RESEARCH HIGHLIGHTS

When we think of our solar system, we usually assume that it ends at the outermost known planet - Neptune. However, researchers note that there are several thousand celestial bodies that move beyond Neptune’s orbit, with potentially tens of thousands of smaller objects that have diameters slightly more than 100 kilometers. According to Jülich Supercomputing Centre’s (JSC’s)  Prof. Susanne Pfalzner, these objects do orbit our sun, just not the same way as other objects in our solar system. 

"Surprisingly, many of these so-called trans-Neptunian objects (TNOs) move on eccentric orbits that are inclined relative to the common orbital plane of the planets in the solar system," she said. Up to now, these irregular, inclined orbits have remained an unsolved mystery.  

Recently, Pfalzer, PhD student Amith Govind, and Prof. Simon Portegies Zwart from the Netherlands’ Leiden University employed JSC’s JUWELS supercomputer to run more than 3,000 computer simulations to investigate these orbits and made a striking discovery: The team found that a close flyby of another star could explain why these trans-Neptunian celestial bodies orbit our sun in the manner that they do today. The team published its results in Nature Astronomy and Astrophysical Journal Letters. 

Simulations chart the way

The most common theory for the behavior of our solar system comes from the so-called “Nice model,” named after Nice, France, where the theory was developed. The model proposes that planets in the solar system started off much more compact and slowly drifted outward from the sun. The Nice model suggests that TNOs formed in between larger planets but were ejected further out into the system as the planets themselves moved away from the sun. Pfalzner noted that while this model would be possible to explain TNOs closer to the solar system, it does not account for extremely distant TNOs—such as the dwarf planet Sedna—nor would it explain why many of these objects have orbits that are nearly perpendicular to the planetary orbits in our solar system.  

To better understand how TNOs came into their distant, irregular orbits, the researchers had to run a host of simulations that could both explain how a star passed by our solar system and the exact trajectories of the TNOs that were influenced by it. “We needed to run 3,000 simulations in order to try out different masses of the star, different distances of approach, and different orientations of the flyby, all of which require significant computational resources,” Pfalzer said. “After we found the best match for the flyby compared to observations, we then had to investigate whether the paths of individual TNOs changes after the flyby in the long-term, which is even more computationally intensive.” Pfalzner noted that having a GPU-based system like JUWELS was essential for the team to efficiently run such a large volume of precise, high-resolution simulations. 

The team feels confident that simulations provided great details about the star that kicked off this TNO motion. "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," Govind explained. "This star flew past our sun at a distance of about 16.5 billion kilometers. That's about 110 times the distance between Earth and the Sun."

HPC helps chart new research avenues

The team’s findings not only helped explain how distant TNOs came into our Sun’s orbit, but also provides a hypothesis for closer phenomena. Pfalzner, Govind, and JSC researcher Frank Wagner closely observed their simulation sets and noticed that some of the TNOs were actually hurled into our solar system after the star flyby into the region hosting the giant outer planets Jupiter, Saturn, Uranus, and Neptune. 

"Some of these objects could have been captured as moons by the giant planets," says Simon Portegies Zwart. "This would explain why the outer planets of our solar system have two different types of moons." In contrast to the regular moons, which orbit close to the planet on circular orbits, the irregular moons orbit the planet at a greater distance on inclined, elongated orbits. Until now, there was no explanation for this phenomenon. "The beauty of this model lies in its simplicity," says Pfalzner. "It answers several open questions about our solar system with just one single cause."

Pfalzner also indicated that without access to world-class, public HPC infrastructure and expertise, researchers could not make these kinds of advancements. She pointed out that end-to-end support on the system, including the use of visualization resources and support in porting and scaling applications, is standard support at a facility like JSC. She also indicated that the team made its results publicly available so that other research teams could test and, hopefully, reproduce the team’s results. “Considering the large computational effort involved, it should become a common aim for all researchers to make their results data available to other scientists,” she said. 

-Regine Panknin

The original version of this article appeared on the Forschungszentrum Jülich website.

Funding for JUWELS was provided by the Ministry of Culture and Research of the State of North Rhine-Westphalia and the German Federal Ministry of Education and Research through the Gauss Centre for Supercomputing (GCS).