Neptune is the eighth and farthest known planet from the Sun. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, and slightly more massive than fellow ice giant Uranus. Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. Being composed primarily of gases and liquids, it has no well-defined solid surface. The planet orbits the Sun once every 164.8 years at an orbital distance of 30.1 astronomical units. It is named after the Roman god of the sea and has the astronomical symbol , representing Neptune's trident.

नेपच्यून हा ग्रह सूर्यापासून आठवा व सूर्यमालेतील सर्वांत बाहेरचा ग्रह आहे. या ग्रहाला हिंदीत व मराठीत वरुण असे म्हणतात. नेपच्यूनचा शोध ४ ऑगस्ट १९६४ रोजी लागला. हा ग्रह दुर्बिणीनेच पाहता येतो.