Something doesn’t quite add up about the orbits of our solar system’s eight planets and their hundreds of moons, a new study hints.

According to the research — which compared more than 100 simulations of our cosmic neighborhood’s early history — there may have once been two extra giant planets careening around the outer solar system, helping to reshape the orbits of the other planets before ultimately being kicked out into interstellar space.

The infant solar system was constantly being reshuffled. One drastic rearrangement, called the giant planet instability, may have caused the giant planets — Jupiter, Saturn, Uranus and Neptune — to migrate from their initial positions, ending up farther from the sun.

This migration, first proposed in 2005 by astronomers in Nice, France, as a part of the "Nice Model," likely started due to extra-close encounters between the planets and leftover planetary building blocks called planetesimals. Additionally, modifications proposed in 2011 suggested there may have been a fifth giant planet whose gravitational encounters with its neighbors ultimately tossed it out of our cosmic neighborhood.

However, astronomers don't know much about this missing planet. To find out more, the new study "systematically tested the effects of giant-planet close encounters on the orbital stability of their satellites," Matthew Clement, a research scientist at the Johns Hopkins University Applied Physics Laboratory and the study's first author, told Live Science in an email.