Uranus was the first ever planet to be discovered with the help of a telescope. But do you know, before Sir William Herschel, it was not recognized as a planet? When ancient astronomers first observed Uranus, they mistook it for a star.
The earliest of such exploration was possibly done by the Greek astronomer Hipparchus in 128 BC. But some theories also suggest that it was Ptolemy who first observed the planet.
Like terrestrial planets, Jupiter and Saturn (classical planets), Uranus is visible to the naked eye and is the third-largest in size, the fourth-largest in terms of mass, and one of the least dense celestial bodies in our Solar System. Below are some of the most intriguing facts about this ice giant.
Table of Contents
First Recorded Observation: In 1690 by John Flamsteed
Equatorial Diameter: 51,118 km
Mass: 8.68 x 1025 kg
Density: 1.27 g/cm3
Surface Gravity: 8.69 m/s2
Orbital Period: 84.02 years or 30,687 Earth days
Satellites or Moons: Ariel, Titania, Oberon, Umbriel, and Miranda (largest moons). 27 in total.
17. Uranus was discovered by Sir William Herschel
A replica of the telescope that Herschel used to discover Uranus
When William Herschel first spotted Uranus on 26 April 1781, he labeled it as a possible comet. However, in his official letter to Nevil Maskelyne, Astronomer Royal at that time, Herschel reported that his discovery is “likely to be a regular planet moving in a circular orbit around the Sun.”
Astronomers including Anders Johan Lexell and Johann Elert Bode were among the first to accurately measure the planet’s orbital characteristics which helped the international scientific community to confirm its planetary status. Uranus was finally accepted as a planet of the solar system in 1783.
16. Uranus Distance From the Sun
The average distance between Uranus and the Sun is approximately 20 AU or 2.8 billion km (the distance between the earth and the Sun is 1 AU). Like most of the planets in our solar system, Uranus revolves around the Sun in an eccentric orbit (0.046), thus the distance between them varies throughout the year. The difference between its maximum distance (apogee) and minimum distance (perigee) is perhaps the largest of all known planets (1.8 AU).
15. It Is the Coldest Planet In the Solar System
Uranus, despite being much closer to the Sun than Neptune, is the coldest planet in the solar system. The lowest temperature recorded at Neptune’s cloud tops (standard surface temperature for gas giants) is 55 K (−361 °F), whereas, the temperatures at Uranus’ tropopause can reach as low as 49 K (-371 °F).
The reason behind the planet’s extremely cold weather is its mysteriously negligible internal heat, which appears to be lower than all giant planets including Neptune. In other words, unlike other planets with incredibly hot cores, Uranus barely radiates any excess heat into space.
While researchers are still not sure why Uranus’ internal heat is so low, few hypotheses have been proposed to help shed some light into the case. One possible explanation is that the planet was hit by a super-massive body at some point during its history which expatriated most of its internal heat.
14. How Many Moon Does Uranus Have?
Uranus and its four major moons | Image Courtesy: ESO
Uranus has a total of 27 known moons or natural satellite, the names of which are based on various characters from the popular works of Alexander Pope and William Shakespeare.
For the most part, Uranian moons are irregular and insignificant in size. The combined masses of five largest moons of Uranus, namely Titania, Oberon, Umbriel, Ariel and Miranda (in descending order of size) would be less than half of the mass of Triton, Neptune’s largest moon.
However, unlike Triton, which was captured by Neptune in the early ages of the solar system, all major Uranian moons are likely to have formed in the accretion disk that once surrounded the planet.
Titania has a diameter of 1,578 km, making it the eight largest natural satellite in the solar system, slightly larger than Saturn’s second largest moon Rhea. Miranda, the smallest of the five major Uranian moons, on the other hand, has a diameter of just 470 km.
13. It Has Only Been Visited Once
Artist’s impression of Voyager 2 space probe | Image Courtesy: NASA/JPL
Uranus is perhaps the least visited outer planet (along with Neptune) in the solar system. The first and only space probe to ever make a close approach to the planet was Voyager 2 in 1986 when it came within 81,500 km of the planet’s topmost cloud layer.
During its time in the Uranian system, the probe took valuable information, including multi-wavelength images and radio interferometry data, critical for our current understanding of the planet.
Since Voyager 2, no other probes have been sent to study the planet and as it stands, there are no such plans in the near future either. However, on multiple occasions, researchers have proposed mission concepts to Uranus including the canceled Uranus Pathfinder. Other missions such as MUSE and ODINUS are currently under consideration.
12. Uranus Rotate On Its Side
Comparison of the tilt of rotational axis | Image Courtesy: Wikimedia Commons
All planets in the solar system rotate on their axis with some level of tilt. For example, Earth has an axial tilt of 23.44°, the highest of all terrestrial planets. Likewise, Jupiter and Saturn are tilted by 3.13° and 26.73° respectively.
The highest axial tilt of any planet in the solar system belongs to Uranus, which is 97.77°. As a result, the planet’s equator is located where other planets have their poles. This interchange of equator and poles causes unique seasonal changes on the planet.
While there are few plausible explanations for such drastic axial tilt of Uranus, none of them are proven to be the case so far.
11. A Day Last Here For 42 Years
Uranus revolves around the Sun once in every 84 years while it takes only 17 hours and 14 minutes to complete a rotation on its axis. But due to the unique axial tilt of the planet, one of its poles continuously faces the Sun for about half of the Uranian year while the other remains in complete darkness. In other words, a day in one of Uranus’ poles lasts for about 42 years.
10. Seasonal Variations in Uranus Is Unique And Extreme
Uranus’ extreme axial tilt appears to have a major effect on its weather. While there is currently not enough information to accurately determine the exact nature of seasonal variation on the planet, photometric data taken over the course of more than 50 years (half Uranian year) has revealed significant periodical changes in brightness of the planet at solstices and equinoxes.
The data indicate that the visible pole of the planet brightens before the solstice and darkens after the equinox, however, the process is inconsistent.
In the 1990s, Hubble witnessed brightening of Uranus’ northern hemisphere, while its southern hemisphere was darkened considerably right after the solstice. Then, it was predicted that the condition would reverse after the next equinox, which indeed happened in 2007.
9. How Many Rings Does Uranus Have?
Voyager 2 image of Uranian rings (nine of them) | Image Courtesy: Nasa
Like all gas and/or ice giants in the solar system, Uranus also has its own intricate ring system, though not as extensive as that of the planet Saturn.
So far, 13 distinct rings have been identified around the planet, first nine of which were discovered back in 1978. Two more rings were identified in 1986 from the images that were taken by Voyager 2. Then in 2005, two outermost rings of Uranus were discovered with the help of Hubble Space Telescope.
These rings are usually composed of extremely dark microscopic particles and dust (their bond albedo doesn’t exceed more than 2%). Except for extremely narrow ones, all the rings are at least a few kilometers wide.
Multiple studies have indicated that these rings are much younger than the planet itself and most probably have resulted from collision and fragmentation of relatively small natural satellites or moonlets.
8. Uranus is One of The Least Dense Planets
Interior of the planet Uranus
The four inner planets are differentiated from the outer ones not just by their distance from the Sun, but also by their size, density, and mass. While the outer planets are much greater in size and mass, their overall density is significantly lower than that of the terrestrial planets.
Uranus, with a density of 1.27 g/cm3 , is the second least dense planet in the solar system, only after Saturn. Like other giant planets, Uranus’ structure can be divided into three distinct layers; a rocky core, an icy mantle, and a thin outer layer of hydrogen/helium.
The term “icy mantle” here is not used in the conventional sense, instead, the planet’s mantle is comprised of dense fluid volatiles such as water, ammonia, and methane.
Due to its low density, Uranus also has a relatively low surface gravity of 8.69 m/s2(0.886 g). In other words, you would only experience 89% of the gravity on Uranus provided that you are able to find a surface to stand.
Some Quick Facts
7. Do you know, Uranus can actually be seen without the help of a telescope? Yes, the apparent magnitude of the planet, that lies between 5.3 and 6.03, makes it possible for an unaided human eye to spot Uranus in the night sky. However, factors like light pollution (photopollution) can make it impossible to do so.
6. Herschel initially wanted to name the planet George’s Star or the “Georgian Planet”, to honor King George III. However, due to the lack of general consensus, other names were suggested for the planet including “Neptune.” It was Johann Bode, a German astronomer who proposed the name Uranus, Latinised version of Ouranos, the Greek god of the sky.
5. Uranus had been observed on numerous occasions, long before it was officially recognized as a planet. One such explicit sighting was done by John Flamsteed in 1690, who observed Uranus at least six different occasions. However, due to the planet’s low luminosity, he mistook it for a dim star in the Taurus constellation.
4. Uranus is one of the only two planets in the solar system that rotates counter-clockwise (east-to-west) the other being Venus. However, the distinctive sideways rotation of Uranus is what makes the planet absolutely unique.
3. On Uranus, winds can reach up to the speeds of 900 km/h (560 mph). In comparison, the fastest ever wind speed recorded on earth was 408 km/h or 253 mph (cyclone Olivia).
Uranus Dark Spot | Image Courtesy: NASA
2. In 2006, the Hubble space telescope in close coordination with the Keck observatory discovered a small dark spot in the winter (northern) hemisphere of the planet. This dark spot, also known as Uranus Dark Spot (UDS), is in many ways similar to dark spots on the planet Neptune.
1. The Voyager 2 mission was solely responsible for discovering essential and earlier unknown facts about the icy planet. For example, it helped uncover the fact that Uranus’ magnetic field is misaligned with its rotational axis unlike others and not to mention the unique seasonal characteristics of the planet which are caused by its unique tilt.