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So let's say it's a big planet, like a Jupiter. Not talking about our Solar System. This is hypothetical. But say a Jupiter-esque planet in a solar system suddenly disappears. And by disappears I mean just that. It's there one moment and gone the next. No large chunks spiraling out to hit other planets. No debris of any kind.

Its gravitational pull is therefore gone. And it had a good chunk of the mass of the solar system. Now that 20% or so of total solar system mass is no longer pulling.

What would happen to the other planets in the solar system? If that missing planet had moons, what happens to those moons?

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    $\begingroup$ Welcome to Worldbuilding! Here are some vaguely related questions you may find interesting and useful: worldbuilding.stackexchange.com/q/34341/627, worldbuilding.stackexchange.com/q/49432/627, worldbuilding.stackexchange.com/q/6518/627. By the way, I hope that 20% figure doesn't include the mass of the central star. Can you clarify if that's the case or not? Thanks. $\endgroup$
    – HDE 226868 ♦
    Commented Mar 20, 2017 at 23:57
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    $\begingroup$ My friend showed me a 'game' called Universe Simulator and it can simulate exactly that. You can add or delete planets, or suns, or black holes, or quasars, and see the paths of all the other planets and moons. Check YouTube for gameplay videos. $\endgroup$
    – Chloe
    Commented Mar 21, 2017 at 5:14
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    $\begingroup$ @kingledion you mean theoretical or hypothetical? $\endgroup$
    – xDaizu
    Commented Mar 21, 2017 at 13:08
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    $\begingroup$ A planet 20% of the mass of the solar system is a binary star system. That's a red dwarf. Jupiter is three orders of magnitude smaller than the sun. It's not even one percent of the mass of the solar system. $\endgroup$
    – Jim W
    Commented Mar 21, 2017 at 14:56
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    $\begingroup$ I though I read somewhere that the big planets where partly responsible for the en.wikipedia.org/wiki/Milankovitch_cycles the Earth experiences by affecting the tilt and the eccentricity of the orbit. I which case, it would be a few thousands of years before we feel big effects. $\endgroup$
    – njzk2
    Commented Mar 21, 2017 at 15:13

4 Answers 4

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I know you said this is not a real planet, but I'm going to do some math with our Solar System to give you some idea of what the effects might be. Also, I'm ignoring your statement of 20% of the mass of the solar system. An object that was 20% of our solar system would be 1/4 the mass of the sun, and most definitely undergoing fusion: not a planet.

Effect of Jupiter on the other planets

Lets calculate the effects of Jupiter's gravity on Mars (the closest planet to it) compared to the effects of the Sun. The distance from the Sun to Mars is around $2.28\times10^{11} \text{ m}$ and the standard gravitational parameter ($GM$) of the Sun is $1.33\times10^{20}\text{ m}^3\text{s}^{-2}$. The acceleration of the Sun on Mars is $$\frac{GM}{r^2} = 0.00256 \text{ m/s}^2.$$

The distance from Jupiter to Mars is around $5.50\times10^{11} \text{ m}$ at nearest approach and the standard gravitational parameter ($GM$) of Jupiter is $1.27\times10^{17}\text{ m}^3\text{s}^{-2}$. The acceleration of Jupiter on Mars is $$\frac{GM}{r^2} = 0.000000420 \text{ m/s}^2.$$ That is about four orders of magnitude smaller than the effects of the Sun, and two orders of magnitude smaller than the effect of the moon on the Earth.

While the presence of lack thereof of Jupiter will certainly change things in the Solar System in the long run, its effects at the present time would be negligible. For example, if Jupiter were 10 times bigger and 10 times closer to Mars, its effects would be similar to the effects of the moon on Earth. It is hard to imagine a solar system where a planet's disappearance would affect the other planets on timescales shorter than millions of years.

Effect of Jupiter on its own moons

This would obviously be significant. According to this handy-dandy calculator, the escape velocity from Jupiter's orbit to the galaxy is 18.47 km/s. Thus, any satellite lined up so that its speed in the solar frame of reference was 18.47 km/s more than that of Jupiter would end up leaving the Solar System.

The orbital speed of a a moon relative to the main planet is $$v\approx \frac{2\pi a}{T}$$ for a low eccentricity orbit. Looking at the list of the moons of Jupiter, the farthest satellite that could potentially end up ejected from the solar system would be Thebe with an orbital speed of about 23.9 km/s. The other moons, including all four Galilean mooons, are not going fast enough to get ejected, no matter what orientation in orbit they are in.

Now keep in mind that Jupiter itself is moving at about 13 km/s. So if a satellite's orbit has it going the opposite direction from Jupiter's direction of motion, it would begin a retrograde orbit of the sun. For example, if Io at 17.3 km/s were in such a path when Jupiter disappeared, it would suddenly be orbiting the Sun the wrong way at 4.3 km/s; much too slow. This means it would fall into the inner solar system gaining speed until it shot back out to the former orbit of Jupiter.

Of course, Io would have a rough ride trying to follow this orbit for a few million years; inevitably it would interact gravitationally with the much larger Venus, Earth or Mars, and end up being thrown about again. It could even collide with one of the inner planets.

All in all, the moons would get thrown willy-nilly about the solar system. While it is relatively unlikely that any of them will be ejected from the Solar System based on their initial velocity, they could end up being slung out of the Solar System by the outer planets, caught into orbit of a different gas giant, hurled into the Sun, or they could collide with just about anything.

Conclusion

While the immediate effects of Jupiter's disappearance would not cause any harm, the medium-term effects of its many moons and Trojans flying around the solar system are potentially cataclysmic. Assuming your disappearing planet had as many co-orbital elements as Jupiter, this would basically be a repeat of the Late Heavy Bombardment, and I would hate to be in a non-spacefaring civilization in that system during the next few million years.

To be more clear, with the four quite sizable Galilean [former] moons roaming the solar system at will for millennia, there will be significant perturbations of the Asteroid belt, various Trojans and Centaurs and whatever. All these new flying rocks will be the biggest and most proximate problems, added on top of the hundreds of medium sized moons to moonlets that Jupiter will release directly.

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  • $\begingroup$ Wouldn't the disappearances of such a large planet throw off orbital resonances? I'd imagine that those effects would start showing up on the order of decades. $\endgroup$
    – Joe Kissling
    Commented Mar 21, 2017 at 1:12
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    $\begingroup$ @JoeKissling Perhaps comparing Jupiter and the Sun's effects on Saturn would be more appropriate, given that Saturn is the one locked in resonance. You are welcome to do it. Still, I think the flying moons will be of greater concern a lot sooner than a slowly receding Saturn. $\endgroup$
    – kingledion
    Commented Mar 21, 2017 at 1:24
  • $\begingroup$ I agree, just adding a comment. Rouge moons are certainly a bigger threat. $\endgroup$
    – Joe Kissling
    Commented Mar 21, 2017 at 1:33
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    $\begingroup$ @JoeKissling And now consider the ones not wearing make-up.:) $\endgroup$
    – DRF
    Commented Mar 21, 2017 at 7:46
  • $\begingroup$ I love the idea of Jupiter leaving with a Scattershot $\endgroup$
    – xDaizu
    Commented Mar 21, 2017 at 12:51
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The moons would probably careen off tangentially at the point they were at when the body disappeared. Whether or not those bodies leave the solar system depends on their orbiting velocity and direction. They may leave or settle into new orbits.

Now your planet is quite massive. Does the mass of your solar system include its sun? If so, then your gas giant is absurdly large. Our Jupiter is 1/1000th the mass of the Sun. Your Jupiter is enough to affect not just the other planets to a considerable margin, but the star itself. Your planets were likely experiencing highly irregular orbits caused by 2 massive bodies. Their orbits will likely stabilize if your star has enough mass. Whether this affects the ecology / seasons of your planets is up to you.

The solar system itself will experience a trajectory change as well. The sun was likely wobbling because the orbit of this massive planet. I would wager that the sun will change trajectory through space. The whole system will also lose momentum as it travels through space, but the magnitude of its velocity will remain constant..

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  • $\begingroup$ At that size, this object may very well have it's own solar system to it, not so much moons, but earth size or larger planets. $\endgroup$
    – Joe Kissling
    Commented Mar 21, 2017 at 1:02
  • $\begingroup$ Or, maybe the stellar system only includes a few other planets of mercurian mass. $\endgroup$
    – can-ned_food
    Commented Mar 21, 2017 at 1:34
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On top of Kingledion's answer, pointing out that effects would be visible on long term, there are few other consideration to be made.

  1. Jupiter, together with Saturn, helps keeping the inner solar system safer, by "sweeping" asteroids and comets. With Jupiter gone, one could expect an increased rate of asteroids impact on the inner solar system.
  2. Jupiter's gravity is the main reason for having the asteroids belt instead of a planet. The present time is probably too late for having, Jupiter gone, the formation of a new planet, but depending on the evolution phase during which Jupiter was MIA the outcome could have been different.
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  • $\begingroup$ Some asteroids in the Belt could merge - or maybe not - but definitely the combined mass of all asteroids is way smaller than that of any present planet. However, without Jupiter Ceres might clean its orbit and qualify for the official definition of planet even with its present mass. $\endgroup$
    – Pere
    Commented Mar 21, 2017 at 9:33
  • $\begingroup$ @Pere I find that relatively unlikely in this scenario. The main reason is that there are now 4 other objects in the solar system much larger (50-150x mass) than Ceres. It is more likely that one or more of them would settle into orbits between Mars and Saturn, potentially clearing the asteroid belt. $\endgroup$
    – kingledion
    Commented Mar 21, 2017 at 14:25
  • $\begingroup$ @kingledion I agree that it's unlikely even without the Galilean ex-satellites moving around. Possible, but unlikely. $\endgroup$
    – Pere
    Commented Mar 21, 2017 at 14:28
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Consider how Neptune was found:

Small "irregularities" in the orbit of Uranus got people looking. Neptune is smaller than Jupiter and farther out, but the difficulty in proving Neptune's existence should be a sign how small the direct effects will be.

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