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After more than 25 years of studying exoplanets, and centuries of looking around our own neighborhood, scientists have a pretty good idea of how solar systems are supposed to work. Generally, you start with a big cloud of gas, which collapses to form a disk.

Then, over time, you get a bunch of planets and asteroids and debris. This idea has been backed up by thousands of observations and countless models, but sometimes, scientists still find solar systems that throw them for a loop. From gigantic planets too close to their stars, to those in unfathomably wide orbits, the discovery of seemingly impossible solar systems has puzzled astronomers.

According to their well-tested ideas, these systems shouldn't exist at all. But they do. So, in no particular order, here are three of them, and what they mean for science.

First, there's the star HR8799, which you can add to our ongoing list of hard-to-remember names. It's 129 light-years from Earth, and it's orbited by four gas giants. The latest planet was announced in 2010, and it's around 7 times the mass of Jupiter, orbiting 14.5 astronomical units, or AU, from its star.

In other words, it orbits about fourteen and a half times farther from its star than the Earth does the Sun. That's roughly equivalent to halfway between the orbits of Saturn and Uranus, but for such a big planet, that's considered relatively close. By itself, this isn't all that unusual.

But the story is made more complicated because the other three planets around this star orbit much farther away. And that makes the system as a whole pretty tricky to explain. Right now, gas giants are thought to form in one of two ways.

One is accretion. In this method, particles of dust clump together to make a solid core, whose gravity attracts a thick atmosphere over millions of years. The other is by fragmentation, where small patches of gas form planets directly over just 10,000 years.

But neither of these models can explain the formation of all four siblings in this solar system. Accretion can explain the inner planet, but not the outer three. Since that trio is so far from the star, the gas to make their atmospheres would have dissipated in the time it took a rocky core to form.

On the flip side, fragmentation can explain the outer three planets, but not the inner one. At 14.5 AU, it's thought that the gaseous disk around the young star would have been too hot and fast-moving to form a planet. And while it's possible that both fragmentation and accretion would have made this system, scientists think it's unlikely, since among other things, the planets are all a similar size.

Until we get a better look, we won't know for sure what happened. But based on a study published in 2013, we at least have a hypothesis. That year, research into the atmosphere of the outermost planet revealed that it had a comparatively oxygen-poor composition.

And that actually lent support to the accretion theory, at least, a modified version of it. Instead of a slow-forming rocky core, scientists think a core made of water ice could have formed, depleting the disk of oxygen. This core could take shape relatively quickly, attracting an atmosphere before the gas dissipated.

So we probably don't need to throw out our models just yet. Now, sometimes, there are even bigger planetary puzzles we need to solve, like in the case of this planet. I'm just gonna call it 2MASS for short, because let's be honest:.

It's 100 light-years away, so you probably won't need to say its full name all that much. This object was discovered in 2009, but at the time, scientists thought it was a rogue planet wandering around without a parent star, which happens sometimes. Then, in 2016, new analyses revealed that 2MASS wasn't rogue after all.

Its motion was actually linked to a tiny red dwarf star. But its orbit was unlike anything we've seen elsewhere. 2MASS is separated from its star by roughly 1 trillion kilometers, nearly 7,000 times the distance between Earth and the Sun. Scientists think there's no way this system could have formed from a disk of gas and dust, because the two pieces are just too far apart.

And they still don't know for sure how these objects maintain their unbelievably long-distance relationship, or how they got together in the first place. However, one of the researchers who discovered the system suggested that a random filament of gas could have pushed the once-rogue planet and the star together in the same direction. So again, it looks like the models can stay.

Finally, TRAPPIST-1. It's probably one of the most famous discoveries of the last few years, and we've talked about it more than once on SciShow Space. This system is about 40 light-years away, and it contains seven small, rocky planets orbiting their star, a cool red dwarf, incredibly closely.

In fact, all seven are closer than Mercury is to our Sun. But how all the planets got so buddy-buddy is a bit of a mystery. For the planets to accrete that close in, you'd need a very dense disk of gas.

But if it was dense, you'd expect the planets that grew from it to be much bigger than they are. There's always the possibility that the planets formed farther out, in a more empty region, and migrated inward, but that doesn't explain their regular small size, either. So, in 2017, a third explanation was proposed.

Scientists from the University of Amsterdam suggested that pebble-sized debris first migrated inward to form small planets. Those small planets disturbed the disk of debris around them, and that disturbance caused them to move into their current positions before they got too big. Like with the other mysteries, we'll need more observations to be sure.

But since everyone is interested in TRAPPIST-1 because of its potential habitability, there's likely more data to come. These three solar systems, and other neighborhoods like them, have forced scientists to rethink their models of planetary formation, but not in a way that completely breaks astronomy. Because that's the thing about research: Every new finding that contradicts your theories isn't Earth-shattering, or solar system-shattering, in this case.

Sometimes, it just means you're missing a piece of the puzzle, and it's up to clever and thoughtful research to figure it out. Of course, finding that missing piece isn't always easy when your puzzle is hundreds of light-years away. But maybe that's what makes it fun.

Thanks for watching this episode of SciShow Space! Scientists find seemingly impossible things out there all the time, but some mysteries are larger than others. If you want to learn about three planets that just straight-up shouldn't exist, you can watch our episode about them. [♪ OUTRO].