字幕表 動画を再生する 英語字幕をプリント Shira Polan: In 2015, astronomers noticed something strange going on nearly 1,500 light-years from home. A star named KIC 8462852, aka Tabby's Star, kept dimming, as though something was passing in front of it. Since then, scientists have been trying to figure out what that something might be. A swarm of comets flying by en masse? An alien megastructure? Then, in 2019, astronomers proposed a new, more plausible explanation that might finally solve the mystery of Tabby's Star. There are really two mysteries centered on Tabby's Star. First, the star keeps brightening and dimming in a bizarrely sporadic pattern. While very young stars occasionally do this, astronomers have never seen this behavior in a star of Tabby's age. Second, the star is slowly but surely getting dimmer over time, in a steady decline that astronomers have never observed in another star of its type. Clearly, something fishy is going on. Some researchers have suggested an advanced alien civilization is to blame. If aliens built a megastructure called a Dyson Sphere to siphon off the star's energy, that could explain how the star is a whopping 10% dimmer than it was just a century ago. But there are some holes in that idea. For one, you'd expect an orbiting megastructure to make regular, predictable trips around the star, just like the International Space Station makes regular trips around the Earth. But, so far, there is no clear evidence that the dips occur on a consistent schedule. Plus, when it comes to any theory about advanced alien civilizations... Brian Metzger: It's a little hard to test it. Polan: That's Brian Metzger, an astrophysicist at Columbia University. He studies what he calls... Metzger: Things that go bump in the night. Polan: Or strange phenomena in the universe, including the odd behavior of Tabby's Star. Metzger says another theory has problems as well. You see, some experts suggest a massive swarm of comets is the real culprit. This has the advantage that, unlike aliens, we're positive comets exist. And if they were orbiting the star, they'd explain the random blips, but... Metzger: A lot of the explanations run into the problem of: Why is this relatively rare? Like, why don't other stars show this behavior? Polan: Basically, there would need to be something special about Tabby's Star to attract so many comets to congregate at the same time at the same place. But as far as astronomers can tell, Tabby's Star is just like any other star of its type. Luckily, Metzger and his team may have a new solution. In 2019, they published a paper in the Monthly Notices of the Royal Astronomical Society that argued for a different culprit: the dusty remains of a dying moon. According to Metzger's models, a gas giant may have orbited the star millions of years ago. And, like Jupiter and other gas giants, it hosted a few icy moons. Now, the planet and its moons would have continued to orbit Tabby's Star to this day, if not for one problem. Metzger says a second nearby star may have exerted a tiny gravitational pull on the planet. Over time, it slowly tugged the planet out of its orbit and towards Tabby's Star until, eventually, the planet fell in and was destroyed. But the planet's moons may have escaped this fate, instead getting trapped into orbit by Tabby's gravity. Over time, the constant radiation could have melted off chunks of these icy moons, resulting in a colossal cloud of moon dust around the star. Now, this plan makes sense for a number of reasons. First: Metzger: Dust or small particulate grains is a really effective way to block the light of a star. Polan: Unlike an alien superstructure, dust floats around in a blob that's constantly morphing and changing its shape, which could explain the irregular dips. And if enough of it accumulated around the star, over time, it could lead to that mysterious 10% dimming astronomers observe. The moon part of Metzger's hypothesis works well, too. Metzger: So, you need an object which is very massive and has a lot of dust or has a lot of materials that it could survive for a very long time, that it could feed this giant cloud of debris. Polan: Meanwhile, you'd need a remarkably large swarm of unusually giant comets in order to get the same job done. Metzger: We would need something much bigger than an ordinary comet to explain the dipping around Tabby's Star. Polan: Plus, unlike an alien machine, we see moons all the time. And while we haven't seen this exact scenario before, it's based on something we have observed with Jupiter-like planets. Metzger: So, we have lots of evidence from other planetary systems that the Jupiters can be driven into their stars. Polan: And... Metzger: If you think Jupiters are being driven close to their stars, you have to say, "What happened to their moons?" Polan: So, mystery solved? Not quite. While Metzger says he's happy with his team's explanation, he still has plenty of work ahead of him. Metzger: You know, I think it's important to, you know, moving beyond what we're working on now. The next thing is obviously to make some predictions that can be tested. Polan: After all, theoretical models only get you so far. That being said, it's still probably not aliens.