I have a prelude to a story which involves a rogue planet entering the solar system, However I am concerned as to its realism. All of this is supposed to be based on real physics and rules; there's nothing special like magic, insane technology and hopefully I don't require adding anything special to make it realistic. Here's how it goes:
While searching for planet nine, scientists eventually detect something. However on closer analysis, this is no solar object. This is a rogue planet; an ice giant with 1 major moon. Worse still, its trajectory takes it on a close flyby of Jupiter, which is expected to tear its moon (about 2/3 the mass of Earth) out of orbit and into a flyby trajectory of Earth! The object does so, and although it does not collide with the planet, it does irreparably alter Earths orbit, and Earth is expected to be inhospitable to humans in about a thousand years. Thankfully, this would-be species killer also offers refuge. It eventually settles into a stable orbit in the asteroid belt, capturing Ceres as a close moon and flinging the remaining asteroids all over the place. It also heats up due to an abundance of greenhouse gasses, and with a hot, active core (previously heated by tidal forces) for a magnetic field, Liquid water eventually melts and the planet becomes habitable. The planet (after thawing) has a breathable atmosphere, however it is also much thicker than Earths. Its surface gravity is about 5/7 Earths gravity and atmospheric pressure is about 6 Earth Atmospheres.
Meanwhile the ice giant is flung into an elliptical orbit, with its perihelion at around Mars's orbit, and its Aphelion just outside of Sedna's perihelion. Eventually though, through interacting with the other giant planets, settles into a metastable orbit between Jupiter and Saturn. Although expected to only last a few million years, this keeps it away from humanity in the meantime.
Not all is good, though. As the once rogue moon thaws in the embrace of Sols energy, something once frozen wakes up, and is not happy to see another species trying to take over their planet, regardless of its necessity to humanities survival.
NOTE: A lot of people seem to think the Ice Giant is the planet that becomes habitable. This is not true; The MOON of the Ice Giant is what stabilizes in the asteroid belt and becomes habitable.
The story is, obviously, about the incoming war between Humanity, driven by the simple need to survive, and the aliens, driven by self defense. Here's a rundown of the aliens. If there's somthing that'd render humanities war completely hopeless or the survival of the aliens impossible, let me know as I'm concerned for that realism too:
The aliens have 300 years more advanced tech than humanity and survived the rogue phase by doing some alterations to their genetics so they could hibernate for the millions of years needed. They did the same alterations to important plants/animals in their ecology as well, specifically their own food chain. But most of the rest of life; like bacteria and viruses; died in the rogue phase, except for some arctic microbes and deep sea life, neither of which I think should pose much of a biological threat. I'll go into more detail of the match up later when I make the post asking if/how humanity would win.