The Pentagon investigates what are now called UAPs or Unexplained Aerial Phenomena.

STEVE INSKEEP, HOST:

What can we learn from reports of UFOs? The Pentagon is investigating what are now referred to as UAPs - or unexplained aerial phenomena.

(SOUNDBITE OF ARCHIVED RECORDING)

SCOTT BRAY: We have detected no emanations within the UAP task force that would suggest it's anything non-terrestrial in origin.

LEILA FADEL, HOST:

Naval intelligence official Scott Bray testified last week at the first public congressional hearing into UFOs - UAPs in a half-century. In human speak, he's saying, there's no proof of aliens swooping into our atmosphere.

CAROL CLELAND: I think, for them to concede there was evidence of non-terrestrial origin, they'd have to come down and say hi.

INSKEEP: Carol Cleland directs the Center for the Study of Origins at the University of Colorado Boulder and says we need to see all the government's 400 UAP files before making a final conclusion.

CLELAND: The preliminary assessment just gives a report. It says, well, they're traveling really fast. And they're stopping cold. And they're moving against heavy winds and doing all of this stuff that we can't explain by natural processes.

INSKEEP: Now, Cleland believes that life could be out there. So more experts, she says, should weigh in.

CLELAND: I think that there is probably extraterrestrial civilizations out there. The question is, are they close enough? Because given current physics, traveling between stars, even the nearest star to us, takes light years. It's not easy to get between stars.

FADEL: Cleland is also a professor of philosophy who wonders whether worlds beyond our scientific reach will have escaped the self-destructive perils that planet Earth seems determined to flirt with.

(SOUNDBITE OF JOHN WILLIAMS AND LONDON SYMPHONY ORCHESTRA'S "FLYING THEME (FROM 'ET THE EXTRA-TERRESTRIAL')") Transcript provided by NPR, Copyright NPR.