Why do we have 4 ‘rock’ planets in a row then 4 ‘gas’ planets in a row?

Question

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

Answer 1

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.