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April 18th: Uranus: Not Just the Butt of Jokes

Date: April 18, 2010

Title: Uranus: Not Just the Butt of Jokes


Podcaster: Emily Temple-Wood

Links: World book, Uranus, Universe Today, Uranus, NASA’s page on Uranus, Nine Planets:

Description: Uranus provides fresh joke fodder for anglophone kids all over as they learn about the solar system, but it’s a little bit more complicated than that. The 7th planet and its system, only studied up close once, still has many interesting features that intrigue scientists today.

Bio: Emily Temple-Wood is a high school student from Illinois who loves astronomy and plans to be a physicist.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by The Planetary Society, celebrating 30 years of inspiring the people of Earth to explore other worlds, understand our own, and seek life elsewhere. Explore with us at planetary.org.


This is the 365 Days of Astronomy podcast for April 18 2010. My name is Emily Temple-Wood; I’m a high school student from Illinois in the United States. Today, I’m going to be talking about Uranus. It’s one of the more mysterious planets in our solar system, a fact that isn’t helped by its unfortunate name.

We’ve known about the 7th major planet’s existence since 1781, more or less, when William Herschel announced his discovery of this first modern-era planet. Herschel originally intended to call his discovery “Georgium Sidus”, or George’s Star, in honor of his royal patron. This name didn’t stick, and the astronomical community adopted a name to fit the Greco-Roman nomenclature we’re familiar with, naming the planet after the father of the Titans. Despite its discovery in 1781, Uranus is visible to the naked eye under dark skies, at approximately magnitude 5.3. In fact, it was classified as a star before Herschel determined its planethood! John Flamsteed and Pierre Lamonier mapped it as 34 Tauri – they didn’t notice its motion and thus simply assumed that it was a fairly dim star.

As we now know, Uranus is one of the two ice giants, the other being Neptune. They lie in distant orbits and have blue (or blue-green) atmospheres, due to the absorption of red light by methane. They’re called the ice giants because they contain high proportions of “icy” materials: water, methane, and ammonia. Uranus is the colder of the two, and in fact, it’s the coldest major planet in the solar system, because its core has cooled so much that it doesn’t radiate heat any more, like the other gas giants do.

At first blush, this atmosphere may seem far less exciting than Jupiter’s or even Saturn’s, but it’s got plenty going on underneath its smooth surface. First of all, Uranus possesses cloud bands much like Jupiter’s and Saturn’s, though they tend to appear and disappear. It also has the huge, powerful storms that grace the other planets, though they are not visible well in the visible spectrum – you have to look in the infrared to find evidence of them. Any way you slice it, wind speeds of 250 m/s can’t be boring!

Now, let’s talk about the Uranian moons. There have been 27 discovered so far, and they’re unique in the solar system in that they’re named after characters in works by William Shakespeare and Alexander Pope’s poem “Rape of the Lock”, as opposed to mythological personages. The 5 largest are Titania, Oberon, Ariel, Umbriel, and Miranda, but, relatively, we don’t know very much about them because they have only been seen up close very briefly. They’re also among the smaller moons of the solar system – their collective mass is less than half of that of Triton, Neptune’s largest moon – and are tidally locked to their parent planet. We do know that 18 of those moons are classified as “regular” (including the 5 largest), and 9 are classified as “irregular”. This just refers to the irregular satellites’ greater eccentricity (or more elliptical orbits).

From Voyager 2, we know that the 5 major moons are quite interesting. Titania, named for the Queen of Fairies in Shakespeare’s Midsummer Night’s Dream, is the largest moon, and is relatively smooth, possibly due to geological activity. It has about ½ the diameter of Earth’s moon, and was discovered along with Oberon by William Herschel. Oberon, Titania’s king, is dominated by craters, and has mountains of unknown height. Ariel, named for a sylph in Pope’s Rape of the Lock and a spirit in Shakespeare’s The Tempest, is small and the brightest of the Uranian moons. It’s been battered by craters, and is covered by huge canyons, much like those on Mars. Umbriel, also named for a fairy in Pope’s poem, is pockmarked with craters, the darkest of Uranus’s moons. The last major moon, Miranda, was discovered by Gerard Kuiper (of Kuiper Belt fame) in 1948, almost a century after Ariel and Umbriel. It’s one of the more interesting moons. Its surface is mixed up, with different geological features all thrown together. There are several theories about this: one says that the moon broke up then reformed under its own gravity; another says that incomplete differentiation gave it the odd features.

Those moons were seen in the only mission humanity has mounted to observe Uranus closely: Voyager 2, which observed the planet in 1986 (I wasn’t even alive then!) before flying past Neptune and onward to the heliopause and, eventually, interstellar space.

Before Voyager arrived, we had no idea if Uranus had a magnetic field or not. It turns out that it does, but – like the rest of the planet – it’s weird. It’s tilted about 60 degrees to its axis, and isn’t generated in the planet’s center. Making it even weirder, its magnetotail is in a corkscrew shape behind the planet, something that doesn’t occur anywhere else. As I said, weird.

We do know a little bit more about the Uranian rings. There are 9, and all are narrow and dark, in contrast to Saturn’s bright, wide rings. The prevailing hypothesis is that the rings formed from the debris of several smaller moons that collided fairly recently, instead of forming with the planet.

Uranus also just reached its equinox. Just like on Earth, its poles have extended periods of daylight and darkness. Unfortunately for any imaginary Uranians who dwell at the poles, their periods of midnight sun and, well, midnight, last about 42 years each, more or less. That’s because Uranus is tilted about 98 degrees on its axis, basically turned over on its side. Compare that tilt to Earth’s, around 23.5 degrees. One hypothesis addressing this states that a planet-sized object – about the size of Venus – smashed into Uranus during the Solar System’s birth, permanently changing its tilt.

Uranus, like fellow solar system objects Ceres, Pallas, Pluto, Neptune, Mercury, Earth, the Moon, and the Sun, also gave its name to an element: uranium. In case you’re wondering, the others are cerium, palladium, plutonium, neptunium, mercury, tellurium, titanium (both of which are named for Earth), selenium, and helium. It’s in good company!

Uranus is commonly viewed as the most boring planet in the Solar System, or the one the most ripe for jokes. Though the latter may be true, Uranus isn’t boring at all. Its history and much of its system are still shrouded in mystery, and it will probably stay that way for a while yet: no new missions are planned to visit the planet. However, when we do, what new mysteries may be uncovered?
Thank you for listening to the 365 Days of Astronomy podcast.

End of podcast:

365 Days of Astronomy
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