PDA

View Full Version : Binary Planets



SkepticJ
2005-Mar-08, 07:09 PM
Did I forget to think of something here?--> http://img.villagephotos.com/p/2005-3/965110/MathScience.jpg

I'm sorry for the crudeness of the drawing but could two planets orbit each other like this? The plane of orbit about the twin suns is perpendicular to the co-orbiting planets you see here. What would a rocky ring look like around twin 0.8 or so gee rocky worlds? The ring is supposed to be made of rocks from a broken proto planet and is made of chunks from about like a pebble to Phobos sized hunks of rock. Any BA here?

jfribrg
2005-Mar-08, 07:54 PM
Did I forget to think of something here?--> http://img.villagephotos.com/p/2005-3/965110/MathScience.jpg

I'm sorry for the crudeness of the drawing but could two planets orbit each other like this? The plane of orbit about the twin suns is perpendicular to the co-orbiting planets you see here. What would a rocky ring look like around twin 0.8 or so gee rocky worlds? The ring is supposed to be made of rocks from a broken proto planet and is made of chunks from about like a pebble to Phobos sized hunks of rock. Any BA here?

Is this 2 planets orbiting each other and both orbiting a binary star system? The two planets appear to be retrograde with each other, raising a question of how they formed. Presumably they would have formed separately and then had a close encounter. I'm not sure how a ring could be stable in a binary planetary system. The particles in the ring would quickly impact the planets or be ejected from the system, but would not stay in a stable orbit unless the ring is very far from the two planets.

jnik
2005-Mar-08, 07:56 PM
I'm sorry for the crudeness of the drawing but could two planets orbit each other like this? The plane of orbit about the twin suns is perpendicular to the co-orbiting planets you see here.
I can't really parse out the drawing, but maybe I can answer this. It's fairly unlikely to have a significant component of angular momentum normal to the angular momentum of the system as a whole. So having two bodies whose mutual orbits are out of the plane of their orbit about the sun (or suns) suggests a capture event or some sort of catastrophe rather than normal formation.

Padawan
2005-Mar-08, 08:19 PM
Also, you will have to consider the Roche limit and see if the planets are too close (which I think they might do, but I'll have to know about the mean density of the objects).

eburacum45
2005-Mar-08, 09:37 PM
These two planets in the Orion's Arm imaginary universe orbit each other and occupy each other's Roche nodes; I have assumed that they will also spin independently of each other, aligned along their long axis.

http://img168.exs.cx/img168/5412/billandbull3tk.jpg

incidentally they orbit the relatively nearby star Rana.

SkepticJ
2005-Mar-08, 10:04 PM
I'm sorry for the crudeness of the drawing but could two planets orbit each other like this? The plane of orbit about the twin suns is perpendicular to the co-orbiting planets you see here.
I can't really parse out the drawing, but maybe I can answer this. It's fairly unlikely to have a significant component of angular momentum normal to the angular momentum of the system as a whole. So having two bodies whose mutual orbits are out of the plane of their orbit about the sun (or suns) suggests a capture event or some sort of catastrophe rather than normal formation.


The planets orbit each other about a central center of gravity; like Pluto and Clarion but their plane of orbit is 90 degrees from being level with their orbit plain about their parent stars. I drew it to explain the idea, not make it harder to understand.

What is it about binary planets that would make a ring around them not work?


What is it that I need to know to make them plausible? What do I not know? Wow, is that a big question!

What's a Roach limit?

Bathcat
2005-Mar-09, 07:50 AM
I think the difficulty with a ring around a pair of binaries might be that the changing gravitational effects of the two planets would tend to disrupt the ring. At different points in their orbits they would end up tugging and twisting the ring out of round, so to speak, and bits would either spiral down to the planets or go careening off into space.

If the ring were far enough away from the double planets that the planets could be considered as a single point mass it might be more stable, but I don't know if a far-away ring is really feasible.

Then again, if the ring only encircled one of the two planets...take Saturn. How big would Titan have to be before it would become fatal to Saturn's rings?

Or, to look at it another way, what would the lifespan of a debris ring be under various configurations of planetary systems? I suppose that you don't have to have a ring that has persisted for billions of years, just one that's persisted long enough -- two or three million years? -- for the purposes of your scenario. It might be that in the long run a ring system around binary planets would be unstable, but do you really need to invoke billions of years?

How old are Saturn's rings? Jeff Cuzzi says probably a few hundred million years, and they will only last a few hundred million more. (reference (http://science.nasa.gov/headlines/y2002/12feb_rings.htm) at NASA site.) If you had a moon get smashed up by an impact (one possible origin for Saturn's rings), that could give you a ring that might be long-lived enough for your purposes.

----

If I recall, Roche's limit is essentially just the closest a planet can orbit another planet (or star, I suppose) without being torn apart by tidal forces. It depends on the nature of the planets, though -- obviously artificial satellites aren't torn apart no matter how closely the orbit Earth. They're too small for tidal forces to be significant. Same for a skyscraper-sized hunk of rock.

cyswxman
2005-Mar-09, 08:25 AM
I seem to recall some reading postulating that the Earth-Moon (Luna) system is actually a binary planet system, though I can't recollect the reasoning.

Romanus
2005-Mar-09, 05:40 PM
If the two planets were close to their stars (say, 1 - 2 AUs, if we're talking solar-mass stars), their mutual orbit would probably not be stable over billions of years, due to the tidal torque exerted by the two stars on their orbit. This problem would only get worse the farther apart the planets are.

Try out the satellite integrator on this page:

http://janus.astro.umd.edu/AW/awtools.html#explore

The ring probably wouldn't be stable either, and the planets would probably be tidally-locked with respect to each other. Of course, like the Moon they'd still have to rotate to keep the same face toward each other.

As to whether it's possible, I don't know if there has been much,if any simulation of binary planet formation. From what little I know of the core-accretion process, I doubt it's common; they'd have to either accrete simultaneously (in the chaotic and violent environment of the protoplanetary disk, and later heavy bombardment), or capture each other, neither of which is easy. With stars, it's easier to account for close doubles because they're gaseous bodies.

However, because there's a lot we don't know about planetary formation, and because there are an *awful* lot of stars and planets out there, I wouldn't rule it out. 8)