If we discount dwarf planets after the asteroid belt all planets are gas, is there a specific reason or is it just coincidence
In: 2019
Question seems to have been answered very well, but I wanted to add a “fun” fact. According to many theories, Jupiter is credited as Earths protector, often dominating the outer solar system with its gravity and soaking up any leftover material that might otherwise fall inward and threaten to impact Earth. We’ve even witnessed many such impacts on Jupiter.
But in the early days, it’s thought that Jupiter’s orbit would have migrated inward and may have ejected other proto-planets or even whole planets out of the solar system. But then comes the true hero, Saturn, which pulled Jupiter back out to its now stable orbit.
The ordering of our solar system is considered to be VERY rare and it plays a big part as to why life was able to form on our planet. I believe most star systems have what is called a ‘Hot Jupiter’ that orbits close to its host star which prevents rocky planets from forming (it essentially steals all of the materials in the area during the stars formation and even if a rocky planet forms, the hot Jupiters gravity usually ends up ejecting it from the system entirely).
We actually haven’t found a single star system similar to ours where all of the rocky planets orbit close to the star and the gas giants orbit further away.
Most star systems are binary, they have 2 stars. Jupiter is the “would be” second star but it did not accumulate enough matter to become a star. That is why Jupiter is so much bigger than the other planets, it is basically a failed star.
Once the Sun formed it sucked in most of the matter of the inner solar system due to its extremely strong gravity, and its solar winds pushed lighter elements outward.
The rest of the matter that Jupiter was not able to capture formed into Saturn, Uranus, and Neptune in the outer rings. That is why we have 4 big gas and ice giant outer planets.
The 4 small inner rocky planets are the made from the heavier elements not captured or pushed away by the Sun.
I suspect the combined gravity of Saturn, Uranus, and Neptune kept Jupiter from moving closer to the Sun, thus protecting the tiny inner planets from being absorbed or flung out of the solar system. I read a theory that said Saturn actually used to be closer to the Sun than Jupiter, but as Jupiter moved inward it pushed Saturn outward and that Saturns gravity helped slow Jupiters inward progress and keep it where it is now.
Our solar system is just amazing and everything happened perfectly right to result in Earth being the paradise that it is.
Most of these comments are off topic, do not address the question properly, or just completely wrong.
Tl;dr: Planets form out of a disc and gas and dust around a newborn star. To build a gas planet, you need a big rocky core first. But you need that big rocky court to form before all the gas is gone. Further out in the disc, water is in its solid form as ice, and the ice helps. Rocky particles stick together to form rocky cores. Big enough fast enough before the gas is gone.
PhD astrophysicist here, with an expertise in exoplanets. The sun and the planets formed from a large cloud of gas and dust. By the time the sun formed at the center of that cloud, there was a disc of leftover gas and dust surrounding the sun that we call a protoplanetary disc.
To build a planet, you need to take very small particles, have them collide and stick together and grow massive enough to eventually start attracting and retaining gas as well. Jupiter is about 300 Earth masses, the majority of which is hydrogen and helium gas. To attract such a significant amount of gas, it is thought that you need to build a ball of rock of about 20 Earth masses.
But the problem is that this process of building a rocky core is relatively slow compared to the average lifetime of the gas in the disc. The other replies stating that lighter elements were pushed away by the solar wind while the rocky material remained in the inner parts of the disc are inaccurate. They are accurate in that the gas does have a limited lifetime in the disc, but the gas being pushed out from the inner disk only is inaccurate.
In the protoplanetary disc, as you move, outwards temperature decreases. Eventually you reach the temperature that the water is in the form of ice. It is thought that this ice helps the rocky particles stick together to grow these rocky protoplanets fast enough to attract enough gas to form a gas giant before the gas disappears in about 10 million years.
Other replies are also mentioning that there are many known exoplanet systems where the gas giant planets are close to the star. The existence of these planets doesn’t mean that the solar system’s architecture is random. Those gas giant planets close to their stars likely migrated there after they formed.
Nobody knows. Some astronomers think that a planetary system like ours, with small planets near the star and large planets farther out, is not typical. When we thought that our arrangement was typical, we had come up with ideas of why it should be that way, but now we know that not all planetary systems have that architecture.
It might be random, and this question is like asking why, in a family of four kids, the oldest two are boys and the youngest are girls. There is, in the vast majority of cases, no particular reason why a family should have a particular arrangement of sex and birth order. But if you’ve only met one family, you have no way of knowing that all families don’t follow that pattern.
The primordial Solar system was basically acting like a giant centrifuge. Light stuff got expelled outwards. Gases are much lighter than rock, metals and minerals.
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