At the furthest edges of our solar system lies the vast Oort Cloud, a hypothetical giant shell full of billions of frozen comets. Formed 4.5 billion years ago, the cloud is a frozen museum of the chemistries of neighboring stars.
The Oort Cloud lies some six trillion miles from the Sun. We don't have any direct proof that the cloud actually exists, but its existence is likely based on the frequency at which comets invade our home in the inner solar system. Some of these comets would have formed along with the rest of our solar system about 4.5 billion years ago, and then they were gravitationally ejected to the furthest reaches of our Sun's influence, right on the edge of the interstellar medium.
But our Sun formed inside a nebula very close to other stars, all of which were undergoing the same formation processes. Astronomer Hal Levinson suspects the Sun swapped comets with its neighbors, and that many of the comets now in the Oort Cloud are actually refugees from other star systems.
His mathematical models make a particularly persuasive case. Based on how often comets enter the inner solar system, he predicts there are some 400 billion comets out in the Oort Cloud. But if you only work with the raw material available during the Sun's formation, you only get 6 billion comets. Even if those numbers are way off, that sort of massive discrepancy makes a very good case argument that some of those comets came from outside our solar system.
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Levinson thinks we may have already seen some of the comets that came from other stars. Those with very long orbital periods likely originated outside our solar system. That could well include 1997's Hale-Bopp Comet, which was the most widely observed comet of the twentieth century and perhaps all of human history. Considering it won't be seen again until the year 6200, its orbital period is probably long enough to make it a good extrasolar candidate.
Now, this isn't just a cool bit of trivia that our solar system is home to travelers from other stars. Because any object formed around a star carries the chemistry of that star with it, any alien comets in the Oort Cloud would provide tremendous insight into what other stars are like without having to go to the immense time and trouble of building an interstellar probe. The Oort Cloud is a long way away, to be sure, but it's a lot closer than any star - and, if we're willing to wait a little while, it will send us a comet from another star hurtling our way soon enough.
[via Discovery News]