This episode is supported by Cheddar.
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When astronomers look out into the galaxy, they see all kinds of planets orbiting other stars.
But the most common type might be the worlds the size of our solar system’s ice giants, Uranus and Neptune.
And that’s great! It means we have two prime examples right in our own backyard,
so we can take a close-up look at what these exoplanets might be like.
Unfortunately, we know less about Uranus and Neptune than almost anywhere in our solar system.
Which is not so great.
We’ve already done an episode about Neptune and its mysteries, so let’s jump back and
check out three of our biggest, seemingly most basic, questions about Uranus.
The first question is one astronomers have been asking for decades:
Why is the planet lying on its side?
Relative to the plane of its orbit around the Sun, Uranus is tilted about 98°,
four times more than Earth.
What’s more, whatever happened must have occurred early on because the planet’s rings
and moons are tilted by the same amount.
The accepted idea is that, during the solar system’s early days,
Uranus was hit by an object several times larger than Earth.
That knocked the young planet over, and the moon- and ring-forming
disk that surrounded it followed suit.
But a giant impact is kind of the default theory in planetary science.
Some computer simulations published in 2011 suggest it might have actually taken more
than one impact to create what we see today.
And one 2009 paper points out that a collision might not have been necessary at all.
If any evidence of what truly happened still survives, it’s probably deep within the
planet’s interior, hiding under all those smooth outer layers.
The only way to figure out what’s down there is through gravity measurements,
and those can only be made by a spacecraft in orbit.
As the satellite circles Uranus, the extra gravity from denser areas would speed it up
a tiny bit, while less dense areas would slow it down.
Over time, the pattern of these changes could hint at the planet’s internal structure.
And if Uranus seems weirdly lumpy on the inside, it might give us clues about its ancient past.
Amazingly, the planet itself might not have had the Uranian system’s most troubled past.
That honor could belong to its moon Miranda,
which appears to be among the solar system’s most tortured objects.
Less than 500 kilometers across, Miranda has canyons more than 20 kilometers deep.
It’s covered in cracks, and its surface seems to have been pushed and pulled from all directions.
To top it all off, its orbit is also tilted relative to Uranus’s equator over ten times
more than any of its other major moons.
There are all sorts of hypotheses about what might have happened, from yet another impact
to tidal heating and gravitational interactions with the other large moons.
But all planetary scientists have to go on is a few images from the Voyager 2 spacecraft in 1986.
And those are on just one side of Miranda.
A long-term Uranus orbiter could build a global map of the moon’s surface, which would enable
geologists to reconstruct the order in which these different events must have occurred.
Finally, lots of pictures could also help solve the case of the weird Uranian rings.
Today, we know of 13 of them, and they’re pretty cool by themselves.
Some are dark, and others have hints of color like red and blue.
One even seems to contract and expand, about five kilometers over a few hours.
They’re very different from the rings around Saturn.
Instead of being broad and icy, Uranus’s rings are narrow and dusty.
The dust is probably bits of rock knocked off nearby moons by meteor impacts, but no
one’s really sure why the rings are so narrow.
Normally, as ring particles bump into each other,
they tend to spread out in a process called diffusion.
One way of confining that material into a narrow ring
is through a pair of moons, called shepherd satellites.
One shepherd orbits inside the ring and one outside, and together their gravity keeps
the band trapped in a narrow region.
That’s the case for one of Saturn’s rings, but only one of the many narrow Uranian rings
seems to have its own shepherds.
We’re not positive what’s going on with the other ones.
Of course, it is possible that astronomers just haven’t found all the moons that are out there.
When the Cassini spacecraft arrived at Saturn, it found many new moons,
including one embedded directly in the planet’s rings.
And back in the ‘80s, Voyager 2 found ten Uranian moons of its own.
An orbiter at Uranus might turn up a similar collection and, even if it doesn’t, long-term
observation of the rings’s motion could help scientists figure out what’s going on.
Now, if you’ve been paying attention, you might have noticed I’ve said one word
over and over again: orbiter.
That’s the key to solving these mysteries and many more.
Only with careful, repeated observations can researchers build a complete picture of what’s
going on in the Uranus system.
And only an orbiter can make that happen.
The good news is, that’s exactly what planetary scientists plan to do.
Every ten years, the research community gets together to create a roadmap for the coming decade.
And in their most recent version, which was released in 2011, a mission to Uranus was
listed as one of the most important objectives.
Of course, that’s no guarantee, and even if a mission started development tomorrow,
we’d still be many years from launch and many, many years from getting results.
But things are moving in the right direction.
And that’s critical, because understanding Uranus isn’t just about our own solar system.
It could be key to the history of planets all over the galaxy.
Thanks to Cheddar for supporting this SciShow Space video.
If you subscribe to SciShow Space, you might also really like Cheddar, which recently launched
their YouTube Channel where they make videos that cover
science, technology, innovation, business, and news.
I just watched their video about the best-case asteroid hitting Earth scenario,
which sounds like a bad day, but it’s a really fun video.
I learned about the difference between an asteroid hitting land vs. water, and how to
embody Bruce Willis and his roughneck pals when it comes to the potential end of the world.
Check out all their videos at YouTube.com/Cheddar,
and we’ll link to the asteroid video in the description.
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