Which Planet Takes the Longest to Orbit the Sun?

Neptune has the longest orbital period of any planet within the solar system. The eighth and most distant planet from the sun, it takes Neptune approximately 165 years to complete a single orbit.

It has an elliptical orbit with an average distance from the Sun of 2.8 billion miles, approximately 30 times more distant than the Earth. This distance makes Neptune invisible to the naked eye from the surface of Earth.

As of 2014, Neptune has completed only one orbit since its existence was first mathematically inferred by French astronomer Alexis Bouvard in 1846. German astronomer Johann Galle subsequently used Bouvard's calculations to spot Neptune with a telescope on September 23, 1846.