An object comparable in size to a planet that potentially visited the solar system might have irrevocably altered our celestial vicinity by modifying the orbits of the four outer planets, according to a recent study. These results could provide insights into the unusual characteristics of these planets’ trajectories.
For many years, astronomers have questioned how the planets of the solar system originated. Nevertheless, the majority of theories concur on the manner in which the planets should orbit: in circles that are organized concentrically around the sun and exist in a shared plane. (Viewed edge-on, they would appear as a single line.) Despite this, none of the eight planets, including Earth, possess perfectly circular orbits. Furthermore, the paths of these planets do not align exactly on the same plane.
In contrast to Mercury (whose orbit is the most oblong and tilted among our planetary family), the trajectories of the four outer giant planets — Jupiter, Saturn, Uranus, and Neptune — exhibit slight variations from the ideal orbits. However, elucidating these minor inconsistencies has proven difficult, remarked Renu Malhotra, a planetary scientist at the University of Arizona in Tucson and one of the co-authors of the recent study.
“[T]he challenge for theoretical astrophysics has long been determining how the orbits later became out-of-round and tilted from their mean plane by an appropriate amount,” she mentioned in a message to Live Science. While earlier investigations have emphasized how interactions between these planets reshaped their orbits, Malhotra remarked, “these theories do not align with certain significant details of the observed orbits.”
A stellar visitor
In an effort to resolve this enigma, Malhotra and her team evaluated a less-explored theory: that a star-sized passing object modified these planets’ paths around 4 billion years ago.
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Employing computer simulations of the four outer planets, the research group conducted 50,000 simulations of such encounters, each lasting 20 million years, while varying certain aspects of each visitor, including its mass, velocity, and proximity to the sun. The investigators also broadened their exploration compared to prior research by including objects considerably smaller than stars — as small, indeed, as Jupiter. They also examined scenarios with extremely close passes, concentrating on instances where the intruding object came within 20 astronomical units (AU) of the sun. (One AU is about 93 million miles, or 150 million kilometers, which is approximately the average distance from Earth to the sun.)
While most simulations resulted in conditions very different from the present solar system, the researchers discovered that in approximately 1% of the simulations, the passing object adjusted the giant planets’ orbits to closely resemble their current configurations. The intruders in these near matches plunged directly into the solar system, traveling far beyond Uranus’ orbit, and some even brushed against Mercury’s path. Moreover, these intruders were relatively small, ranging from two to 50 times the mass of Jupiter.
“This range encompasses planetary masses to brown dwarf masses,” Malhotra stated. (Brown dwarfs, often referred to as “failed stars,” are peculiar celestial objects that are heavier than planets but not as massive as stars.)
Since many close-matching simulations involved the planet-like object traversing the inner solar system, the team conducted an additional 10,000 simulations that included the terrestrial planets as well. In these instances, too, the flybys that previously modified the giant planets’ orbits to their current states replicated the solar system’s present arrangement.
The simulation yielding the most accurate results featured an object eight times the mass of Jupiter passing as closely as 1.69 AU from the sun. This places it just slightly farther than Mars’ current orbit of 1.5 AU from the sun.
The simulations indicate that a single substellar object’s flyby was adequate to modify the trajectories of the giant planets. Given that observations imply substellar bodies are relatively abundant in the universe, encounters with such objects could be more frequent than flybys of stars.
The study, which has yet to be peer-reviewed, was published in the arXiv preprint archive in December.