Name: 小惑星が太陽系外からやってきた! (An Asteroid Visited Us From Outside the Solar System!)
Uploaded: 2021-01-14T10:41:46.000Z
Duration: 5 min 3 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

Even though we spend tons of time studying them,

it's impossible for us to visit other star systems, at least, right now.

But recently, a visitor from another system came to see us.

On October 19, astronomers at the University of Hawaii noticed something new in the sky,

and at first, it seemed like a regular asteroid, less than half a kilometer wide.

Except, when they mapped its path through space,

they found that it's moving way too fast to be orbiting the Sun.

That means, as far as anyone can tell, this newly discovered asteroid is the first one

we've ever seen from outside the solar system!

The asteroid is currently known as A/2017 U1,

and astronomers have searched for objects like it for decades.

They expect that, when new planetary systems form, the planets' gravity should occasionally

slingshot some leftover rocks into interstellar space.

And every once in awhile, one of those lonely rocks should enter another star system, like

ours, carrying chemical clues about the neighborhood it came from.

But we've never actually seen one of these interstellar tourists before, which is why

At first, astronomers thought that U1 was just a Near-Earth Object, our name for any

large rock whose orbit passes near Earth's.

But when they calculated its speed and trajectory, they discovered that it can't be an NEO:

Those all orbit the Sun, and U1 is just going too fast to be held,

or even slowed down, by the Sun's gravity.

And we don't know of anything along its path that could've sped it up so much.

Which means it must've fallen toward the Sun from somewhere else,

probably from the direction of the constellation Lyra.

Right now, U1 is rocketing out of the solar system at about 44 kilometers per second,

or almost twice as fast as the average asteroid.

And unfortunately, it'll probably never come back.

Researchers all over the world will spend the next few weeks

frantically pointing telescopes at U1, hoping to learn as much as they can

about its trajectory, composition, and behavior before it leaves for good.

After a month or so, it'll be so far from the Sun

that even our best telescopes won't be able to see it any more.

Now, it's completely possible that all this research will end up revealing a much more

normal explanation of U1's impressive speed.

After all, it was just discovered, so we hardly know anything about it.

And even if it is from somewhere else, we might not have enough time with U1 to learn

much more about where it came from or what it's made of.

But for now, this rock seems like it's the closest we've ever been

Now, as far as we know, everything else around the Sun has been here since the beginning.

And one of those rocks is the dwarf planet Ceres in the asteroid belt, where NASA's

Dawn spacecraft has been orbiting for more than two years.

Two new studies, published this month, have used data from Dawn to support the idea that

Ceres used to have oceans, by finding ways that

the ocean's remnants could still be affecting its surface.

The first study, published in the Journal of Geophysical Research: Planets, used measurements

of Ceres's gravity and shape to calculate the density of rocks on and beneath its surface.

When there are denser rocks, or something with a lot of mass, like a mountain,

the gravitational pull of that spot will be stronger.

And that can help us figure out what an object is made of.

Using this data, the researchers found that Ceres's crust just isn't dense enough

to be made of regular rocks, which we basically already knew.

But in this new paper, the scientists proposed that there's water-ice mixed in with the

rocks, along with salts and something known as clathrate hydrate, or trapped gas bubbles

And the second study, published in Earth and Planetary Science Letters,

It confirmed and added to those findings by looking more closely at how Ceres's surface

We know that harder rocks like iron tend to keep their shape

better than softer or waterlogged rocks do, so the more flattened areas on Ceres's surface

are probably made of softer, less dense rock.

Except, these researchers also found that Ceres is made of pretty strong stuff.

So the only way they could reconcile Ceres's low density with its strong crust was if the

crust was made of a combination of rocks, ice, salts, and clathrate hydrate.

And if that sounds familiar, that's because that's exactly what the first study's

authors found, based on different measurements and methods.

All that water in Ceres's crust supports the idea that Ceres originally had oceans

on its surface that have since frozen or escaped into space.

Which is pretty awesome for a dwarf planet.

The second study's authors also pointed out that Ceres's insides seem weaker than

the outer crust, which could mean that there's still some liquid water leftover after all.

But before you get your submarines ready, we still need more data to be sure.

Fortunately, NASA announced that they'll be extending the Dawn mission a second time,

and it'll be closer to Ceres than ever before

until it runs out of fuel sometime late next year.

So, our interstellar visitor U1 might be speeding away from the solar system, but we still have

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