The first early astronomer to propose a sun-centered solar system was

Nicolaus Copernicus

Explanation:

Nicolaus Copernicus was the first early astronomer to propose a sun-centered solar system. He was the one to first stated that the sun was at the center of the universe, this theory is called heliocentrism. Heliocentric model is the astronomical theory in which the Earth and the other planets revolve around the Sun at the center of the solar system.

answer;

d. are made mostly of liquid

explanation;

-the general   composition and structure planets includes; they have a thick gas and liquid layer covering a small,solid core. mostly the gases are hydrogen and helium atmosphere and layers of liquid hydrogen. for instance neptune and uranus are examples of the outer planets, they have a core covered by ice and ammonia.

-neptune's atmosphere is hydrogen and helium with a bit of methane so there is no oxygen to breath and the interior is made up of frozen water and ammonia with a rock and iron core. thus, there is no liquid water or solid surface to survive.

the five characteristics of adolescence are biological growth and development, and undefined status, increased decision making, increased pressure and the search of self.