How Fast Do Rogue Planets Move?

[Vondy]

I have been able to find how fast planets generally move in their orbits in our solar system, but have not been able to find any decent information on the movement/velocity of rogue planets. While it seems likely that planets knocked from their orbit would retain a velocity similar to their orbital velocity, it appears not all rogue planets necessarily had an original orbit. Also, their movement could be affected by the gravity of stellar objects (sped up or slowed down). An additional question I have pertains to how to speed up a rogue planet's velocity.

In summation:


how fast do rogue planets generally move? (all theory, of course)
what could increase/decrease a rogue planet's velocity?
if you were to knock a planet out of its orbit for an interstellar trip...




a) how would you do it?
b) how would you increase its velocity?

I know some scientists have discussed "how to move the earth," as well as the fact that its possible some rogue planets may have atmosphere and non-frozen water (though probably a hydrogen rich atmosphere and massive amounts of vulcanism), so the more "hard sci-fi" the answers the better...

[Diamond Spear]

Unless there is some reason you have to have hard numbers I'd just say that is moves at the speed of plot and leave it there. Other than that I'm afraid I have no good answers for you.

[Thia Halmades]

He beat me too it.

[eternal_sage]

the only good "real" way to do increase its velocity is the good old fashioned sling shot method. what this requires is a LARGE body (since we are talking about a planet, a sun will do, we humans do it for probes using Jupiter or Saturn)

you take said planet, have it enter the sun's gravity well, it turns from its current trajectory, and basically begins to "fall" into the sun. of course, it misses, but the extra velocity it gains easily exceeds escape velocity for the sun in question.

here's the wikipedia link:
http://en.wikipedia.org/wiki/Gravitational_slingshot

as for knocking it out of it original orbit, very few possible methods (that are in any way Hard SF) that aren't very determental to the planet in question. however, collisions with other rogue planetoids (the pool ball effect, if you will) is one possiblity, in which case the velocity would be determined by some simple physics calculations based on the original planetoid's velocity and mass, new planetoid's velocity and mass, and the angle of the collision. of course, a strike that strong will likely destroy all life on the planet.

another possible scenario is a supernova (also your best bet if you want high speed) in which a star goes supernova (explodes) expending all its energy in a very short time. the planet would easily be at 1/10th the speed of light, but would likely be destroyed by the explosion. to really be possible, the planet in question would need to be fairly far away (beyond Mars might be enough, but maybe farther than that) which puts it out of the traditional habitable zone, but gives it a small chance of survival. it would still likely loose its atmosphere, and probably slow down somewhat from the lessened energy (it is an explosion effect after all ;) ).

your best bet would be to have the life you wanted to exist on the planet get there by being transported to underground bunkers of some sort prior to the cataclysim, then reterraforming the planet afterwards. maybe the residents of a planet in the habitable zone realize their sun is about go pop, so they use their advanced tech to transport many of themselves and their flora/fauna to a rocky world (moon?) orbiting around the same distance as Neptune, which has the requisite vulcanism to keep them warm. after the explosion maybe they could terraform it, or maybe they would just continue living underground (much more likely to survive that way).

however, note that most vulcanism is driven by gravitational stresses, so unless the journey was short (a couple of hundred years, for example) then planet will still likely be barren by the end of its journey.

here is a list of the closet stars. at speeds of 1/10 the speed of light, a planet could reach here in less than 2 centuries.
http://en.wikipedia.org/wiki/List_of_nearest_stars

here is a list of stars with confirmed planets. it also lists distances, which could be helpful.
http://en.wikipedia.org/wiki/List_of_stars_with_confirmed_extrasolar_planets

[eternal_sage]

oh, and as for knowing how fast its going, he said he is going for a hard sci-fi game, which means this stuff matters. as little hand waving as possible should be done, because hard sci-fi is all about the crunchy data and real life uses. of course, as stated above, this kind of thing is highly unlikely, but the above is the best, hardest, most scientific way i know of getting the desired effect.

oh, and remember, no sun = no photosynthesis

[Wyrm Ouroboros]

how fast do rogue planets generally move? (all theory, of course)
what could increase/decrease a rogue planet's velocity?
if you were to knock a planet out of its orbit for an interstellar trip...




a) how would you do it?
b) how would you increase its velocity?


A planet finds the orbit where its speed and mass is balanced against the gravitational pull of the primary. Whether that's a natural planet evolving in a regular elliptical orbit, or a rogue planet captured by a star in whatever orbit it's captured into, as long as the planet is captured, it'll be in an orbit that will balance those three items. The balance does not have to be equal across the orbit -- it could be a regular planetary elliptical, cock-eyed like Pluto, or a further extreme ellipse akin to a cometary orbit. (Not likely, but possible.) However, the orbital 'tV * M = tG' where 'tV' is the total velocity across its orbit, 'M' is the planetary mass, and 'tG' is the total effect of gravity across its orbit.

All you have to do is figure that equation out. ;)

As for the rest -- gravitational manipulation is, really, the only effective way to do it. Any 'force applied', whether that be sudden (big explosion) or minute (immense solar sails) is going to either shatter the planet (big explosion) or be either so minute or require such thorough integration with the planet (solar sails) that you'd be better off just taking the planet apart piece by piece and moving it that way. :P

Altering the stellar 'G' changes your equation. What effect you get depends on where in the orbit you are...

[Nyrath]

I have a vague memory of an article by Dr. Robert Forward about two neutron stars orbiting each other being used as a gravity catapult.

Runaway stars can have velocities around 100 km/sec
http://en.wikipedia.org/wiki/Runaway_star

[Vondy]

oh, and remember, no sun = no photosynthesis

This is actually key to the motives for moving the planet. The rogue planet was home to a race of "benevolent" aliens who found it bombarded by a race of "malevolent" aliens with asteroids containing virulent spores and microbes that needed the nearby sun to reproduce quickly (terraforming bio-bombs). Their navy didn't have sufficient numbers to interdict the assault, and the planet was too far from her sister worlds to receive timely aid. The resident aliens took a gamble.... space billiards.

[Nyrath]

Just for interest, keep in mind the sinister star Gliese 710. It is apparently heading straight for the Sun at 24 km/s.

Of course it isn't going to get here for about 1.5 million years, and it appears that the closest it will come to the Sun is about one light year, but isn't it a sinister coincidence?

http://en.wikipedia.org/wiki/Gliese_710
http://jumk.de/astronomie/special-stars/gliese-710.shtml
http://www.solstation.com/stars2/gl710.htm

[Vondy]

Altering the stellar 'G' changes your equation. What effect you get depends on where in the orbit you are...

Drop the planet into a lower orbit and shoot a gravitonic implosion device into the star on the way by...? :D

[Vondy]

here is a list of the closet stars. at speeds of 1/10 the speed of light, a planet could reach here in less than 2 centuries.
http://en.wikipedia.org/wiki/List_of_nearest_stars

here is a list of stars with confirmed planets. it also lists distances, which could be helpful.
http://en.wikipedia.org/wiki/List_of_stars_with_confirmed_extrasolar_planets

I was thinking about Tau Ceti as it is the same class of star as Sol.

[Vondy]

One other question. If you did this with a gas giant, would it take its moons with it?

[Lord Fyre]

how fast do rogue planets generally move? (all theory, of course)

Well, given that a "rogue" planet would need to move stealthfully, I cannot see it* moving too fast.

(* - Of course I can't see it, it made its Concealment roll.) :D

[Captain Obvious]

I have a vague memory of an article by Dr. Robert Forward about two neutron stars orbiting each other being used as a gravity catapult.

Runaway stars can have velocities around 100 km/sec
http://en.wikipedia.org/wiki/Runaway_star

Coincidentally, I recently reread an old article I have squirreled away that mentions this very thing (or something similar...I think this used two white dwarfs). The article mentioned something about doing this to quickly boost a hypothetical interstellar ship to 1/150 c without turning the passengers to goo.

Although it would seem vanishingly unlikely that a random planet would fall into a binary star system at the exact trajectory to get that boost, so unless the planet was intentionally shot off by sapients, this would seem to be an upper limit, if not well above.

[The Monster]

Rogue planets can get moving pretty fast if you can get a Wizard planet to cast Haste on it.



:whistle:

[Narratio]

There is no real upper limit to it's speed. Once you've got it accelerating (and it's still in one piece) it could go to anything you liked including .999999 whatever of light. The problem of course lies in maintaining planetary cohesion. Put any sideways pull on the planet and it'll not only change course, it'll change course in an uneven manner as that sideways pull is felt at different levels across it's mass, exactly the same as an airplane, just much, much bigger. So stresses are set up, big earthquakes, possible restart of vulcanism and, assuming a really big sideweays pull, change in shape of the planet and possible ripping apart.

The stress math is funky and depends upon what your planet is made of.

[SteveZilla]

Coincidentally, I recently reread an old article I have squirreled away that mentions this very thing (or something similar...I think this used two white dwarfs). The article mentioned something about doing this to quickly boost a hypothetical interstellar ship to 1/150 c without turning the passengers to goo.

Yeah, but how do they stop at the end of the trip? :D

[SteveZilla]

however, note that most vulcanism is driven by gravitational stresses, so unless the journey was short (a couple of hundred years, for example) then planet will still likely be barren by the end of its journey.

I beg to differ. AFAIK, volcanism here on Earth is caused (either directly or indirectly) by convective heat from the core which likely has (I would think) a large quantity of radioactive isotopes.

[Captain Obvious]

Yeah, but how do they stop at the end of the trip? :D

I don't know. Maybe they spend most of the trip decelerating.

[Maur]

There is no real upper limit to it's speed. Once you've got it accelerating (and it's still in one piece) it could go to anything you liked including .999999 whatever of light. The problem of course lies in maintaining planetary cohesion. Put any sideways pull on the planet and it'll not only change course, it'll change course in an uneven manner as that sideways pull is felt at different levels across it's mass, exactly the same as an airplane, just much, much bigger. So stresses are set up, big earthquakes, possible restart of vulcanism and, assuming a really big sideweays pull, change in shape of the planet and possible ripping apart.

The stress math is funky and depends upon what your planet is made of.

Well, you might get it up to any given speed, but unless you continue to apply force on it to keep that speed, then the drag it will feel moving through interstellar space will either tear it apart or slow it back down.

[Cancer]

If the planet is to remain bound in the system, then the thing to compare it to is comet orbits, both long- and short-periods, though the long-period comets are at the barely-bound-to-the-Sun limit. This means an absolute maximum speed (relative to the Sun) of about 60 km/sec, IIRC. Any faster than that and it's going to exceed the escape velocity of the star and be gone, unless in that single pass you slam into something.

(Of course, collisions could occur between objects at up to twice that 60 km/s speed, if they were both bound, but happened to be in oppositely-directed orbits....)

Gravitational interactions and orbital change can also be a slow, literally chaotic, and complex process. That "literally chaotic" means that predictions are difficult to make. But that process works slowly, working over thousands or even millions of orbits, and while I can imagine ejecting a planet from a star system in that way, you certainly can't capture one that way.

To deorbitize something in one go, you need a near-collision with another body of roughly the same (or greater) mass. Otherwise you have a hard time carrying the needed energy and momentum into or away from the deorbitized target.

If you have more patience, long-term resonances with a distant, more massive object can do the trick.

EDIT: An interstellar object will, in general, pass through the Solar System once, on a hyperbolic orbit. It's not bound to the Sun; its trajectory will get bent as it passes by, but an encounter with just the Sun is not enough to cause it to become bound. 2-body interactions can't take enough energy & momentum from such an object to cause it to become bound.

If you have a close pass of a 3rd object, though, now you can repartition the orbital energy and shuffle momentum around to cause the interstellar guy to become bound. He'll be on a nasty, inclined, highly inclined orbit, most likely, nothing like the near-circular orbits we have in the Solar System now.

I'll look for numbers, references and post links. Wont happen til next week, probably.

[Cancer]

One other question. If you did this with a gas giant, would it take its moons with it?

If you did it one blow, that is, a close encounter with another object of similar or greater mass, then no, the moons would get scattered; maybe the closest, most tightly bound one or two would go with it, but that seems unlikely.

If you do it slowly, by a resonance effect that pumps up the orbital energy and eccentricity over the course of 10^7 years or so, then the moons would, for the most part, go with their host planet.

[Lucius]

If you're looking for a practical, hard science way to move a planet from one star to another (with people included (intending them to survive)) then check this out:
http://www.schlockmercenary.com/d/20030803.html

It worked for Larry Niven.....


Schlock Mercenary
Note: Building a gas-giant colony ship is not as difficult as it looks.
Build a fusion candle. It's called a "candle" because you're going to burn it at both ends. The center section houses a set of intakes that slurp up gas giant atmosphere and funnel it to the fusion reactors at each end.
Shove one end deep down inside the gas giant, and light it up. It keeps the candle aloft, hovering on a pillar of flame.
Light up the other end, which now spits thrusting fire to the sky.
Steer with small lateral thrusters that move the candle from one place to another on the gas giant. Steer very carefully, and signal your turns well in advance. This is a big vehicle.
Balance your thrusting ends with exactness. You don't want to crash your candle into the core of the giant, or send it careening off into a burningly elliptical orbit.
When the giant leaves your system, it will take its moons with it. This is gravity working for you. Put your colonists on the moons.
For safety's sake, the moons should orbit perpendicular to the direction of travel. Otherwise your candle burns them up. They should also rotate in the same plane, with one pole always illuminated by your candle (think "portable sunlight"), and the other pole absorbing the impact of whatever interstellar debris you should hit (think "don't build houses on this side")
Whether or not your gas giant heats up to the point that it ignites and turns into a small star depends largely on how much acceleration you're trying to get out of your candle. Remember, slow and steady wins the race!

Addendum to Note: Larry Niven suggested that such an arrangement could be used to move rocky worlds from one orbit to another, and he wrote a novel entitled A World Out of Time in which the Earth was moved with the help of giant candle they'd shoved up Uranus. I'm not making this up.


Of course, you want to be more careful than the rogue AIs in Schlock Mercenary were...

Lucius Alexander

The palindromedary quotes Edna St. Vincent Millay:

My candle burns at both ends
It will not last the night
But Ah! My foes, and Oh! My friends,
It gives a lovely light.

[SteveZilla]

To deorbitize something in one go, you need a near-collision with another body of roughly the same (or greater) mass. Otherwise you have a hard time carrying the needed energy and momentum into or away from the deorbitized target.

EDIT: An interstellar object will, in general, pass through the Solar System once, on a hyperbolic orbit. It's not bound to the Sun; its trajectory will get bent as it passes by, but an encounter with just the Sun is not enough to cause it to become bound. 2-body interactions can't take enough energy & momentum from such an object to cause it to become bound.

I think that it is possible for an object that comes from outside the system to be trapped without needing to interact with a third object. The key is that it either comes in below the combined system's (the two objects in question) escape velocity, or it is not to far above that excape velocity so that tidal braking can bring it into a trapped orbit.

But I could be wrong. :)

[Cancer]

I think that it is possible for an object that comes from outside the system to be trapped without needing to interact with a third object. The key is that it either comes in below the combined system's (the two objects in question) escape velocity, or it is not to far above that excape velocity so that tidal braking can bring it into a trapped orbit.

But I could be wrong. :)

Well, tidal interactions are inherently short-range: the strength of tides goes as the inverse cube of the distance. So to get appreciable tidal interactions, you do have to have a close encounter...

One of the things I need to look up when I can get to a library is what the velocity dispersion among rogue planets is likely to be. (This takes a library because the number is unknown; but there is likely to be papers that make predictions about numbers sort of like that are likely to be.) That's important here, because if that dispersion is large, then having something come in not too far above escape velocity gets pretty unlikely. And to get an encounter, you have to pass pretty close to the star (where the main planets are), no more than 10 AU or so; that's a pretty well-aimed shot. Doesn't rule it out; just makes it less and less likely to happen.

[Curufea]

One way to move a planet is to change its orbit from elliptical to a slingshot - slow the planet enough such that it falls towards its sun in a slingshot.

It would mean you'd only be able to move it in the plane of the ecliptic, though - and it may take half a year (of that planet) or so before the planet is on its way.

On a side note - the Puppeteers of Known Space by Larry Niven, have "The Rosette (http://en.wikipedia.org/wiki/Pierson%27s_Puppeteer#Homeworld_.E2.80.94_The_Flee t_of_Worlds)" - 5 planets and a sun they have moving away from the galactic core (to escape the explosion).

[Sundog]

One method of getting a rogue planet is to have a system change from a sigular to a close binary. If this were to happen to Sol, Mercury would be slingshotted, Venus would have a seriously perturbed orbit, and Earth would have a slightly disturbed orbit.

Of course, this would necessitate a close encounter with another star at just the right angles and velocities for mutual capture, which isn't exactly likely.

[Choldath]

There was a PC game some time ago that allowed you to set up your own solar system. I can't remember what it is, but it looks a bit like this.

http://dan-ball.jp/en/javagame/planet/

[BobGreenwade]

There was a PC game some time ago that allowed you to set up your own solar system. I can't remember what it is, but it looks a bit like this.

http://dan-ball.jp/en/javagame/planet/

Thanks for reviving that link.

Funny thing... I set one up just for giggles, put as many satellites in as I could just to see how many would survive, and just checked back after five minutes. Two of the bodies I put in have become moons.

I never had that happen the first time the link made the rounds.... :eek:

[SteveZilla]

One method of getting a rogue planet is to have a system change from a sigular to a close binary. If this were to happen to Sol, Mercury would be slingshotted, Venus would have a seriously perturbed orbit, and Earth would have a slightly disturbed orbit.

Of course, this would necessitate a close encounter with another star at just the right angles and velocities for mutual capture, which isn't exactly likely.

Not to mention what the extra solar radiation would do to us and our climate. Think we have Global Warming now? Just wait until the solar energy (Optical, Radio, Thermal, UV, Gamma...) received by the Earth gets increased 25% from the capture of a small second star (a second G-type would roughly double our received radiaion).

But on the up side, the Northern (& Southern) Lights would be spectacular!

[Nyrath]

Not to mention what the extra solar radiation would do to us and our climate. Think we have Global Warming now? Just wait until the solar energy (Optical, Radio, Thermal, UV, Gamma...) received by the Earth gets increased 25% from the capture of a small second star (a second G-type would roughly double our received radiaion).
It depends upon how close the second star is.

In the Alpha Centauri system, the distance between Alpha Centauri A and Alpha Centauri B varies from 11 AU to 35 AU (i.e., between the distance betwix Sol and Saturn, and the distance betwix Sol and Pluto).

This means that from the viewpoint of a person on a hypothetical planet at 1 AU from either star, the other star will appear as a brilliant star, but will not contribute anything detectable for solar heat.

http://en.wikipedia.org/wiki/Alpha_Centauri

[SteveZilla]

Anthing close enough to slingshot Mercury out of the system (per Sundog's post) would be (I would think) close enough to make a noticable difference in received solar energy.

[Captain Obvious]

There was a PC game some time ago that allowed you to set up your own solar system. I can't remember what it is, but it looks a bit like this.

http://dan-ball.jp/en/javagame/planet/

Heh...awesome...I set up a binary star and got a planet going in a figure 8 loop around them.

[Michael Hopcroft]

This is actually key to the motives for moving the planet. The rogue planet was home to a race of "benevolent" aliens who found it bombarded by a race of "malevolent" aliens with asteroids containing virulent spores and microbes that needed the nearby sun to reproduce quickly (terraforming bio-bombs). Their navy didn't have sufficient numbers to interdict the assault, and the planet was too far from her sister worlds to receive timely aid. The resident aliens took a gamble.... space billiards.

But wouldn't that have eliminated them as well? Conditions that eliminate photosynthesis should, sooner or later, eliminate all other life as well for several reasons -- most notably that without photosynthesis the oxygen in what atmosphere is retained gets replaced with carbon dioxide and animals that haven't already starved or frozen suffocate.

This of course assumes that the planet in its new condition can retain an atmosphere at all. If it strays too close to the sun, the gases boil away. If it strays too far, they freeze. Either way makes life impossible.

The only way for anything to survive would be in massive temperature-controlled shelters under the planet's surface, and to hope you can provide enough food and oxygen for a long enough period. It becomes a matter of trying to hold out until rescue comes, and hoping it comes at all.

Such shelters defy physics, but then so does jolting a life-bearing planet out of orbit without killing everything.

[Basil]

There was a PC game some time ago that allowed you to set up your own solar system. I can't remember what it is, but it looks a bit like this.

http://dan-ball.jp/en/javagame/planet/
OK, that instantly became my favorite webtoy. :D

[Sundog]

Anthing close enough to slingshot Mercury out of the system (per Sundog's post) would be (I would think) close enough to make a noticable difference in received solar energy.


Actually, the information I posted was based on the conditions in the Alpha Centauri system.

Gravitational perturbations apparently have a worse effect the closer you are to one of the primaries.

[tkdguy]

Heh...awesome...I set up a binary star and got a planet going in a figure 8 loop around them.
Most of my planets collided with one another! :hush:

Edit: I even managed to destroy the main planet (the earth?) :ugly:

[SteveZilla]

Actually, the information I posted was based on the conditions in the Alpha Centauri system.

Gravitational perturbations apparently have a worse effect the closer you are to one of the primaries.

I never would have figured that. I would have thought that for any planet orbiting "between" the stars (i.e., around one of the two binaries), the closer the orbit is to the Binaries' Center Of Mass, the more perturbed it would be. Case in point would be the 'figure 8' orbit, IMO.

[Maur]

But wouldn't that have eliminated them as well? Conditions that eliminate photosynthesis should, sooner or later, eliminate all other life as well for several reasons -- most notably that without photosynthesis the oxygen in what atmosphere is retained gets replaced with carbon dioxide and animals that haven't already starved or frozen suffocate.

Killing photosynthesis does not kill all life. We have thermophiles living in the oceans that do not appear to get any of their energy from the sun. They get it from the sulphides ejected by the black smokers.


This of course assumes that the planet in its new condition can retain an atmosphere at all. If it strays too close to the sun, the gases boil away. If it strays too far, they freeze. Either way makes life impossible.

I'd be careful about using the word impossible when it comes to life. We've used it many times only to find our assumptions were incorrect. While life as we know it might not survive in such conditions, some might survive. Case in point, we've found bacteria frozen in ice for 25,000 years that came back to functioning life as soon as it was warmed up.


The only way for anything to survive would be in massive temperature-controlled shelters under the planet's surface, and to hope you can provide enough food and oxygen for a long enough period. It becomes a matter of trying to hold out until rescue comes, and hoping it comes at all.

Such shelters defy physics, but then so does jolting a life-bearing planet out of orbit without killing everything.

No, it doesn't defy physics. It just defies our current level of technology.

[keithcurtis]

Cancer, have you ever tried to work out the feasibility of the Bronson Bodies from When Worlds Collide?

Keith "I must have read that book a dozen times" Curtis

[Cancer]

Cancer, have you ever tried to work out the feasibility of the Bronson Bodies from When Worlds Collide?

Keith "I must have read that book a dozen times" Curtis

I must sheepishly admit I've never read that book, so I have only a vague idea of what those are. Never thought about them in a physical-reality sense.

[keithcurtis]

Spoilers ahead!

The book is far different from the movie. In the book, there are two planets, not a planet and star. Bronson Alpha is a gas giant, roughly the size of Neptune. Bronson Beta is a terrestrial planet/satellite. They were torn from their original orbit presumably by the close passage of another star. The original inhabitants of Bronson Beta saw their doom for centuries, but earth had no such luck. Putting aside the amazing coincidence of these planets coming to our solar system (the characters often ponder if it is divinely-directed, a la the Flood), the motions are fairly well described.

The planets arrive from the southern (and fairly unwatched, in the early 20th century) skies, roughly 90° from the ecliptic. They pass through the solar system on a parabolic(?) course, whipping around the sun. Thus they pass by the earth twice, at six month intervals. On the first passage, Bronson Alpha destroys the Moon. Whether this was a direct impact or if it is torn apart by tidal forces is unspecified. Earth is devastated, of course and civilization collapses. It is on the second pass that the end comes. A few hastily-constructed rockets brave the passage between Earth and Bronson Beta, which has been thawing all this time. Bronson Alpha impacts the earth with a glancing blow, which of course destroys it utterly. The gravitational forces of the masses in question cause Bronson Alpha to continue out into space, while Bronson Beta takes an eccentric orbit about the sun, canted 90° to the ecliptic while varying in distance from the sun somewhat between the distances described by Venus and Mars.

Unlikely premise in the extreme, but again, the characters note this in the book, some becoming extremely religious. It's a great read, and probably my first experience with an Apocalyptic novel.

Keith "Recommends the sequel, too" Curtis

[Nyrath]

Cancer, have you ever tried to work out the feasibility of the Bronson Bodies from When Worlds Collide?
My gut feeling is that it is believable as far as the initial scenario goes: Neptune sized body with Earth-sized moon whizzes past Earth causing earthquakes, loops around the Sun, then obliterates Earth.

The part about the Earth-sized moon magically escaping its primary's gravity and magically decelerating into a perfect circular orbit one AU from the Sun is more or less impossible, AFAIK

[Cancer]

... And you only get one encounter. Yes, the parabolic orbit could intersect Earth's orbit at two points, but it's utterly impossible to have the two bodies have close passes at both of those points in one orbit.

[Clonus]

If you actually have a species of people who decide to move their planet to deal with a biocontrol problem, then obviously they can move their planet as fast as you want them to.

[Cancer]

True, but at that point you depart from standard orbital mechanics, and you're magically moving the planet around to suit your plot. Also, those people are making a conscious, premeditated choice to destroy an inhabited planet and everything that lives on it, because there's no natural way to get two close passes in a single orbit. You can't have that happen by accident, only by willed action and use of trans-human tech.

[Maur]

If they can get a planet moving then they aren't using orbital mechanics that a normal star system operates under since they are applying a force that isn't normally there.

[Vondy]

But wouldn't that have eliminated them as well? Conditions that eliminate photosynthesis should, sooner or later, eliminate all other life as well for several reasons -- most notably that without photosynthesis the oxygen in what atmosphere is retained gets replaced with carbon dioxide and animals that haven't already starved or frozen suffocate.

This of course assumes that the planet in its new condition can retain an atmosphere at all. If it strays too close to the sun, the gases boil away. If it strays too far, they freeze. Either way makes life impossible.

The only way for anything to survive would be in massive temperature-controlled shelters under the planet's surface, and to hope you can provide enough food and oxygen for a long enough period. It becomes a matter of trying to hold out until rescue comes, and hoping it comes at all.

Such shelters defy physics, but then so does jolting a life-bearing planet out of orbit without killing everything.

Species survival: this is totally dependent on what assumptions are in play about the technology available to the inhabitants and whatever measures they took prior to playing space billiards. I didn't propose any yet, so I don't have a problem to deal with at this point. Presumably, however, if one plans to move a planet, let alone has the ability to do so, one has a plan to ensure one's survival (as that was the motive for moving the planet).

As for the atmosphere: current theories accept the possibility of rogue planets (ejected from systems) that have atmospheres and surface water, though those atmospheres aren't likely to be friendly to humans, and the planet is likely to have significant vulcanism. Also, atmospheres don't burn off overnight. A one-time pass close to a primary is very different from a close orbit (a long term situation). What's more, the survivability of surface water would be dependent on atmospheric density, albedo, and a host of other factors. Again, a question of which "assumptions" are in play.

At this point I'm only concerned with the feasibility of moving a planet from a system within 12 LY to the Sol system over a short period (100-300 years). After that I can work backwards and come up with a "believable" set of assumptions.

[SteveZilla]

If one is playing Billiards with one's world (as opposed to using the Gas Giant "Candle" method), then one has no way to do a midway course correction... Better get it 100% right on the first try, and that there are no unknown variables.