Dr. Avi Loeb, the distinguished astrophysicist and chair of Harvard's Astronomy Program, just published a book called Extraterrestrial: The First Sign of Intelligent Life Beyond Earth.
In the book, Dr. Loeb calls out several specific peculiarities about the interstellar object Oumuamua. He proposes a rather shocking hypothesis:
I submit that the simplest explanation for these peculiarities is that the object was created by an intelligent civilization not of this Earth.
In interviews 1, 2, 3: Dr. Loeb mentions several peculiarities. Here are three:
Oumuamua has a unique light curve. Uhoh has a nice analysis and some great graphics in this answer: Could asteroid 'Oumuamua actually be round?. Dr. Loeb claims that the high aspect ratio implies this is not a natural object. He uses an analogy of a thin piece of paper falling through the air having a less extreme aspect ratio.
Oumuamua showed acceleration unaccounted for by orbital mechanics after its close pass to the Sun similar to a comet. A comet gets its push due to evaporation of gasses, but there was no evidence that Oumuamua had a tail of gas. Dr. Loeb proposes that Oumuamua may have been designed as a light sail.
Dr. Loeb claims Oumuamua's velocity as it entered the Solar System is slower than 1 out of 500 near star systems in an LSR reference frame.
Does the evidence support Dr. Loeb's hypothesis that Oumuamua is most likely of extra-terrestrial origin?