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View Full Version : Rogue moon/planet. My feeble attempt at research



Stadred
2004-Oct-02, 05:31 AM
Good evening all. I'm trying to do what the Bad astronomer recommends and ask someone when I don't know something. Since this seems to be a good collection of intelligent people, I figured I'd ask you guys. Go easy on me, I'm a newbie/longtime lurker.
Background: I'm writing a novel for nanowrimo www.nanowrimo.org. and I just wanted to do a little research.

The entire concept revolves around a ship that went and got itself stuck in a rogue moon, and ends up coming to visit our little solar system. The ship getting itsef stuck is science fiction that I can deal with. The rogue moon and such I want to try to base on science fact.

From what I understand, it is possible for there to be rogue moons (Supernova ejecta?) wandering the void between stars. But how unlikely is it for one to encounter our solar system (I am aware that space is mostly empty)? What would be the evidence of it's approach? Increase in comets from the kupier belt?

I'm intending on having the moon get pulled into a stable orbit (or stable enough that earth, say, 100 years in the future could stabalize it) around Jupiter. Is that even possible? Or would it just slam into the planet?

I know that if such an occurance would happen, it would cause a huge stir. That's readily apparent. But could an astronomer be able to calculate that the moon would end up orbiting Jupiter when it was out beyond the orbit of Neptune, or would the moon have to come closer for such math? I can't handle that math, but could someone else handle it?

Hmmm.. That looks like that's it. If anyone has any useful info, I'd appriciate it. I'm no astronomer, I'm a computer science major. I'd just like to have some fact in this little novella of mine...

Thank you for your time.
/Stadred[/url]

umop ap!sdn
2004-Oct-02, 06:52 AM
I'm intending on having the moon get pulled into a stable orbit (or stable enough that earth, say, 100 years in the future could stabalize it) around Jupiter. Is that even possible? Or would it just slam into the planet?

Depends on its orbital velocity and trajectory. It might pass Jupiter and get its orbit perturbed somewhat (most likely), slingshot around Jupiter (less likely), get caught in orbit (less likely yet), or if its path is exactly right, slam into the planet (least likely). Jupiter has several outer moons which I believe are captured asteroids, so no it's not impossible.


But could an astronomer be able to calculate that the moon would end up orbiting Jupiter when it was out beyond the orbit of Neptune, or would the moon have to come closer for such math? I can't handle that math, but could someone else handle it?

They would be able to tell that it is likely to pass near Jupiter, but probably there would be too much uncertainty at that distance. It all depends on how accurate the observations are and how much computing power the astronomer has at his/her disposal, because all objects in the solar system will gravitationally affect the rogue moon.

Evan
2004-Oct-02, 06:56 AM
Hmmm..

First, a single body drifting through space is not a moon, rogue or otherwise.

Second, a body cannot be captured in a stable orbit by another body alone. Possible short term exceptions exist due to aerobraking in an atmosphere but stability is not likely. In the case of atmospheric encounter the periapsis is within the atmosphere so the orbit will disintegrate.

Third, any body entering our solar system from outside will be hyperbolic. This means that it will enter with velocity high enough to escape the solar system as it is not in orbit around any component of our system. Again, it would have to be slowed by some interaction with a body in our system that involved something other than just gravity. It is remotely possible that a combination of "reverse gravitational slingshot" passes could slow it enough to remain in our system but that would be less likely than me finding a solid gold brick in my driveway.

As far as the math goes, there are no determinate solutions to the three body problem in the general case. There are for two bodies but that is easy. One body cannot capture another (excepting impact). The reason for this is simple. Any body approaching another from a significant distance must have some velocity on a vector toward that body. This is added to the velocity imparted by the mutual gravitational attraction of the bodies. The approaching body then either impacts the "target" body or the atmosphere (if one exists) or passes the "target" body with enough velocity to escape.

darkhunter
2004-Oct-02, 07:02 AM
Welcome to th e Board!

I don't know how likely the rouge moon (I would personally say planetoid--a moon it in orbit around anouther, larger body) would be to enter the solar system from inetersteller space, but I thnik that odds would be pretty low. Space is BIG as well as empty...

That's not saying it's a bad idea to use for a basis of a story, though--it can be a very good one...

Evidence for it's identification would most likely come from an astronomer (profesional or amature) looking for asteroids or comets. It would gain immediate scientific interest because it would be moving much faster than any other bodies out there, and it would have a hyperbolic orbit (http://www.daviddarling.info/encyclopedia/H/hyperbolicorb.html). (as far as I know, there have been no objects observed that came from outside the solar system, but I'm not an expert)

Unfortunatly, it would not stabilise itno any sort of solar orbit unless it entered the solar system at just the right plane, angle, and trajctory to take it on a tour of several of the planets and use a garitational assist (or sling shot) (http://www.wordiq.com/definition/Gravitational_slingshot) to slow it down. Of course, this would highten the already great scientific insterest in the planetoid....

The crashed spaceship: How long ago was the crash? are the crew still alive? Remember, space is big--if the crash occured near the crews home system (perhapes when they investigated the planetoid the zipped through their star system) it and the crew would be millions (if not billions) of years old.... Voyager 1 (http://voyager.jpl.nasa.gov/) has been traveling 27 years and technically is still in the solar system....

Again--welcome to the board! There will most likely be some more folks along to give more answers/idies/corrections if I made any mistakes...

edit=spelllling

Evan
2004-Oct-02, 07:25 AM
Capture is possible in the case of impacts with other objects already in orbit around the primary. That was likely the case with some of Jupiter's moons. It would be pretty catastrophic.

Andreas
2004-Oct-02, 04:18 PM
There are for two bodies but that is easy. One body cannot capture another (excepting impact). The reason for this is simple. Any body approaching another from a significant distance must have some velocity on a vector toward that body. This is added to the velocity imparted by the mutual gravitational attraction of the bodies. The approaching body then either impacts the "target" body or the atmosphere (if one exists) or passes the "target" body with enough velocity to escape.
You forgot that tidal forces can dissipate energy. I have no idea however if that is a viable capture mechanism for a big rock passing by Jupiter. Certainly not on the first pass, I'd suspect.

Evan
2004-Oct-02, 04:32 PM
Tidal forces won't change the orbit or trajectory of an object with hyperbolic velocity passing another by a measurable amount.

There are two effects to consider there. The one that might matter is if the object passes within Roche's limit and is pulled apart. There isn't time for that to happen. Even if it did then the pieces will have different resulting trajectories and will have differing velocities relative to the body encountered. Capture is still unlikely, probably impossible.

The other type of tidal effect is that which serves to slow the rotation of an object in orbit, such as our moon. That is a tiny effect and would play no part in a close pass taking only hours at most.

BTW, "supernova ejecta"????

Supernovas don't burp out moons, only 100 million degree plasma, X rays, gamma rays, neutrinos etc.

Tobin Dax
2004-Oct-02, 09:32 PM
BTW, "supernova ejecta"????

Supernovas don't burp out moons, only 100 million degree plasma, X rays, gamma rays, neutrinos etc.

SNe do "burp out" solid dust grains, or they form from the gasses eventually. The only possible huge chunk of material a supernova could eject would be a (very likely radioactive) hunk of metal (i.e. radioactive aluminum (http://www.astro.uiuc.edu/features/0211snr.shtml)).

Evan
2004-Oct-02, 10:19 PM
Supernovas do NOT burp out anything solid. The boiling point of tungsten is 5555, far, far below the temperatures found in a supernova.

The temperature at the core of a supernova is estimated to reach 100 billion degrees. There isn't the slightest possibility that a supernova "burps" out anything more solid than plasma. None, nada. Eventually, after many millions of years, more like billions, the elements formed in a supernova will begin to coalesce into a proto star system. Then, and only then, will solid material begin to form from the remanants of the supernova. Our universe is on the third or fourth cycle of such formation.

The link you posted is referring to radioactive aluminum plasma.

Stadred
2004-Oct-03, 04:13 AM
Okay, then what force would cause a large body to exit the solar system that spawned it?

Evan
2004-Oct-03, 04:31 AM
What? Please elucidate.

Stadred
2004-Oct-03, 06:34 AM
Like I said before, I understand that it can be possible for a rogue body to exist, something the size of a planet, 'roaming' the space between stars. But I've never understood how they got there in the first place. What gave them enough velocity to escape their star? Apparently a supernova is right out, but what natural event could eject a large body from it's solar system? And how fast would that thing be travelling?

You'll excuse my lack of knowledge. I'm just a poor comp sci major, and this group is the best resource I can find.

darkhunter
2004-Oct-03, 08:56 AM
What size/type is your planetoid? If it is like a comet, it could have been ejected from the original system when the home star passed another one (they don't have to be that close, just close enough to tug it out of its original orbit to a hyperbolic one...it would then leave the system.

Somthing bigger could have been ejected by a gas giant in the original system--small nudges adding up to alter the orbit enough to throw it out of the system. In a forming system, many of the planetoids are initially in "unstable orbits" as they are peturbed by all the other planetoids. Initially, many of the planetoids are "deorbited" and fall into the sun, and many more ejected from the system until the main planets eventually stabilize out...

Link 1 planetary formation (http://stardate.org/resources/ssguide/planet_form.html)
Link 2 Planetary Formation (http://www.google.com/search?q=planetary+formation+eject&hl=en&lr=&ie=UT F-8)
Powerpoint Presentation of planetary formation (http://geology.wcupa.edu/mgagne/scb210/planets.ppt)


------------
[fiction mode]
Or, for the purpose of your story, it was for some reason moved out of it's original system by an alien civilization--but they would need a good reason from their point of veiw that would be , well, alien to us....
[/fiction mode]

Tobin Dax
2004-Oct-03, 07:12 PM
darkhunter just reminded me of another possiblity. There was a paper a few years on planet formation around Alpha Centauri A and B. If the orbits were in the wrong positions when the planets formed, some fell into the sun and some got ejected. So it seems possible that a close binary system (say a separation of less than a few tens of AU) could eject a "rogue planet" early on.

Sticks
2004-Oct-03, 08:12 PM
It could be that on this piece of rock was a moon around another world and on it there was somer kind of nuclear dump that exploded and flung the moon in to deep space.

Mind you that plot does sound kind of familliar :lol:

Russ
2004-Oct-03, 09:15 PM
I'd like to point out that there are several moons orbiting planets in our system that are thought by professional astronomers to be captured objects. I'd say that is pretty clear indication that your scenario is a good possibility. It may require interaction with one or more planets before settling into orbit around Jupiter and possibly interaction with an existing Jovian moon.

As far as how your "moon" got to be a rogue; gravitational interaction between forming planets in a forming stellar system could easily eject an object about the mass of our Moon. I have read scientific speculation, though I don't remember where, that Jupiter and the other gas giants did this very thing, many times, as our system was forming.

I must point out that the chances of your scenario happening are small to :lol: astronomical numbers of decimal places. :lol:

Stadred
2004-Oct-04, 03:58 AM
Please. Astronomically small odds are what Sci-fi is all about! Thank you all for the help, I have a pretty good idea where to go from here. I owe you one!

Evan
2004-Oct-04, 03:31 PM
"Astronomically small odds are what Sci-fi is all about!"

I disagree. That is what science FANTASY is about. Hard science fiction is all about extrapolating what we know to be true and projecting how PLAUSIBLE scenarios might develop. I suggest you read DRAGON'S EGG by Robert L. Forward and MISSION of GRAVITY by Hal Clement.

Ilya
2004-Oct-05, 01:30 AM
"Astronomically small odds are what Sci-fi is all about!"

I disagree. That is what science FANTASY is about. Hard science fiction is all about extrapolating what we know to be true and projecting how PLAUSIBLE scenarios might develop. I suggest you read DRAGON'S EGG by Robert L. Forward and MISSION of GRAVITY by Hal Clement.

Funny you should mention "Dragon's Egg" -- for the odds of a neutron star passing THAT close to the Solar System (and not having been discovered as of yet) are pretty close to zero.

Evan
2004-Oct-05, 02:28 AM
Could be my memory is failing but I don't recall the neutron star in Dragon's Egg being anywhere close to our system. It was 20 years ago.