After the discovery of Uranus, astronomers were surprised to find that some unseen force seemed to "pull" the planet away from its path round the sun. It was only in 1846, that this "unseen force" was identified in the form of Neptune. It was Neptune's gravitational force that tugged at Uranus.
Neptune is only slightly smaller in size than Uranus and in several ways appears to be similar to it. Indeed, that's why two are referred to as the "Twins" of our solar system. Neptune is also largely made up of gas. It has two moons of it's own. Neptune takes 165 earth-years to make one revolution round the sun. Therefore, it will not be back in the position where it was discovered (in 1846) until the year 2011. Voyager II reached Neptune on August 25, 1989.