Science-fact Question

What do you mean to be captured so that it orbits the Earth like the moon does? I'd think that this is extremely unlikely, first because, while there are comets and asteroids that sometimes cross the path of the Earth's orbit, we don't generally capture them. I'd be more worried about something that size in our vicinity hitting us, as it would essentially be like the impact that is thought by many scientists to have formed the moon (and it essentially liquified the Earth for a while).

But I don't think Eris is likely to be visiting our vicinity of the solar system any time soon, as its orbit is elliptical and ranges between something like 37-97 AU out there--from approximately the distance of Pluto to about three times further out than Pluto.

Seems like it belongs in the science fact forum...

Liosse has a good point. You can contact a mod and ask them to move it.

I'm pretty sure this could happen. Capturing a moon is something most planets in the solar system have done. It doesn't happen very often, of course.

Achieving geosynchronous orbit would be trickier. All geosynchronous orbits around Earth have to have an average orbital radius of 42164km. That's a bit more than 1/10th the distance to the Moon.

Using Eris' mass, and that radius of orbit, the acceleration due to such a body would about 20x larger than the acceleration we feel due to the Moon.

This would still be too small for us to feel, but it would really screw up the tides.

The new moon would also look over 60x larger in the sky than our Moon or our Sun does. So, depending on the moon's reflectivity, the new moon is going to light up the night and darken the day (during eclipses.)

As for climate change, that's not my field. I don't have any feel for what it would do.

I will note that a body as small as Eris wouldn't keep its own atmosphere if it's orbiting Earth. The Sun would heat the gas molecules enough they would be lost to space eventually.

However, if the new moon is coming in from colder regions (which is what you expect if it's being captured from outer space), then it could still have a (cold, still) atmosphere at first. It would only start have a dynamic climate when it began warming up.

As written, your question is a bit too broad. There is no doubt that moons have been captured, but you did not define what you mean by synchronous. As for effects on the atmosphere, that would depend on size and distance. The Moon has kept the Earth's atmosphere relatively thin; if it were closer, then would the atmosphere be too think?

If you determine a diameter and mass, then it would be easier to figure out what it would do.

I take it that you do not mean geosynchronous, but you wrote "similar to the Moon around the Earth". Similar in what way?

I would guess that you want a moon with a period of roughly a month. Eris is larger than the Moon, so it would settle into an obit outside the Moon. I don't think that you want a monster moon like that. You might want to think of what characteristics it should have, then you can describe the moon based on those.

Is the captured moon orbiting in the same direction the world is spinning in? Where is it orbiting in relation to the moon? At what point in the earth's history did this happen? Does the rogue have it's own moon?how far away is it? How big is it? Rogue planets can giants be gas after all. . .
( I've been watching the Universe, and such lately. . .)

King Neptune said:

The Moon has kept the Earth's atmosphere relatively thin; if it were closer, then would the atmosphere be too think?

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Curious about this bit. Googled it, and didn't come up with any explanation, and my husband (who teaches astronomy) wasn't sure about this either. Not saying you're wrong, but is there a reference here, as it's an interesting notion that could make the odds of finding Earthlike planets in the habitable zone of distant stars a lot less (unless moons like ours are also commonplace).

I do know the moon is thought to have stabilized our axial tilt (around 23 degrees), which is something that definitely affects the Earth's climate and seasons, though. The Earth's rotation is also slowing down, and the moon plays a role in this as it spirals outwards. At some point, the moon and Earth will phase lock to one another, and a "day" and a month will be the same length (and longer than a month is now), and the moon will start to spiral inwards again. This will definitely affect weather, though it is likely that in a billion years or more, there will be much more significant changes in the atmosphere due to the sun growing warmer and the oceans growing smaller and outgassing and all that.

The Earth and moon will eventually collide, but by then, they'll likely be charred cinders from the Sun's demise anyway.


Capturing another satellite would likely wreak havoc with all these dynamics.

Interestingly, though, Mars has no substantial moon, and it has a similar axial tilt (about 25 degrees) and a similar day length (24 and a half hours).

Roxxsmom said:

Curious about this bit. Googled it, and didn't come up with any explanation, and my husband (who teaches astronomy) wasn't sure about this either. Not saying you're wrong, but is there a reference here, as it's an interesting notion that could make the odds of finding Earthlike planets in the habitable zone of distant stars a lot less (unless moons like ours are also commonplace).

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I can't think of any specific discussion of it in any major book, but there are many comments in descriptions of Venus that suggest that the atmosphere would be different, if there were a moon that stripped away part of the atmosphere, but I can't find any now. This is a little about it:
http://atramateria.com/earth-like-planets-with-moons-not-so-rare/
I may have gotten that idea from something comparing Earth and Venus that concluded that the Moon stripped away the excessive atmosphere that Earth would otherwise have. That makes sense, but it would be hard to prove.

King Neptune said:

I can't think of any specific discussion of it in any major book, but there are many comments in descriptions of Venus that suggest that the atmosphere would be different, if there were a moon that stripped away part of the atmosphere, but I can't find any now. This is a little about it:
http://atramateria.com/earth-like-planets-with-moons-not-so-rare/
I may have gotten that idea from something comparing Earth and Venus that concluded that the Moon stripped away the excessive atmosphere that Earth would otherwise have. That makes sense, but it would be hard to prove.

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Ah, okay. I always heard that the Earth's atmosphere was likely stripped away at the time of the moon's formation, but that was such a long time ago, and it had plenty of time to evolve and change since then. There are atmospheric tides caused by the moon (as well as oceanic), but it's my understanding that the effect is tiny. Didn't know the moon was supposed to be stripping atmosphere away. I know solar wind does, and that the magnetic fields are proposed to be somewhat protective (though recent evidence suggests that current rates of loss are similar for Venus and Mars, so maybe this was more important when the sun was younger).

It seems like there is still disagreement about the size of the so-called Goldilocks zone (which is debated, even for our own sun), the role of things like planetary mass, magnetic fields and whatnot on atmospheric composition and density as well. I'll admit I'm not up on all the competing models about how all these factors might interact to allow for a reasonably dense, but not too dense, atmosphere with a hospitable mixture of gasses with or without a sizable moon.

Roxxsmom said:

Ah, okay. I always heard that the Earth's atmosphere was likely stripped away at the time of the moon's formation, but that was such a long time ago, and it had plenty of time to evolve and change since then. There are atmospheric tides caused by the moon (as well as oceanic), but it's my understanding that the effect is tiny. Didn't know the moon was supposed to be stripping atmosphere away. I know solar wind does, and that the magnetic fields are proposed to be somewhat protective (though recent evidence suggests that current rates of loss are similar for Venus and Mars, so maybe this was more important when the sun was younger).

It seems like there is still disagreement about the size of the so-called Goldilocks zone (which is debated, even for our own sun), the role of things like planetary mass, magnetic fields and whatnot on atmospheric composition and density as well. I'll admit I'm not up on all the competing models about how all these factors might interact to allow for a reasonably dense, but not too dense, atmosphere with a hospitable mixture of gasses with or without a sizable moon.

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The more I think about it, the more I think that the Moon was primary in making Earth's atmosphere as thin (compared to Venus') as it is. Now, wwith the think atmosphere the outermost part of Earth's atmosphere reaches beyond that Moon; although demaned little of it is that far out. If the rates of loss for Mars, Venus, and Earth are about the same, then adjust Earth's rate for no giant Moon; the atmosphere would reach farther out and be thicker, and there probably would be as much loss of atmosphere as Venus had.

I can't prove it without taking a time machine back, then running a movie on the Earth as it goes from thoickly clouded to thinly clouded. I would also look at pre-Moon versus post Moon.

Boy I would LOVE to win that lottery. There have been a lot of ideas here but here are some facts that should be considered.

• While such an occurrence is possible it is so extremely unlikely that I can guarantee you it won't happen. (Hence my opening statement above, a lottery with equivalent odds would make somebody a multi-trillionaire at least). However, it is much more possible that such an occurrence could be artificially induced. This could add something to your plot.
• Most of the moons that have been captured by other planets were captured in the solar system's early history when interplanetary debris was far more common.
• Earth's own moon was very likely not captured; but formed by an earth-asteroid collision which sent material into orbit around the earth which eventually formed the moon in through the same process as the planets (through accretion).
• The odds of capturing an Eris sized object by itself is extremely unlikely since Kuiper objects (like Eris) never come close to the sun. Kuiper objects that do come that close become comets because they eventually lose mass due to the sun's heat and solar winds.
• A body that large is almost guaranteed to collide with earth rather than enter orbit around it. (This kind of collision is believed to have caused the present moon).
• Sedna is the only known dwarf planet sized body with an orbital period long enough (11,400 years) to avoid becoming a comet quickly and it doesn't come any closer to the sun than Pluto.
• Add to the above odds the odds of a geosynchronous orbit and we have a near impossibility (without some intelligent force interceding). NONE of the moons in our solar system have a geosynchronous orbit.