Author: Fraser Cain

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Uranus has a mass of roughly 14.5 times that of Earth, which makes it the least massive of the giant planets. Astronomers know that it’s mostly made of various ices, like water, ammonia and methane. And they theorize that Uranus probably has a solid core.

The core of Uranus probably only accounts for 20% of the radius of Uranus, and only about 0.55 Earth masses. With gravity of all the outer mantle and atmosphere, regions in the core experience a pressure of about 8 million bars, and have a temperature of 5,000 Kelvin. That sounds hot, like as hot as the surface of the Sun, but keep in mind that the core of Jupiter is more like 24,000 K – much hotter. The core of Uranus has a density of about 9 g/cm³, which makes it about twice as dense as the average density of the Earth.

For astronomers, Uranus has an unusually low temperature; and that’s a mystery. One ideas is that the same impact that knocked Uranus off its rotational axis might have also caused it to expel much of its primordial heat. With the heat gone, Uranus was able to cool down significantly further than the other planets. Another idea is that there’s some kind of barrier in Uranus’ upper atmosphere that prevents heat from the core to reach the surface.

We have written many stories about Uranus on Universe Today. Here’s an article about a dark spot in the clouds on Uranus, and here’s an article about the composition of Uranus.

If you’d like more info on Uranus, check out Hubblesite’s News Releases about Uranus. And here’s a link to the NASA’s Solar System Exploration Guide to Uranus.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

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Uranus is a ball of ice and gas, so you can’t really say that it has a surface. If you tried to land a spacecraft on Uranus, it would just sink down through the upper atmosphere of hydrogen and helium, and into the liquid icy center.

When we look at Uranus, we see the blue-green color that seems to come from the surface of Uranus. This color is light from the Sun reflected off Uranus’ surface. The atmosphere of Uranus contains hydrogen and helium, and most importantly, it has relatively large amounts of methane. This methane absorbs color in the red end of the spectrum of light, while photons at the blue end of the spectrum are able to reflect off the clouds and go back into space. So the full spectrum of the Sun’s light goes in, the red and orange end of the spectrum is absorbed, and the blue green end of the spectrum bounces back out. And this is why the surface of Uranus has its color.

But let’s imagine that the surface of Uranus was actually solid, and you could walk around. You might be surprised to know that you would only experience 89% the gravity that you feel back on Earth. Even though Uranus has 14.5 times more mass than Earth, it has 63 times the volume of Earth. Uranus is the second least dense planet in the Solar System, so it has a relatively weak gravity on its surface.

We have written several articles about Uranus for Universe Today. Here’s a story about what’s inside a gas giant, and here’s one about two new moons discovered for Uranus.

If you’d like more info on Uranus, check out Hubblesite’s News Releases about Uranus. And here’s a link to the NASA’s Solar System Exploration Guide to Uranus.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

We understand the weather on Earth. The Sun heats the air at the equator, causing it to rise. The warm air goes to the poles, cools down and sinks, and then circulates back. Scientists call this Hadley Circulation. The weather on Uranus works very differently. This is because Uranus is tilted over onto its side, rotating at an angle of 99-degrees.

Over the course of its 84-year orbit, the north pole of Uranus is facing towards the Sun, and the south pole is in total darkness. And then the situation reverses for the rest of the planet’s journey around the Sun. Instead of heating the clouds at the equator, the Sun heats up one pole, and then the other. You would expect the pole facing the Sun to warm up, and to have air currents move towards the other pole.

But this isn’t what happens. The weather on Uranus follows an identical pattern to what we see on Jupiter and Saturn. The weather systems are broken up into bands that rotate around the planet. While Uranus has a completely different tilt from Jupiter and Saturn, it does have internal heat rising up from within. It appears that this internal heat plays a much bigger role in creating the planet’s weather system than the heat from the Sun.

Although less than Jupiter and Saturn, the wind speeds on Uranus can reach 900 km/hour, and seem to be changing as the planet approaches its equinox – when the rings are seen edge on.

We have written many articles about the weather on Uranus for Universe Today. Here’s one that talks about how stormy the planet can get. And here’s one about the discovery of a dark spot in the clouds on Uranus.

If you’d like more info on Uranus, check out Hubblesite’s News Releases about Uranus. And here’s a link to the NASA’s Solar System Exploration Guide to Uranus.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

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The orbit of Uranus takes 84.3 year to complete one revolution around the Sun. In other words, 1 Uranian year is 84.3 Earth years.

Like the rest of the planets in the Solar System, Uranus doesn’t have a perfectly circular orbit. Instead, it follows an elliptical path around the Sun. Astronomers call a planet’s closest approach to the Sun perihelion. The perihelion for Uranus is 2.75 billion km, or 18.4 astronomical units (1 AU is the distance from the Earth to the Sun). The most distant point of orbit is called aphelion. The aphelion of Uranus is 3.00 billion km, or 20 astronomical units. On average, Uranus orbits at a distance of 2.88 billion km, or 19.2 AU.

Uranus is unique among the planets in the Solar System because of its axial tilt. While Earth is tilted at a mere 23.5 degrees, Uranus has rolled over completely sideways, with an axial tilt of 99-degrees. This has a significant impact on the planet’s seasons. The north pole of Uranus experiences 42 years of complete darkness, followed by 42 years of sunlight, where the Sun never dips below in the horizon. Astronomers aren’t sure why Uranus is flipped over sideways, but they think an impact from a protoplanet early in its history gave it the momentum it needed to roll over.

We’ve written many articles about Uranus for Universe Today. Here’s an article about how we got to see the planet’s rings edge on, and another about how the atmosphere of Uranus can be more violent than previously believed.

If you’d like more info on Uranus, check out Hubblesite’s News Releases about Uranus. And here’s a link to the NASA’s Solar System Exploration Guide to Uranus.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

So far, only one spacecraft has ever captured an image of Uranus: NASA’s Voyager 2 spacecraft. So let’s take a look at some pictures of Uranus, some captured by Voyager 2, and others seen from Hubble, and even here on the ground.

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This is the classic image of Uranus, taken by NASA’s Voyager 2 spacecraft during its 1986 flyby of Uranus. During this mission, Voyager 2 came within 81,500 kilometers of the cloudtops of Uranus. Until then, the best pictures of Uranus came from telescopes on Earth. Nothing compared to the pictures of Uranus sent back by Voyager 2.

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This beautiful picture of Uranus might look like it was captured by a space telescope, but it was actually taken from the powerful Keck telescope located on Hawaii’s Mauna Kea. This image shows Uranus in the infrared spectrum, which reveals the detailed cloud patterns in the atmosphere of the planet.

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This Uranus pic was captured by the Hubble Space Telescope. On the left is the image itself, and then on the right are the names of all the moons captured in this photograph of Uranus.

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This beautiful image of Uranus was captured by NASA’s Voyager 2 spacecraft curing its 1986 flyby of the planet. It was actually three images captured at different wavelengths and then combined on computer.

Like these pictures of Uranus? Here are some images of Pluto, and photographs of the Sun.

If you’d like more info on Uranus, check out Hubblesite’s News Releases about Uranus. And here’s a link to the NASA’s Solar System Exploration Guide to Uranus.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

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The mass of Uranus is 8.68 x 10²⁵ kg.

Want to put that in perspective? That’s 14.536 times more than the mass of Earth. That makes Uranus a pretty massive world, but it’s actually just a tiny fraction of Jupiter. Jupiter is 21.9 times more massive than Uranus.

Even though Uranus is much more massive than Earth, it has a fairly low density. in fact, it’s the second lowest density in the Solar System; only 1.27 g/cm³. If you could stand on the surface of Uranus (you can’t… don’t try), would experience only 89% the force of gravity that you would experience on Earth.

If you could peer inside Uranus, you would discover that most of the planet consists of ices, like water, ammonia and methane.

Here’s an interesting article about the mass of Mercury. And here’s an article about the mass of Jupiter.

If you’d like more info on Uranus, check out Hubblesite’s News Releases about Uranus. And here’s a link to the NASA’s Solar System Exploration Guide to Uranus.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

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The Earth’s axis is tilted about 23.5 degrees. This is why we have seasons on Earth. But the axis of Uranus is tilted so far it’s hard to imagine how it might have even happened. The axis of Uranus is tilted at an angle of 98-degrees compared to the Sun’s orbital plane.

While the rest of the planets in the Solar System can be thought of like spinning tops, Uranus is more like a rolling ball going around the Sun. During the point of the Uranian solstices, one pole faces the Sun continuously, while the other pole faces away. Only a thin strip of the surface of Uranus experiences any kind of night/day cycle. Uranus’ poles experience 42 years of continuous sunlight, and then 42 years of continuous darkness. During the time of the equinox on Uranus, the planet’s equator is facing the Sun, and so it experiences day/night cycles like we have here on Earth.

What could have caused Uranus to be tilted over on its side like this? Astronomers think that a large protoplanet smashed into Uranus billions of years ago. This collision set the planet tumbling. Eventually it settles into its current axial tilt.

Here’s a cool article on Universe Today about mysteries of the Solar System, including the question, why is Uranus tilted? And here’s an article about images of Uranus and Neptune captured by Hubble.

Here’s the same question posed to “ask a scientist”, and here’s an article from the Planetary Society Blog.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

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Sure, Uranus is big… but how big is it? There are so many ways we can measure the size of Uranus, so let’s look at all of them.

First, let’s take a look at diameter. The diameter of Uranus is 51,118 km across. For comparison, this is about 4 times bigger than Earth.

Now, let’s look at volume. The total volume of Uranus is 6.833×10¹³ km³. Again, for comparison, you could fit 63 Earths inside Uranus, and still have room to spare.

Next, mass. The mass of Uranus is 8.68×10²⁵ kg. This is about 14.5 times more massive than Earth. This sounds like a lot, but this makes Uranus the smallest of the outer planets, smaller even than Neptune.

How about surface area? The surface area of Uranus is 8.1×10⁹ km². This would let you flatten out almost 16 Earths to cover the surface area of Uranus.

How big are the other planets? Here’s how big Jupiter and Saturn are.

If you’d like more info on Uranus, check out Hubblesite’s News Releases about Uranus. And here’s a link to the NASA’s Solar System Exploration Guide to Uranus.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.