Proponents of the extraterrestrial hypothesis suggest that these unexplained reports are of alien spacecraft, though various other hypotheses have been proposed.

I must stress that the Air Force does not dismiss an extraterrestrial hypothesis based on our scientific analysis.

In the course of all these adventures and frenetic activities I have come to reject outright the popular extraterrestrial hypothesis.

In addition to the extraterrestrial hypothesis, a variety of possible explanations for flying saucers have been put forward.

He seemed to regard the extraterrestrial hypothesis not as unimpeachable fact, but as a working model.

Tombaugh was probably the most eminent astronomer to have reported seeing unidentified flying objects and to support the extraterrestrial hypothesis.

He tried to avoid the kinds of open-ended speculation that had led to Sign's personnel being split among advocates and critics of the extraterrestrial hypothesis.

Alternative explanations, such as the extraterrestrial hypothesis, are dismissed by many academics as being pseudoscientific.

He also considered the extraterrestrial hypothesis to be viable.

Present evidence surely does not amount to incontrovertible proof of the extraterrestrial hypothesis.