Simple Question about the Solar System/Planetary Systems

Main Question or Discussion Point

In our solar system, the more massive planets orbit the Sun in between less massive planets. I was wondering if it was by coincidence or if it's dictated by some universal law in all planetary systems. One would think more massive planets would be closer to the sun but I'm clearly missing something very simple.

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The planets orbits are elliptical by nature. They travel in Mathematical calculated paths that agree with the observed. The inner solid planets do not have a very large gaseous outer layer because of being bombarded by the Solar Wind tearing away at the upper atmosphere. The Solar Wind disapates more as you get by the rocky planets.

That still doesn't explain why Jupiter and Saturn wouldn't/couldn't be the farthest planets away from the sun.

Chronos
Gold Member
Think about the minimum distance those gasses can cool enough to condense.

Think about the minimum distance those gasses can cool enough to condense.
I'm not sure if it's the way you worded it but I don't fully understand still. Are you saying that Jupiter and Saturn's distance from the sun is the perfect temperature for hydrogen and other gases to be pulled in gravitationally without being overheated by the Sun's rays? Are the outer planet's location too cold for the same to happen?

Drakkith
Staff Emeritus
In our solar system, the more massive planets orbit the Sun in between less massive planets. I was wondering if it was by coincidence or if it's dictated by some universal law in all planetary systems. One would think more massive planets would be closer to the sun but I'm clearly missing something very simple.
Given the abundance of "Hot Jupiters" and other exoplanets close to their stars I think that it's simply coincidence.

As far as I know, the traditional view is this: There are two competing factors in the formative stages of a solar sytem. One is that the protoplanetary disk grows more tenuous as you move out from the star. Thus, close to the star, there is lots of stuff which could form into planets, far from the star, there is less. The other one is that planet formation proceeds more readily as you move out from the star. Thus, close to the star, there may be lots of stuff but it'll have a hard time coalescing into a planetary body, far from the star, there may be less stuff but it'll have an easier time of coming together.

The explanation for the first factor is fairly obvious, I think. The explanation for the second factor is temperature, as Chronos already hinted. For example, beyond a certain distance from the star, water freezes, which gives you ice/snow to work with in assembling a planetary core, in addition to the rocky materials closer in. There is a lot more water than there is a rocky stuff, and ice/snow sticks together a lot better than rocky stuff - and since adhesion is crucial to get to the point at which gravity can take over, the latter is quite crucial. This distance lies between Mars and Jupiter, which should come as no surprise. Once a planetary core exists, gravity can begin to draw in at atmosphere, which is again easier at lower temperatures.

Keeping the first factor in mind, this accounts for our solar system pretty well. As Drakkith said, though, it does a terrible job of matching the abundances of exoplanets that have been found so far. The way to reconcile the two involves additional considerations such as observation bias, migration of planetary orbits, and additional factors in planet formation which happened not to have much effect in our solar system but did in those others. Does that help?

Or maybe not coincidence. There have been people that have pointed out that solar systems with hot Jupiters are unlikely to have intelligent life develop because the hot Jupiter will disrupt the inner star system.

Drakkith
Staff Emeritus
Or maybe not coincidence. There have been people that have pointed out that solar systems with hot Jupiters are unlikely to have intelligent life develop because the hot Jupiter will disrupt the inner star system.
How does that make it not coincidence?

How does that make it not coincidence?
Weak anthropic principle: We would not exist in a solar system with a hot Jupiter.

Drakkith
Staff Emeritus
Weak anthropic principle: We would not exist in a solar system with a hot Jupiter.
What does intelligent life have to do with the formation of gas giants?

What does intelligent life have to do with the formation of gas giants?
The problem is limited to hot Jupiters. According to the current models the formation of a hot Jupiter destabilizes the orbits of potentially inhabitable planets. But in the light of Kepler-47 the models might need to be updated.

The problem is limited to hot Jupiters. According to the current models the formation of a hot Jupiter destabilizes the orbits of potentially inhabitable planets. But in the light of Kepler-47 the models might need to be updated.
Also, if situated in larger distances, wouldn't the gas giants act as "guardian angels", taking care of many stray asteroids or comets that would otherwise *might* crash into planets with life?

Also, if situated in larger distances, wouldn't the gas giants act as "guardian angels", taking care of many stray asteroids or comets that would otherwise *might* crash into planets with life?
That is what happened with our own solar system. Jupiter and Saturn work as "space dust cushions" so we don't get obliterated by asteroids and different sort of space dust pulled by our Sol gravity.
Regarding comets, it is a little bit different. Actually, Jupiter and Saturn worked as gravity magnets to pull in the comets from the Oort cloud. Comets where the main source for water during the early stages of our planetary formation process, so comets reaching our solar system habitable zone made it possible for the water to melt and become liquid.

On regards of the "coincidence" part others were discussing. Mainly, yes, we can say that our solar system got a series of "coincidental" factors that made it possible for intelligent carbon-oxygen based life to evolve.
Our habitable zone is large enough. Sol is a pretty stable G star, and has been for 5 billion years, more than enough time for life first, and then intelligent life, to evolve. We have the huge Jovian external-planets sheltering us from bad dust and also working to straight the comets (good dust) orbits. There are no O stars in the vicinity, or any other dead photon cannon type of star or stellar source. And so on.

But, that doesn't mean that set up wouldn't be pretty normal in the rest of our galaxy, or even the universe, at least the part that is around our same 5 to 6 billion age. The outskirts of the universe with their dead-barely dead 15 to 16 billion old galaxies and stars, could be really different.
May be also the other way around, and our stellar system setup is a pretty odd one. Perhaps, there are Astronomers and Astrophysics right now, somewhere around a binary star, looking at our Sol system and saying that our system is inhospitable to life (they can't see the inner planets...), and there are also religious fanatics there saying they are the only planet to harbor life, because their god created them as its image and its only creation, and there is only one son of god and if that son of god provided atonement for them, he couldn't possibly have died as well for another creation somewhere else, and the radio waves coming from our Sol system, and being captured by their radio-observatories, are just considered as: space noise, by the scientists, and "a devil's lie", by the religious people. We never know.
Only one thing is certain about the universe: physics laws will apply equally everywhere.

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It is not necessarily a coincidence if you consider the base core accretion model, which is the favoured (but not perfect) way of explaining how planets form.

The first step is to build up a rocky body. This is the same for the inner terrestrial planets and the outer gas and ice giants. These bodies are built quicker closer to the star. There is an equation but it's not overly intuitive, so just trust me :). Whilst the inner bodies form quicker, they do not reach the same mass because there is a lot more "solid" (a pretty lose term since we're basically talking about dust) material beyond the snow line (about 4 times more dense).

So you have three cases:

1) Inner planets form quickly but don't have a lot of solid material so they can't get huge. You need to get huge if you want to suck in a heap of gas like Jupiter has (at least 10 Earth masses or so).
2) Jupiter and Saturn form slower but with enough time to suck up a lot of gas.
3) Neptune and Uranus form too slowly so when they finally get big enough to suck in the gas, most of it has gone!

Of course, this comes with the standard planetary formation theory disclaimer of...we really know **** all about this stuff and holes can be found in every model. Furthermore, we only have a rough estimate of the cores of these giant planets.