Is there a reason why we have four “rock” planets lined up followed by four “gas” planets in a row, or is it merely coincidence?
In: Planetary Science
There is sun wind that blow away the gas of the planet that close to the sun, so we have rock planet. The further from the sun, the weaker the sun wind, so we have gas planet
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Yes. It’s called the “frost line” and it is the distance from the sun beyond which it is cold enough for gases like hydrogen, helium, methane, etc. will condense and form thick atmospheres like you find in the gas giants.
Gases are far more abundant than “rocks” near to a star gases become dispersed further away they start to clump together the frost line is where the gas planets start to form as it get cooler. https://youtu.be/Mf8-bOBfG98
Coincidence.
However, the reason the gas giants are in the outer solar system is because when the sun originally ignited, it pushed all the extra gasses to the outer solar system, forming gas giants in the outer solar system.
We have observed gas giants close to their parent star (called a hot Jupiter) and we believe these planets formed in much the same way as the gas giants formed in our solar system, but various orbital interactions dropped these gas giants into a lower orbit.
Fill a glass with cold water. Add a heaping scoop of Tang (or whatever). Give it a good whirl. What do you see?
You get a pile of the ~~bigger~~ denser bits in the middle.
ipso facto.
(I actually have no idea, thanks for asking)
I take it you’ve never seen a centrifuge in action.
the early solar system was the sun and a swirling disk of matter swept up by gravity from earlier supernovae and other sources. just like our own planet, and just like in a centrifuge, denser matter collects toward the middle and gasses remain toward the outside.
the dense rocky matter eventually started clumping together as small rocks smashed together to form bigger rocks and gravity took over. the gasses did the same in essence.
unrelated, we do have the kuiper belt and oort cloud which are rocks captured from other solar systems or thrown off during novae or captured comets.
To clarify what other’s have said. There’s a reason the gas ones are further out than the rocky ones. But the fact that it’s 4 to 4 is a complete coincidence.
Contrary to every other answer (at the moment), the answer is really that we don’t know. The explanations given by other commenters reflect the consensus in the 90’s and early 00’s, but our telescopes have gotten a lot better since then and we now know that our solar system is extremely unusual–[in its star, the fact that there is only one star, and the composition and ordering of its planets.](https://www.youtube.com/watch?v=_Tju7EaSfmM)
Most star systems have two (or more) stars orbiting each other, and most stars are dimmer and less massive than our Sun. Many/most exoplanets (planets in star systems outside our own solar system) are either “hot Jupiters” (gas giants very close to a star, sometimes even closer than Mercury is to the Sun) or “super Earths” (rocky planets that are much more massive than Earth), and most star systems appear to have fewer than the 8 planets we have. Furthermore, [recent work](https://arxiv.org/abs/2301.02374) suggests that ordering of planets in star systems is usually either star –> gas giants –> rocky planets, all gas giants, all rocky planets, or a random mix of gas giants and rocky planets. Star –> rocky planets –> gas giants appears to be the *rarest* order, which obviously conflicts with the explanations given by other commenters.
What all this means is that there may have been something unique or unusual about how our solar system formed that caused it to have the planet order that it does, or it could be a random coincidence–we don’t know. But the previous explanations for why it has the order it does are clearly only part of the story (if they are correct at all), because most star systems *don’t* have the order and type of planets that we do.
The reason is Saturn. Following a model for solar systems that is most likely a Jupiter sized gas gain forms and gets closer to the start and clear all orbits. The current hypothesis is the because a second gas gain formed – Saturn, it pulled on Jupiter in the other direction and there was a chance for orbits for rocky objects to exist. So the question moves to – why was there a second gas giant right after the first one. Probably chance.
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