I have a prelude to a story that involves a rogue planet entering the solar system. However, I am concerned about its realism. All of this is supposed to be based on real physics and rules; there's nothing special like magic or 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 Earth's orbit, and Earth is expected to be inhospitable to humans in about a thousand years. Thankfully, this would-be species killer also offers a 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 the Earth's. Its surface gravity is about 5/7 Earth's gravity and atmospheric pressure is about 6 Earth's Atmosphere.

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 for humanity's 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 about the match up later when I make the post asking if/how humanity would win.

  • $\begingroup$ Note: if we here in Worldbuilding can't seem to get that it's the rocky moon, then there is a huge chance that your readers won't get it. I think you're juggling too many balls for people to understand them all. $\endgroup$
    – NomadMaker
    Dec 28, 2020 at 19:00
  • $\begingroup$ @NomadMaker lol good point. I intend to make this expressly clear in a less technical setting. The ice giant is supposed to get flung into an orbit among the other outer planets, while the moon is meant to be thrown into the asteroid belt, capture Ceres, and become habitable. Naming the objects will help a lot as well once I figure out how human culture would name them. This is just a prelude idea, I haven't made the rest of the story yet. $\endgroup$ Dec 28, 2020 at 19:22
  • $\begingroup$ It will take eons for tidal forces to melt the core of that moon/planet to be. The energy will have to come out of Ceres' orbit, i.e. it's potential energy. And an object that is already mostly cold does not deform well to begin with, so it's probably never going to happen. $\endgroup$
    – Karl
    Dec 28, 2020 at 19:56

3 Answers 3


Lets look at this step by step:

  • Flyby with Jupiter to capture into the solar system: Totally believable.

    The rogue planet must pass in front of Jupiter, and it will make its orbit elliptical.

  • Tearing away the moon: Totally believable.

  • Encounter of the moon with earth: Totally believable.

  • Destruction of earth's climate: Totally believable.

    The encounter would make earth's orbit elliptical, causing significant variation of sunlight that it receives over the course of a year.

  • Prognosed inhabitability in 1000 years: Not believable.

    If the effects are large enough to force humans off the planet, they won't survive their first year. I believe that we humans would be able to adapt to a vastly different climate over the course of 1000 years, we have enough intelligence for that. But we cannot adapt infinitely fast, developing the technology takes time. I find it hard to believe that there could be a change caused by a single event that would make life impossible in 1000 years when we had all time in the world to develop means to deal with it, while not killing us off within the first 10 years it takes to develop breakthrough technologies.

  • Stabilizing in the asteroid belt: Highly unlikely.

    To stabilize a heavy object in a circular orbit, it needs to interact with lots of material already near that orbit, to get rid of its orbit eccentricity. Also, it would need to get rid of any orbital inclination. Wikipedia says about the asteroid belt:

    The total mass of the asteroid belt is estimated to be 2.39×1021 kilograms, which is just 3% of the mass of the Moon.

    That's not nearly enough to circularize the orbit of an ice giant.

I don't think that it makes any sense to look much farther. Especially considering that I find it highly unlikely that a thawed ice giant could turn into a habitable world: You absolutely need a rocky surface for habitability, and it would take much longer than 1000 years to boil enough water off the planet to turn the remaining atmosphere into something breathable.

I think, you need to change some core parts of your story. Sorry.

  • $\begingroup$ A bit handwavey, and need some extremely unlikely coincidences, but possible maybe something like this. The new alien planet first enters a highly elliptical orbit (mercury-jupiter distances) around the sun. The close sun passes provide rapid heating for surface thawing. But its orbit will intersect earth's orbit soon (small number of years to prepare), After encountering Earth, earth gets ejected to extreme orbit and new planet settles in close to Earth's old orbit, maybe a bit further out.... I think that will satisfy physics(if not probability), and should work well with the plot. $\endgroup$
    – PcMan
    Dec 28, 2020 at 10:14
  • $\begingroup$ @PcMan Yes, something like that could work. It would require the ice giant to pass very close to the sun to provide sufficient heating (I'd go well below Mercury orbit where the interaction with the solar wind would also lower the perihelion slowly), causing it to loose such tremendous amounts of mass that its mass drops to somewhere near earth mass. The encounter with earth would then basically swap the orbits of the two bodies (parabolic encounter trajectory). After that, the old earth would need to be removed from the system, either by crashing it into Jupiter or the Sun. $\endgroup$ Dec 28, 2020 at 10:53
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    $\begingroup$ @Karl The orbit would still be quite elliptic, with an apogee somewhere between the orbits of the former Mars and Jupiter (i.e. somewhere in the asteroid belt, the heavier the former ice giant, the further out, because Jupiter is where the former giant is coming from), and the perigee no further out than Mars (because that's where the collision happens). Not exactly the recipe for habitability. Plus, it would take many thousands of years for the surface to reform a crust after such a cataclysmic collision. A lava planet is not exactly the recipe for habitability either. $\endgroup$ Dec 28, 2020 at 14:52
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    $\begingroup$ Yea, if anything, the moon of that ice giant could be(come) habitable. If it survives the crash of Mars into its mother planet, and the following outgassing. It'd need a Minineptune with Supertriton companion, and some highly efficient greenhouse effect. $\endgroup$
    – Karl
    Dec 28, 2020 at 15:20
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    $\begingroup$ @InfiniteDelta Even if it's "just" the rocky moon that is supposed to stabilize in the asteroid belt, the available mass in the belt is just way too little to circularize its orbit. Remember, there's only about 3% the mass of our own moon available, which by itself is already only 1.2% the mass of our home planet. You said, you want the rocky moon to be heavy enough to kick earth off its orbit, so it must be about the same weight. Consequently, you cannot have it circularize in the asteroid belt. You could make it swap places with Mars, though. $\endgroup$ Dec 28, 2020 at 19:38

At a first glance it looks problematic, since you state

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.

If this ice giant exchanges momentum with Jupiter and Saturn, they will be somehow displaced out of their current orbits, since we are not talking about a gravity assist to something flimsy as a Voyager space probe.

With Jupiter and Saturn out of their current orbits, I think you can surely forget about Trojans in the asteroid belt (I assume you were placing your moon there, as they are less bothered by the large neighbor), and expect some serious havoc in the inner solar system too.

  • $\begingroup$ I don't mind this. Earth is already going to be inhospitable, as long as the planet (now in the asteroid belt) is habitable. Asteroids falling from the sky was already expected. Its not a instant mass extinction event as many people believe. As for the movement of Jupiter & Saturn, I'd like some clarification on which moon you're referring to. The Ice giants former moon or Ceres? $\endgroup$ Dec 28, 2020 at 8:19

You have some big problems.

Lets start with orbital mechanics 101: Your orbit includes the point where you last changed your orbit. Your last major encounter is with Earth, therefore the orbit of the moon intersects Earth's orbit (that doesn't mean it encounters Earth again, merely that distance from the sun.) There's no way the asteroid belt can drag it's orbit out there, there simply isn't enough mass out there to catch it.

Note that the situation is a bit better if you send the moon into the asteroid belt and let the ice giant do the flyby of Earth but even then you can't circularize well enough.

Second, orbital mechanics again: Whatever you do to Earth's orbit is done. After the flyby you can't change it other than by some other encounter and to get another encounter you must have flung it's orbit to reach either Venus or Mars, either one of which is going to cause some pretty drastic climate effects immediately.

Third: My gut says that flinging Earth that far isn't an option, but I haven't worked it out. Two factors:

First, a sufficiently close flyby results in the destruction of the world with the smaller Roche limit (generally this means the lesser world but density enters into the picture--a rocky world can destroy a somewhat greater ice or gas world.)

Second, even if you don't cross the Roche limit that doesn't mean you escape unscathed. The Roche limit is the point where the tide destroys a world, it will fling the oceans about well beyond that point.

I don't believe you can get a flyby close enough to produce that sort of orbital change while staying beyond the point both worlds are scoured clean of life.


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