Interstellar Visitor May Have Scrambled Our Solar System

Theorbits of Jupiter, Saturn, Uranus and Neptune may have been altered by the fly-by of an enormous object from deep space, billions of years ago.

According to a pre-print research paper, which is yet to be peer-reviewed, an object 8 times the mass of Jupiter—which itself is 318 times greater than Earth's mass—may have swept through our solar system about 4 billion years in the past.

This may explain the strange properties of the orbits of our solar system's planets, which are not quite perfectly circular, and all lie on slightly different planes.

"[T]he puzzle for theoretical astrophysics has long been to figure out how the orbits later became out-of-round and tilted from their mean plane by not too much and not too little," paper co-author Renu Malhotra, a planetary scientist at the University of Arizona in Tucson, told Live Science.

In the paper, the researchers describe how they modeled the orbits of the four outer planets and performed 50,000 simulations of a large object passing through the solar system to determine if such a cosmic visitor may be responsible for the discrepancies in the planets' orbits.

While the majority of these simulations resulted in a very different solar system to the one we know, around one percent resulted in the planets orbiting in a very similar manner to how they do today.

In these simulations, the orbits were warped by the passing of an object between two and 50 times Jupiter's mass, which plunged deep into the inner solar system as it traveled.

"This [mass] range includes planetary masses to brown dwarf masses," Malhotra said.

Brown dwarfs are astronomical objects that are larger than planets but smaller than stars. They are sometimes referred to as "failed stars" because they lack sufficient mass to sustain the nuclear fusion of hydrogen in their cores. Brown dwarfs typically have masses between about 15 and 75 times the mass of Jupiter

The researchers performed even more simulations, this time including the orbits of the inner planets (Mercury, Venus, Earth and Mars), and found that the most likely scenario involved an object eight times the mass of Jupiter zipping nearly as close to the sun as Mars's current orbit.

This study suggests that the currently slightly strange orbits of the outer planets may have been a result of just one visit by an object like this, which may be more of a frequent occurrence than the fly-by of a star.

"Our numerical simulations indicate that there is approximately a 1-in-10³ to 1-in-10⁴ chance of the necessary encounter parameters being realized with random encounters within an open star cluster having properties expected for where the solar system formed," the researchers wrote in the paper.

"Given that the estimated population of sun-like stars in the Galaxy is on the order of 10¹⁰ and that stars are commonly formed in open star clusters, the 1-in-10⁴ chance is not negligible. In other words, we don't need to look for a needle in a haystack to find a suitable encounter."

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Reference

Brown, G., Malhotra, R., & Rein, H. (2024). A substellar flyby that shaped the orbits of the giant planets. arXiv. https://doi.org/10.48550/arXiv.2412.04583