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We've already learned a lot about the solar system, but sometimes the most fascinating things are what we DON'T know.

Hosted by: Michael Aranda
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[SciShow intro plays]

Michael: In the grand scheme of things, we haven’t been at the space-exploration game very long, but we've already learned a ton about the solar system. We’ve sent probes to planets and asteroids and comets. We know what they look like, what they're made of, their temperatures, atmospheres, and so much more. But you know what’s even more amazing? What we don’t know.

The truth is, there's still a lot we don't understand about our little corner of the universe. So let's look at just a few unsolved mysteries of the solar system.

[1: What causes the Sun's magnetism?] First off, the Sun's magnetic field. Magnetic fields everywhere are created by the movement of charged particles. On Earth, for example, a flow of charged particles deep inside the outer core of our planet generates the magnetic field that makes your compass point north, and protects us from dangerous solar radiation. Now, we know that the Sun has a magnetic field too.

Maybe that's not surprising. After all, the Sun's made of plasma—a kind of gas in which electrons and ions have separated and are free to move around -- a recipe for a magnetic field. But we still don't know exactly how it works, or where it forms. Does it start near the solar surface, or deep inside the Sun? How do the different layers affect each other?

Getting to the bottom of this matters, because it'll help us understand everything from solar flares, to the northern lights,. Plus, it could help us predict what the magnetic fields of other stars might be like. But above all, unlocking the secrets of the Sun’s magnetism will help us figure out why our star is so... inconsistent.

The Sun follows an 11-year cycle. At the peak of this cycle, the Sun is brighter, and there are more solar flares and sunspots. We call this peak the solar maximum. But what's interesting is the way the Sun's magnetic field changes during the cycle. The lines of its magnetic field get more and more messy as it nears the solar maximum, and then a series of explosions -- known as coronal mass ejections -- smooth it out again. The best we can tell, the field lines start out running straight from pole to pole, like they do on Earth.

But then, because of the Sun spinning, they get wrapped around it like cotton candy. Eventually these stretched and pulled field lines “snap” like a rubber band stretched too far, producing explosions and calming the field back down to where it started. But all of this is based on what we can observe on the surface of the Sun.

What we can’t figure out is how these phenomena are created by what’s happening beneath the surface. Maybe they’re caused by forces between the outer layers of the Sun that are churning in convection currents, like pots of hot water, and the parts below them that aren’t. Maybe it’s more about the motion in the convection currents themselves. We still have a long way to go before we'll understand where exactly the field originates. To get our answers, we'll need to look much deeper.

[2: Why is Venus so different to Earth?] Now a little further out from the Sun: the stormy planet Venus. Venus has always been a bit puzzling. It's been described as Earth's twin. It's a roughly similar size, and it’s well inside the Sun's so-called habitable zone, where liquid water could be a thing. But it turns out... not so much. In many ways, Venus is more like our evil twin. It's a planet of unrelenting storms, raging at 300 kilometers an hour, and a runaway greenhouse effect that's given it an average temperature of 462 degrees Celsius. That is hot enough to melt lead.

So, why is it so different from Earth? And what got that greenhouse effect started? Well, we know what's causing the greenhouse effect today. The atmosphere is 95% carbon dioxide. That's a powerful greenhouse gas, the same gas that's the main cause of climate change on Earth. When you consider that Earth’s atmosphere only has 0.04% CO2, you can see why 95% might be a problem.

The question is, why does Venus have so much? Scientists think Venus was once a lot like the Earth, with liquid water and not so much CO2. But at some point, it got warm enough that the water evaporated, and since water vapor is a powerful greenhouse gas, too, this just made the heating worse. Eventually it got hot enough that carbon that had been trapped in rocks was released, which ended up filling the atmosphere with CO2.

The million dollar question is: What got the heating started in the first place? Was it because the planet had a little too much CO2 to start with? Was it maybe a tad too close to the Sun? Or could it have been because of some catastrophic event? It's anybody's guess.

Despite all the questions we have about Venus, we've only sent three missions there, so we have a lot more exploring to do. In future missions, we could study its atmosphere, to better understand the weather patterns, and figure out what chemical reactions happen in each layer. We could look for hotspots to see if there have been active volcanoes recently. We could even search for signs of past life, and study the planet's geology.

[3: The Storms of Uranus] Now for another stormy place, this time on the outer reaches of the solar system: Uranus. When you get caught in a thunderstorm, it might be sticky and uncomfortable. But that’s nothing compared to some of the storms in the rest of our solar system. And for the longest time, Uranus wasn't seen as particularly crazy in the storm department.

That is, until 2014, when astronomers got a surprise. They found clusters of gigantic methane storms sweeping across the planet. Before that, storms on other planets were thought to be driven by energy from the Sun. But the Sun’s energy just isn’t strong enough on a planet as distant as Uranus. And as far as we know, there isn't any other source of energy to drive such huge storms. The only thing that scientists are pretty confident about is that the storms on Uranus start in its lower atmosphere, unlike Sun-driven storms, which occur higher up.

Beyond that, though, the actual cause remains a mystery. Maybe we’re totally wrong about what's going on in the middle of Uranus. The atmosphere could be much more dynamic than it seems from the outside, generating heat that’s powering these storms.

And it could be a lot hotter in there than we think, too. It’s possible there’s an atmospheric layer trapping heat inside the planet, making the upper atmosphere cooler, and masking its true inner temperature. The secret may lie in how the different parts of the atmosphere interact. We just can't say for now. At the very least, these storms have taught us that there's a lot more to Uranus than meets the eye.

[4: Why does the Kuiper belt end suddenly?] Now we head out beyond the planets we know and love, to the Kuiper belt. The Kuiper belt is a disk of frozen bits of water, methane, and ammonia. It starts at the orbit of Neptune -- 30 astronomical units from the Sun -- and keeps going to about 50 AU from the Sun. But there's one thing about the Kuiper belt that's a huge mystery.

Once we get to 50 AU, the belt just... stops. It ends all of a sudden, something the astronomers call the “Kuiper cliff”. This isn’t easy to explain, but we have a few ideas.

It could be that the belt really does continue, but the objects become so small that we can't see them. But this idea doesn't fit with what we know about how the solar system formed. If anything -- because of the complex interactions of the outer planets’ orbits -- we'd actually expect objects to start getting larger again at that distance.

A more exciting idea is that the objects may have been pulled away by the gravitational attraction of an as-yet-undiscovered planet. Such a planet—which would be the ninth planet in the solar system—could be the size of Earth or Mars. Sadly though, it's tough to see anything that far out, so we might be waiting a while for the answer.

[5: Is the Oort Cloud a thing?] So the Kuiper belt is pretty far away, but there's one part of the solar system that's even more out there: the Oort Cloud. We all have a picture in our minds of the solar system as a flat disk. But astronomers have hypothesized for a long time that the disk might have a spherical shell around it.

This shell, the Oort Cloud, is thought to be made up of icy rocks — water, methane, ethane, carbon monoxide, hydrogen cyanide, and other nasty stuff — extending out as far as 2 light-years from the Sun. Why do we think it's a thing? Well, every so often, we spot long-period comets -- comets whose orbits take longer than 200 years -- and when we trace back their paths, they seem to come from sources a long way out in every direction. Our mathematical models for how the solar system forms tell us the cloud should be out there, too.

As the mess of the early solar system collapsed into the disk we know today, we'd expect small icy objects to be thrown into an outer shell by the gravity of Jupiter and the other gas giants. But even if it makes sense for it to be there, we've never actually observed the Oort Cloud. Being so far away, with so little light, we just don't have the technology to see it.

That means for the moment we have no way of proving that it exists, or if it does, how big it is. As these mysteries show, we don't have to go far to find puzzles-a-plenty right on our own celestial doorstep. And really, these five mysteries are only the beginning.

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