The solar system is pretty crowded, so it would be quite hard to add an extra planet without disturbing the other planets. It's more or less as full of planets as it could be and keep them in nice stable circular orbits. So it's just as well it stays out there. Indeed there may well have been other planets that got ejected from the solar system, hit the other planets etc. May even have ejected gas giants. What we see may be the ones that are left at the end of that process.
The whole thing quietened down after a few hundred million years. Finally ending after the "late heavy bombardment" about 3.8 billion years ago with some big objects still left for a while after that. But by 3 billion years ago the solar system had quietened down and was much as it is now.
In other words, its opportunity to come into the inner solar system was well over three billion years ago.
If you wanted to do it now, well I think you could try making it into a double planet or a moon of one of the other planets, that would be the best bet. E.g. try to make it a new huge moon of Jupiter, and sneak it in quickly past the orbits of Saturn, Uranus and Neptune. With mega technology perhaps you could do that?? I think that would be more likely to succeed than trying to make it into a new planet in its own right. It wouldn't add that much to the mass of Jupiter, so the combination of Jupiter with an ice giant moon like this might still be a stable solar system (you'd need to do a model to check this).
But that's assuming some vast amount of energy available. There are ideas for shifting Earth's orbit by using an asteroid in flybys of Earth and Jupiter. I don't know if a similar approach could move this planet using e.g. dwarf planets instead of asteroids to move it? The idea was just to move Earth out a little, over billions of years. This would be a far bigger task than that, larger object, much larger change in its orbit.
Also, that's not yet in the habitable zone.
To put it in the habitable zone I think your best bet is to put it in Earth's orbit and then have Earth, Mars, Venus and Mercury all orbiting it. That way you have four planets all in the habitable zone. And though it would be quite a challenge to have those four planets orbiting each other, it should be easier to have them all orbiting a planet much larger than any.
Whether that's a stable solar system or not I don't know. But you could try it out in universe sandbox and see what happens to get a first idea, of whether it is possible.
I wouldn't want to limit what technological future beings on Earth could do, perhaps a million years of future technology development from now. Maybe they would be able to do such things. I think well beyond us, probably, even given millions of years to do it.
Incidentally their hypothesis about where it came from is that it came from closer to the solar system originally, got ejected from the solar system, but because this happened early on when the solar system was still forming, it got slowed down by the gas and dust and ended up in this orbit instead of leaving the solar system already. If that's true (and if the planet exists at all of course) then it actually started off closer to the sun and was ejected and ended up where it is now.
THE NAME "PLANET NINE"
Just to add - it's of course natural to call it "planet nine" as that's what everyone calls it. But I think that's a poor name for a planet X myself. Why not call it "Planet X"?
Or find some new name, like Nemesis and Tyche as was done for previous planet X candidates. None of them were called Planet + some number.
There is no way that we can know it will be planet 9. What if other planets are found closer to the sun? There's another proposed "Planet X" which orbits between 100 and 200 au from the sun to explain the "Kuiper cliff" which if it exists would be Mars or Earth sized. So what if we find this supposed "planet 9" and then find that planet?
Or what if it is not a single planet, but several in similar orbits?
And what if someone now comes up with another planet X. This is only the last of many, by no means proved, it may well fall into the dust of history and lead to another planet X. Do we call the next one "planet 10"? Or should we number the planet X candidates sequentially, so Pluto is "Planet X 1", or maybe even Neptune is "Planet X 1", Pluto is "Planet X 2" - in a system like that it might well be "Planet X 9" but you'd get disagreements about how many candidate planet X's we've had so far that are distinctive enough to need a new number.
If it exists, it's close to a borderline planet already according to the IAU definition Robert Walker's answer to Is it clear whether the putative "ninth planet" has cleared its neighbourhood?
Or, what if the IAU changes its definition again, as many astronomers are asking it to do - to call Ceres and Pluto both planets as well - then it would become planet 11 or maybe some much higher number, to take account of Sedna etc and Neptune would be planet 9.
And it's a silly name I think, like a name from a sci. fi. story, we never call Earth "Planet 3" :).
I'm not suggesting you change the title of your question :). I'd call it "Planet X" but people know what you mean if you say "planet nine" so it's become something of a "fait accomplis". But I do think it is a very silly name for a planet X candidate.
See also my
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