system
Given the following post, respond as if you're commenting on Reddit:
human
Why are terrestrial planets with the same mass as our gas giants so rare?

Looking at the exoplanets that have been discovered, terrestrial planets on average are far less massive than gas planets. What stops gravity from pulling all that matter into a solid? Why are the densest planets normally far less massive than the least dense?
gpt
a gas giant is very, very, very large.

it's also very very heavy.

a planet forms from a disc around a star, where smaller rocks are attracted to larger rocks and then even more rocks are attracted to those rocks.

more often than not, multiple planets form from these discs, but so far apart that their gravity never interacts with any other.

in order to coalesce into one single planet the mass of jupiter, the protoplanetary disc would need to be very thin, but at the same time have a huge amount of matter in it.

a star needs to have less gravity for the disc to be thinner, but more gravity for there to be more material.

so it's basically impossible, since a star can't both be heavier and lighter at the same time.

(for example, if you took mercury, venus, earth, the moon, mars, phobos and deimos and smashed them together, they wouldn't even be close to jupiter or saturn)