<p>This is for a novel.</p>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>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.</p>

CC BY-SA 4.0
2018-10-17T05:59:51.130Z
"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
CC BY-SA 4.0
2018-10-17T06:02:42.510Z
What is a "sharp angle"?
ProfRob
CC BY-SA 4.0
2018-10-17T06:04:19.513Z
Very closely related https://astronomy.stackexchange.com/questions/24810/could-the-dinosaurs-have-seen-the-asteroid-that-killed-them
ProfRob
CC BY-SA 4.0
2018-10-17T08:36:37.080Z
You also need to specify the relative velocity, otherwise any answer is possible.
ProfRob
CC BY-SA 4.0
2018-10-17T14:04:21.583Z
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
CC BY-SA 4.0
2018-10-17T20:06:25.070Z
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
CC BY-SA 4.0
2018-10-17T21:35:12.443Z
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
CC BY-SA 4.0
2024-02-11T01:43:28.733Z
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
<rogue-planet>
Detecting a Rogue Planet from Earth
EvilSnack
# Detecting a Rogue Planet from Earth

(Asked by: username_0 on 2018-10-17)

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.

**Tags:** rogue-planet

> **Comments:**
> **username_1**: "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?  
> **username_2**: What is a "sharp angle"?  
> **username_2**: Very closely related https://astronomy.stackexchange.com/questions/24810/could-the-dinosaurs-have-seen-the-asteroid-that-killed-them  
> **username_2**: You also need to specify the relative velocity, otherwise any answer is possible.  
> **username_3**: 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.  
> **username_4**: 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.  
> **username_5**: I'm voting to close this question as off-topic because it falls within the scope of "Questions that are purely hypothetical".  
> **username_1**: 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.  

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## 0 answers

