An international team of astronomers has concluded that the early Solar System likely contained another ice giant that was eventually ejected into interstellar space. The gravitational influence of this “lost” planet may have dramatically altered the orbits of Uranus’s moons and left a lasting imprint on the modern architecture of the Solar System.

The findings were published in the journal Icarus.

What the Simulations Revealed

The researchers analyzed more than 1,000 computer simulations of Solar System formation and selected 122 of the most accurate models—those that best reproduced the current arrangement of the planets.

The conclusion was surprising: the probability of preserving the present-day moon systems of Jupiter and Uranus without the involvement of an additional large planet was extremely low, less than 15%. In only one of the examined scenarios did the moons survive a period of intense dynamical instability.

The effects were particularly dramatic for Uranus. When the hypothetical fifth giant planet passed close to it, its gravitational pull disrupted the orbits of Uranus’s moons, triggering numerous collisions. Rather than being scattered into space, the resulting debris formed a vast icy disk from which a new generation of moons later emerged.

The Nice Model and the “Lost” Planet

According to the widely accepted Nice model, about 4–4.5 billion years ago the orbits of the outer planets were unstable. Jupiter, Saturn, Uranus, and Neptune repeatedly interacted, exchanging energy and altering their trajectories. Adding another ice giant—similar to Uranus or Neptune—to these simulations significantly improves the ability to explain the Solar System’s current configuration and the survival of its moon systems.

If the hypothesis is correct, one of the Solar System’s largest planets was ejected into interstellar space billions of years ago and is now wandering somewhere within the Galaxy.

In Brief

Astronomers suggest that the early Solar System may have contained a fifth ice giant that was later expelled beyond its boundaries. Its gravitational influence could have triggered large-scale collisions among Uranus’s moons and helped shape their current orbits. Computer simulations indicate that without this “lost” planet, the likelihood of preserving the present-day satellite systems is extremely low. The new study provides additional insight into the turbulent history of Solar System formation.