https://www.vice.com/en_us/article/y3wgg5/alien-fossils-could-be-hidden-across-earth-and-we-must-find-them-scientist-says

Whether or not we’re alone in the universe is one of humanity and science’s most enduring questions. But now a new paper is asking whether or not we’re even alone on Earth. 

The theory was published this month in the International Journal of Astrobiology and proposes the possibility that sub-micron sized alien fossils or minerals may be floating through space or even buried deep under our planet's oceans or ice sheets as the result of asteroid impacts on other planets. The paper’s author and University of Tokyo professor of astronomy, Tomonori Totani, told Motherboard that devising ways to find these particles on Earth could help identify alien biosignatures that established methods—like searching radio signals for so-called technosignatures or analyzing the atmospheres of exoplanets—miss.