We are leaving the gas giants, and we will move towards the icy worlds of our Solar System, visiting its seventh planet Uranus. Uranus got its name from the Greek god of the sky Uranus, father of Cronus (Saturn), and grandfather of Zeus (Jupiter). Images from Uranus show a light blue planet with no features that may attract interest. So, below we will see all the fun facts about Uranus.
Discovery of Uranus
Prior to its recognition as a planet, Uranus was known, but it was always considered as a star. The reason for this is its dimness and its slow orbital period. The first person who ever recorded Uranus was probably Hipparchus, who recorded Uranus as a star in 128 BC. These results were later incorporated into Ptolemy’s Almagest.
The first verified record of Uranus dates back to 1690 when John Flamsteed observed Uranus and recorded it as 34 Tauri (i.e., a star in a catalog). The French astronomer Pierre Charles Le Monnier observed Uranus between 1750 and 1769, but he was unable to recognize its planetary nature.
Uranus was recognized as a planet a few years later, when on the 26th of April 1781, William Herschel reported the discovery of a new nebula or a comet. It took almost two years of data analysis, after which Uranus was identified as a planet.
Planet Uranus facts
The mean radius of Uranus is ~ 25,362 km (~ 3 times that of Earth). This makes Uranus the third-largest by size. The mean density of the planet ~ 1.27 g/cm3, and the mass of Uranus is 8.68×1025 kg (~ 14.5 times more massive than Earth). Thus, Uranus is the fourth most massive planet. All the above combined correspond to a surface gravity of ~ 0.886g (8.69 m/s2).
The planet is located at a distance 19.2 times greater than Earth’s distance from the Sun, or 2.87×109 km. Uranus needs around 84 years to complete an orbit around the Sun, and one day in Uranus (i.e., orbital period) lasts 17 hours and 14 minutes.
As a gaseous planet, Uranus has no solid surface, so its temperature increases as you move towards its interior. So, if you wonder what is the temperature of Uranus, at the upper layers (0.1 bar pressure) of its clouds is -220°C (-364°F) while, at a pressure of 1 bar the temperature is -198°C (-325°F).
Uranus has 27 natural satellites (moons) that are as small as 9 km in radius (Cupid), up to 789 km (Titania). Their names come from characters that appear or mentioned in the works of William Shakespeare and Alexander Pope. Finally, like all gaseous planets in our Solar system, Uranus has a faint ring system composed of thirteen faint rings.
Structure of Uranus
The current model for the structure of Uranus suggests that the planet consists of three layers. Thus, there is a rocky core (silicate/iron-nickel), a mantle made of icy material, and an outer gaseous hydrogen/helium envelope. The low density of the planet (only Saturn has lower), points towards a small rocky core. It probably has a mass half that of Earth, and it contains 20% of the planet. The density here is around 9 g/cm3, it has a pressure of 8 million bars (~ 7.9 million greater than Earth’s surface). The temperature reaches 4,725°C (8,540°F).
The mantle is the main component of Uranus and contains most of its mass (~ 13.5 times that of Earth). It is composed of a hot and dense fluid comprising water, ammonia, and other volatiles. Such fluid has high electrical conductivity, and it is commonly referred to as a water-ammonia ocean.
The outer layer of Uranus has low mass (~ 0.5 Earth’s) and extends for the last 20% of the planet. The most abundant element in the atmosphere of Uranus is hydrogen, followed by helium. The third most abundant gas is methane (2.3%), which is responsible for the cyan color of the planet. Traces of other hydrocarbons (i.e., ethane, acetylene) are also found in the atmosphere of Uranus. These are most likely produced through photolysis of methane by ultraviolet radiation from the Sun. Finally, observations have shown the presence of water vapor, carbon monoxide, and carbon dioxide in the upper atmosphere. These can only originate from external sources such as in-falling dust and comets.
Moons of Uranus
Uranus has 27 known satellites. The main five are Miranda (radius 236 km), Ariel (radius 579 km), Umbriel (radius 585 km), Titania (radius ~789 km, the eighth largest in the Solar System), and Oberon (radius 766 km). The other 22 are relatively small and their radii range between 9-80 km. The total mass of the Uranian satellites is the lowest amongst the giant planets. Their composition is roughly 50% ice and 50% rock. The most interesting features of these satellites are the fault canyons of Miranda that reach a depth of 20 km. They appear to have terraced layers and they have a chaotic variation in surface ages and features. Finally, Ariel seems to have the youngest surface since it has the fewest impact craters, while Umbriel has the oldest.
Rings of Uranus
William Herschel was the first person who mentioned the ring system of Uranus in 1789. This is considered doubtful since the rings are faint. Furthermore, for almost two hundred years, nobody
else has reported anything for a ring system. Yet, Herschel made an accurate description of the epsilon ring’s size, its relative angle towards Earth, its red color, and its appearance changes as Uranus traveled around the Sun.
The ring system of Uranus was discovered on the 10th of March 1977, by James L. Elliot, Edward W. Dunham, and Jessica Mink. The discovery can be considered accidental since their initial plan was to use the occultation of the star SAO 158687 by Uranus to study its atmosphere. Their data analysis showed that the star disappeared five times for short periods before it disappeared behind Uranus. Thus, they concluded that a ring system must be responsible for this effect. Through further observations, they discovered four more rings, which were imaged directly in 1986, when Voyager 2 flew by Uranus. Voyager 2 discovered two more, bringing their number to eleven. In December 2005, the Hubble Space Telescope detected a pair of two unknown rings, bringing their total number to thirteen. These are far from Uranus, so they are referred to as the outer ring system.
The rings of Uranus are composed of dark particles that vary in from micrometers to a fraction of a meter. The epsilon ring is the brightest one. All rings are a few kilometers wide apart from two. The material from the rings could be part of a satellite (or satellites) that was shattered by impact(s). Thus, the resulting debris that remained under the gravitational influence of Uranus formed the ring system.
Exploration of Uranus
Because of its immense distance from Earth, no probe has ever visited Uranus, apart from Voyager 2. On the 24th of January 1986, NASA’s Voyager 2 flew by Uranus at a distance of 81,500 km, on its way to Neptune. Voyager 2 studied the chemical composition and the structure of the atmosphere of Uranus and checked its unique weather features. It also sent back to Earth detailed images of the five largest satellites and discovered 10 new ones. Through imaging of the ring system (nine were known back then), Voyager 2 discovered two more. Finally, it also conducted studies on the magnetic field of the planet (we had no actual information).
Fun facts Uranus
So let us see some interesting facts that characterize Uranus.
- Herschel named Uranus Georgium Sidus (George’s star), in honor of King George. The name Uranus was adopted in 1850.
- Uranus is the first planet that was discovered using a telescope.
- Uranus is an ice giant. This a distinct class of planets, and they differ from the gas giants like Jupiter.
- Earth has an axial tilt of 23.3° with respect to its orbital plane. This tilt is responsible for the seasonal changes that we experience. On Uranus, the axial tilt has a value of 97.77°.
- Uranus has seasons too. Each lasts around 20 years.
- The lowest temperature recorded in Uranus is −224°C (-371.5°F). This makes Uranus the coldest planet in the Solar System.
- Even though Uranus appears to be featureless, in 2006, a dark spot similar to the Great Dark Spot of Neptune has been detected.
- Wind speed on Uranus can reach up to 900 km/h.
- The internal heat that is emitted from Uranus is lowest amongst the gaseous planets. It is also lower than Earth’s.
- Uranus’ atmosphere smells like a rotten egg. Responsible for this is the hydrogen sulfide (i.e., the gas that gives rotten eggs their distinctive odor), that is present in its atmosphere.