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Here’s a simple question with a not-so-simple answer:
What's it like at the edge of the solar system?
You'd think that space would basically be the same anywhere,
except punctuated by different kinds of stars and objects.
But that's not actually true.
The region inside our solar system is distinctly different from the cold, black void outside of it.
But we didn’t know much about those differences until NASA launched the
Interstellar Boundary Explorer, or IBEX, ten years ago.
For almost a decade, IBEX has been mapping out the size and shape of our nice,
calm region of space, all without leaving the safety of Earth’s orbit!
And in that time, it’s shown us a lot,
both about interstellar space and about how special our neighborhood is.
It’s even debunked at least one popular hypothesis.
Now, it’s easy to imagine a few different definitions for the edge of the solar system.
You could figure out how far the Sun’s light goes or where the influence of its gravity ends,
or you could even mark the orbit of the farthest object that travels around it.
But those all have their own problems.
Like, the Sun’s light actually travels an infinite distance.
And the farthest object orbiting the Sun is far beyond what we can see,
so we can’t actually measure it.
So to get at an idea of the solar system’s boundary,
scientists often use what’s called the heliosphere.
The heliosphere is the region of space dominated by the Sun’s solar wind,
a stream of electrically-charged particles that pours nonstop out of its surface in all directions.
The farther away from the Sun you get, the weaker the solar wind is
and, eventually, there’d be more particles from interstellar space than from the Sun.
That change happens at a place called the heliopause,
and it’s a convenient definition for the solar system’s edge.
This is the boundary IBEX is studying, along with a little help from its friend Voyager 1.
Back in 2012, Voyager 1 became the first human-made object
to cross this border and enter interstellar space.
It found the heliopause to be about 120 times farther from the Sun than the Earth is,
but it could only take measurements of one tiny spot.
Fortunately, that’s where IBEX comes in.
Even though its orbit never takes it beyond the Moon,
IBEX has spent the last decade mapping the entire heliopause.
It does this by measuring particles called energetic neutral atoms, or ENAs.
ENAs form at the heliopause when the solar wind slams into the interstellar medium,
the collection of gas that fills in the gaps between star systems.
It’s sometimes called the ISM for short.
When the solar wind’s electrically-charged particles strike the neutral molecules
of the ISM, they sometimes steal electrons and become neutral themselves.
Then, those freshly-minted ENAs can suddenly travel in straight lines
because, without charge, they’re no longer deflected by magnetic fields.
A tiny portion of them bounce back towards Earth,
and an even tinier portion strike the detectors carried by IBEX.
Seriously, these events are rare:
Sometimes, ENAs strike the spacecraft at a rate of only one per hour.
Given that, it’s pretty impressive that it took just six months for
IBEX to make its first major discovery, which happened in October 2009.
The spacecraft created our most comprehensive map of the heliopause ever.
And in doing so, it found that one spot was just a bit different than all the others.
In one very narrow region of space, a lot more ENAs were being made than anywhere else.
Scientists had long expected to find at least a little variation in the production of ENAs
across the heliopause, but this spot was especially surprising.
Astronomers aren’t exactly sure why it’s happening,
but it probably has to do with the influence of interstellar magnetic fields.
In the years since, this ribbon-like region has also continued to change, suggesting that
the solar system is embedded in a surprisingly dynamic environment.
Interstellar space might not look like much, but there’s probably a lot going on out there.
IBEX also took some of the first measurements of oxygen, neon, and hydrogen atoms
coming into our neighborhood from interstellar space.
And by comparing the ratio of oxygen and neon atoms it detects,
it’s shown that our solar system is oxygen-rich
compared to the surrounding galactic neighborhood.
Since oxygen is a key component of water, and water is a key component of life here on Earth,
this could be a big deal for understanding where we came from.
Not to mention that we need oxygen to, you know, breathe.
But as for why we have so much extra oxygen, scientists aren’t sure.
It could be dumb luck.
Elements are spread a little unevenly through the galaxy,
and we might’ve just ended up in an oxygen-rich area.
And it’s always possible that we’re not really special after all.
Oxygen in the ISM could just be locked up in things like ice
that don’t produce energetic neutral atoms.
Probably IBEX’s biggest discovery to date, though,
is actually something it didn’t find: a bow shock.
Bow shocks are the space equivalent of a sonic boom, and scientists thought one would form
where the heliosphere slammed into the interstellar medium.
But instead, IBEX showed that the Sun is actually moving more slowly relative to
the interstellar medium than we’d previously thought.
Combine that with the direct measurements of these interstellar magnetic fields
from the Voyagers, and the math just doesn’t work out for a bow shock.
So score one for more peaceful interactions with outer space!
A decade in, and IBEX has lasted about five times longer than it was designed to.
And with no end in sight, it’ll keep circling the Earth with its eyes on the
solar system’s literal final frontier.
Thank you for watching this episode of SciShow Space, which is produced by Complexly.
Complexly also produces a new channel called Nature League,
where my friend Brit talks about all things nature,
and she has a really great video about organisms that can survive without oxygen
that you might want to check out, and we’ll link to it in the description. Thanks!
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