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Could a terrestrial planet (1x diameter and mass up to 1.5x diameter and mass) hold a Titan-like (dense atmosphere, relatively large size, weather) realistically in its orbit? If not, how large would the planet have to be to sustain such a moon?

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  • $\begingroup$ Titan has a dense atmosphere only because it is so far away from the Sun. If it was at the same distane from the Sun as Earth it would have no atmosphere. $\endgroup$
    – AlexP
    Mar 24 at 14:08
  • $\begingroup$ It probably could have an atmosphere, just not one based off of or similar to Titan's atmosphere. $\endgroup$
    – Madman
    Mar 24 at 15:01
  • $\begingroup$ it seems like once you have that big of a moon orbiting a pretty standard terrestrial the orbit would just stabilize to make a binary planet instead $\endgroup$
    – zackit
    Mar 24 at 16:21
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Could it have a Titan size moon? Well, maybe. Titan's only twice as big as our own moon, after all... not like ten times bigger or anything. Our moon does seem to be a bit unusual, as it wasn't formed at the same time as the Earth, but in a subsequent collision with another planet. It wouldn't be beyond the realms of possibility for a larger moon to have been formed the same way, but it wouldn't be that likely.

A binary planet (or just a planet with a really big moon) could be a thing that forms naturally, Pluto and Charon exist, for example. If Titan orbited the Earth at the distance our Moon does now, the barycentre of the two bodies would be above Earth's surface (~8500km away from Earth's centre, and Earth only has a radius of ~6400km) which is one way of classifying the arrangement as a binary system rather than a mere moon-planet relationship.

Could it have a Titan-like atmosphere? Well, probably not.

Take a look at this diagram that shows atmospheric escape as a function of planetary temperature and surface gravity. It has some technical issues, but it shows the relationships we need well enough.

Atmospheric escape as a function of temperature and surface gravity

Titan can hold onto a thick atmosphere because it is cold, and it is cold because it is a long way from the sun. Bring it as close as the Earth and it will heat right up and all that atmosphere will merrily blow away into space.

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    $\begingroup$ Xenon. According to that dadblasted buzzkill of a chart, Earth moon Titan can have an atmosphere of xenon. And I want a lot of it, so it is under a lot of pressure, so I am neutrally buoyant in it, so I can swim thru the air using giant fans (between 3 to 5 fans total), wearing my xenon swimming costume. Illustrations welcome. Make sure I look good, though. $\endgroup$
    – Willk
    Mar 24 at 21:17
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    $\begingroup$ @Willk given Titan's low surface gravity and thick atmosphere, you might be able to fly by muscle power alone. Xenon would be denser than the current mix, so it should be easier. Awkward to get enough of it, though. $\endgroup$ Mar 24 at 21:51
  • $\begingroup$ As regards muscle power alone, you probably think that because of how I look in my xenon swimming costume. But those are actually floaties disguised as glutes and pecs. $\endgroup$
    – Willk
    Mar 24 at 22:33
  • $\begingroup$ So Ganymede could in theory hold onto an O2–N2 atmosphere, hmm… $\endgroup$
    – JohannesD
    Mar 24 at 23:11
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    $\begingroup$ @JohannesD At its current temperatures, hundreds of degrees below the freezing point of water, and thus the life that would be required to produce atmospheric oxygen, sure. $\endgroup$
    – nick012000
    Mar 25 at 1:04
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I don't see why not.

For a visual comparison, this image can help, it compares the sizes of Earth, Moon and Titan:

enter image description here

Titan is 50% larger (in diameter) than Earth's Moon and 80% more massive. It is the second-largest moon in the Solar System after Jupiter's moon Ganymede, and is larger than the planet Mercury, but only 40% as massive.

What matters for an orbit is the velocity around the attractor, if it is right, the body will orbit the attractor (as long as its mass is smaller than the one of the attractors).

What would likely be affected would be the center of mass of the system, with a more pronounced "wobble" of the main planet while orbiting around it.

A different problem is how such a system could form, but that's matter for another question.

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Titans primary atmosphere results from being 9.54 AU from the massive ball of plasma undergoing nuclear fusion at the center of the solar system, with jeans escape (It's just a slide which gives you an idea about the mechanics of it, but the basic idea is that any gas on the upper end of the energy curve would have enough energy to reach orbit and escape a probably rocky or icy bodies atmosphere). As for the size of the moon, that's perfectly plausible, but due to just how cold titan is, so cold that cryovolcanism takes place that being that water on titan is the eqivlinat of magma on earth(paper), so the size of the moon would be plausible, as would an atmosphere, but a titanian one, no.

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  • $\begingroup$ So you basically said the same thing as Starfish Prime, but with less detail or pictures? $\endgroup$
    – JDługosz
    Mar 24 at 21:55
  • $\begingroup$ I wasn't paying attention, but I think I MIGHT have posted my answer before starfish prime. MIGHT. $\endgroup$
    – Madman
    Mar 26 at 5:54
  • $\begingroup$ Somebody(who wasn't me) decided to upvote it, which I doubt would have happened if Starfish Primes answer was available, that being on top of the fact that there's really only one answer to the question $\endgroup$
    – Madman
    Mar 26 at 13:57
  • $\begingroup$ No, this answer was added later. It's no longer showing "hours", but you can sort by Oldest. $\endgroup$
    – JDługosz
    Mar 26 at 14:32
  • $\begingroup$ Oh, and welcome to Worldbuilding! $\endgroup$
    – JDługosz
    Mar 26 at 14:32

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