Which planet orbits the sun approximately every 84 years?

Uranus.

The planet that completes an orbit around the Sun approximately every 84 years is Uranus. Discovered by William Herschel in 1781, Uranus is the seventh planet from the Sun and marks a significant leap in our understanding of the solar system beyond the classical planets visible to the naked eye. Its discovery expanded the known boundaries of our solar system and introduced a new category of planets – ice giants – due to its unique composition and characteristics.

Uranus’s orbit is markedly different from the inner planets, not only in duration but also in the nature of its axial tilt. One of the most striking features of Uranus is its extreme axial tilt of about 98 degrees, meaning it rotates nearly on its side relative to the plane of the solar system. This peculiar orientation leads to extreme seasonal variations, with each pole getting around 42 years of continuous sunlight followed by 42 years of darkness.

Uranus’s composition is primarily made up of ices, such as water, methane, and ammonia, alongside hydrogen and helium, which is why it’s classified as an ice giant. The presence of methane in its atmosphere gives Uranus a distinct blue-green color, as methane absorbs red light and reflects blue and green light. This planet is also enveloped in a layer of clouds, and while it appears to be a smooth, featureless world through most telescopes, closer observations have revealed dynamic weather patterns, including storms and winds that can reach speeds of up to 900 kilometers per hour (560 miles per hour).

Despite being visible to the naked eye under optimal conditions, Uranus’s dimness and slow orbit meant it was not recognized as a planet until the modern era. Its long orbital period means that it moves slowly through the zodiac constellations, spending about seven years in each one. The lengthy orbit of Uranus around the Sun underscores the vastness of our solar system and the diversity of planetary environments within it, providing a broader context for understanding planetary formation and behavior in the cosmos.