We have idea of the solar system’s past: it was full of violence and chaos. However, we are still investigating how brutal this event was. Current models suggest that at some point after the formation of the giant planets, they underwent a phase of such extreme instability that one or even two bodies the size of Uranus or Neptune were ejected into interstellar space. If such a scenario comes true, we could find clues in the most unexpected places in the solar system, such as the moons of Jupiter and especially Uranus.
A recent article published in Icarus analyzed 122 possible scenarios for such instability to assess how the satellite systems of “left behind” planets would respond. The scientists concluded that it would be extremely arduous to explain the current characteristics of Uranus’s moons without some episode of violent instability. This type of instability only appears in models where there were more giant planets than we see today.
The authors indicate that Uranus’s moons were most likely destabilized at least twice in the past: first by an impact that tilted the planet, and then by a close encounter between the giant planets during a time of instability. This chaos, fueled by the presence of one or more planets that were later ejected, would destroy and rebuild the moon system to the state we see 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 slightly closer to the Sun and closer to each other. After millions of years, they migrated towards their current orbits.
However, there are details of this model that do not agree with observations. First, 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 did not get to where they are today.
It is therefore possible that the Solar System at one point had more planets and they “pushed out the others.” According to this hypothesis, the puzzle of the solar system fits better. The problem is that these bodies, if they existed, are gone – they were thrown away and left no physical traces or fragments. This leaves the concept of lost planets as a hypothesis awaiting sufficient evidence to be confirmed.
Extraordinary Moon
Modern Icarus the study tested the missing planets hypothesis using Uranus’s moons as direct evidence. A total of 122 simulations of the evolution of the Solar System were used. In 85 percent of scenarios, the Uranus lunar system collapses. In only a few scenarios did its moons survive, and in all of them the lost and ejected planets hypothesis fit very well.
The report points to Miranda, the smallest moon in the main Uranus system. Astronomers believe it is the most unusual object in the solar system. It’s patchy, as if sewn together from scraps, too icy for its size, and quite miniature compared to Uranus’ other moons. It is also geologically vigorous.
Astronomers believe Miranda is the remnant of a larger body. The study confirms this thesis and suggests that this is the clearest example of traces of planetary instability.