What would we be left with if we parked Saturn into Earth's orbit?

Factual Questions
1

Obviously we’d stick it on the other side of the sun, ‘six months’ away, because interplanetary billiards would be painful.

Now, according to my research (about three minutes on Wikipedia) “Saturn’s interior is most likely composed of a core of iron–nickel and rock (silicon and oxygen compounds). Its core is surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium, and finally, a gaseous outer layer.”

Okay, so the iron/nickel cire sounds familiar, as do silicon and oxygen. I have no idea about metallic hydrogen, but I’m fairly sure liquid hydrogen and helium will turn into gases as Saturn warms up in its new, relatively balmy neighborhood.

So what happens then? Do most of the gases just float off into space? Or will the hydrogen manage to meld with the oxygen and create water?

After a suitable amount of time for this thawing to occur and so forth, what are we left with? Do we basically have another Earth? Rocky core, tectonic plates, ocean, breathable atmosphere?

Will it be bigger than Earth? Or lose enough mass to bring it down to size?

Will there be an effect on Earth? Like, from all that gas boiling off Saturn, will Earth tend to hoover some of it up as we follow it around and around?

What about all of Saturn’s moon? The rings, and the moons that are just ice I assume will melt and go away, but aren’t some of them rocks? As Saturn loses mass, will they start shooting off into space?

2

Just addressing interplanetary billiards …

If you (or Musk) magically dropped Saturn “6-months” away from Earth with the correct orbital velocity, pretty quickly they’d collide and Earth would be comprehensively destroyed.

“Pretty quickly” might be millenia, so past your personal bedtime, but it’d be fast on astronomical timescales.

I hate it when that happens.

3

Jupiter is still going to be a significant factor in the orbital mechanics. Saturn won’t sit in the inner orbit that long. There seems to be a current understanding that the gas giants have naturally moved to relatively stable orbits where they currently are from closer in positions. I would guess that would happen again before any huge changes happened to Saturn. The Earth won’t escape unscathed. Any fate, from ejection from the solar system, capture as as a moon by Saturn, collision, or slung into some horrendous eccentric orbit are on the menu.
The solar wind will slowly remove gas from the upper atmosphere. And the planet will slowly warm. But on extraordinary long timescales.
Saturn orbits at about 9.5 AU. So dropping it at 1AU roughly means 100 times as much heat from the Sun. Saturn has an internal heat source that is roughly three time as much as it’s current insolation. So heating of Saturn would really only be about 25 times current energy input. The upper atmosphere will reach an equilibrium and not that much additional energy will be getting down to the planet. Given there is an existing heat source in the core, things may hardly change.
For an ideal question where orbits don’t change and the Sun lasts forever in its current form, eventually you might expect to be left with the rocky core. The hydrogen would just join the solar wind out into interstellar space, and maybe in some long future event get hauled into a star generation event. Or not. Other than helium there is near enough to no other element for the hydrogen to worry about.

4

Which is why the opposite side of the sun is the wrong place to drop Saturn. The best place is where Earth is in either of Saturn’s Lagrange 4 or 5 points. That is, 60 degrees ahead or behind Earth. Of those two, L4 would be best, since it’d be visible in the evening sky rather than L5 where it’d be in the morning sky. But either one of those would be a fairly stable orbit.

5

Not really.

Lagrange points are places where the gravity well is pretty flat. In fact, rather than a gravity well, it’s better to describe the Lagrange “point” as the flattish top of a gravity hill that slopes down and away in every direction. Which slope gets quickly steeper as you move away from the “summit”. Anything wandering any appreciable distance away from the summit will quickly find itself plummeting “downhill” to someplace else.

Meanwhile, as the various bodies move relative to each other, the exact “point” of zero gravitational potential migrates around a small neighborhood. Where “small” might be a few hundred miles.

If there were only the three primary bodies in the problem, all were true point masses, and all masses and other energy fluxes were perfectly eternal and all orbits perfectly circular and never-changing, then Lagrange points would be true points that are eternally stable.

The actual real-world Lagrange region is a place where a small machine with active station-keeping can use very little fuel only intermittently to stay balanced atop that very large radius beach ball. But something the size of a planet is not going to stay in the Lagrange region over astronomical timescales.

6

I’m wondering if the relocation of Saturn into the earth’s orbit would destabilize the asteroid belt and once again turn the inner solar system into a shooting gallery. Opinions?

7

You’re describing the L1, L2 and L3 points†. L4 and L5 are different. Otherwise there wouldn’t be a huge number of Trojan asteroids at Jupiter’s L4 and L5 points. There are estimates that the number of Trojans is roughly the same as the number of Main Belt asteroids.

Very few Trojans are within a few hundred miles of the actual Lagrange Point. Their orbits relative to the point make long kidney bean shaped loci that are usually hundreds of thousands or even millions of kilometers long.

Putting Saturn on the opposite side of the Sun would put Earth in the L3 Lagrange Point, which as you say, is unstable in the long term. Putting it 60° behind or ahead of the Earth puts Earth in Saturn’s L4 or L5 point, respectively.

† Actually even that description is not quite right. AIUI, the gravity gradient around L1, L2, and L3 makes them saddle points, rather than the top of a hill. That is, it’s uphill in two directions and downhill in the other two.

8

Yes, L4 and L5 are more like dips in the gravity well, where anything close will eventually gravitate (so to speak) to the L-point, circling (or beaning) that point. There is a “gotcha” with L4 and L5 that the second body needs to be significantly smaller than the first and significantly larger than the third body, but with Saturn 95 times Earth this should not be a problem. (Asimov wrote a short story that depended on this point - with two not too different stars in a dual system, there is no L4/L5 point.)

9

The good news, though, is that the interplanetary billiards wouldn’t be painful at all. If you started with Saturn on the opposite side of the Sun from the Earth, then the Earth would end up in a horseshoe orbit with respect to Saturn. The two would still “collide” periodically, in the sense of exchanging momentum, but the collision would be purely gravitational and hence gentle.

What oxygen? There are surely oxygen atoms in the soup of stuff that makes up Saturn, but they’ve all already joined with other elements. The Earth is the only place known in the Universe with significant amounts of elemental oxygen, and in fact we would be very interested in finding any other such places, since it would be a strong indicator of life.

10

That’s what would happen if the orbits of the two planets are not exactly the same. One will orbit a little bit faster than the other, so will eventually catch up and do sort of an orbit swap with the other. It’s called co-orbiting. A couple of Saturn’s moons do this (Janus and Epimetheus).

But that won’t work if Saturn is put into the exact same orbit as Earth. Neither planet will catch up to the other. Eventually things will go bad, although I don’t think we can predict how.

11

Good catch. Thank you.

12

They can’t go bad so long as they stay in the exact same orbit. If things were to go bad, it would start with the orbits becoming slightly different.

13

Yes, but just being in slightly different orbits may not be enough to ensure a stable co-orbit situtation. I’m not even sure it’d work for planets in even slightly elliptical orbits. Janus and Epimetheus are in circular orbits so it works for them. But with ellipticity, the orbit-swap interaction may not be the same from one orbit to the next, so things may start to go haywire.

14

Untrue. Saturn would be comprehensively destroyed and Earth would be 100 times larger. We’re the home team and we declare Ship of Theseus rules to be in effect.

15

If Saturn were in the position of Earth’s orbit around the Sun, wouldn’t a Jovian planet being close to the Sun result in a “hot Jupiter” type of planet, or would it have to be much closer to the Sun for such a result?

16

What about the rings? Would there be enough disruption to leave significant debris in Earths path?

17

The rings are made of what passes for gravel in the neighborhood, but what we would be more inclined to call “ice” down here. Put Saturn, with its 11 hour day, at 1 AU, fling those frosties around, and see what you have left. Not gonna be a problem.

18

Collisions and such notwithstanding, wouldn’t a lot of Saturn just boil away and eventually make its way back to where it came from, driven by the solar wind? ISTM the solar system is doing (or perhaps has done ) a sort of fractional distillation thing, where the lighter and more volatile elements and compounds get driven outwards, condensing in the colder, outer regions, leaving the heavier and less volatile stuff closer in.

Or did that already only happen as a one-time deal during the formation of the solar system, not an ongoing process?

19

Well, wait a minute: Jupiter already does not orbit the Sun, so if you moved Saturn to 1 AU, if I undestand the principle of inverse squares, it would really not be orbiting the Sun. In that situation, the L3 position would be extremely unstable for old Terra.

20

Jupiter orbits the Sun. The fact that the center of mass of the J-S system is outside the Sun is irrelevant and doesn’t change the fact that Jupiter orbits the Sun.

I don’t think you understand correctly. Moving Saturn to 1 AU would make the center of mass of the S-S system be closer to the center of the Sun than it is now.

Saturn has a pretty good magnetic field, not quite as strong as Earth’s but close. So I expect that would protect its atmosphere from the solar wind. I’m pretty sure Saturn’s gravity should also keep the hydrogen in its atmosphere from escaping even with the greater heating in its new location.