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This is for a novel.

A rogue planet, is, by a one-in-a-zillion freak chance, on its way to collide with Earth. It has been drifting in interstellar space since its original formation (billions of years ago), and thus will be about as cold as it can get. It is approaching at a sharp angle to the ecliptic, and at its nearest point it will pass within the orbit of the Moon.

I'm currently envisioning its mass to be roughly equal to Neptune, but the only real constraint is that even if it does not collide with the Earth, its passage will end life on Earth with near certainty.

My question is: How close can such a planet come and still have a reasonable chance of not being detected from Earth by modern astronomy? Assume that the astronomical community is at its usual level of vigilance. I don't want the failure to detect it earlier to be due to some fluke; the oncoming collision is flukey enough.

I assume that this will depend on its composition, whether predominantly rocky or predominantly gaseous. I'd prefer the answer to be based on whichever type can get closer without being detected.

EvilSnack
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  • "the only real constraint is that even if it does not collide with the Earth, its passage will end life" Does that mean it's moving fairly slowly? – PM 2Ring Oct 17 '18 at 05:59
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    What is a "sharp angle"? – ProfRob Oct 17 '18 at 06:02
  • Very closely related https://astronomy.stackexchange.com/questions/24810/could-the-dinosaurs-have-seen-the-asteroid-that-killed-them – ProfRob Oct 17 '18 at 06:04
  • You also need to specify the relative velocity, otherwise any answer is possible. – ProfRob Oct 17 '18 at 08:36
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    Related: https://astronomy.stackexchange.com/questions/18924/has-the-existence-of-earth-sized-rogue-planets-been-confirmed and https://worldbuilding.stackexchange.com/questions/51612/what-are-some-methods-to-locate-a-rogue-planet-in-deep-space. And note that there are many questions about rogue planets on Wordlbuilding.SE that you could benefit from. –  Oct 17 '18 at 08:46
  • World building is a very good place for novelists, in general; when you just need something to be believably accurate, and based on facts-- but not perfectly accurate and based on math. – Magic Octopus Urn Oct 17 '18 at 14:04
  • It would be detected probably in order of the Pluto distance. It will be known that it will cross the Earth's orbit in some decades, but that we will have a collision, only months or years before the crash. We will have some tens of years for doing something. It won't be enough to save the Humanity, but it will be enough to build some Martian colony. The resulting collective psychosis will probably avoid this, so the Humanity will die. – peterh Oct 17 '18 at 20:06
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    I'm voting to close this question as off-topic because it falls within the scope of "Questions that are purely hypothetical". – Chappo Hasn't Forgotten Oct 17 '18 at 21:35
  • There's some info on detecting a small rogue body at https://astronomy.stackexchange.com/q/39837/16685 and links therein. Some of that info is relevant to this question. – PM 2Ring Feb 11 '24 at 01:43

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