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  • [♪ INTRO]

  • By now, planets around other stars are old news.

  • In the last couple of decades, astronomers have found thousands of these exoplanets,

  • and now, it's time to start learning more about them.

  • And one of the things researchers are looking for are rings.

  • After all, our solar system isn't just a star and eight blobs.

  • Most of our neighborhood planets have moons, and half of them have rings.

  • So far, exoplanet rings, or exorings, have been pretty elusive.

  • But we've already found what might be the first set of exorings, and if we find more,

  • we'll have a treasure trove of new information.

  • In our solar system, astronomers hunt for planetary rings using a technique called stellar

  • occultation, where something passes in front of a background star,

  • blocking out some or all of its light.

  • Since we can measure changes in brightness very accurately, astronomers can use occultations

  • to spot things too faint or far away to be picked up otherwise.

  • For example, we knew that Pluto had an atmosphere

  • decades before New Horizons arrived because of occultations.

  • They were used to discover the rings of Uranus, Neptune, and the minor planet Chariklo.

  • Outside of our solar system, stellar occultations are so useful that they've been the most

  • successful technique for discovering exoplanets.

  • It's usually called the transit method when used in planet hunting,

  • but the basic principle is the same: as a planet passes in front of its star as seen from Earth,

  • we detect a small dip in the star's light.

  • That dip is called the lightcurve, and all transiting planets have lightcurves with basically the same shape.

  • So if astronomers see an unusual pattern, it might mean there's something

  • extra blocking the star's light, like a ring!

  • Finding a ring would be awesome, but what it could tell us might be even cooler.

  • During its 13-year mission, NASA's Cassini spacecraft revealed that Saturn's ring system

  • is intricately related to both the planet and its moons.

  • A gap, for example, could mean an unseen moon is embedded within the ring.

  • Occultations of Saturn's rings revealed the narrow Keeler gap

  • well before Cassini discovered a tiny moon named Daphnis orbiting inside it.

  • On the other hand, a moving pattern of higher density, called a density wave,

  • could reveal the presence of a moon orbiting outside of the rings.

  • Nearly all the fine detail in Saturn's rings is due to the gravitational sway of its many moons.

  • With an exoplanet, if astronomers knew the mass of that moon, something that's possible

  • with the transit method, the wave's shape could even reveal the density of the ring.

  • But wait! There's more.

  • Cassini found that special density waves in Saturn's inner rings

  • actually revealed the motion of material within the planet.

  • Now, researchers are probably a long way from repeating those observations in another star system.

  • We're still working on spotting whether rings exist at all;

  • it's a lot harder to see what's happening in their internal structure.

  • But it's a tantalizing hint of what the future might hold.

  • And the path to that future is already in motion,

  • because astronomers might have found the first set of exorings back in 2012.

  • The rings were found orbiting a planet with a name so bad

  • I'm not even going to try to say it, so let's call it J1407 b for short.

  • In the world of exoplanets, everything is over the top, and this system is no exception.

  • At dozens of times the mass of Jupiter, J1407 b is so large that it's tough to even be

  • sure whether it's a planet or a failed star called a brown dwarf.

  • The rings are no slouch, either.

  • Based on their data, the researchers think that a total of 30 rings stretch about

  • 120 million kilometers, a span more than 200 times larger than the rings of Saturn.

  • One study indicates that a gap between two of the rings

  • could be caused by a moon up to 80% the mass of the Earth.

  • With their gigantic size and high density,

  • some scientists wonder if it's even right to call them rings at all.

  • Instead, we might be seeing the remnants of a circumplanetary disk,

  • the disk of material around a planet that gives rise to moons early in its life.

  • Our solar system went through that process billions of years ago,

  • but the J1407 system is only about 16 million years old.

  • It's probably not worth worrying about whether to call this material a ring or not

  • until we have other examples to compare it to.

  • Which begs the question: if planets are common in the galaxy and rings are common around

  • planets in our solar system, where are all the exorings?

  • Why have we found only this one example?

  • Part of the answer is that we need a new generation of technology to see them.

  • Just finding an exoplanet is still a pretty tricky task and rings are even harder.

  • But geometry is also working against us.

  • The transit method, which is our best, and possibly only, way to find exorings,

  • only works when we're looking at a star system edge on.

  • But models of planet formation suggest that planetary rings should also be edge-on

  • in that scenario, making them nearly impossible to detect.

  • After all, Saturn's rings become almost invisible to a spacecraft

  • orbiting the planet itself when viewed edge-on.

  • From hundreds of lightyears away, we'll need to be looking at really big rings in a

  • really obvious configuration to have a chance of seeing them anytime soon.

  • That would be a pretty unusual situation, so it makes sense that we'd be having

  • so much trouble finding exorings even if they're all over the place.

  • But, hey, if there's one thing exoplanets are awesome at, it's being unusual.

  • So the hunt is on!

  • Thanks for watching this episode of SciShow Space,

  • and special thanks for our patrons on Patreon for making it possible!

  • If you'd like to help support our team and help us make more episodes like this,

  • you can go to patreon.com/scishow.

  • [♪ OUTRO]

[♪ INTRO]

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太陽系外惑星にはリングがあるのか? (Do Exoplanets Have Rings?)

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    林宜悉 に公開 2021 年 01 月 14 日
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