In 1849, we discovered an object between Mars and Jupiter called Hygiea.

For a while, we thought it was just another big asteroid in the asteroid belt,

but on Monday, a team announced in Nature Astronomy

that it might actually deserve a different title.

Based on their observations, Hygiea is probably a dwarf planet!

And it might also have a different origin story than once thought.

The requirements for being a dwarf planet are pretty simple:

You need to orbit only the Sun, be massive enough that your gravity pulls you

into a roughly spherical shape, and not be able to clear your orbit.

In other words, your gravity has to be weak enough that you can't push

similarly-sized objects out of your path.

As a relatively light object living in the asteroid belt,

Hygiea already checked two of these boxes.

But until now, we couldn't tell whether it was spherical, because we were busy

looking at other things, and because it's very dark and hard to see.

Then came this week's big announcement!

To make the new discovery, astronomers used the

European Southern Observatory's Very Large Telescope

to get the best images yet of Hygiea.

And we can now say for sure that it is really round!

That means it checks all the dwarf planet boxes!

Though, just to be clear, Hygiea isn't formally a dwarf planet yet

some official conversations will need to be had about it first.

But if it is reclassified, it will officially replace Ceres as the smallest dwarf planet

in our solar system.

So, this is all great; I mean, humans love categorizing things.

But these observations have greater significance for the solar system's history, too.

Because this team didn't just find that Hygiea is spherical.

They also noticed something weird about the object's surface.

Previous research has shown that Hygiea is the largest member in a family of

asteroids that all formed from the same parent body.

The working hypothesis has been that this parent body was hit

by a bunch of impacts that knocked off smaller asteroids until,

eventually, Hygiea was all that was left behind.

Based on this, Hygiea should be covered in big craters.

But the authors of this paper imaged 95% of the thing's surface,

and there are no giant craters to be found.

To solve this new mystery, the authors ran simulations to see what conditions could

have led to this scenario.

And they suggested the original parent body didn't survive all those impacts after all.

Instead, the impacts were likely so big that the parent body was totally destroyed!

Lots of the pieces just went flying off to form all of the smaller asteroids in the family,

but enough were big and close enough that they reformed into Hygiea!

So, keep an eye out for an official announcement,

and maybe we'll get to categorize Hygiea as a dwarf planet soon.

And speaking of big collisions…

In 2017, scientists had the chance to directly observe something

amazing for the first time: the collision of two neutron stars.

Neutron stars are incredibly dense, neutron-rich objects,

and it's normally really hard to see what happens when they run into each other.

Partly because the events are pretty rare,

and partly because there aren't any neutron stars around here.

But we finally got a chance to do it when two stars collided

only 130 million light-years away.

Which in astronomy terms, is much closer than it sounds.

Last Wednesday in the journal Nature,

one team published their analysis of data from that collision.

And they found the best evidence yet of something we've suspected for a long time:

Neutron star collisions produce really, really heavy elements.

So, some context.

Stars form elements through nuclear fusion, which is pretty straightforward:

They fuse together lighter elements to create heavier ones.

At first, this process just involves turning hydrogen into helium.

But over a star's life, as more particles get smashed together,

it can make elements as heavy as iron.

But there are tons of heavier elements out there,

like copper and lead, that stars can't make.

The pressures and temperatures inside them just aren't high enough.

So to get these elements, you need something more.

Traditionally, scientists have believed these atoms come from supernovas:

hotter, more powerful explosions that happen when some stars die.

And that still seems to be true!

But according to this new paper, supernovas aren't the only events

that make heavy elements: Neutron star collisions can, too.

In the paper, the team found evidence that the collision was creating

the heavy element strontium - a lot of it.

In fact, about five Earth masses!

They suggest it came from a process called rapid neutron capture.

Essentially, that's where you have so many neutrons in one place that they just sort

of stick to the particles around them.

So you end up with a bunch of nice, heavy atoms.

If the atom is unstable, some of those neutrons

can break down into protons and electrons.

And that leads to heavier elements.

This is cool, because we need heavy elements to make things like planets.

So knowing that supernovas aren't the only creators out there

tells us something valuable about the makeup of the universe.

And another fun fact: This study officially confirms that neutron stars are actually

made of neutron-rich matter.

That was another case where we had a lot of math to support that assumption,

but no physical confirmation.

So, this work provides really clear, hard evidence

for two fundamental aspects of astrophysics.

You know, no big deal.

Thanks for watching this episode of SciShow Space News!

And an extra-special thanks to this episode's President of Space, Matthew Brant.

Matthew is one of our longtime patrons on Patreon

one of the people who supports SciShow

and helps us keep making content like this.

So, thanks for everything, Matthew!

SciShow is so much better because of patrons like you.

If you want to learn how to support the show

and become our next President of Space, you can go to patreon.com/scishow.

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