So in a Sci Fi story I'm working an alien planet appears in Earth's orbit I'm wondering how far would it have to be to not be a threat to the Earth but still get the benefits of the sun?
As DeeDee said, it depends on size. It also depends on how it entered Earth's orbit. Technically, it just needs to be far enough away to not affect our atmosphere or affect the moon.
Also maybe depends on what you mean by "in Earth's orbit". Do you mean orbiting the Earth? That would be... problematic. Or following the same orbital path as the Earth? Or...?
Yeah sorry I should probably have said the sun's orbit. Also to answer the question on the size the planet is roughly the same size as Earth.
Any big object would cause weird perturbations. Orbits of planet’s tend to evolves and move slowly until they come to a point of resonance and that resonance is extremely fragile. Jupiter is the anchor that holds all of the planets where they are so the best thing to do would be to put your planet in orbit around Jupiter, not the sun. Putting a new planet in orbit around the sun would cause the orbits to degrade over the next tens of thousands of years, where adding a small amount of mass to Jupiter’s gravity well would barely alter anything. Also, what benefit would such a civilization have by orbiting the sun? I would assume any civilization that can move a planet between stars is not bound by the energy constraints of a star.
And contrary to the previous answers, the size of the planet really doesn't matter. If it's big enough to be called a "planet," it's big enough to screw with t If you're set on having the planet orbit our sun, the best place to put it without violently disrupting the solar system would be at one of Jupiter's LaGrange points, which are a third of an orbit behind and in front of it. These are places where the gravitational pull of Jupiter is essentially zero (it's cancelled by the pull from the sun.) Jupiter is 5 times the distance of Earth from the Sun, so the closest approach of your new planet would be about 460 million miles, and at it's furthest (when it's on the opposite side of the sun) is about 500 million. You'd be able to see it with the naked eye on a very clear night and it'd be bright in binoculars. With a moderately powerful amateur telescope, you'd be able to resolve it's disc as well as major features if there are any.
Actually, there is a theory that such a planet existed in Earth's orbit. Another similar system is thought to have been spotted recently by astronomers.
I assume you are referring the Theia? It's the best fit for all of the data for the question of where our moon came from. It was about the size of Mars and was obliterated when it slammed into the Earth. In the early solar system there were dozens or even hundreds of protoplanets, and lots of them shared orbits. They either fall into the sun, get ejected by a larger planet, or merge in monster collisions (thought to be why Uranus is sideways, Venus rotates backwards, and we have a really big moon.)
@newjerseyrunner is his usually technically astute and always correct self. I agree, however in the near term, at the LaGrange points of Jupiter, it would have negligible effect on the Earth, though orbital disruptions will occur over the long haul. If the span of your story is years to decades, no effect. Beyond that watch out for comets especially being put onto a collision course with Earth.
*Asteroids. Comets come from extremely deep space, hundreds to hundreds of thousands of times further than Jupiter. They're effected more by the nearest stars than the inner solar system. Asteroids would be a hazard though, Jupiter holds onto a lot of asteroids in it's LaGrange points, some of which are quite large. Putting a planet there would disperse all of them, but it'd still be extremely unlikely that any of them would head to Earth in the next million years or so, space is really big and Earth is really tiny.
As in 'orbiting the Sun at the same distance and in the same plane'? Pop it on the opposite side of the Sun from us and and we'd be fine. Makes it a bit of a bugger to visit, mind you.
Unfortunately, that would create a gravitational symmetry, which is not stable and would collapse instantly. That'd produce tidal friction which would cause the inner planets to go into chaotic orbits. Their orbits are dependent on resonances. As Venus's orbit naturally degrades, it gets gently pushed back into it's position every time it passes by the Earth. It's orbit is such that it's entirely dependent on passing Earth in a 13:8 ratio. If you added another earth on the opposite side, that resonance would be broken.
That is dependent on mass and gravity tides. Technically no matter where it finds a spot, it could be destructive at any level. The bigger it is the likely hood it tosses us (if not a couple other planets) out of the solar system. A shell world could be substantially larger than the Earth, while being of the same mass roughly, but it would still have to be like newjerseyrunner said. Question being is are you using a rogue planet, or one that is actually moved into our system by an advanced species? The former is pretty much so far fantastical in all sense of the word, that it is damn near infinitely impossible for it to sync up with the rest of the planets in harmonious orbit around the sun. The latter is more plausible, as they would have the advanced tech and knowledge to move and place a planet, without disrupting the natural flow of the system to any real degree.
... I can't give this thread enough likes. This post dilated the edges of my mind beyond human understanding.
I mean, any new introduction into the solar system would completely throw off Earth's orbit, which is exactly where it needs to be. If you introduce a new planet, it'll either pull Earth away from the Sun, which would deep freeze us all, or drag us closer to the Sun, which would boil our planet. If you actually do want to introduce a new planet, it can't be a logical one. Luckily enough, you're writing science fiction! That means that you can come up with a whole slew of reasons for why a new planet hasn't indirectly caused to damnation of all human life on Earth! I'm not sure how @newjerseyrunner came to his conclusion about putting it in Jupiter's LeGrange point, but that should still cause problems, right? If the planet being close to the Sun would degrade the planets' orbits, how does putting it near Jupiter mean that the orbits would be fine? It's still adding an extra gravitational force, it would just pull the planets away from the Sun, still killing everyone. It wouldn't just "add to Jupiter's mass", that planet isn't adding to Jupiter, it has it's own gravity. Everything would be pulled, especially Jupiter. Even if it did just add to Jupiter's mass, the orbits are still extremely delicate, and the planets' orbits would still be thrown off. It would only cause the orbits to become more elliptical, which would eventually kill all life on Earth. So yeah, tons of problems. I really can't see a realistic explanation as to why your planet can exist without causing major problems in the long term. Even if you introduced the planet, then took it away, the planets' orbits wouldn't correct themselves, they'd be irreparably changed forever, becoming more and more elliptical with each revolution around the Sun until they either fall into the Sun or escape the solar system. Granted, I did only get a C in Astronomy, but, unless you plan on using a Science Fiction-y excuse, there's no way your planet exists. But, again, that's fine because you're writing Science Fiction! That means that, however the planet got there, you could make some super genius or alien entity that created the solution to the problem! But, yeah, don't expect for there to be a real-life answer to your dilemma, because there just isn't one, which is why Science Fiction is such a fun genre.
I misspoke. It would add to Jupiter's gravity well. Purely in a mathematical sense though. The reason it's best at Jupiter's orbits is because orbital resonances is based on mass distribution. Adding a medium sized mass to an already massive gravity well doesn't alter the ratios as much as if you say added a medium mass object to another medium gravity well. It would be destructive though, but not as much.
