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## Rogue Planets

Hello, I am new to the forum. I have a question about planets escaping from thier parent star after the red giant phase or other star death. Shouldn't there be planets roaming around the galaxy all by themselves due to orbital shifts during the late stages of a star? If so, what are the odds of one perturbing our solar system in the future or having done so in the past?

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Planets can also be ejected in the beginning of a star's life. Intergalactic space is probably full of them.

As for the odds, here is a link to a 'stellar encounter calculator'. It will compute for you how many stars are likely to pass within a specified distance to the sun over a specified time. For example, over the 10 billion-year lifetime of the sun, there is a 1/10 of 1% chance that another star will pass as close as 40 AU, breaching the orbit of Pluto. If there's 1000 rogue planets per star, then we should expect 1 visitor over the course of the sun's lifetime.

Rogue comets and asteroids are probably much more plentiful, although they'd probably have to pass closer than Jupiter for us to notice them. So if there were 1 million rogue comets per star, we'd expect about 20 of them to breach Jupiter's orbit over the sun's lifetime. There would need to be about 1/4 trillion comets per star before we could expect 1 per millennium to breach Jupiter's orbit.

calculator: http://orbitsimulator.com/gravity/articles/cse.html

3. A good question and a good answer. This is an example of this forum at its best.

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I think I should have said 'interstellar space', the space between the stars within our galaxy, not 'intergalactic space' the space between galaxies.

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## Is it that simple?

Originally Posted by tony873004
Planets can also be ejected in the beginning of a star's life. Intergalactic space is probably full of them.

As for the odds, here is a link to a 'stellar encounter calculator'. It will compute for you how many stars are likely to pass within a specified distance to the sun over a specified time. For example, over the 10 billion-year lifetime of the sun, there is a 1/10 of 1% chance that another star will pass as close as 40 AU, breaching the orbit of Pluto. If there's 1000 rogue planets per star, then we should expect 1 visitor over the course of the sun's lifetime.

Rogue comets and asteroids are probably much more plentiful, although they'd probably have to pass closer than Jupiter for us to notice them. So if there were 1 million rogue comets per star, we'd expect about 20 of them to breach Jupiter's orbit over the sun's lifetime. There would need to be about 1/4 trillion comets per star before we could expect 1 per millennium to breach Jupiter's orbit.

calculator: http://orbitsimulator.com/gravity/articles/cse.html
Why should such an event not be possible anytime?
Since we do not know the number and maximum size of relevant objects in interstellar space perhaps we cannot say much about the frequency of such events(since there must be billions of planetary systems such events must be expected to happen sometimes in our galaxys histroy)? We can imagine some sort of "domino" or "avalanche effects", making it even more difficult to calculate. An event were an object approach the outer parts of our solar system, with the effect some Trans Neptunian objects approach the sun, in their new path changing the orbits of other objects (perhaps asteroids, other TNOs, or large moons of outer planets). So, we come to the question: Perhaps it is not so simple to see how often a planetary system like ours may eject its planets?

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I didn't mean to give the impression that it was that simple. It's that simple for stars. If we know the density of stars in the sun's region of the galaxy, and we know how fast they move relative to each other, than a mathematical model can tell us how often they encounter each other. That's what the linked calculator does. For rogue planets and comets, you have to guess how many there are. I simply chose numbers arbitrarily to demonstrate how the calculator worked. That's why I said "...if there were 1 million rogue comets", and "if there's 1000 rogue planets per star". I have no idea how many there are.

We can set an upper limit. Mankind has never seen one, and if there were hundreds of trillions of them per star, we'd expect to see about 1 every year. So its safe to say that on average, a star ejects far fewer than hundreds of trillions of planets or comets. If there were a few visitors over human history, then we could do some statistics on that. If historically we observed 1 rogue planet per century, that might give us a rough estimate as to how many there actually are. But historically, our count is zero, so all we can do is set an upper limit.

There is some speculation that some TNOs, such as Sedna, had their orbits altered by the gravity of stars that passed through the outer Kuiper Belt. There's even some speculation that Sedna might have once been part of another star or brown dwarf system, and was captured by the sun during a close stellar encounter. Early in the sun's history, it may have been part of a star cluster, as stars seem to form in clusters. So close stellar encounters were probably much more frequent then.

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Another question may be how best to find them. To look for eclipses of very bright far away "pointlike" objects? Or to see if anything passes in front of a dust cloud or far away galaxy(or may the effect be too small to be detectable, if the "planet" is far outside the solar system and not particularly big)? There is some radiation from our "own" gas planets - enough to be detected ligth years away?

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Originally Posted by jhwegener
Another question may be how best to find them. To look for eclipses of very bright far away "pointlike" objects? Or to see if anything passes in front of a dust cloud or far away galaxy(or may the effect be too small to be detectable, if the "planet" is far outside the solar system and not particularly big)? There is some radiation from our "own" gas planets - enough to be detected ligth years away?

We would likely find them in the same way we found thousands of asteroids, comets, KBO's and minor planets. Take multiple images and look for objects that have changed position between frames. If you have enough observations, you can calculate an orbit, or confirm it was just passing through.

9. That would not work well for objects far outside the solar system, they are too faint. The most likely methods is microlensing of the stars in the background. Surveys of this type, like OGLE, are online already.

10. Originally Posted by tony873004
Planets can also be ejected in the beginning of a star's life. Intergalactic space is probably full of them.

As for the odds, here is a link to a 'stellar encounter calculator'. It will compute for you how many stars are likely to pass within a specified distance to the sun over a specified time. For example, over the 10 billion-year lifetime of the sun, there is a 1/10 of 1% chance that another star will pass as close as 40 AU, breaching the orbit of Pluto. If there's 1000 rogue planets per star,

That seems rather improbable. Most planetary system formation papers have a small number (<20) of planets forming then some subset of these being ejected. Did you have any reason to think 1,000 per star is reasonable?

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Originally Posted by timb

Did you have any reason to think 1,000 per star is reasonable?
None at all. All the numbers I chose were arbitrary, to demonstrate how to use the calculator. I was simply saying that if there were 1000, then over the sun's life, we'd expect to get visited by 20 of them. That doesn't mean I think there is a thousand. The real number is probably significantly lower by a couple of magnitudes, meaning the sun probably never has, or never will have a rogue planet visitor.

13. Originally Posted by rusheagle
Hello, I am new to the forum.
Welcome!

I have a question about planets escaping from thier parent star after the red giant phase or other star death. Shouldn't there be planets roaming around the galaxy all by themselves due to orbital shifts during the late stages of a star?
The process the Sun will go through leading up to and including the red giant phase will indeed cause the orbits to shift outward, somewhat, due to the loss of mass that the Sun will be blowing off. Of course, Mercury and Venus will likely get gobbled, though Earth may only get completely torched.

However, stars like the Sun have tremendous gravitational influence even during the red giant phase. The Oort Cloud supposedly has as many as 10 trillion cometary bodies that are under the gravitational hold of the Sun. But this extends perhaps to 50,000 AU (almost 1/4 the distance to our nearest neighbor). The larger planetary bodies are much, much closer to their host star and are not likely to float off due to the red giant phase (which is probably much more orange, btw. ).

If so, what are the odds of one perturbing our solar system in the future or having done so in the past?
Tony has provided some great stuff in this regard.

A great source of rogues likely comes from places like OMC-1 in Orion. As many as 30,000 stars per cubic light year may be forming. [This was from 2006 source and may have been revised since.] With such proximity, many planets and brown dwarfs are expected to be fleeing the scene, and not by choice.

14. Weather on a rogue gas giant would be interesting. If one were in the upper atmosphere, the only thing one would see, aside from stars, are massive lightning flashes from below. But given the upper atmosphere's temperature might be near absolute zero, the contrast between that and the hot liquid hydrogen interior could cause ferocious weather-maybe 2,000 mph winds, and thunderstorms the size of earth or larger.

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## Thanks everyone

I had been away for a while and just got a chance to check back on all the interesting answers. The absolute hugeness of space seems to be a nice safety net to prevent anything like this from happening. It might be a rather interesting find someday. I know rogue stars have been spotted fleeing the galaxy at insane speeds.

Thanks again and in particular for the calculator.

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