A Visiting Star Might Have Reshaped the Outer Solar System

Billions of years ago, the close pass of another star might have sculpted the outer regions of our solar system.

According to a paper in today’s Nature Astronomy, some puzzling peculiarities of the solar system’s most distant worlds, known as trans-Neptunian objects (TNOs), can be explained by a close stellar flyby. Really close: The star would have passed 110 times the distance between Earth and the Sun (110 au). For reference, the closest star to our Sun currently is Proxima Centauri, and it’s more than 2,000 times farther away.

The Nice model of solar system formation (named after the French city where it originated) holds that TNOs’ orbital properties resulted from an early phase when the orbits of the four giant planets were unstable, and they migrated about the solar system. But some mysteries remain.

For example, the discovery of distant Sedna, whose orbit takes it as far as 937 au from the Sun, heralded a population of other objects like it in huge, strongly elongated orbits whose closest points lie way beyond Neptune. The Nice model can’t easily explain these objects. It also can’t explain TNOs in apparently undisturbed orbits as well as those in highly inclined orbits.

Susanne Pfalzner, Amith Govind (both at the Jülich Supercomputing Centre, Germany) and Simon Portegies Zwart (Leiden Observatory, The Netherlands) have now carried out thousands of detailed computer simulations to study the gravitational effects of a close stellar encounter on TNO dynamics. According to the researchers, such a flyby most likely happened in the first 10 million years of the Sun’s history, when it was still part of the star cluster it was born in.

By varying the mass of the star, its flyby distance, and its trajectory’s inclination with respect to the plane of the solar system, the team was able to identify what kind of encounter best explains TNOs’ current orbital properties. A star with 80% of the Sun’s mass passing by the Sun at 110 au at a steep angle of 70° “gives a near-perfect match” to the orbits of outer solar system worlds, they write.

“The simulation also produced a population of TNOs in retrograde orbits,” says Pfalzner. “I was a bit worried at first; only later did I learn that a few retrograde TNOs have indeed been discovered recently.”

In a separate paper due to be published in Astrophysical Journal Letters, Pfalzner and her colleagues even argue that dozens of small, retrograde moons that orbit the giant planets may actually be TNOs, scattered inward by the stellar flyby.

However, Alessandro Morbidelli (Collège de France), one of the originators of the Nice model, is “not impressed” by the new results. “The structure of the outer solar system is not so mysterious as the authors pretend,” he says. “The Sedna population is consistent with that expected from [objects] scattered from the giant planet region while the Sun was still embedded in a cluster. All the rest […] are consistent with the giant planet instability scenario,” which presumably happened much later.

But recent results from the Japanese Subaru telescope on Mauna Kea, Hawai‘i, and from NASA’s New Horizons mission seem to bolster the stellar flyby scenario. The computer simulations predict that distant TNOs are relatively common. And indeed, Subaru has uncovered more TNOs than expected at large distances from the Sun, a finding supported by readings from the dust counter on New Horizons.

According to Pfalzner, the future Vera C. Rubin Observatory will provide an observational test of the flyby theory. As it discovers many more TNOs, including ones on distant and retrograde orbits, Rubin’s Legacy Survey of Space and Time will hopefully reveal the true origin of the various dynamical groups in the Kuiper Belt. As the team writes in their paper, “[the results] presented here can only be a first step.”