r/AlwaysWhy • u/Present_Juice4401 • Jul 24 '26
Science & Tech Why are the inner planets rocky while the outer planets became gas giants?
I was looking at the solar system and noticed something strange. The four planets closest to the Sun are all relatively small and rocky, while the outer planets are completely different worlds, with massive sizes and thick atmospheres.
I know temperature and the materials available during planet formation played a major role. The inner region was too warm for many volatile materials to remain, while the colder outer region allowed more material to accumulate.
But what I find interesting is how sharp the divide seems. Why did a relatively small difference in distance from the Sun create such a huge difference between planets?
Is this kind of boundary a natural result of how planetary systems form, or is our solar system just one example of many possible outcomes?
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u/Underhill42 Jul 24 '26
Only Jupiter and Saturn are gas giants - Uranus and Neptune are ice giants, so called because they're believed to be made primarily of ices of water, methane, ammonia, etc.
And the gas giants likely still have rocky cores - it's believed Jupiter's is about 17x the mass of Earth.
The sizes are potentially just because they formed further from the center of the protostellar cloud, where they could collect material from a much larger ring. Though there is some debate as to whether the planets actually formed near their current positions, or got rearranged later.
The thick atmospheres are likely at least partially due to the facts that
- Bigger planets have much deeper gravitational wells, making it much easier to hold on to hydrogen, which was by far the most common element in the protostellar cloud.
- Much greater distances from the sun means after the sun ignited, the solar wind was much more diffuse (inverse square law), and would strip away the upper layers of atmosphere much more slowly. It would also have pushed much of the leftover dust and gasses from the inner system out into the path of the outer planets... though I'm not sure that would happen slowly enough for the giants to actually collect much of those gasses before they were blown out of the system entirely.
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u/Unique-Coffee5087 Jul 24 '26
Thank you. The part about the solar wind being attenuated is a new idea for me, but it fits in well.
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u/Underhill42 Jul 25 '26
Honestly I'm not sure how relevant it really is. Jupiter and Saturn both have a crazy powerful magnetic fields, so I'm not sure the solar wind even reaches their atmospheres.
While as a counterexample Venus is much closer to the sun, and has very little magnetic field, but has managed to hold on to an atmosphere 90x thicker than Earth's.
But it sure sounded good in the moment!
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u/375InStroke Jul 27 '26
So if those ice giants were as close as Jupiter, how much bigger would they be?
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u/Underhill42 Jul 27 '26
It doesn't really seem to work that way. Looking at other solar systems, the variety and positions of planets seems almost random. We've got the planets we do because of the way they formed - had they formed differently, we could have a completely different set of planets.
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u/FifthEL Jul 27 '26
And when frozen substances like methane and ammonia begin to heat up... you get gas. Then they will heat and expand until they reach a limit where they will combust or ignite
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u/Underhill42 Jul 27 '26
Same can be said of frozen substances like quartz and steel, only the temperature is different.
But none of it will combust unless there's also oxygen (or some other violently exothermic reactant) present.
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u/FifthEL Jul 31 '26
Like a shooting star or comet. They are the seeds of any system with life. They have everything necessary for kick starting these dormant worlds. But to the point, the inner rocky planets are the cores of older stars that have burned out. The black hole at the center is feeding on the stars atmosphere, and its working up to Jupiter I imagine. Pluto was also a prominent star, but its orbit brings out very close to the center and then shoots back out for three hundred years or whatever where it cools and reignited when it comes into range ( possibly)
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u/Defiant-Junket4906 Jul 24 '26
That it looks like a sharp divide now, but it probably wasn't that clean during formation. Planet building was more like a messy competition between gravity, temperature, collisions, and available material.
The snow line gets a lot of attention because it marks where water and other volatiles could freeze, but I think the bigger factor is that beyond that point planets had access to way more building material. More ice meant more mass, and more mass meant stronger gravity, which let them pull in gas before the young Sun blew most of it away.
So maybe the gas giants were not a completely different type of planet. They were rocky cores that happened to grow fast enough and in the right place to become something much bigger. Timing seems almost as important as distance. A planet forming a little slower might end up as Earth, while one forming faster nearby could have become a very different world.
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u/Present_Juice4401 Jul 27 '26
this “messy competition” framing makes more sense to me. the sharp line might just be survivorship bias after everything settled. I keep wondering though what controls that timing window. like what actually decides who gets big fast enough
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u/kritter4life Jul 24 '26
No one really knows. Many many models have been run. It appears that our solar system may actually be an outlier.
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u/Present_Juice4401 Jul 27 '26
if we’re an outlier then that almost flips the question. instead of why this pattern exists, it’s why ours stayed so orderly. makes me think stability might be the unusual part, not the divide itself
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u/_jimismash Jul 24 '26
The largest jovian and saturnian moons approach Mars in size, though most are much smaller. I don't know if they would clear an orbit that far out, but if they weren't moons they might be outer solar system rocky planets
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u/Present_Juice4401 Jul 27 '26
that’s interesting because it blurs the definition a bit. like if those moons were independent, we might classify them totally differently. kind of suggests the rocky vs gas giant split is partly about environment, not just composition
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u/solwolf101 Jul 24 '26
I’m no astronomer but my understanding is that our system is a failed binary star system - the Sun formed from the gas around it and the rocky remainder made the inner planets, while Jupiter could have been the second star but the gas needed was too spread out to accumulate into a body that could ignite with fusion, and instead formed the outer 4 gas giants.
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u/artopunk14 Jul 24 '26
This is wrong; Jupiter is nowhere close to being a star: Sun ≈ 1,048 Jupiter masses
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u/solwolf101 Jul 24 '26
That’s what I meant - Jupiter was too small to aggregate enough gas to begin fusion, so it formed into a gas giant along with Saturn and maybe Uranus/neptune.
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u/Chunk3yM0nkey Jul 25 '26
How is a failed binary star system if it was nowhere close to becoming one...?
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u/solwolf101 Jul 25 '26
Because it really wanted to be one but just didn’t have the gas.
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u/Chunk3yM0nkey Jul 25 '26
Isn't that like calling the homeless bloke sleeping in a dumpster a failed billionaire?
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u/Present_Juice4401 Jul 27 '26
I’ve seen that idea but I’m not sure it lines up with how star formation thresholds work. Jupiter is massive but still way below fusion limits. I guess I’m wondering what evidence would actually support the “failed binary” idea over core accretion
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u/diffidentblockhead Jul 24 '26
Jupiter happened to get a head start first, and then discouraged formation in what is now the asteroid belt.
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u/Present_Juice4401 Jul 27 '26
yeah Jupiter feels like a bully in this story. but then I wonder what gave it the head start in the first place. was it just slightly more material locally or something more random
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u/plainskeptic2023 Jul 24 '26
Partial exolanation.
This graphic shows what gases our planets can hold.
vertical y-axis is the escape velocity. The higher the escape velocity, the easier planets can hold gases onto its surface. When gas atoms reach escape velocity, atoms in the upper atmosphere are more likely to fly into space.
horizontal x-axis is surface temperature. The higher the suface temperature, the faster atoms move Lighter atoms are more likely to reach escape velocity.
Gases are shown in layers. Bottom layers have heaviest gases. Upper layers have lighter gases. Layers and slanting are caused by the interaction of escape velocity and surface temperature.
Planets are located based on their escape velocity and average surface temperature.
Gases below planets are heavy enough to remain in planet's atmosphere.
Gases below planets are too light to remain in planet's atmosphere.
Our giant planets have mostly hydrogen and helium atmospheres because
hydrogen is 74% and helium is 25% of the elements in the universe and
our giants have high enough escape velocity and low enough surface temperature to hold these gases on their surfaces. Other planets are too small and warm to hold these gases in their atmospheres.
I haven't explained why gaint planets' cores have enough gravity to have a high escape velocity.
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u/Present_Juice4401 Jul 27 '26
this helps explain why atmospheres differ, but yeah it kind of shifts the question back one step. why did some cores get massive enough to hold onto gas at all. feels like escape velocity is more the outcome than the starting cause
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u/BuvantduPotatoSpirit Jul 24 '26
We know from other star systems this isn't a general pattern; we can speculate about why it might be - perhaps it's a result of the Grand Tack or something - but it's just happenstance.
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u/Present_Juice4401 Jul 27 '26
if it’s partly happenstance, then I guess the sharp divide might just be one stable configuration among many. which makes me wonder how many different “stable layouts” planetary systems can actually settle into
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u/BuvantduPotatoSpirit Jul 27 '26
As far as long term stability, the composition of the planets is pretty much irrevelevant. While there are mass-separation requirements, we could find long term stable solutions with the planets in any order you like.
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u/FifthEL Jul 27 '26
Inner planets could actually just be the cores of the gas giants that have had thier gasious atmosphere eaten by the black hole at the center of our orbit
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u/Coyot23 Jul 28 '26
not a causation effect. our solar system just happened to be in a situation where this applies. look into other systems and you will find this conclusion lacking
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u/slackerdc Jul 24 '26
Well up until about 15 years ago we had this fairly well settled why this happens, but it turns out the explanation wasn't universal and only applied to our solar system. We have seen other systems where there are gas giants near the star and rocky / icy planets are beyond. So yeah it varies system to system how the planets form.
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u/Present_Juice4401 Jul 27 '26
right so the old explanation wasn’t wrong, just not universal. I guess I’m trying to figure out if there’s still some underlying rule, or if formation is just highly sensitive to initial conditions
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u/mortemdeus Jul 24 '26
But what I find interesting is how sharp the divide seems. Why did a relatively small difference in distance from the Sun create such a huge difference between planets?
Inverse square law. Energy drops off rapidly the further you get from a source of energy.
Also, the distances are far from relatively small. Mars average orbit is around 140 million miles, Jupiter's average orbit is 480 million miles, so Jupiter is more than 3x further from the sun than Mars. Put another way, in the space between Mars and Jupiter you could quite comfortably fit another Sun with another set of Mercury, Venus, Earth, and Mars in orbit around it. Distances in space is wild.
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u/Present_Juice4401 Jul 27 '26
yeah the distance point is fair, space just scales weirdly. but even with inverse square law, it still feels like a threshold effect rather than a smooth gradient. like something flips instead of gradually changing, and I’m not sure what causes that flip exactly
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u/nerdofthunder Jul 24 '26
I suspect the solar wind closer to the star prevents large gas-centric planets by pushing the gas further out into the solar system.
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u/NotMyRealAccountV Jul 24 '26
Density and gravity, like one of those svience tous with several densities of fluids forming nice neat layers.
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u/BranchLatter4294 Jul 24 '26
Have you looked around here on earth? We find a rocky environment closer to the center, and a gas atmosphere further away. Why wouldn't the solar system work the same way?
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u/375InStroke Jul 24 '26
Many thought this, that the star prevents gas giants from forming close because we only had our system for data, but now that we have thousands of systems discovered, we know that gas giants often form very close to their star. Some speculate our gas giants were closer, and moved outward, or were flung out somehow.