According to the textbook version of Solar System formation, planets should orbit the Sun in elliptical orbits, all lined up in the same plane. Jupiter, Saturn, Uranus, and Neptune don't follow this script. Their orbits are a little more elliptical and tilted relative to each other, not dramatically, but enough to puzzle astronomers for decades. Standard formation models predict the giant planets emerged from the protoplanetary disk on the same plane as the rest of the planets. Instead, something seems to have pushed them off course.
Atacama Large Millimeter Array image of HL Tauri showing its protoplanetary disk (Credit : ALMA)
A new study by researchers Garett Brown, Hanno Rein, and Renu Malhotra proposes a provocative answer. Billions of years ago, an interstellar intruder passed through our Solar System and gravitationally shoved the giant planets into their current configuration. Not a star, but something called a substellar object, a rogue planet or brown dwarf between two and fifty times Jupiter's mass, wandering the Galaxy without a stellar anchor.
The researchers ran 50,000 computer simulations spanning 20 million years each, varying the intruder's mass, speed, and trajectory. Most produced solar systems nothing like ours. But in roughly one percent of simulations, a single close encounter reproduced the orbital characteristics astronomers observe today. The winning scenario involved an object about eight times Jupiter's mass swooping within 1.7 astronomical units of the Sun, barely beyond Mars's current orbit, at a velocity between one and three kilometres per second.
That's remarkably close for such a massive visitor. The gravitational disturbance during this flyby would have excited the giant planets' eccentricities and tilted their orbital planes, nudging them from idealised circles into the slightly wonky paths they follow now. The researchers estimate roughly a one in 9,000 chance that such an encounter occurred during the Solar System's residence in its birth cluster, when stars were packed more densely and close passes were more common.
Artist impression of a brown dwarf. Such an object may have been responsible for the adjustment of the orbits of the outer planets (Credit : NASA/JPL-Caltech)
Previous theories attributed the planets' orbital quirks to internal dynamics; resonances between planets, migration through the protoplanetary disk, or gravitational interactions that played out over millions of years. These mechanisms can certainly alter orbits, but they struggle to explain the specific pattern of eccentricities and inclinations observed. The flyby hypothesis offers a cleaner explanation, one dramatic event rather than a complicated sequence of internal adjustments.
Importantly, the simulations also included Earth and the other terrestrial planets. The flybys that successfully reproduced the giant planets' orbits left the inner solar system largely intact. Rocky planets survived the encounter and acquired orbital characteristics similar to what we observe, suggesting Earth's habitability wasn't compromised by this ancient near miss.
The findings carry implications beyond our solar system. Substellar objects appear relatively common in the Galaxy, rogue planets and brown dwarfs untethered to stars, drifting through interstellar space. If such encounters can reshape planetary architectures, then the diversity of exoplanet systems discovered in recent years might partly reflect similar close calls with passing wanderers.
The research doesn't dismiss internal perturbations entirely. Brown, Rein, and Malhotra acknowledge that a combination of internal and external influences likely shaped the Solar System's final form. But their simulations demonstrate that a single substellar flyby provides a likely, efficient mechanism for generating what we see today, perhaps just a coincidence that left permanent fingerprints on our planetary neighbourhood.
Source : A substellar flyby that shaped the orbits of the giant planets
