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We have an idea of what the solar system's past was like: it was violent and chaotic. However, we are still investigating how violent it was. Current models suggest that at some point after their formation, the giant planets went through a phase of instability so extreme that one or even two bodies the size of Uranus or Neptune were ejected into interstellar space. If this scenario occurs, we could find clues in the most unexpected places in the solar system, such as the moons of Jupiter and, above all, those of Uranus.

A recent article published in Icarus analyzed 122 possible scenarios of such instability to assess the reaction of satellite systems of the "left behind" planets. The researchers concluded that it would be extremely difficult to explain the current characteristics of Uranus' moons without an episode of violent instability. And this type of instability only appears in models where there were more giant planets than we see today.

Most likely, the authors point out, the moons of Uranus have been destabilized at least twice in the past: first by the impact that tilted the planet, then by close encounters between giant planets during the instability. This chaos, fueled by the presence of one or more planets which were then ejected, would have destroyed and rebuilt the lunar system as we see it today.

The solar system and chaos

Jupiter, Saturn, Uranus and Neptune have not always had their current positions in the solar system. According to the planetary instability model, they were born a little closer to the Sun and closer together. After millions of years, they migrated to their current orbits.

But some details of this model do not match observations. On the one hand, the current orbits of Jupiter and Saturn are eccentric, while there are specific structures such as the Kuiper Belt that apparently should have prevented Neptune from moving to its current position. In the simulations, the planets have not reached their current position.

It is therefore possible that at one time the solar system had more planets, and that these were the ones that “pushed the others”. In this hypothesis, the solar system puzzle fits better. The problem is that these bodies, if they existed, disappeared: they were ejected and left no physical trace or fragment. This leaves the idea of missing planets in the realm of hypothesis, until enough evidence is accumulated to confirm it.

The Unusual Moon

The new Icarus study tested the missing planets hypothesis using the moons of Uranus as direct evidence. It used a total of 122 simulations of solar system evolution. In 85% of scenarios, the Uranus lunar system collapsed. Its moons only survived in a handful of scenarios, and in all of them the hypothesis of lost and ejected planets fit very well.

The report points to Miranda, the smallest moon in the major Uranus system. Astronomers consider it the most unusual in the solar system. It is uneven, as if sewn from pieces, too icy for its size and quite small considering the rest of Uranus' moons. It is also geologically active.

Astronomers believe that Miranda is the debris of a larger body. The study reinforces this idea and suggests that this is the clearest example of traces of planetary instability.

This work does not yet solve the mystery of the missing planets, but it shows that the moons could serve as witnesses to the chaos of the solar system. Such data, along with other independent data on unusual structures such as Trojans, Jupiter asteroids or the mere presence of the Oort Cloud, will one day tell what happened to these missing bodies, if they actually existed.

On the other hand, a mission dedicated to Uranus, like the one being discussed by NASA and ESA for the 2040s, could confirm whether Miranda is indeed a body rebuilt after the chaos. If this is the case, then moons could be the key to understanding how many worlds the solar system actually has.

This story originally appeared in WIRED in Spanish and has been translated from Spanish.