Uranus: A family portrait (I)

In every sense, my dad is the coolest planet in the Solar System. Where else do you see polar lights in unexpected places and diamonds rain?

Hubble Space Telescope picture of my dad and some of my siblings. Credit: E. Karkoschka (Univ. of Arizona), NASA

I think I told you I belong to quite a large family. We lead a quiet life, about 20 times further away from the Sun than you on planet Earth. Despite being numerous, most of the time, we are simply there, in the shade, minding our own business.

You have heard about my dad, of course, but probably do not know much about him. No wonder; unlike some of his siblings, he does not like to show off. Think of this: You Earthlings didn't even know about him before inventing the telescope!

He is great! Four times bigger than auntie Earth, in fact (and 14 times as massive). He's said to be an "ice giant", but that's a bit exaggerated. You would say he's a sort of iceball floating in space! Instead, he is a ball of gas mixed with liquid and ice –supposedly on top of a rocky core, but Daddy still bears many secrets, even to his offspring.

Isn't it gorgeous when observed in infrared light? Credit: Lawrence Sromovsky, University of Wisconsin-Madison/W.W. Keck Observatory

Cool, not dull

To me, my dad is the coolest planet in the Solar System! I am not only speaking of his temperature (about –195 ºC on average, even lower than that of Uncle Neptune), but also of his many unique characteristics. Think of his colour: that wonderful, uniform aquamarine. All thanks to the methane in his atmosphere –you may complain of the smell, but who's there to feel it anyway. And below the misty clouds, it is said that an ocean could be hidden. I have asked Dad many times about it, but he never confirmed nor denied it.

Methane storms in Uranus, observed with the Keck telescope. Credit: Imke de Pater (UC Berkeley), Larry Sromovosky and Pat Fry (U. Wisconsin), and Heidi Hammel (AURA)

Better not joke about his sideways rotation axis, for example. Although he doesn't like to talk about it, I've heard that it was caused by a bad encounter with another body, long time ago, when he was very young. The other dude must have been big, to cause such an effect! As a result, if you Earthlings were to settle on Uranus (which you can't, unless you find a way to build a colony in a planet without a solid surface), you would experience much more extreme seasons than on your own home planet.

He is also a bit ashamed of his weird magnetic field. All his siblings are wrapped in sort of magnetic "bubbles" (call them "magnetospheres") that are quite symmetrical and more or less aligned with their rotation axes. On the contrary, his is tilted and lopsided. If you ask me, I don't think that's a big issue: What if his auroras do not glow in the poles, like with my aunts and uncles? They are impressive anyway!

Composite image combining observations taken with the Hubble Space Telescope, the Voyager 2 probe and the Gemini Observatory of an aurora of Uranus, in line with its equatorial rings. Credit: NASA

Diamonds in the sky

Wealth is always associated with Uncle Saturn... Sure, he loves to boast about his amazing rings. But do you know what? He's not that special. Uncles Jupiter and Neptune also have rings, and so does my dad, too.

The difference is that Dad's rings are just like everything in him... discreet. They are made of small, dark particles that do not shine that much. Yet, it did not take you Earthlings so much to find out about them. Short after Dad was discovered to you in 1789, rumour had it that such rings existed. However, this was not confirmed until 1977, when a group of astronomers noticed that a star was being hidden by something close to Uranus. Since then, you have counted 13 rings... Let me keep to me if there are still more!

Dad's so modest, in fact, that he has a secret deep inside his atmosphere: Diamond rains! Remember the methane? Part of it it's been compressed to form those brilliant rocks you go so crazy about on Earth. Shhhh... Don't tell anybody. We don't want a Diamond Fever with a horde of mining spacecrafts back here in the Outer Solar System!

To be continued...