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Our solar system is full of rocky debris from its formation 4.6 billion years ago, including comets, meteors, and millions of asteroids. Most of those asteroids hang out in the Asteroid Belt between Mars and Jupiter.

But plenty of them follow less typical orbits. And earlier this month, astronomers found the first asteroid orbiting closer to the Sun than Venus, giving it the tightest orbit of any asteroid we've ever seen. Astronomers discovered this rock at the Palomar Observatory in southern California.

The camera that captured it had been scanning the sky for short-lived signals like the kind you get from supernovas, but it picked up something interesting inside our solar system. It was an asteroid, which astronomers dubbed 2020 AV2. The entire thing is about the size of a few city blocks, and it zips around the Sun every 151 days.

That's the shortest year of any asteroid we've found so far. And it's really exciting to find something this close to the Sun because it is not easy. See, the closer something orbits the Sun, the closer to the Sun it'll appear in the sky.

And the Sun has a way of outshining things. Like, we only ever see Mercury and Venus right after sunset or right before sunrise, because that's the only time sunlight isn't overpowering them. So no asteroid that orbits at a similar distance is going to appear in a pitch-black sky, which is where telescopes do their best work.

These asteroids are only visible for a short time during dusk and dawn, which doesn't leave a lot of time to actually find them. In fact, we've only ever found 21 asteroids orbiting the Sun closer than Earth, and they make up a small population called Atiras. Since this is the first Atira to live inside Venus's orbit, astronomers have crowned it as the first member of the so-called Vatira class of asteroids.

Yep! They just slapped a V for Venus on the front of it. In any case, scientists aren't sure if it's part of a larger population of Vatira asteroids or if it's special, but astronomers will be looking for more like it.

Because these tightly-orbiting asteroids may hold some clues about the evolution of the solar system. Right now, astronomers don't know exactly how 2020 AV2 ended up with such a small orbit, but it probably didn't start out that way. It's more likely that it migrated from somewhere farther out.

That means that it and any other Vatiras could help astronomers figure out what was happening in our solar system in the past that sent these space rocks plunging toward the Sun. It can be hard to think about a time when our solar system was different than the way it is today, but there's a ton of evidence showing how it has evolved over the last few billion years. And recently, some scientists have been looking specifically at Earth, trying to understand the origins of our protective magnetic field.

This week in the journal PNAS, scientists published the latest evidence that the Earth's early magnetic field was way stronger than we used to think. And what's even more incredible is that the history of our magnetic field can help us understand what was going on under the surface way back when our planet was just a baby. But to figure out what Earth's magnetic field was like in the past, scientists had to do some pretty amazing sleuthing.

In this study, they started by measuring the magnetic flecks inside minerals. Because all across time, as minerals have solidified, they've locked in details about the magnetic field at the time, including its strength. So if scientists know the age of a rock, they can figure out what the magnetic field was like back when the rock formed.

To look back at when Earth was super young, the authors of this study turned to tiny crystals called zircons, which are the oldest solid bits of Earth we've ever found. And when I say these crystals were tiny, I mean tiny. The crystals themselves are just a fraction of a millimeter across.

And the magnetic flecks inside them, made of magnetite, are even smaller. The zircon crystals this team tested are between 3.3 and 4.2 billion years old. And their measurements of the magnetite showed that right around 4 billion years ago, the strength of the magnetic field more than doubled compared to a hundred million years or so earlier.

Then, around half a billion years later, it faded to its previous strength. That unusual spike was a bit of a mystery because the whole reason Earth has a magnetic field today is because of the really intense heat coming from its solid inner core, which churns the liquid in its outer core. The thing is, back in those early days, Earth's core was completely molten.

It hadn't cooled down enough to form the inner core yet. But the team has an idea about what might have happened. They suggest that a mineral called magnesium oxide, which was dissolved in liquid core, began crystallizing as things cooled down.

These chunks of magnesium oxide would have been less dense than the surrounding matter, so they'd likely have shot upward, stirring the liquid metal around them in a way that could generate a planetary magnetic field. Then, around half a billion years ago, all that magnesium oxide finished floating out of the core. The team suggests that this nearly left Earth without a protective magnetic field, completely vulnerable to radiation from the Sun.

But we may have gotten really lucky. Because the team's previous research shows that right around the same time, the inner core likely solidified. And it started producing enough heat to churn the liquid in the outer core.

Scientists don't completely agree on how old Earth's inner core is, but if this team is right, that timing may have worked out almost perfectly to protect life on Earth. It seems like, just like the asteroids in tight orbits around the Sun, our magnetic field is another sign that the stability we seem to enjoy on Earth now is the result of a much wilder past than we often imagine. Thanks for watching this episode of SciShow Space News!

While you're here, we wanted to let you know that our January pin of the month is available now. This month's pin is of Explorer 1, the first satellite the U. S. ever launched into space!

And it's only available during the month of January, so if you want one, grab yours now. You can check it out at DFTBA.com/SciShow or find the pin in the merch shelf below. [♪ OUTRO].