. Astronomers aren’t certain how large the Kuiper Belt population is, but it could contain tens of thousands of objects larger than 100 km in diameter.

Some of these objects have unusual orbits and are called Trans-Neptunian objects (TNO). In new research, a team of astronomers suggest that these orbits, and some other evidence in the Solar System, support the idea that another star passed by our Solar System and drove these objects into their current orbits. The star may have disturbed some objects so strongly that they were driven into the inner Solar System and took up residence as moons around the giant planets.

These results are in two new papers. One is published in the journal Nature and is titled “Trajectory of the Stellar Flyby Shaping the Outer Solar System.” The second is published in the Astrophysical Journal Letters and is titled “Irregular moons possibly injected from the outer solar system by a stellar flyby.” Susanne Pfalzner, the lead author of both, is from Jülich Supercomputing Centre, Forschungszentrum (Research Center) Jülich, Jülich, Germany.

“The beauty of this model lies in its simplicity. It answers several open questions about our solar system with just a single cause.”

Susanne Pfalzner, Jülich Supercomputing Centre, Forschungszentrum Jülich, Germany

While Neptune marks the outermost boundary of planets in our Solar System, an entire population of objects exists beyond it. “However, several thousand celestial bodies are known to move beyond the orbit of Neptune,” said Pfalzner. “Surprisingly, many of these so-called trans-Neptunian objects move on eccentric orbits that are inclined relative to the common orbital plane of the planets in the solar system. “

Pluto is the most well-known TNO because it used to be considered a planet. Its orbit is inclined by 17 degrees relative to the ecliptic, an imaginary plane that Earth follows as it orbits the Sun. In the ecliptic, Earth is considered to orbit the Sun at zero degrees, and none of the other planets are inclined by more than only seven degrees.

Pfalzner and her co-researchers used simulations to try to understand how some objects are inclined. They ran more than 3,000 supercomputer simulations in their effort. They wanted to investigate the idea that a passing star could be responsible, and their work showed that it could.

“Our exhaustive numerical parameter study consists of over 3,000 individual simulations modelling the effect of a stellar flyby on a planetesimal disk surrounding the Sun extending to 150?au and 300?au, respectively,” the authors write in their research.

There are three distinct populations of TNOs:

• the cold Kuiper belt objects moving on nearly circular orbits close to the plane,
• the Sedna-like TNOs orbiting at large distances (rp?>?60?au) on highly eccentric orbits (e?>?0.5),
• TNOs with high inclination (i?>?60°).

Any theory on the formation of the Solar System has to explain these three groups, according to the authors. “While only three Sedna-like objects and few highly inclined TNOs are known so far, they are the make-or-break test for any outer Solar System formation theory,” they write.

This isn’t the first time scientists have wondered if a stellar flyby can explain these puzzling parts of our Solar System. But this question has been dismissed because stellar flybys were thought to be rare. However, as we get more powerful telescopes, we’re discovering that they’re more commonplace. “However, recent Atacama Large Millimeter Array observations reveal that close stellar flybys seem to be relatively common,” the authors write.

The flyby hypothesis has gained renewed interest, but it’s difficult to study because the flyby parameter space is so large, and predictions are vague.

These researchers have made important progress, though, and their simulations can explain a lot.

“Even the orbits of very distant objects can be deduced, such as that of the dwarf planet Sedna in the outermost reaches of the solar system, which was discovered in 2003. And also objects that move in orbits almost perpendicular to the planetary orbits,” Pfalzner said. Sedna has an extremely wide orbit and takes 11,400 years to complete one orbit around the Sun. Its orbit is also wildly eccentric.

According to Pfalzner and her colleagues, a stellar flyby can also explain two Solar System objects with very oddball orbits. 2008 KV42 has a retrograde orbit, meaning it orbits in the opposite direction than the planets. 2011 KT19‘s orbit is tilted 110 degrees, meaning it effectively follows a polar retrograde orbit.

What kind of star could’ve shaped these objects’ orbits?

“The best match for today’s outer solar system that we found with our simulations is a star that was slightly lighter than our Sun – about 0.8 solar masses, “explained Pfalzner’s colleague Amith Govind. “This star flew past our sun at a distance of around 16.5 billion kilometres. That’s about 110 times the distance between Earth and the Sun, a little less than four times the distance of the outermost planet Neptune.”

The irregular moons are one of the Solar System’s puzzles. Everything in the Solar System formed from the solar nebula, which means barring outside influence, everything should share orbital similarities. “The origin of these irregular moons is still an open question, but these moons have a lot in common with the objects beyond Neptune (trans-Neptunian objects—TNOs), suggestive of a common origin,” the authors write.

The passing star could’ve disrupted distant objects and sent them careening into the inner Solar System, where the giant planets captured them into their orbits.

“Some of these objects could have been captured by the giant planets as moons,” says co-author Simon Portegies Zwart from Leiden University. “This would explain why the outer planets of our solar system have two different types of moons.”

Irregular moons have unusual orbits that can be inclined, “highly elliptical, sometimes retrograde, and sometimes at great distances from their planet. All four giant planets host irregular moons, like Saturn’s Phoebe and Neptune’s Triton. “The beauty of this model lies in its simplicity,” says Pfalzner. “It answers several open questions about our solar system with just a single cause.”

“A stellar flyby can simultaneously reproduce the complex TNO dynamics quantitatively while explaining the origin of the irregular moons and the colour distributions of both populations,” the authors write. Their simulations show that the flyby would’ve sent 7.2% of the TNO population into the inner Solar System. Many of them would’ve followed retrograde orbits, though most would’ve been subsequently ejected from the Solar System, and only a handful were captured by planets.

Could this flyby have impacted the appearance of life? That’s a purely speculative question, but since life is so rare and unexplained, it needs to be asked. It’s possible that some objects disturbed by the flyby crashed into Earth or other planets, possibly delivering prebiotic material and volatiles. At the same time, Earth’s orbit could’ve remained undisturbed. “However, how much prebiotic material originally contained in an injected TNO would survive impact on a terrestrial planet would require further studies,” the authors write.

The simulations were able to explain critical things about the Solar System that are in need of explanations. However, there needs to be more evidence before the work is conclusive.

The team’s predictions may be verified when the Vera Rubin Observatory (VRO)comes online. The VRO is expected to discover around 40,000 TNOs.