The fabled Planet Nine — or Planet X, if you will — may be a former “rouge planet” that our sun captured long back. This is a theory put forward by astronomers James Vesper and Paul Mason from New Mexico State University during a conference at the recently concluded meeting of the American Astronomical Society in Grapevine, Texas.

“Rogue, or free-floating, planets may be abundant in the galaxy. Several have been observed in the solar neighborhood. They have been predicted to even outnumber stars by a large fraction, and may partially account for dark matter in the disk of the galaxy, as a result of circumbinary planet formation,” the researchers said in a statement. “We speculate that if rouge planets are abundant as predicted, Planet 9 may be a captured rogue.”

According to a NASA estimate, there may be up to twice as many free-floating Jupiter-mass planets as stars — a number that adds up to hundreds of billions of rogue planets in the Milky Way alone.

Planet Nine, if it exists, is believed to be several times more massive than Earth and is locked in an extremely elliptical orbit around the sun — one that, at its closest point, is at least 200 times farther away from the sun than Earth. A recent study even stated that this planet, whose existence was hypothesized last year by Konstantin Batygin and Mike Brown from the California Institute of Technology, may be responsible for giving the orbits of the planets in our solar system their inexplicable tilt with respect to the solar equator.

In order to reach their conclusions, Vesper and Mason ran 156 simulations of possible encounters between our sun and rogue planets. They found that in 60 percent of such encounters, the planet would be ejected from the solar system, but in the remaining 40 percent, it would be captured by our star’s gravity — in some cases, even kicking out one or more of the native planets.

“It is certainly plausible that Planet 9 is captured object,” Batygin told Gizmodo. “Without knowing the precise orbit, it is difficult to decisively confirm or refute rogue capture as Planet 9’s origin story, but it’s certainly possible.”

However, until we gather definitive evidence for the existence of Planet Nine — which some say may happen as early as next year — the rogue planet hypothesis would remain just one of the many possible origin stories being mulled over by astronomers. Another explanation for the planet’s putative orbit states that the planet may have once had a tight orbit around the sun, but was later booted out to where it is now due to Jupiter’s and Saturn’s gravitational influence.