We think that our solar system lost at least one large planet in the past. Besides that, we thought we observed a planet disappear in the Fomalhaut system. Could it be evidence of extraterrestrial star wars?
Planets form in protoplanetary disks, and in their youth, many collisions and orbital instabilities can eject them, cause them to collide, or cause them to fall into stars and be destroyed. We think that our system hosted one more large planet located either between Saturn and Uranus or between Uranus and Neptune, and might have been similar in mass to Uranus and Neptune, making it an ice planet.
Robust evidence for its existence comes from computer simulations of the evolution of the orbits of the four largest planets. They don’t reproduce their current position unless a fifth world is added and ejected.
Furthermore, this banishment might have saved the inner four planets, including Earth, from instability caused by Jupiter's slow outward migration. When the fifth planet is added, its ejection propels Jupiter rapidly outwards, saving Earth, Venus, Mercury, and Mars from a sorry fate. Additionally, it sends Neptune moving outwards, and it can then capture a trans-Neptunian object, such as its moon Triton, which is thought to have been an independent dwarf planet.
It orbits Neptune in the opposite direction to other moons, and its destiny is sealed. It will fall into Neptune in about 3.5 billion years and be destroyed.
The ejection of this hypothetical fifth large planet probably occurred 3.9 to 4.1 billion years ago, and it coincides with the late heavy bombardment of Earth by asteroids, as seen in the geological record on our planet and even on the Moon and other worlds of our system. This might be additional evidence that something was going on at the time, and asteroids in the Kuiper belt and elsewhere were disrupted.
This lost planet is now a rogue world that jets through our galaxy alone without a star, probably very far away, because our system is also thought to have moved about 16,000 light-years from the place where it was born, closer to the galactic center, in the last 4.5 billion years.
Fomalhaut is one of the brightest stars in the sky, but it is a triple star system. Its main component is almost twice as massive as the Sun, an A-type star 440 million years old. This means that it's already in a quarter to a third of its lifespan, which is 1.5 to 2 billion years.
Since this system is as young as the solar system was at the time of the ejection of the fifth large gas planet, it's also as unstable, and we see absolutely vast debris disks there. We thought we had identified a planet in it, but it turns out we witnessed, for the first time, a collision of large asteroids that left a cloud of dust so large it mimicked a planet, which eventually disappeared as the dust dispersed.
We also need to update our models of such debris disks because, within a few years, we witnessed another asteroid collision in Fomalhaut. According to models, they shouldn’t happen more often than once every tens to hundreds of thousands of years, so either the models are wrong, or there is an extraterrestrial star war there.
Just joking. This system is too young to host advanced civilizations. It took almost 4 billion years in the solar system for multicellular life to emerge, and humans appeared even later.