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
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.
Latest Answers