Astronomers say a close encounter with a mysterious—and massive—interstellar object early in our solar system’s history could explain the current orbits of several of Earth’s largest planetary neighbors.

The findings, detailed in a new study, point to a visit by a substellar object early in the solar system’s history that might offer astronomers an explanation for the eccentricity and inclination of the orbits of the giant planets Jupiter, Saturn, Uranus, and Neptune. The findings, if confirmed, offer a bold new perspective on planetary formation dynamics.

Specifically, what the research proposes is that at some time in the ancient past, a hyperbolic flyby of the solar system by a substellar object—something comparable in magnitude to a massive brown dwarf—might help account for the observed behavior of some of our solar system’s largest planets.

The theory, put forward by researchers Hanno Rein and Garett Brown of the University of Toronto at Scarborough and Renu Malhotra with the University of Arizona’s Lunar and Planetary Laboratory, challenges current leading theories about the characteristics our solar system’s giant planets display, which largely hold that they result from internal interactions between planets within the solar system.

“The modestly eccentric and non-coplanar orbits of the giant planets pose a challenge to solar system formation theories,” the team writes in a new paper that appeared on the preprint server arXiv.org, “which generally indicate that the giant planets emerged from the protoplanetary disk in nearly perfectly circular and coplanar orbits.”

Taking a very different approach from the traditional protoplanetary disk model, Brown, Malhotra, and Rein demonstrate in their new study “that a single encounter with a 2–50 Jupiter-mass object” could also account for phenomena long observed by astronomers. Such an object, they say, could “excite the giant planets’ eccentricities and mutual inclinations to values comparable to those observed.”

By running numerical simulations, the team found a plausible scenario for this encounter, shedding light on the dynamics of planetary systems both within and beyond our own.

In their study, the team identifies a hypothetical interstellar object that demonstrates a series of specific values. Specifically, this mysterious ancient visitor would have had a mass of close to 8.27 Jupiter masses, and at its closest came to within 1.69 astronomical units (AU) of our early solar system, traveling at a velocity of 2.69 km/s and an inclination of 131 degrees.

Such an encounter, the team says, would be rare, but not infeasible. Bolstering their argument, they point to the conditions present in an open star cluster—in other words, the very sort of environment in which our early solar system likely began to take shape.

The chances of such an event are still low, at around just one in a 10,000 chance. However, the researchers argue that this offers a significant enough probability to warrant consideration.

Notably, the team’s approach differs from past approaches in that it presents a perspective on the solar system’s evolution that incorporates external influences. In the past, most studies have instead focused on isolated interactions between local materials that gave rise to planets over long periods, all of which help to account for the secular architecture of the solar system.

By contrast, the team’s novel approach introduces the hypothetical entry of a substellar mass object—in other words, an object with less mass than a star, but still possessing enough mass to significantly influence the orbit of large planets in our solar system.

In simulating the interactions between such a hypothetical object and our solar system’s planets, the researchers believe they present an efficient mechanism for generating the observed orbital configurations of the giant planets.

Additionally, a key discovery arising from the team’s simulations is that the while the effects of such a close encounter would have been dramatic, to say the least, terrestrial planets would likely be able to survive nonetheless. This, in addition to “acquiring secular mode amplitudes similar to those observed today.”