[ ♪ Intro ].

Before the 1990s, we had no idea how the solar system compared to the rest of the galaxy. We'd never found a planet around a foreign star, so for all we knew, our solar system was special.

But in 1992, radio astronomers published their discovery of the first two exoplanets, and soon after that, the floodgates opened. In the years since, powerful telescopes have revealed more than 3500 exoplanets in the galaxy, and research now suggests that nearly every star has at least one planet. So in that respect, our solar system is just another face in the crowd, but we're still kind of a weird one.

Because out of thousands of other systems, none look quite like ours. We could be special after all. The question is why.

Scientists have had a basic understanding of how the solar system formed since at least 1755, when the philosopher Immanuel Kant proposed the nebular hypothesis. He leaned on the work of other scientists of his day, and hypothesized that the Sun and planets evolved out of a cloud that collapsed under its own gravity to form a disk. Then, to make a 4.6-billion-year-long story short, that disk became the plane where all of the planets orbit today.

That basic idea has stuck around so long because it's really good at broadly describing the solar system, and extrasolar systems, too. It tells us why planets generally sit in the same disk and orbit in the same direction. It also explains why we sometimes see disks around newly formed stars.

But it doesn't explain everything. Take Mars. It's puny.

It has only about one-tenth the mass of Earth, but that shouldn't be the case. The nebular model tells us that, back when Mars was forming, there should have been enough material in Mars's part of the disk for it to grow much larger. And most simulations suggest that it should be closer to Earth's mass.

So something doesn't add up. And Mars isn't the only outlier. The solar system's planets are wildly different in size, ranging from little Mercury to giant Jupiter, but exoplanets don't usually vary much in size within one system.

Scientists sometimes describe them as “peas in a pod,” because their sizing and spacing are so even. So all the variation we have here is not normal. And not only do we have these uncommon features, but we're also missing some of the most common ones, like super-Earths.

These are planets more massive than ours, but less massive than something like Neptune. More than half of Sun-like stars seem to have one, but there's nothing like that here. So our solar system has left us with a lot of mysteries.

Fortunately, we're starting to get to the bottom of them with the help of computer simulations. As far as we can tell, a lot of our weirdness is probably Jupiter's fault. Thanks, Jupiter!

Yeah, if you thought Jupiter was a majestic, swirly wonderland, well, yes, it is. But it's also a huge troublemaker. Based on evidence from meteorites, scientists believe that Jupiter was born early in the formation of the solar system, and that it may have roamed around a lot before settling in where it is today.

In 2011, a team of scientists proposed a hypothesis about this called the grand tack model. There's still some debate about it, but it's based on simulations that suggest swirling gas around our young Sun swept up Jupiter and whisked it inward. It got to around where Mars orbits today before a newborn Saturn reined it in.

While Jupiter hung out there, its gravity likely robbed a lot of the material that would have formed Mars. Eventually, when the gas cleared out from the disk, it released its hold on the two giant planets, and they were free to wander back toward the outer solar system. Some astronomers also believe that, as it and Saturn threw around the material in the disk, they interfered with other planets as well.

Which could explain the wide range of sizes. Jupiter might even be to blame for destroying any super-Earths. According to scientists building on the grand tack model,.

Jupiter might have thrown off the orbits of would-be super-Earths and the material that would have formed part of them. If that's true, it could have created debris that slowed the whole mess down and sent it spiraling into the Sun. We'll probably never know for sure if that's what really happened, but what is pretty clear is that the planets didn't just neatly form in one place and stay there.

For some reason, our solar system seems to have had much more drama than others. But wait, there's more. If all that sounds violent, the objects past Neptune may have had an even wilder shakeup.

Out there, objects have bizarre, irregular orbits, and generally aren't in the same plane as the planets. But, if we trust the nebular model, and believe they evolved out of the same disk as Earth, something must have happened to make them that way. Some researchers think they could have been stirred up by a passing star.

For a long time, the chances of that seemed nil. But in 2018, a group of researchers published a paper looking into it more. They knew that the Sun was born in a cluster of stars that split up after about 10 million years.

So they created a model based on the Orion Nebular Cluster, a similar cluster thought to have at least as many stars as the one our Sun was born in. After running simulations on it, they made the case that during those early years, the probability of a star passing nearly as close Pluto was more like 20 or 30 percent. And that could have easily scattered objects into the orbits we see today.

Finding the answers to these questions is important for understanding our origins, but not just in a “we seek to understand the cosmos” kind of way. Whatever happened in the early solar system is a big part of the reason that Earth exists, and that it's in just the right orbit for life. So if we understand how the solar system got the way that it is, we can use that information to figure out how many other systems might have planets like ours, or why they don't.

And who knows? Maybe one day, we'll discover another solar system that's weird in all the right ways. Thanks for watching this episode of SciShow Space!

You know, over the last few years, we've made more than 500 videos, because there's a lot to say and a lot to explore out there. If you haven't any questions we haven't answered, though, we'd love to hear from you. So feel free to leave your suggestions in the comments, and we'll consider them for a future video. [ ♪ Outro ].