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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.
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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.
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[♪ OUTRO]