Uranus is a very weird planet. Here's why astronomers want to probe it

By Genelle Weule for ABC

Of all our planets, Uranus has had a tough time in the reputation stakes over the years.

Its name alone is the butt of jokes, and NASA's announcement last week that it wanted to target Uranus with a probe was a comedian's field day.

But planetary scientists are over the moon our seventh planet is finally going to get its day in the sun.

The fact that it's so far away from the Sun - 2.9 billion kilometres - is one of the reasons it has been so lonely and misunderstood.

Unlike its flashy siblings Jupiter and Saturn, Uranus has only had one visitor - a brief flyby by the Voyager 2 spacecraft in 1986, said Amy Simon, one of the NASA scientists who has been advocating for a mission to the planet.

"It's one of the strangest planets in the Solar System and yet we haven't been back there," Dr Simon said.

"So the time is right for us now to really go explore the whole system.

"This really is huge, unexplored territory."

Helen Maynard-Casely, an Australian planetary scientist at ANSTO who studies ice giant planets, agrees.

"It's just a shame that Uranus and Neptune have been ignored so much," Dr Maynard-Casely says.

"As soon as we start studying one of the two, it will reveal so much about the other one."

So let's see what makes this planet so cool (apart from the fact that at a very chilly minus-224 degrees Celsius, it holds the record for the coldest place in the Solar System).

It took 70 years to name (it could have been called George)

All the inner planets have been known since ancient times because they are easy to spot in the night sky, but Uranus was the first planet to be "discovered", and even then astronomers thought it was a comet or a star.

If William Herschel had his way in 1781, our seventh planet would have been called Georgian Sidus, or George's Star, after his patron King George III.

Seventy years later astronomers settled on Uranus.

While many people pronounce the planet as Ur-Anus, astronomers prefer to say Uran-us.

"I think there are many ways of pronouncing it but if you go for Ur-Anus, people snigger," says Jonti Horner, a planetary scientist at the University of Southern Queensland.

"It would have been a lot easier if Herschel had his way and it was called George."

But Uranus is much more regal. It was named after the Greek god of the sky, Ouranos, father of Saturn and grandfather of Jupiter.

Summer lasts for 42 years at the poles

Unlike all the other planets, Uranus is tipped on its side at an angle of 97.77 degrees.

"What that means is that it rolls along its orbit rather than spinning," Professor Horner says.

"It's a really very weird, very different place."

Its extreme tilt means that each pole continuously faces towards the Sun for 42 Earth years.

"Averaged through the course of the year, the poles get more sunlight and more energy than the equator, which is very different to other planets," Professor Horner says.

We don't know why the planet - and its bevvy of moons and rings that circle the equator - are on an angle.

One hypothesis, says Dr Simon, is that it was smashed into by something roughly the same size in the early Solar System.

But that doesn't explain how Uranus got its moons.

"It has all these small moons that are clearly in that same orbit and they had to have formed around the same time [as the planet]," Dr Simon says.

This sets it apart from its ice giant neighbour, Neptune.

"If you go to Neptune, its moons are not actually its original moons, they were captured."

It has extreme weather

Thanks to the planet's extreme tilt, it has extreme weather. Not that you'd realise that by looking at the images captured by the Voyager 2 spacecraft.

"It was just bland, blue, pretty, but nothing spectacular," Dr Simon says.

Part of that was actually a consequence of Voyager's cameras.

"They were not that advanced and they could not see red wavelengths," she says.

We also only got to see the planet's southern hemisphere during its summer.

"When Voyager flew by, one side was illuminated, we couldn't see the other side at all," she says.

If other planets such as Jupiter or Neptune are any indication, weather patterns are more pronounced at the equator, Professor Horner says.

"So when Voyager flew past Uranus, it was maybe when the weather was least interesting," he says.

In the 36 years since Voyager, we have glimpsed some of this weather through the lenses of the Keck Telescope and the Hubble Space Telescope.

These telescopes have revealed massive storms caused when regions that haven't been heated for 42 years warm up as the seasons change.

But all our existing telescopes, including the new James Webb Space telescope, are still limited in what they can tell us, Dr Simon says.

"These planets are still very far away, so we can see details [with a space telescope] clearly better than we could with a small telescope on Earth, but it's still not as good as getting there."

It's a giant blue ice slushy

Around four times the size of Earth, we think Uranus is a giant ball of dense fluid of "icy" materials - water, methane and ammonia - around a small rocky core.

Its atmosphere is hydrogen and helium mixed with significant amounts of ammonia and methane, which gives it its blue colour, as well as water and other gasses.

Its atmosphere is hydrogen and helium mixed with significant amounts of ammonia and methane, which gives it its blue colour, as well as water and other gasses.

But we don't know exactly what these layers look like, Dr Maynard-Casely says.

Understanding more about this could help us work out why the planet is so cold.

"It is colder than Neptune, we know that it's putting out less heat than we expect," Dr Maynard-Casely says.

"Is there a layer within the interior that's stopping the heat from escaping it?"

What's going on in the layers could also help explain why the planet has a very strange magnetic field.

"Its magnetic field doesn't seem to originate right in the centre like the Earth and magnetic fields of most other planets," Dr Maynard-Casely says.

"It looks like the magnetic field is coming from this from the mantle, the bit around the core."

One hypothesis, she says, is that the water and ammonia in the layers is under high pressure, which is changing the subatomic particles within them.

"Putting a spacecraft into orbit would enable us to find a lot more about how the density changes through the planet," she says.

"It gives you a lot better idea if there are three layers … and then those transitions between them.

"Or are there lots and lots of layers? Or is it sort of just a gradual change all the way down to the middle?"

Its Shakespearean moons may be water worlds

Unlike other planets, whose moons are named after characters from Greek mythology, Uranus's 27 moons are named after the works of William Shakespeare and Alexander Pope.

And like Uranus, Voyager only gave us fleeting glances of parts of these small worlds, which have names like Ophelia, Cordelia, Bianca and Belinda.

"Because Uranus is tilted on its side, it automatically comes in on an almost polar orbit, which is not great for getting to all those satellites that are in orbit at the equator," Dr Simon says.

Yet we suspect some of them may be - or may have been - water worlds.

"It's incredible to think that something so far from the Sun can potentially have a habitable environment," Dr Simon says.

"It just kind of blows your mind."

Miranda - the fifth-largest moon named after the character in The Tempest - is covered with cracks and fissures.

"For a long time, Miranda was thought to be an object that was shattered and reconstructed from big lumps, but more recently people are thinking it's evidence of cryovolcanism," Professor Horner says.

"But to learn more about it, we need to get up there and spend time."

Miranda is odd, Dr Maynard-Casely agrees.

"It's the smaller of the round satellites, but it's got some really quite dramatic features.

"Are they something that's bumped into it, or are they actually about tectonic activity?

"It could also be that there's actually radiation damage on the surface."

Radiation damage from the Sun, Uranus itself, or from somewhere outside our Solar System, has the potential to change the structure of water into a glassy state known as amorphous ice.

"Because none of these moons have atmospheres, we do anticipate that there will be quite a lot of this amorphous ice on the surface," she says.

A mission for a new generation

It will take a while before Uranus is restored to its position as the "God of the Sky" (until then expect a couple more decades of jokes).

Because Uranus is so far away and takes 84 years to orbit our Sun, it is very tricky to get to.

"It's really the mission for the next generation of scientists because it is on such a long timescale," Dr Simon says.

If NASA can meet its 2031 or 2032 deadline, it will take 12 years for the mission to reach its destination using the gravitational pull of Jupiter to fling it out into the outer Solar System, Dr Simon says.

"It starts to get harder after that [deadline]. Then you can't use Jupiter and have to do Earth and Venus flybys," she says.

Not only will NASA send an orbiter to spend time flying around the planet and its moon, it will plunge a probe through the planet's atmosphere.

Equipped with modern technology, the mission will open our eyes, just like the Galileo mission did for Jupiter and Cassini did for Saturn.

"If we think about what we knew about Jupiter and Saturn from the Voyager spacecraft, and then compare that with what we know from Galileo and Cassini, we've learnt a huge amount more," Professor Horner says.

"It's the difference between having a single short flyby, and having months or years of continual observations."

- ABC