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In 2017, astronomers discovered 'Oumuamua — the first definitive interstellar visitor to our solar system. But definitive evidence of space rocks that don't just visit but *join* our solar system is a little more elusive.

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Thanks to Brilliant for  supporting this SciShow video.

Brilliant is offering a 30 day  free trial and 20% off an annual premium subscription for people who  sign up at In 1979, the English astronomer  Fred Hoyle said outer space was “only an hour's drive away if your  car could go straight upwards”.

And sure, that’s technically true. But the rest of space is pretty darn remote. It would take that very same car 400 billion hours to reach the next star over.

So it’s almost impossible for scientists to get up close and personal with interstellar objects, limiting our understanding  of the greater universe. But every so often, the rest  of the universe comes to us. Every so often, our solar system  gets an interstellar visitor.

And maybe, just maybe, that  visitor might stick around. [♪ INTRO] In October 2017, researchers in Hawai’i caught sight of something zooming across the night sky. And the way this body was zooming across the sky was highly unusual. Because based on its trajectory, it didn’t have an orbit that we’d expect  for an asteroid or comet.

See, basically every body in our solar system goes around the Sun in a path that’s  shaped like a squashed circle. But this orbit was hyperbolic. It  looked a lot more like the path a spacecraft would take on a  voyage out into deep space.

So to the scientists analyzing  this strange new body, a hyperbolic orbit meant one thing: this thing didn’t come from our solar system. It wasn’t a Kryptonian spaceship due to collide with a patch of Kansas farmland, of course. But it was a visitor from  elsewhere in the Milky Way.

And by analyzing its trajectory,  astronomers determined that it was merely passing  through our solar system. It was already on its way out,  and would never be coming back. Eventually, this object got  an official name: ‘Oumuamua.

It also got an official designation: 1I, because it was the very first  interstellar object, or ISO, that humanity had definitively spotted. And boy, did people have thoughts about ‘Oumuamua. Was it an alien spacecraft up  to some nefarious business, or was it a much more boring  hunk of plain ol’ rock?

All signs point to the  latter, but astronomers might take umbrage with my calling  an interstellar rock “boring”. After all, ISOs are pieces of somewhere else in the universe delivered to our cosmic doorstep! That makes them an invaluable resource for figuring out if our solar system  is something run-of-the-mill, or actually a massive weirdo  and we just don’t know it, yet.

Maybe we just think other systems are built out of the same stuff as ours… because this is what we’re living with. ‘Oumuamua is already further  from the Sun than Uranus is, and should reach Neptune’s orbit in late 2024. So it’s too far out for astronomers  to continue collecting data on. But that doesn’t mean they can’t work the data they do have to update our vision  of this interstellar visitor.

For example, they initially thought ‘Oumuamua was long and roughly cylindrical, kinda like a cigar. But it turns out it’s probably  more of a pancake in shape. And rather than being a comet or asteroid, some researchers hypothesize that it’s actually a giant hunk of nitrogen ice  that chipped off a frigid, Pluto-like exoplanet millions of years ago. ‘Oumuamua may be the first recognized  ISO, but it isn’t the last.

Back in 2019, an amateur astronomer discovered Comet Borisov, also known as 2I. And just like with ‘Oumuamua,  it was its hyperbolic orbit that tipped us off about  its interstellar origins. As you might have guessed,  Comet Borisov is, well, a comet.

It’s got all the hallmarks we’d expect: the fuzzy halo-like shape around the  main body, and a tail in its wake. But there’s something just  a bit odd about it, too. It has way more carbon  monoxide gas than we’d expect, and more than we’d seen in any other comet that got within 300 million kilometers of the Sun.

And that’s not just a fun fact. It actually gives us hints about where Borisov came from. We don’t know its exact origins, but the amount of carbon monoxide suggests it was made somewhere very cold, and rich in carbon.

It probably formed in the  outskirts of its home system, somewhere where it was cold enough  for carbon monoxide to freeze. And then, it stayed that cold until it got here. ‘Oumuamua and Comet Borislov are two examples of interstellar “tourists”: objects that merely pass through our solar system. But we’re pretty sure our system  is home to another type of ISO.

Interstellar “immigrants” that  wind up settling down here. Unfortunately, we don’t have absolute confirmation of any interstellar immigrants  the way we do tourists. But we do have some pretty solid candidates.

First up is Asteroid 514107,  a.k.a. Kaʻepaokaʻāwela. It shares an orbit with Jupiter,  but goes around the Sun backwards.

Backwards compared to all the major bodies in the solar system, and  most of the minor ones, too. And according to research, it appears to have been flexing that weird orbit for  the last 4.5 billion years. It’s not definitive proof,  but that backwards motion is pretty solid evidence that Kaʻepaokaʻāwela came from beyond our solar system.

And if that’s true, it likely moved  here right at the very beginning! But that’s not all. Another ISO  candidate is Comet Hyakutake, or the Great Comet of 1996.

In  this case, it’s not the orbit that suggests it has an interstellar  origin, but the chemical composition. See, Hyakutake seems to have a lot of ethane gas, and comparatively little methane gas. Which might not sound like  a big deal to you or me, but astronomers don’t normally see that ratio in the comets that are native to our system.

So maybe this one just happens  to have an unusual composition. Maybe a closer look at  other comets will eventually reveal Hyakutake isn’t all that unusual after all. Or maybe, it comes from a  different stellar system, where that particular ethane/methane  mix is totally normal.

We’ll never know for sure if either  of these space rocks really is an ISO, but the hunt for interstellar  immigrants hasn’t stopped there. It’s a pretty difficult hunt, though, because most interstellar asteroids and  comets are probably pretty small. And while we do have some hints  as to what types of orbits we should be looking for,  it’s really hard to track the trajectory of something that small  unless it’s super close to Earth.

So it’s sort of like our solar  system’s version of “Where’s Waldo?”. And that’s not all. Earth  may also be getting struck by interstellar meteors that are even smaller.

Unfortunately, the actual evidence  for this is even spottier, because a meteor’s extra small  size makes it more susceptible to the gravitational pulls of  other bodies in our solar system. In other words, a meteor that  appears to have a hyperbolic orbit could have been knocked into that  shape by getting too close to, ike, Saturn or something. No  interstellar origin needed.

But we can be reasonably  sure that our solar system is home to at least some interstellar immigrants. The exact estimate varies from paper to paper, but our solar system could host  thousands of ISOs at any given time. Most of these will move on eventually, but the time they spend here could vary from a few decades to a few million years.

And from my limited human perspective, that sounds like more than tourism! And while astronomers continue their search for the first definitive interstellar immigrant, we can all wave goodbye to a  space rock that was born in our solar system, but is packing its  proverbial bags to emigrate right now. Back in 1980, Comet Bowell had a  gravitational run-in with Jupiter.

And that run-in gave the rock a big enough speed boost for its orbit to turn hyperbolic, so it’s been shooting off toward  interstellar space ever since. But if you want to wish it a bon voyage, you shouldn’t keep waving until it’s finally gone. Astronomers often describe the Voyager 1 probe as having encountered interstellar space.

But it’s still a good three centuries  from entering the Oort cloud… the outermost boonies of our solar system. And Comet Bowell won’t hit Voyager 1’s current distance record until the  next century rolls around. But when it eventually leaves home, maybe it’ll bring a little bit of our solar system to some far flung aliens and teach  them about where it came from.

If you want to learn more about  how stuff moves around in space, you’ll want to understand  vectors and other math concepts. After all, vectors are just  descriptions of movement through space. And Brilliant has tons of content all about them!

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Thanks to Brilliant for  supporting this SciShow video! [♪ OUTRO]