What is the earth made of? How does earth's composition compare to other planets?
Earth is made mostly of silicate rock in its crust and mantle, iron-nickel metal in the core. As will be explained, this is like some other planets -- but very unlike others.
There are two kinds of planets in our Solar System.
Terrestrial planets -- Mercury, Venus, Earth, Mars. These are relatively small and dense, and basically made up of similar materials to Earth -- silicate rock overlying an iron-nickel core. Two large moons in our Solar system have this composition too, our Moon and Jupiter's moon Io.
On Earth and Venus, large portions of both the rocky mantle and the metallic core are molten. Smaller bodies like Mars are believed to be almost or fully solidified.
Jovian planets -- Jupiter, Saturn, Uranus, Neptune. These are much bigger and more massive, but less dense, than the terrestrial planets. The Jovian planets are vastly different in composition from the tertestrial ones. They are all made mostly of gas, mainly hydrogen and helium. Compared with Jupiter and Saturn, Uranus and Neptune contain more "ices", compounds like water and ammonia that become icy at low temperature and pressure.
But deep inside the hot, high-pressure interiors of these Jovian planets, these gases and ices are not in their familiar form. The hydrogen and helium "gases" are condensed to hot liquids and, further down, an exotic form of hydrogen that is molten metal. The "icy" components inside Uranus and Neptune are condensed to hot liquids as well. Underneath it all there may be rock-iron cores after all, but at temperatures in the tens of thousands of degrees Celsius they would bear little resemblance to our terra firma.
There is a third type of body inder the Sun, made up of ordinary solid ices (mostly water) either throughout, or as thick layers on rocky bodies that are otherwise much like the terrestrial planets. No planets are made of this icy composition, but most of the Solar System's moons are. So are dwarf planets like Pluto and Ceres. Several larger representatives, like Jupiter's moon Europa, have large amounts of liquid water under the ice -- and maybe life to go along with it.