Journey Through the Solar System: Planet Uranus

Uranus
Uranus as seen in infrared light. Its atmosphere has storms churning around and the planet is encircled by a thin set of rings. NASA

 

The planet Uranus is often called a "gas giant" because it is largely made of hydrogen and helium gas. But, in recent decades, astronomers have come to call it an "ice giant" due to the abundance of ices in its atmosphere and mantle layer.

 

This distant world was a mystery from the time it was discovered by William Herschel in 1781. Several names were suggested for the planet, including Herschel after its discoverer. Eventually, Uranus (pronounced "YOU-ruh-nuss") was chosen. The name actually comes from the ancient Greek god Uranus, who was the grandfather of Zeus, the greatest of all gods.

The planet stayed relatively unexplored until the Voyager 2 spacecraft flew past in 1986. That mission opened everyone's eyes to the fact that gas giant worlds are complex places. 

 

Uranus from Earth

Uranus
Uranus is a very small dot of light in the night sky. Carolyn Collins Petersen

Unlike Jupiter and Saturn, Uranus is not readily visible to the naked eye. It's best spotted through a telescope, and even then, it doesn't look very interesting. However, planetary observers do like to search it out, and a good desktop planetarium program or astronomy app can show the way. 

 

 

Uranus by the Numbers

Rim Of Uranus
Space Frontiers - Stringer/Archive Photos/Getty Images

Uranus is very distant from the Sun, orbiting at some 2.5 billion kilometers. Because of that great distance, it takes 84 years to make one trip around the Sun. It moves so slowly that astronomers such as Herschel weren't sure if it was a solar system body or not, since its appearance was about like an unmoving star. Eventually, however, after observing it for some time, he concluded it was a comet since it did appear to be moving and looked slightly fuzzy. Later observations showed that Uranus was, indeed, a planet. 

Even though Uranus is mostly gas and ice, the sheer amount of its material makes it quite massive: about the same mass as 14.5 Earths. It's the third-largest planet in the solar system and measures 160,590 km around its equator. 

Uranus from the Outside

Uranus
A Voyager view of Uranus showing a visible light view (left) of the nearly featureless-looking planet. The right view is an ultraviolet study of the polar region that was pointed toward the Sun at the time. The instrument was able to look through the hazy upper atmosphere and see distinct cloud structures surrounding the planet's south polar region.

The "surface" of Uranus is really just the top of its enormous cloud deck, covered by a methane haze. It's also a very chilly place. Temperatures get as cold as 47 K (which is equivalent to -224 C). That makes it the coldest planetary atmosphere in the solar system. It's also among the windiest, with strong atmospheric motions that drive giant storms. 

While it doesn't give any visual clue to atmospheric changes, Uranus does have seasons and weather. However, they're not quite like anywhere else. They're longer and astronomers have observed changes in the cloud structures around the planet, and in particular at the polar regions.     

Why are Uranian seasons different? It's because Uranus rolls around the Sun on its side. Its axis is tilted at just over 97 degrees. During parts of the year, the polar regions are warmed by the Sun while the equatorial areas are pointed away. In other parts of the Uranian year, the poles are pointed away and the equator is warmed more by the Sun. 

This weird tilt indicates that something really bad happened to Uranus in the distant past. The most like explanation for the tipped-over poles is a catastrophic collision with another world millions and millions of years ago. 

 

Uranus from the Inside

Uranus
Like other gas giants, Uranus is mainly a ball of hydrogen and helium in various forms. It has a small rocky core and a thick outer atmosphere. NASA/Wolfman/Wikimedia Commons

Like the other gas giants in its neighborhood, Uranus consists of several layers of gases. The topmost layer is mostly methane and ices, while the main part of the atmosphere is mostly hydrogen and helium with some methane ices.

The outer atmosphere and clouds hide the mantle. It's made mostly of water, ammonia, and methane, with a large part of those materials in the form of ice. They surround a tiny rocky core, made mostly of iron with some silicate rocks mixed in. 

Uranus is surrounded by a thin set of rings made of very dark particles. They are very hard to spot and weren't discovered until 1977. Planetary scientists using a high-altitude observatory called the Kuiper Airborne Observatory used a specialized telescope to study the planet's outer atmosphere. The rings were a lucky discovery and the data about them were helpful to the Voyager mission planners who were about to launch the twin spacecraft in 1979. 

The rings are made of chunks of ice and bits of dust that were likely once part of a former moon. Something happened in the distant past, most likely a collision. The ring particles are what's left of that companion moon. 

Uranus has at least 27 natural satellites. Some of these moons orbit within the ring system and others farther away. The largest are Ariel, Miranda, Oberon, Titania, and Umbriel. They are named after characters in works by William Shakespeare and Alexander Pope. Interestingly, these little worlds could qualify as dwarf planets if they were not orbiting Uranus.

Uranus Exploration

Artist rendering of Uranus Fly-by
Uranus as an artist imagined it would look as Voyager 2 flew by in 1986. Historical / Getty Images

While planetary scientists continue to study Uranus from the ground or using Hubble Space Telescope, the best and most detailed images of it came from the Voyager 2 spacecraft. It flew by in January 1986 before heading on to Neptune. Observers use Hubble to study changes in the atmosphere and have also seen auroral displays over the planet's poles. 

There are no further missions planned to the planet at this time. Someday perhaps a probe will settle into orbit around this distant world and give scientists a long-term chance to study its atmosphere, rings, and moons.