In DepthPlanetary Astronomy

Alien comets may be common, analysis of 2I/Borisov suggests

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Science  01 Nov 2019:
Vol. 366, Issue 6465, pp. 558
DOI: 10.1126/science.366.6465.558-a

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Summary

The comet is an alien intruder, from another star system. But 2I/Borisov, the second known interstellar visitor after far smaller 'Oumuamua, discovered in 2017, looks remarkably like a normal comet from our own Solar System: an object a few kilometers across spewing carbon monoxide gas, water vapor, and dust. Researchers who announced their analysis this week, the most comprehensive yet, say Borisov could be normal in another way. They say the size of the two objects, along with the rate of their discovery and other factors, suggests that at any given moment about a dozen interstellar visitors at least as large as 'Oumuamua are passing through the Solar System. Large telescopes under construction could catch these visitors more often, perhaps two or three times a year.

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