Currently there are four giant planets in our solar system, but scientists think there may have once been a fifth.

A team of astrophysicists in Toronto have published new research in the Astronomical Journal that suggests that there was a collision with Jupiter 4 billion years ago that resulted in the ejection of an ancient ice giant planet from our solar system, according to a Huffington Post report.

To come to their conclusions, the researchers used computer simulations of the four giant planets in our solar systems as well as their moons. Using a process of elimination, the research team was able to determine how the moons would act had their been a fifth gas giant that was ejected billions of years ago. Jupiter was the only planet that matched the description they were looking for — Saturn was ruled out because it would have thrown the orbit of its moon Iapetus off course, while Jupiter’s moon Callisto would have retained its orbit.

The research team thinks it was an ice giant similar to Uranus or Neptune that was the fifth giant, and at one point it would have been part of our solar system. In fact, a different team of researchers earlier this year surmised that it may have smashed into Neptune as well before it left the solar system.

The findings are important not just for curiosity’s sake, but also because it could better inform scientists on how the solar system evolved, and also how other solar systems in other galaxies may have evolved. It raises an interesting question: is this solar system a unique one not found anywhere else, or is this a typical way for a solar system to evolve?