URANUS

The Tipped-Over Giant

Greenish-blue, haze-covered Uranus is well known in the planetary science community as the planet that orbits the Sun on its side. None of the other planets do this. During its orbit, one pole or the other on Uranus points toward the Sun during the solstices. At the equinoxes, the planet’s equator points toward the Sun. This combination of axial tilt and the lengthy year gives Uranus some interesting day-night scenarios. For example, each pole gets forty-two Earth years of sunlight. During the equinoxes, the equatorial regions have very short days and nights because the Sun never rises very high over the horizon. Uranus is made of a higher percentage of water, methane, and ammonia ices than Jupiter and Saturn, and for that reason it is sometimes called an ice giant.

It’s not impossible to see Uranus with the naked eye, but it’s very dim. However, unlike the other planets, which were known to naked-eye observers throughout history, Uranus wasn’t discovered until the invention of the telescope. The first observer to officially chart it was John Flamsteed (1646–1719) in 1690, and he thought it was a star. This is because it didn’t seem to be moving. Sir William Herschel (1738–1822) spotted it in 1781, and he noted in his observation journal that this object seemed to be a comet. This is because through his telescope, Uranus appeared disk-like and fuzzy, not point-like as a star would look. With more observations and some calculations of the object’s supposed orbit, it became clear that Herschel’s “comet” was really a planet.

Uranus Facts

1. Closest point to Sun: 2.7 billion kilometers (1.7 billion miles)
2. Most distant point from Sun: 3.0 billion kilometers (1.86 billion miles)
3. Length of year: 84 Earth years
4. Length of day: 17 hours (based on rotation of interior)
5. Tilt of axis: 97.77 degrees
6. Gravity: 0.89 Earth’s gravity

The History of Uranus

The gas giants formed in a region of the solar system where temperatures were cold enough to allow hydrogen and various other compounds to freeze. Some recent research suggests that they actually formed closer to the Sun than where they are now and later migrated out to their present positions.

These worlds were built by planetesimals slamming into one another to build bigger ones, just as other worlds in the solar system did. However, the inner worlds are made mostly of rock, while the outer worlds became mixtures of rock and ice at their cores, covered by layers of liquid metallic hydrogen and helium and smothered in thick atmosphere. Large amounts of methane in the upper atmosphere color Uranus a pale blue-green, and there’s a smog layer hiding cloud features from our view.

Uranus wasn’t created with its odd axial tilt. Something had to push it over. What could that have been? The best theory is that the infant Uranus was in a collision with an Earth-sized object that was strong enough to tip the planet on its side.

The Moons and Rings of Uranus

Uranus has twenty-seven known moons. Some are named after characters in Shakespeare’s plays and from the Rape of the Lock by Alexander Pope. Titania and Oberon were discovered in 1787. Ariel and Umbriel were found in 1851, and Miranda was first spotted in 1948. The other moons were found during the Voyager 2 flyby, and more recently through Hubble Space Telescope observations. The five largest moons are Ariel, Umbriel, Titania, Miranda, and Oberon. Each is a small, icy body with many cracks and craters on its surface.

The small moon Miranda looks most unusual. It has a very mottled appearance, with deep canyons splitting its crust. There are also curious-looking, oval-shaped landscapes called coronae. In some places, faults (cracks) in the surface crisscross each other. In other places, Miranda is torn apart by the action of cryovolcanism—ice volcanoes spewing softened and slushy ice across the surface. It’s likely that Miranda, like the other four large moons, has some kind of geological activity going on inside. This activity causes changes to the surfaces of these moons in the same way that tectonism and volcanism change Earth’s surface.

Many of Uranus’s moons orbit in the same plane as its ring system, which was only discovered in 1977. There are thirteen rings, and the system is fairly dark and hard to spot. The rings formed relatively recently in solar system history—perhaps 600 million years ago. They were probably a result of the collision of some of the planet’s moons. The debris was scattered and eventually collected into the flat ring structures we see today. Like Saturn’s rings, which are at most a few kilometers thick, the Uranian rings are fairly thin. They stretch out as far as 98,000 kilometers (nearly 61,000 miles) from the planet and are made up of small water ice chunks and dust particles.

Exploring Uranus

Until the Voyager 2 spacecraft encountered Uranus in 1986 (the only spacecraft to do so), exploration of this distant ice giant was done using ground-based telescopes. As Voyager passed 81,500 kilometers (50,644 miles) above the cloud tops, the probe snapped images, studied the gases in the planet’s atmosphere, charted the peculiar magnetic field, and captured views of five of its largest moons.

The Hubble Space Telescope has also studied Uranus from its vantage point in Earth orbit. The telescope’s long-term mission is to provide frequent monitoring of activity in the Uranian atmosphere, track seasonal changes, and look for more auroral displays (such as the northern and southern lights here on Earth) driven by interactions between Uranus’s magnetic field and streams of charged particles from storms that originate on the Sun and are carried along on the solar wind.

There are no further missions to Uranus underway, although some are on the drawing board. One idea is to mount a joint mission by NASA and the European Space Agency, which would launch in 2022. Another proposal would send an orbiter to Uranus, where it would spend two years studying this distant and still mysterious system.