USA: Color mysteries of trans-Neptunian objects reveal ancient passage of a star past the Solar System

A new hypothesis from scientists at the Julich Center for Supercomputing sheds light on one of the most mysterious features of the outer Solar system — the heterogeneous distribution of colors and orbits of trans-Neptunian objects (TNOs). According to the results of numerical modeling, in the first millions of years after the formation of the Solar System, a star could pass by it at a distance of about 110 astronomical units, which triggered structural changes in the Kuiper Belt and the colored mosaic of its objects.

Modern observations have shown that TNOs-bodies located beyond the orbit of Neptune-have very different orbital parameters and spectral slopes. Some move in elongated, tilted orbits, while others maintain almost circular trajectories. The surfaces of these objects vary from rich red to gray-blue shades. However, neither the distance to the Sun nor the mass of the object explains such a sharp color dispersion.

Modeling suggests that in the primary gas-dust disk, the color of TNOs depended on their initial position: objects that formed closer to the Sun had bright red surfaces, and those that were further away had gray ones. The passage of the star led to a redistribution of bodies: red bodies remained in stable, low-sloping orbits, while gray ones were thrown out on more elongated and inclined trajectories.

Astronomical surveys by OSSOS and DES have found that objects with an orbital inclination of more than 21 degrees and an eccentricity above 0.42 are almost never red. But the so-called "cold" Kuiper Belt, where the orbits are close to circular and lie in the ecliptic plane, mainly consists of bright red objects, which directly corresponds to the model of stellar perturbation.

Moreover, scientists predict that among distant TNOs with perihelions of more than 60 AU and retrograde motion, red objects are practically not found. These hypotheses may be confirmed in the coming years with the launch of the Vera Rubin Observatory (LSST), which is expected to detect tens of thousands of new bodies beyond the orbit of Neptune.

Interestingly, the proposed model explains not only the distribution of TNOs, but also the colors of unstable satellites of giant planets, which are assumed to have been captured from the outer part of the disk. There are also no red objects found among them, which confirms the idea that these bodies formed in more remote, colder regions.

In contrast to the theory of unstable planetary migration, which requires numerous refinements and parameters, the star transit scenario allows us to explain both the dynamics of orbits and the spectral characteristics of objects without introducing additional factors. This reinforces the hypothesis that not only internal processes, but also external gravitational influences — in particular, a close flyby of a neighboring star in the early history of the Solar System-played a key role in shaping the external architecture of the Solar System.