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[♪ INTRO]
About two and a half million years ago, a nearby star went supernova,
and it was so close to Earth that it would have been visible during the day!
After that explosion, a shower of cosmic particles rained down on us.
Scientists think this particle bombardment might have
shaped how planet Earth looks today.
And they were able to track those effects not by looking up to space,
but down to the bottom of the sea.
Although a wave of cosmic rays crashed onto Earth millions of years ago,
they did not come directly from the supernova itself.
Instead, when the star blew up, it created a shockwave that pushed neighboring
atoms like hydrogen and helium, until they eventually got to Earth,
where they rained down for thousands of years as cosmic rays.
And this constant onslaught had consequences for our planet.
Cosmic rays could have kickstarted chemical reactions in our atmosphere,
forming compounds that contain nitrogen.
And many plants use these types of compounds as fertilizers, so when they
eventually got to Earth’s soil, they could have facilitated the growth of plants.
One other thing plants use for growth is carbon dioxide,
a key ingredient for photosynthesis.
If enough carbon dioxide was consumed by these plants,
they could have helped cool down the planet.
That means that it is possible that a supernova hundreds of light-years away
from Earth could have helped trigger a massive climate change.
But it is tricky to know all of this happened for sure,
because the event took place way before human records.
Like way before.
Luckily, scientists were able to track some clues.
See, when the star exploded, it left some evidence on our planet:
Radioactive atoms hidden in the bottom of the sea.
Now, there are a lot of radioactive atoms in space, especially in stars,
which have enough energy to form them.
And when these stars explode some of that debris
takes part in the formation of planets like Earth.
So chemically, our planet is made of similar atoms to those found on stars.
Which is a pain in the butt for some scientists,
because when they try to differentiate, like, new star stuff from the stuff that
makes up Earth, they run into a pickle because those two things are very similar.
However, a key property of these radioactive particles is that they
decay over time because they are unstable.
That’s what makes them radioactive.
So they keep releasing radiation until they become non-radioactive.
And some of those particles have been on Earth for a really long time,
which means that they are no longer radioactive.
So if scientists find million-year-old space particles that are still radioactive,
that means that they were not from our planet’s formation, so they must have
come from another place or event, like the explosion of a supernova.
In essence, scientists were looking for a needle in a haystack,
and against all odds they found it.
The radioactive particle that fit the bill was Iron-60.
Iron-60 is a radioactive isotope of iron, and the 60 in its name comes from the
four extra neutrons it has compared to the most common isotope of iron, Iron-56.
And that makes Iron-60 radioactive.
This isotope has a half-life of around 2.6 million years, so after that amount of time
passes only half of the original iron-60 should still be around.
The problem is /finding/ Iron-60.
Remember, we’re talking about atoms that came from space
more than two million years ago.
To measure Iron-60, you first need to find a sample that has
survived for millions of years with enough Iron-60 left to detect.
And after a very extensive search, researchers found that perfect sample…
at the bottom of the ocean.
More specifically, in a section of the ocean where sediment doesn’t settle,
like on the slopes of seamounts, or in places with a strong current.
In those places, the ocean floor is dominated by something called
ferromanganese crusts, which is where metal particles shift from being
dissolved in seawater to their concentrated solid form.
And they do that very slowly; we’re talking about
a few millimeters of accumulation every million years.
After analyzing samples from those crusts, scientists found traces of Iron-60,
one of the radioactive isotopes from the supernova.
The first study on this, from 1999, proved that we could at least
detect supernova remnants here on Earth.
And scientists have not stopped there.
Since then, they have also found traces of star stuff in fossils,
because all animals take iron into their bodies from their diet!
By looking at where all those radioactive elements ended up in the world,
scientists could determine how far away the supernova was
by modeling the trajectory that that material took to make it to Earth.
They concluded that it happened about 150-300 light-years away, leading them to
their next question…could we see a supernova event like this again?
Well, according to astronomers, yes and no.
It would have to happen pretty close to our planet for a supernova to affect us,
anywhere between 30 to 300 light-years.
But that said, supernovas are fairly common.
The universe is an unimaginably big place.
Even in our Milky Way galaxy, supernovas happen a few times a century.
So it’s not unthinkable that another supernova could happen
close enough to Earth to be noticeable like tomorrow.
If another supernova happened that close, we wouldn’t get hit
directly by X-rays and gamma rays, because we’d be far enough away,
but it would definitely be visible from Earth.
And those accelerated cosmic rays would still pepper our atmosphere for decades.
We are still learning about the effects the last nearby supernova had on the planet,
so it’s hard to predict exactly what future effects could be.
Climate change and an increase in radiation exposure
could all be within the realm of possibility.
But there’s no reason to panic.
There aren’t any stars likely to go supernova
in our immediate celestial neighborhood.
The closest supernova candidate, Betelgeuse, is more than 500 light-years away.
And the math is on our side, because other likely candidates are moving
farther away from us, not closer.
So chances are we won’t see an event like the one
our ancestors witnessed millions of years ago any time soon.
But although we will not see one like that old supernova,
its remnants will still be part of us.
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[♪ OUTRO]