What makes a planet a terrestrial planet?

a. is closest to the sun

b. has few, if any, moons

c. primarily composed of rock and metal

d. lacks rings

Terrestrial planets are Earth-like planets made up of rocks or metals with a hard surface. Terrestrial planets also have a molten heavy-metal core, few moons and topological features such as valleys, volcanoes and craters. 

In our solar system, there are four terrestrial planets, which also happen to be the four closest to the sun: Mercury, Venus, Earth and Mars. During the formation of the solar system, there were likely more terrestrial planetoids, but they either merged with each other or were destroyed.

The definition of "planet" from the International Astronomical Union is controversial. The IAU defines a planet as a celestial body that is in orbit around the sun, has a nearly round shape, and has mostly cleared its orbital neighborhood of debris. Scientists are divided in particular on the third point, with some saying that it's hard to define how much clearing a planet does, while others saying a world like Pluto would clear less than a world like Earth. This means that some astronomers argue that the dwarf planet Pluto should be classified as a planet, along with various other dwarf planets scattered throughout the solar system

c) up

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