字幕表 動画を再生する 英語字幕をプリント We have gotten really good at spotting exoplanets. Usually we see them when they transit in front of their star and cause a drop in light levels, or when their gravitational pull makes their star wobble. But very rarely, it's possible to see them with an optical telescope and we've seen about fifty this way. But that number has actually gone down, because one of the first exoplanets ever directly imaged vanished in 2014. Now new research may explain where it went. Was it destroyed by a Death Star? It was the Death Star, wasn't it? The planet in question was observed orbiting the star Fomalhaut, and so was named Fomalhaut b. Hubble was the first to spot it in 2004 and again in 2006 as a moving dot around its star, and it could have been easily missed. Even though it was estimated to be as massive as three Jupiters and much brighter than other exoplanets, which tend to be too small to reflect enough light for us to see, Fomalhaut b is a billion times fainter than its star. One astronomer called it “one of the most difficult detections in the history of exoplanet science.” Though it was spotted in 2004 and 2006, there was some controversy over whether or not it was actually an exoplanet. Some things just seemed off. For one thing, its orbit was highly elliptical. Fomalhaut, by the way, also has a huge debris belt around it, so a false-color composite image of the system taken by Hubble looks like, there's no other way of saying it, the eye of Sauron. Then there's the fact that it didn't radiate any detectable infrared radiation. Scientists expected to see some heat coming off what they assumed was a large and young planet, but there wasn't any to see. Nonetheless, in 2008 scientists declared it was an exoplanet visible from Earth, causing much debate. And then in 2014, Fomalhaut b up and vanished on us. You ungrateful hunk of space rock, do you know how lucky you are? Pluto would kill to be declared a planet! To find out where it went, astronomers from the University of Arizona returned to the first images taken of it by Hubble in 2004 and tracked its evolution until its 2014 disappearance, presumably while muttering “enhance” the whole time. Over that 10-year period Fomalhaut b actually appeared to expand and fade away. Now, in early May of 2020, they think they have an explanation. The astronomers who noticed the expansion and slow fade concluded that Fomalhaut b the planet never existed in the first place. Plot twist! Instead they suggest it was a cloud of expanding dust that resulted from two massive planetesimals smashing into each other. This hypothesis would explain some outstanding questions about Fomalhaut b nicely, like the elliptical shape of the orbit, and how it would fade as the small dust particles spread out until they fell below Hubble's detection limit. So, Fomalhaut b may not have been one of the 50 or so exoplanets we've directly imaged through a telescope, but it may be something we've seen even less often. We almost never directly observe collisions of massive objects like these, and based on the simulations proposed by the study's authors, Hubble only just missed this collision. It could be a very useful data point that helps us understand how planetary systems evolve. That's assuming this new explanation is in fact what we saw. It's thought a dust cloud like this wouldn't be visible until a decade after the asteroid's crash, disagreeing with the model the astronomers from the University of Arizona put forward for Fomalhaut b's disappearance. Perhaps it's something even stranger. More observations from bigger and better telescopes like the long awaited James Webb could finally put this mystery to bed. So, what I'm hearing is the Death Star...not totally off the table yet. While this new paper suggests Fomalhaut b was the result of two planetesimals colliding, it's not the first time someone proposed the “planet” was actually just a cloud of dust. As our technology improves, we're not going to just look for exoplanets, but signs of alien life as well. Check out Amanda's video on the future of space-observing cameras here. Make sure to subscribe to Seeker for more videos like this, and as always, thanks for watching. I'll see you next time.