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  • [ ♪ Intro ]

  • When stars like the Sun reach the end of their lives,

  • they balloon into red giants and swallow up any planets in their way.

  • In our solar system, that will be the end of Mercury, Venus, and Earth.

  • They then shrink to a dying remnant called a white dwarf,

  • which is sometimes surrounded by a disk of, like, ex-planetary dust.

  • But in an extrasolar system 410 light-years away,

  • scientists discovered a shard of a planet that survived the death of its star,

  • according to a study published last week in the journal Science.

  • They discovered the object in orbit around a white dwarf.

  • In the star's spectrum, they saw a repeated pattern of light dimming and brightening.

  • This told astronomers that something was moving through the star's disk of dust

  • and creating a cloud of gas.

  • That gas emitted a little extra light that was noticeable each time it went around the star.

  • And whatever was causing it seemed to be somewhere between a few kilometers

  • and a few hundred kilometers wide.

  • This makes it only the second solid body ever discovered around a white dwarf.

  • Now, the object's circumstances are pretty grim.

  • Not only is it a lone survivor circling a dead star,

  • but it's sitting in a disk of debris made of the remains of destroyed planets.

  • But the fact that it exists at all is pretty incredible.

  • It's way closer than astronomers expected to find any planetary survivors.

  • It's so close, it's actually thought to be orbiting within the star's original radius.

  • Meaning, if you replaced the white dwarf with our Sun, it would be orbiting inside of the Sun.

  • Yeah. So this thing orbits very quickly.

  • Fast enough that it rings in a new year every two hours.

  • At this distance, the star's gravity is so intense,

  • most planets or asteroids would be ripped apart.

  • But somehow this fragment has held up.

  • Scientists suspect that it's because it's made mostly of heavy metals like iron and nickel.

  • And such a strong chunk of metal could have survived the destruction of its planetary system,

  • the researchers say.

  • Metalis also the word I would use to describe this planetary fragment's, like, whole thing.

  • The researchers' best guess is that it's the metal core of a larger planet that used to orbit farther out,

  • before the death of its star caused all the planetary mayhem.

  • And this fragment of a past planet can tellus more about what kinds of planets make up extrasolar systems.

  • It can also offer us a hint at our solar system's future,

  • and what traces might remain after our Sun dies.

  • This planetary remnant isn't the only interesting thing that has turned up recently in a disk of debris, either.

  • In a preprint of a paper accepted for publication in the journal Astronomy & Astrophysics,

  • researchers announced that they'd discovered a comet around the young star Beta Pictoris.

  • That's right, an exocomet.

  • They found it in data from TESS, the exoplanet-hunting satellite

  • that launched last year as a successor to Kepler.

  • And the fact it's already got an exocomet discovery under its belt is pretty promising.

  • There's been pretty strong evidence for a while that exocomets are out there.

  • Astronomers studying the emission spectra of stars saw that sometimes certain wavelengths of light

  • were getting absorbed for a few hours at a time.

  • So they could infer that something comet-sized was passing in front of these stars.

  • One 2014 study even suggested that there were thousands of exocomets around Beta Pictoris alone.

  • But it was hard to be certain about any of this.

  • The best evidence would be in the form of a light curve:

  • a subtle dip in brightness as one of these comets passed in front of its star.

  • It wouldn't look like the dip that comes from a planet.

  • Planets are basically symmetrical, so the light dims as the planet passes in front and

  • then brightens when it leaves.

  • But comets have tails.

  • So the first half of an exocomet's light curve should look like a planet's,

  • a sharp drop in brightness.

  • But then the drop should reverse more gradually, as light starts to peek through the tail.

  • Astronomers predicted 20 years ago that a comet would have this kind of light curve,

  • but our observations are just starting to catch up.

  • In its 9 years of operation, the Kepler telescope found three potential instances of exocomets.

  • But Kepler paid more attention to older stars, which we think have fewer comets,

  • as the chaos of planetary formation calms down a bit.

  • TESS looks at all kinds of stars, including young ones,

  • which are still surrounded with some building blocks that formed planetary systems.

  • And the fact that it's already found one exocomet

  • gives us a lot of reasons to get excited about the science that's ahead.

  • Now that they know what TESS is capable of,

  • scientists will be on the lookout for more signals like this.

  • With more data, they can start to see what kinds of stars typically have lots of comets, or don't.

  • And that can tell us about the role comets play in the formation of planetary systems,

  • and how common they actually are.

  • So, whether it's in the debris surrounding a young star or the rubble swirling around a dead one,

  • there's a lot of galactic archaeology to do in the years ahead.

  • Thanks for watching this episode of SciShow Space News,

  • which is brought to you with the help of our patrons on Patreon.

  • If you like what we do here and want to help us out,

  • check out patreon.com/scishow to learn how.

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史上最金屬星球碎片 (The Most Metal Planet Fragment Ever)

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    林宜悉 發佈於 2021 年 01 月 14 日
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