When you look at Earth today, it's easy to imagine that it formed as a perfect, fertile planet,

full of everything it needed to support life.

It's a beautiful, big, wet rock.

But scientists are pretty confident that's not what happened.

They've known for a long time that, because of Earth's early conditions,

the key ingredients of life, elements like carbon and nitrogen, have not been here since the beginning.

Popular hypotheses suggest they arrived via meteorites or comets,

but none of those models totally checks out.

So now, there's another idea.

Last Wednesday, in a paper published in Science Advances,

researchers announced that these elements most likely made a more dramatic arrival.

Instead of coming on meteorites, they may have come from a massive collision.

The same massive collision that formed the Moon.

Now it's not surprising that these elements, called volatiles, came from elsewhere.

They have really low boiling points, so when Earth was forming,

it would have been way too hot to hold onto them.

And also, thanks to their chemistry, any volatiles that didn't escape

would likely have been pulled into the Earth's iron core.

So, somehow, they must have been added to the mix later.

Otherwise, we wouldn't be here.

It's just that figuring out that “somehow” is much easier said than done.

Like, even though the idea about Earth getting its volatiles from meteorites is popular,

the numbers have never quite added up.

Earth's ratio of carbon to nitrogen is way higher than that of any meteorite.

So in this new study, a team of researchers at Rice University got creative to try and

understand what happened.

In their lab, they used machinery to put rocks under extremely high pressure and temperature,

squeezing them as if they were around 100 kilometers below the Earth's surface.

They were trying to recreate the environment that would have existed when planets' cores were forming.

And they found something interesting.

In their experiments, a planet with an iron core would normally pull in all the volatiles,

just like on Earth.

But if that core was rich with sulfur, the volatiles were less attracted to it,

so they remained free.

That doesn't mean much for Earth itself, since our planet doesn't have a sulfur-rich core.

But it does mean that a foreign rock with a core like that could have had plenty of

volatiles in its outer layers.

So if an object like this collided with Earth at some point,

it could have contaminated our planet with those elements that earth had long ago lost.

You might be thinking that sounds like a lot of sketchy “could have”s,

but the researchers found that it was surprisingly likely.

They ran around a billion simulations of the evolution of the solar system,

and found that the best explanation for the number and ratio of volatile elements on Earth is a scenario

where an object around the size of Mars collides with our planet.

Now as for the timeline, in the best-fit scenario, the collision lined up with the one that formed the Moon.

It's a promising, and really convenient, idea, but the case isn't closed yet.

This study mainly looked at the chemistry that might have happened during a collision,

but we'll need to learn more about the physical side of how planets grow and evolve.

Still, if proven, this research backs up the idea that, in all likelihood,

we owe our whole existence to the colliding worlds of the early solar system.

Of course, it's not easy to decode the solar system's history billions of years after events took place.

Fortunately, some clues are locked away at the edge of our solar system,

and scientists are starting to uncover them.

On Monday, in the journal Nature Astronomy, researchers announced that they may have indirectly detected

a kilometer-sized rock in the Kuiper belt, the ring of icy objects past Neptune.

If true, it would be the first time astronomers made a detection of an object like this on two separate telescopes,

making it the most convincing detection yet.

These barren rocks might not seem like they have much to do with us,

but they're kind of like long-lost relatives.

Earth and the other planets formed from objects like those.

The difference is that this icy fringe of the solar system wasn't dense enough to form planets,

so it's barely evolved at all in the last 4.6 billion years.

So in the absence of time travel, it's the closest we can get to seeing what things were like

when the planets were first forming.

In the study, scientists were especially interested in finding objects between one and 10 kilometers across,

because rocks like these formed the seeds of our planets.

Unfortunately, objects that size are way faint.

Like, much fainter than Pluto, so even the largest telescopes can't see them directly.

But, in theory, and maybe now for the first time in practice, we can detect them indirectly

by measuring blips in the light as they pass in front of stars.

It's a method called a stellar occultation.

And it's not easy to pull off.

That blip in light is very small and lasts less than a second, and with just one telescope,

it can be embarrassingly difficult to tell between a 4.6-billion-year-old space rock

and, like, a bird that flew past.

So the team in this study set up two identical telescopes on the roof of a school in Japan,

and monitored around 2000 stars for just over a year.

After sifting through more than 100,000 hours of data, they found what they were looking for:

one possible detection of a Kuiper belt object passing in front of a star.

So far, it's just a candidate.

Even though the chances are really small, we can't entirely rule out the possibility

that the signal came from a statistical fluke, or something like an asteroid.

But if it is real, this tiny shadow can still offer some insight.

Making some assumptions about its shape and position, scientists peg its diameter at around 1.3 kilometers.

That supports previous results that suggest there may be more small objects in the Kuiper belt

than some studies previously thought.

And the better we understand how they're distributed,

the better we can understand what kinds of objects grew into the planets and which ones stayed behind.

To get closer to that answer, the team and their collaborators plan to keep looking for

other occultations that can tell us more about these ancient rocks and the history we share with them.

So, between our observations and simulations,

we can start to fill in some of the holes in our solar system's majestic history.

Thanks for watching this episode of SciShow Space News!

And especially thank you to all the people who support us on Patreon,

helping us unpack science news like this.

We love doing what we do and we're very thankful to have you on board.

If you want to support the show and partner up with a bunch of other curious, wonderful people,

you can go to patreon.com/scishow.

[ ♪ Outro ]