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  • [♪ INTRO]

  • 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

  • A/2017 U1 is so exciting.

  • 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.

  • But they don't have long.

  • 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

  • to a piece of another star system.

  • So, thanks for swinging by, U1!

  • 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

  • surrounded by water molecules.

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

  • found the same thing.

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

  • seems to have changed over time.

  • 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

  • lots of cool stuff to study here, too.

  • Thank you for watching this episode of SciShow Space News, which can only exist because of

  • our awesome Patrons on Patreon!

  • Thank you to Patrons like Alaina, Zane, and someone who just goes by UhMike.

  • Not only do you guys have awesome names,

  • you make everything we do at SciShow possible, so thank you.

  • If you want to help make SciShow, you can go to patreon.com/scishow,

  • and we're also conducting an audience survey right now too.

  • If you want to tell us more about yourself, please click on the link in the description.

  • [♪ OUTRO]

[♪ INTRO]

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小惑星が太陽系外からやってきた! (An Asteroid Visited Us From Outside the Solar System!)

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    林宜悉 に公開 2021 年 01 月 14 日
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