
Neptune is the eighth and farthest known from the . It is the in the by diameter, the third-most-massive planet, and the densest . It is 17 times the . Compared to its fellow , Neptune is slightly more massive, but denser and smaller. Being composed primarily of gases and liquids, it has no well-defined.

Natural resources such as , (crude oil) and take thousands of years to form naturally and cannot be replaced as fast as they are being consumed. It is projected that fossil-based resources will eventually become too costly to harvest and humanity will need to shift its reliance to such as solar or wind power. An alternative hypothesis is that carbon-based fuel is virtually inexhaustible in human terms, if o.