The early Solar System may have been far more crowded than it is today. New simulations suggest that two additional giant planets could once have formed alongside the known giants before being flung into interstellar space.
That possibility comes from computer models designed to replay the Solar System’s development from billions of years ago. Researchers ran more than 100 simulations and published the results in Icarus, looking for an early configuration that could match the Solar System seen now.
A larger family of giant planets
The study points to a youthful Solar System that likely included one or two extra planets in addition to Jupiter, Saturn, Uranus, and Neptunus. Those lost worlds are thought to have been comparable in size to Neptunus, or to have fallen into the category of super-Earths and ice giants heavier than Earth.
In many of the strongest simulations, the early Solar System began with five giant planets. About 40 percent of the best-performing runs required six giant planets before the system evolved into its present state.
How two planets may have disappeared
In the six-planet scenario, the extra bodies were destabilized by powerful gravitational encounters. Their interactions with Jupiter, Saturn, Uranus, and Neptunus could have gradually pushed their orbits into chaos until they escaped into interstellar space.
That upheaval likely happened hundreds of millions of years after the Sun formed, when the architecture of the Solar System was still shifting. The violent phase may have sounded destructive, yet it may also have helped shape the stable arrangement that remains today.
Clues from Jupiter and Uranus
The simulations also offered a possible explanation for the survival of satellite systems around Jupiter and Uranus. The researchers found that two extra planets could help account for why those moon systems remained intact.
When only one additional planet was included, some of Uranus’s moons became unstable. With two extra planets in the model, the orbits of the main satellites were preserved more effectively.
Why the finding matters
The research strengthens the view that the young Solar System was much more chaotic than it appears now. It also fits the broader idea that planet ejection may be a common part of how many planetary systems evolve across the universe.
The two missing planets have not been observed directly, but the latest simulations suggest the idea deserves serious consideration. Their possible existence could help explain several unusual features of the Solar System, from orbital stability to the endurance of satellite systems around giant planets.