# Stupid Question: Why are the outer planets of the solar system so far away? **URL:** \ **Category:** Factual Questions\ **Created:** [November 23, 2011, 7:00pm UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903 "2011-11-23T19:00:16Z")\ **Posts on this page:** 13\ **Page:** 2
**Author:** ![Alka\_Seltzer](https://avatars.discourse-cdn.com/v4/letter/a/c89c15/32.png) [@Alka\_Seltzer](https://boards.straightdope.com/u/Alka_Seltzer)\ **Post date:** [November 24, 2011, 10:23am UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903/21 "2011-11-24T10:23:27Z")
> [@Chessic\_Sense](#): > > There are other planets that are supposed to be at those intervals. They’re just inside Jupiter and Saturn. That’s how they got so big in the first place. 😉 What I mean is that big planets clear big orbits. There can’t be anything near them or they’d have been sucked in by now, most likely. This is pretty much correct. You could have another planet fairly close to Jupiter, but only if it was in a resonant orbit. For example, if it went around the sun three times for every two orbits Jupiter completed, the net influence of Jupiter on its orbit would be zero. However, such an orbit would not be stable over millions of years, unless it was also in some kind of orbital resonance with Saturn. > [@Kevbo](#): > > The placement of the largest of a system of planets is random. I’d expect the largest planet will form in a fairly predictable location, just beyond the frost line where there is more material available due to the presence of solid ices. Other processes might cause it to migrate from that location. Planets can only form in circular orbits, as otherwise collisions between planetesimals are too violent for them to coalesce into larger bodies. > [@Jamicat](#): > > It’s a conundrum… > > According to Newton and Kepler’s physics, there should be, relative Mass Bodies in every viable window of space in our solar system. > > Apparently, things seem to got all messed up with all that Einstein’n physics junk. > > Consensus is, outside influence disturbed the system over millenia’ of time. I’m not sure where you read that, but it’s incorrect. There is no reason to expect a solar system full of evenly spaced similarly sized bodies, theory predicts the opposite, and that’s what we actually see. Close to a proto-star, it is too hot for ices to form, so there is less material to form planets. Also, once a body reaches a critical size it can retain hydrogen and helium, and experiences runaway growth as a result. This is what happened to the gas giants. Uranus and Neptune got to this point fairly late compared to Jupiter and Saturn, and are much less massive as a result. When the Sun ignited, any remaining gas was removed from the solar system by radiation pressure, terminating the growth of the gas giants. Relativity had very little influence on the formation of the solar system, it’s difficult to measure its effect on the orbit of the planets other than Mercury. It’s unlikely another star has passed close enough to the solar system since it was formed to affect the planets. The distances between stars is vast compared to the distance between the Sun and Neptune. Passes by other stars do affect objects in the Oort cloud. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [November 24, 2011, 3:09pm UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903/22 "2011-11-24T15:09:36Z")
> [@dtilque](#): > > The answer to the OP has to do with where the planets formed and how they migrated afterwards. The [Nice model](http://en.wikipedia.org/wiki/Nice_model) has the giant planets forming in between 5 and 17 AU and then migrating to their current locations. Possibly Neptune started closer to the sun and swapped positions with Uranus. > > For the inner planets, there’s a model which has them all forming between .7 and 1 AU with Jupiter forming about 3.5 AU out and then some subsequent migrations. It’s described [here](http://www.skyandtelescope.com/community/skyblog/newsblog/105108519.html). This model disagrees with the Nice model as to the distance Jupiter formed and how it moved, so there’s obviously going to have to be some modifications of one or both to get them in sync. I’ve recently read of simulations where they were more likely to obtain a solar system like ours if they began with five gas giants, and one gets ejected from the solar system. Starting with only four, one still usually got ejected, obviously not matching our current system. ETA: [Here’s an article on it](http://www.sciencedaily.com/releases/2011/11/111110142102.htm). I thought the article I read linked to a paper on arxiv, though. ---
**Author:** ![No\_Wikipedia\_Cites](https://avatars.discourse-cdn.com/v4/letter/n/94ad74/32.png) [@No\_Wikipedia\_Cites](https://boards.straightdope.com/u/No_Wikipedia_Cites)\ **Post date:** [November 25, 2011, 2:04pm UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903/23 "2011-11-25T14:04:48Z")
> [@ZenBeam](#): > > I’ve recently read of simulations where they were more likely to obtain a solar system like ours if they began with five gas giants, and one gets ejected from the solar system. Starting with only four, one still usually got ejected, obviously not matching our current system. > > ETA: [Here’s an article on it](http://www.sciencedaily.com/releases/2011/11/111110142102.htm). I thought the article I read linked to a paper on arxiv, though. And if a gas giant gets booted from the Solar System, what happens to it? How long before the free ride on the sun’s energy runs out? Do the gases waft off into space? Is Jupiter’s long lost twin on its way to Alpha Centauri? Probably a bad real estate purchase . . . ---
**Author:** ![Alka\_Seltzer](https://avatars.discourse-cdn.com/v4/letter/a/c89c15/32.png) [@Alka\_Seltzer](https://boards.straightdope.com/u/Alka_Seltzer)\ **Post date:** [November 25, 2011, 2:26pm UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903/24 "2011-11-25T14:26:30Z")
> [@No\_Wikipedia\_Cites](#): > > And if a gas giant gets booted from the Solar System, what happens to it? How long before the free ride on the sun’s energy runs out? Do the gases waft off into space? Is Jupiter’s long lost twin on its way to Alpha Centauri? Probably a bad real estate purchase . . . It wouldn’t “stop”, it would orbit the centre of the Milky Way independantly of the sun. Inertia causes objects to continue moving until they meet an external force. Escaping from the solar system would actually make it easier for a planet to retain it’s atmosphere, as ultraviolet light from the sun tends to strip it away. There are thought to be many rogue planets in the galaxy, some candidates have been detected. Very few of them would be expected to have an encounter with another star system, as the average distances between them are vast. It would cool down in the absence of the sun’s warmth, but not as much as you might think. Jupiter generates as much heat internally as it receives from the sun, as it contracts. When it formed it was much hotter and twice as big. ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [November 25, 2011, 6:15pm UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903/25 "2011-11-25T18:15:42Z")
Recent thread on this issue, discussing the article **ZenBeam** cites: [Jumping Jupiter](http://boards.straightdope.com/sdmb/showthread.php?t=631748&highlight=jumping+jupiter) (every time I see the thread title I hear it in a Norwegian-Minnesotan accent 😉 ) ---
**Author:** ![spacemandave](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@spacemandave](https://boards.straightdope.com/u/spacemandave)\ **Post date:** [November 29, 2011, 2:08am UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903/26 "2011-11-29T02:08:10Z")
I am a planetary scientist, and this happens to be my field of research (planetary orbital dynamics and planet formation). There is no simple answer to why the solar system is arranged the way we see it, and indeed the data coming in regarding exoplanetary system architectures shows us that there is a huge variety of possible outcomes for the messy business of planet formation. However, there are a couple of things that are pretty universal. One is that you tend to be able to pack in more planets in a given space closer to a star compared to farther away. This results from the way that some important gravitational interactions scale with orbital distance. So it’s no surprise that our solar system and many others exhibit Titius-Bode-like progressions (the exact form of the Titius-Bode “law” is completely arbitrary, however). The other universal is that planetary systems seem to undergo large-scale rearrangements early in their histories. Planetary systems can’t know ahead of time if they will be stable over billion-year timescales. As far as we can observe with other stars as well as piecing together a history of our own solar system’s birth from a variety of sources, gas giants form relatively quickly (for an astrophysicist) in less than 10 million years. Rocky planets like Earth can take longer, but less than 100 million years, give or take. Therefore any instability that happens on timescales longer than this won’t factor in to the planet-formation process itself, so planetary systems can form in configurations that are like an orbital mechanics version of a ticking time bomb. When the system goes kablooey, you can eject a planets, rearrange orbits, destabilize asteroids/comets, and generally make a big mess of things. We have good reasons to believe our own solar system went through a period of instability like this about 4 billion years ago, give or take. We think that the gas giant planets formed closer together than we see them today, but at some point in the first 700 million years of solar system history they went unstable. The 5-gas giant planet model that was linked to earlier is part of a research effort understand what happened at this time. It’s under an umbrella of theoretical models called the Nice Model (named after the city in France so rhymes with geese), that are attempting to make sense of the early solar system. There is probably a range of violence that planetary systems undergo. Some systems go so violently unstable that their gas giants don’t even orbit in the same plane as each other and are on very elliptical orbits. Others may be lucky enough to be stable for the lifetime of the star. I’ve seen some work that suggests, in keeping with the Copernican principle, that our own system is likely somewhere in the middle of the violent-to-boring scale of planetary instability, and is probably a rather average type of planetary system. ---
**Author:** ![EdwardLost](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@EdwardLost](https://boards.straightdope.com/u/EdwardLost)\ **Post date:** [December 1, 2011, 9:42pm UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903/27 "2011-12-01T21:42:39Z")
> [@spacemandave](#): > > …gas giants form relatively quickly (for an astrophysicist) in less than 10 million years. Rocky planets like Earth can take longer, but less than 100 million years, give or take. Thanks for the great explanation, Dave. I had thought that gas giants form around a rocky core. How do gas giants form more quickly than rocky planets? ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [December 1, 2011, 10:36pm UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903/28 "2011-12-01T22:36:13Z")
I always assumed Bode’s law had to do with harmonics of the nearby orbits. If 2 orbit periods were harmonic in time, they whould either reinforce or cancel each other. (We see some such effect with “shepherd moons” and Saturn’s rings.) As a result, over time, much of the debris in the solar system is swept into the path of the nearest aggregate mass, forming planets at relatively prime intervals. Jupiter, being most of the non-solar mass of the system, has an excessive effect on he rest of the system. ---
**Author:** ![Alka\_Seltzer](https://avatars.discourse-cdn.com/v4/letter/a/c89c15/32.png) [@Alka\_Seltzer](https://boards.straightdope.com/u/Alka_Seltzer)\ **Post date:** [December 1, 2011, 10:53pm UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903/29 "2011-12-01T22:53:11Z")
> [@EdwardLost](#): > > I had thought that gas giants form around a rocky core. How do gas giants form more quickly than rocky planets? The form around a rocky and icy core, once they reach a critical size they can start pulling in hydrogen and helium. They form outside the frost line, where ices are solid and there is more material available. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [December 1, 2011, 11:02pm UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903/30 "2011-12-01T23:02:12Z")
**Why are the outer planets of the solar system so far away?** If they weren’t so far away, they would be inner planets. 🙂 ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [December 2, 2011, 8:14am UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903/31 "2011-12-02T08:14:02Z")
> [@md2000](#): > > I always assumed Bode’s law had to do with harmonics of the nearby orbits. If 2 orbit periods were harmonic in time, they whould either reinforce or cancel each other. (We see some such effect with “shepherd moons” and Saturn’s rings.) As a result, over time, much of the debris in the solar system is swept into the path of the nearest aggregate mass, forming planets at relatively prime intervals. Jupiter, being most of the non-solar mass of the system, has an excessive effect on he rest of the system. The term you want is resonances, not harmonics, I think. Current thinking is that the planets did not form where they are now located, but rather migrated due to gravitational interactions with planetisimals and the disk of gas and dust. Resonances probably influenced where they formed and how they migrated to but were not the whole story. ---
**Author:** ![Apollo](https://avatars.discourse-cdn.com/v4/letter/a/41988e/32.png) [@Apollo](https://boards.straightdope.com/u/Apollo)\ **Post date:** [December 2, 2011, 11:18am UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903/32 "2011-12-02T11:18:44Z")
Cool Interactive 3D solarsystem. [Link](http://www.solarsystemscope.com/) press space for fullscreen. ---
**Author:** ![Alan\_Smithee](https://avatars.discourse-cdn.com/v4/letter/a/2acd7d/32.png) [@Alan\_Smithee](https://boards.straightdope.com/u/Alan_Smithee)\ **Post date:** [December 3, 2011, 5:04am UTC](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903/33 "2011-12-03T05:04:27Z")
> [@njtt](#): > > **Why are the outer planets of the solar system so far away?** > > If they weren’t so far away, they would be inner planets. 🙂 I’m glad someone else thinks the way I do! [Previous page](https://boards.straightdope.com/t/stupid-question-why-are-the-outer-planets-of-the-solar-system-so-far-away/603903.md?page=1)