Neptune

Type: Ice giant planet
Mass: 1.02 × 10^26 kg
Position in the Solar System: 8
Distance from the Sun: 4.5 billion km

Neptune is the eighth and last planet in the Solar System (sorry, Tombaugh!).

Fun fact: Neptune is the first celestial object—and the only planet in the Solar System—to have been discovered by mathematical deduction rather than direct observation. Irregularities observed in the trajectory of Uranus led astronomer Urbain Le Verrier to predict the existence of an unknown planet. On September 23, 1846, Johann Galle pointed his telescope in the indicated direction… and Neptune was exactly where the calculations had predicted it!