While at first blush Uranus and Neptune appear to be smaller versions of Jupiter and Saturn, they are in fact very different from their giant gas neighbors. Unlike Jupiter and Saturn, which, like the Sun, are predominantly composed of hydrogen and helium, Uranus and Neptune are predominately composed of water, ammonia, and methane ices—that is, they are composed of carbon, oxygen, and nitrogen—plus small amounts of helium, hydrogen, iron and silicates. Uranus and Neptune are therefore closer in composition to the moons of Jupiter and Saturn than to Jupiter and Saturn themselves.
While Uranus is just visible to the naked eye with a magnitude of 5.5, Neptune, which its eighth magnitude, can only be seen with a telescope. Neither planet was known to ancient astronomers; Uranus was discovered in 1781, while Neptune was discovered in 1846.
Despite their massive sizes, 15 and 17 time Earth's mass respectively, Uranus and Neptune are much smaller that Saturn and Jupiter, which are 95 and 318 times Earth's mass respectively. Uranus and Neptune are also very distant from the Sun, having semimajor axes of 19 AU and 30 AU respectively. Their size and their distance from the Sun enable them to cool below the freezing point of their constituent compounds.
The model for the interiors of these ice giants, which is based on their radii and the rotation-induced flattening of the planets at their poles, is of a rocky core of iron and silicates surrounded by a mantel of water, methane, and ammonia ices, with much of the water in liquid form. This is similar to the composition of Saturn's moon Titan and of Jupiter's moons Ganymede and Calypso. The outer envelope of each ice giant is composed of hydrogen and helium. By mass, Uranus is thought to be 25% iron and silicates, 60% methane, water, and ammonia, and 15% helium and hydrogen. The composition of Neptune is similar, estimated at 20%, 70%, and 10% respectively. Neptune has an internal heat source, something that is absent in Uranus.
One aspect separates Uranus from all other planets: it's axis of rotation is nearly in its orbital plane, with an axis tilt of 98°. Because of this, most of the northern and southern hemispheres of the planet are in total darkness or total light for almost half of the planet's 84 year orbit. In contrast, Neptune is tilted by 29.5° to the orbital plane.
The radical difference in composition of Uranus and Neptune from Saturn and Jupiter suggests that these two sets of planets formed through somewhat, if not wholly, different mechanisms. Current theory for the formation of the planets in the solar system is that the young Sun was ringed by a disk of gas that was the remnant of the gas that collapsed to form the Sun. The temperature within this disk was set by solar heating, so that materials with high melting points, such as iron and silicates, precipitated into small asteroids throughout the solar accretion disk, while elements with lower melting points, such as methane, ammonia, and water, only precipitated into small asteroids farther out than the current orbit of Mars. This explains the difference in composition of the inner planets, which are predominately metals and silicates, to that of the two outer giant planets and the numerous moons of Jupiter and Saturn, which are predominately water, methane, and ammonia ices.
The materials that precipitated out of the solar accretion disk formed small particles that gradually adhered to each other, gradually forming large bodies. With time, these bodies became large enough to hold themselves together through their gravity, and to influence the orbits of other bodies in the solar accretion disk. At this point in time, the system of asteroids was similar to the present-day Kuiper Belt of ice asteroids.
At some point in this evolution, the largest asteroids began to collect the smaller asteroids very rapidly to become small planets. These planets continued to collect asteroids less than a kilometer in size until the density of planets was larger than the density of small asteroids. The gravitational interactions among the large asteroids caused them to move in chaotic orbits, and a number of them were expelled from the solar system. As planets were expelled, the gravitational interactions among them weakened, and their scattering and capture of the remaining asteroids circularized and flattened their orbits. The two planets that remained at the end of this process were Uranus and Neptune.1
But why are the two inner giants predominately hydrogen and helium, while the two outer giants are predominately water, methane, and ammonia ices? There are two theories for these differences. The first is that Jupiter and Saturn initially formed cores in the same way as Neptune and Uranus: they swept-up smaller ice asteroids. At some point, however, the masses of these planets were large enough to allow them to accrete large amounts of the hydrogen and helium gas remaining in the solar disk. They are therefore different because they rapidly developed larger cores of water, methane, and ammonia. The second theory is that Jupiter and Saturn formed directly from the collapse of the hydrogen and helium gas within the solar disk. This type of mechanism is possible if the amount of gas in the disk at the orbits of Jupiter and Saturn is very high. The second theory implies that there was insufficient gas farther than Saturn to cause a direct collapse into a planet.
1 Goldreich, P., Lithwick, Y., and Sari, R. “Planet Formation by Coagulation: a Focus on Uranus and Neptune.” In Annual Reviews of Astronomy and Astrophysics, edited by G. Burbidge, A. Sandage, and F.H. Shu. 549–601, no. 42. Palo Alto, California: Annual Reviews, 2004.