Uranus, the seventh planet from the Sun, is an ice giant, a category it shares with Neptune. It resides in the outer solar system and is distinct from gas giants like Jupiter and Saturn because its composition includes a greater proportion of “ices,” such as water, methane, and ammonia, surrounding a small rocky core.
The Planet That Rolls Sideways
Uranus is unique due to its extreme axial tilt of approximately 98 degrees relative to its orbital plane. This causes the planet to orbit the Sun essentially on its side. For comparison, Earth’s axial tilt is 23.5 degrees. The leading theory for this orientation is a massive collision with an Earth-sized body roughly four billion years ago. The impact may have also expelled much of its internal heat, contributing to its surprisingly cold state today.
The Coldest World in the Solar System
Despite being closer to the Sun than Neptune, Uranus holds the record for the coldest minimum temperature measured in the solar system. Temperatures in its atmosphere can drop to a frigid -224 degrees Celsius (-371 degrees Fahrenheit). Unlike Jupiter and Saturn, which radiate more heat than they absorb, Uranus radiates almost no excess heat.
The atmosphere is primarily composed of hydrogen and helium, along with a small amount of methane. This methane gas absorbs red light and reflects blue light, giving the planet its characteristic cyan-blue color.
A History of Discovery and Naming
Uranus holds the historical distinction of being the first planet discovered using a telescope. Sir William Herschel, a German-born English astronomer, officially discovered the planet on March 13, 1781. Herschel initially believed the faint, slow-moving object was a comet, but further observation confirmed it was a planet.
Herschel originally named it “Georgium Sidus” (George’s Star) in honor of King George III. The German astronomer Johann Bode later proposed the name “Uranus” to maintain the tradition of naming planets after figures from classical mythology, specifically the Greek god of the sky.
Moons Named by Shakespeare
The satellite system of Uranus consists of 27 known moons, and their naming convention is unique. Instead of using figures from Greek or Roman mythology, the major moons are named after characters from the works of William Shakespeare and Alexander Pope. This tradition was established by John Herschel, the son of the planet’s discoverer.
The five largest moons—Titania, Oberon, Umbriel, Ariel, and Miranda—are drawn from English literature. For example, Titania and Oberon are the king and queen of the fairies from Shakespeare’s A Midsummer Night’s Dream. This literary theme extends to smaller satellites, including Ophelia, Desdemona, and Puck.
Seasons That Last Decades
The planet’s extreme 98-degree tilt, combined with its lengthy orbital period of about 84 Earth years, creates a bizarre seasonal cycle. Each of Uranus’s four seasons lasts for approximately 21 Earth years.
During summer, one pole is pointed directly at the Sun and experiences 42 continuous Earth years of daylight. The opposite pole simultaneously endures 42 continuous Earth years of complete darkness.