Duna and Ike

[Whirligig Girl]

In Kerbal Space Program, there is a planet called Duna. orbiting it is a Moon called Ike. Ike is very massive compared to Duna, and the two objects are very close together. A Double Planet is a system where two massive objects orbit each other, so Pluto and Charon are a Double Dwarf Planet system. So then my mind collided these two thoughts together.

In real life, what if there was a system like Duna and Ike? Let's assume for the moment their sizes are corrected from the Kerbal Universe to 10.61 times their Kerbal size. Obviously "Barycenters would happen.", but what would happen to the planets themselves?

Note: The above is written as an email to xkcd "What If?". I'm curious to hear other people's thoughts on the idea, in the likely event Mr. Randall Munroe does not get around to answering this question.

Edited October 15, 2014 by GregroxMun

[ZodiaK]

Barycenters would happen, presumably.

[Whirligig Girl]

"Barycenters would happen"

I laughed a little. Yeah, that's probably a given. The question is what would happen to the planets themselves.

[-Velocity-]

"Barycenters would happen"

I laughed a little. Yeah, that's probably a given. The question is what would happen to the planets themselves.

They would orbit each other, tidally-locked, at a fixed distance. Occasionally, they would be hit by asteroids and comets. They'll keep orbiting each other until until some external factor interferes- be it the end of the universe, the red giant phase of their host star, another body making a close approach, etc.

If they're orbiting in a solar system with lots of planets, such systems are not stable over the long term. Not even our own solar system is stable in the long term, it's actually possible that by the time the Sun turns red giant, one (or two?) of the current inner planets could be gone, ejected after a close encounter with another inner planet. It's impossible to predict, because the solar system is a chaotic system and we can't measure the positions of the planets (and other properties) closely enough to predict their orbits a billion, two billion, three billion years from now. Over such long periods, orbital resonances with other bodies (I think Jupiter being the most significant, despite its distance) can pump up the eccentricity of the orbit of an inner planet to the point where "bad things" start to happen.

Edited October 15, 2014 by |Velocity|

[KerikBalm]

I've never heard any claims like this of the inner planets being in orbits unstable within that time frame...

But as to the question... I gues the planets would end up being sort of egg shaped, with the "eggs" pointing at each other. The atmosphere of duna would preferentially accumulate on the ike facing side, as would any liquid if duna had surface liquids (ie, like perpetual high tide).

But I think they are well outside the rochelimit, so you wouldn't get a "rocheworld" type effect.

[-Velocity-]

I've never heard any claims like this of the inner planets being in orbits unstable within that time frame...

Just Google "stability of the solar system". There's been a fair amount of research into it; our solar system is not stable, but most likely, nothing bad will happen before the Sun dies. Most likely.

[EdFred]

If it's on the internet it must be true. They can't put anything on the internet that isn't true.

For the record, I'm a French model.

[-Velocity-]

If it's on the internet it must be true. They can't put anything on the internet that isn't true.

For the record, I'm a French model.

You try my patience. Are you completely helpless at doing any form of independent research? The long term stability of the solar system has been discussed in scientific journals for over a century. The internet didn't even exist and people were talking about it already.

Examples-

Abstract

Large scale chaos is present everywhere in the solar system. It plays a major role in the sculpting of the asteroid belt and in the diffusion of comets from the outer region of the solar system. All the inner planets probably experienced large scale chaotic behavior for their obliquities during their history. The Earth obliquity is presently stable only because of the presence of the Moon, and the tilt of Mars undergoes large chaotic variations from 0° to about 60°. On billion years time scale, the orbits of the planets themselves present strong chaotic variations which can lead to the escape of Mercury or collision with Venus in less than 3.5 Gyr. The organization of the planets in the solar system thus seems to be strongly related to this chaotic evolution, reaching at all time a state of marginal stability, that is practical stability on a time-scale comparable to its age.

J. Laskar, “Large scale chaos and marginal stability in the solar system,†Celestial Mech Dyn Astr, vol. 64, no. 1–2, pp. 115–162, Mar. 1996.

I can't copy this text, it's in an image form, but hopefully it's visible for others-

From:

P. E. Nacozy, "On the stability of the solar system", Astronomical Journal, vol. 81, Sept. 1976, p. 787-791.

As you can see (hopefully), the very first paragraph talks about how mathematical modelling work has been done on determining the stability of the solar system going back to the 19th century.

[Will Fawkes]

A little less condescending if you please. Not everyone does research, or trusts their own objectivity if they do.

[-Velocity-]

A little less condescending if you please. Not everyone does research, or trusts their own objectivity if they do.

Respect is reciprocal; I found EdFred's post quite disrespectful, hence my reply. It does get tiring and annoying when you post correct information about well-known science and physics and you get mocked for it, especially when information on the subject is incredibly easy to come by (such as: http://en.wikipedia.org/wiki/Stability_of_the_Solar_System ). All it takes is a Google search these days and you simply need to look at the source of your information; if it looks like it's from a peer-reviewed journal or a university, it's usually reliable. Even Wikipedia is reliable like 95% of the time. You can then double check with other sources to be sure.

scholar.google.com is a better way to search through reputable publications, though if you don't have a library pass of some kind you will only be able to get abstracts on a lot of the papers.

Keep in mind that not all researchers are reputable, and not all things that look like reputable papers ARE. That's why if something looks or sounds "fishy", it's best to double-check other sources.

[GoSlash27]

For those who may have missed the reference...

Best,

-Slashy

[GoSlash27]

IRT the OP, I would guess that given enough time the orbits would degrade (space being an imperfect vacuum) and both bodies would eventually smoosh together into a single oblong body.

Best,

-Slashy

[Kryten]

It does get tiring and annoying when you post correct information about well-known science and physics and you get mocked for it,

That's rich, coming from someone that doesn't even understand the difference between chaotic and unstable.

[linkxsc]

That's rich, coming from someone that doesn't even understand the difference between chaotic and unstable.

You're just being pedantic. You know what he meant. And its a pretty far stretch the argument you're trying to push with this statement.

At the same time Velocity needs to calm down a bit. It was a mild joke, and there are countless "sources" of bs information about space stuff around on the internet.

Also the things you find when searching google arent the same as I would find.

Also ed was the 1 who made the stupid comment. Not the guy who actually asked for a source.

At the same time, eds comment before was quite uncalled for, the subject is done now though, so how about the actual topic.

Edited October 17, 2014 by linkxsc

[EdFred]

Sorry, having been online longer than 90% of this board has been alive, I grow tired of "Google it" responses.

[hugix]

Duna would slowly tear Ike apart, Ike forming a ring around Duna that slowly disintegrates into Duna's atmosphere.

[KerikBalm]

You're just being pedantic

Chaotic and unstable are not neccessarily the same thing.

The paper he posted also mentions bounded orbits, implying stability.

The wikipedia link says:

"none of the planets will collide with each other or be ejected from the system in the next few billion years, and the Earth's orbit will be relatively stable"

This all started with velocity making a reference to a time scale of 1-3 billion years:

"we can't measure the positions of the planets (and other properties) closely enough to predict their orbits a billion, two billion, three billion years from now. Over such long periods, orbital resonances with other bodies (I think Jupiter being the most significant, despite its distance) can pump up the eccentricity of the orbit of an inner planet to the point where "bad things" start to happen."

Then I questioned the stability within that time frame:

"I've never heard any claims like this of the inner planets being in orbits unstable within that time frame..."

he replied with "nothing bad will happen before the Sun dies. Most likely. "

The sun won't die in 3 billion years (although it will grow quite bright), and the wikipedia link says its stable for basically the same time frame....

So I don't even know what the argument is about anymore... there seems to be agreement that its fine for the new billion years, at which point the sun will be too bright for us on Earth anyway...