AMNA NAWAZ: Since the 1940s, unidentified flying objects, or UFOs, have been a part of our nation's cultural phenomena.

But for the U.S. government, they have been a mystery and something the military has been investigating for decades.

I recently sat down with author Garrett Graff to discuss his new book called "UFO: The Inside Story of the U.S. Government's Search for Alien Life Here and Out There."

Garrett Graff, welcome back to the "NewsHour."

GARRETT GRAFF, Author, "UFO: The Inside Story of the U.S. Government's Search for Alien Life Here and Out There": Thanks so much for having me.

AMNA NAWAZ: So you have written incredible, deeply researched and reported books on Watergate, an oral history of 9/11.

Why UFOs?

Why did you turn your attention to this?

GARRETT GRAFF: So I came at this as a national security writer, as you know.

And what changed for me, what got me interested in the subject was there was this sea change in recent years in Washington around this topic, that you started to hear serious people talking seriously about this subject.

And, for me, there was this one very specific moment in December 2020 where John Brennan, who was just wrapping up the better part of a decade as the former CIA director, White House homeland security adviser in the Obama years, said, basically, like, there's stuff out there.

We don't know what it is.

It puzzles me.

And the phenomenon may end up constituting some new form of life.

And it was a really startling comment to me.

And so for him to be leaving office saying, I still don't know what this thing is, felt to me like a topic that would be fun to dig into.

AMNA NAWAZ: Tell me more about that sea change, though, because you do trace in the book the 75-year history of related military programs.

And there was, for years, a real lack of transparency, I think it's fair to say.

You cite specifically, of course, the 1947 crash outside of Roswell, New Mexico.

But what has happened over the years that's moved this conversation from the fringe to the mainstream?

GARRETT GRAFF: Yes.

So, in the book, I try to trace two threads that historians and journalists normally treat differently.

There's the 75-year, 80-year history of the military's attempt to solve the mystery of UFOs here.

And then there's the evolving science and astronomy of what's known as the search for extraterrestrial intelligence, SETI, out there in the rest of the universe.

And both sides of these conversations have really seen revolutions in the last decade, where there was some blockbuster reporting here by The New York Times and Politico in 2017 that outlined the Pentagon's study of what the government now calls UAPs, unidentified anomalous phenomenon.

The other thing that's really changed in all of the science is our understanding of how big the universe actually is.

And we now understand sort of just how many stars around the universe have planets and how many of those planets would be habitable by things that we recognize as life.

And sort of as part of that, we have seen the science really shift from the possibility that life exists to the probability that there's actually likely a lot of life and probably a lot of intelligent life out there across the universe.

AMNA NAWAZ: You talked about this specifically.

You said that the math is on the side of the aliens.

What does that mean?

GARRETT GRAFF: So we now understand -- and this is a huge sea change -- that as late as the 1990s we didn't know that there was a single planet outside of our own solar system.

We now believe that, almost every star, effectively every star has planets, and that as we understand the scale and the scope and the breadth of the universe, once you adjust for what they call the Goldilocks zone, sort of planets that would be at the temperature and ability to hold water and atmospheres of oxygen that we would recognize, there are potentially a sextillion habitable planets across the universe.

So that's a billion trillion habitable planets across the universe.

AMNA NAWAZ: All this made me think about -- you probably know this very well - - the Pale Blue Dot photo.

GARRETT GRAFF: Yes.

AMNA NAWAZ: Right?

It's taken by NASA, the Voyager 1 back in 1990, that small dot from a distance of some four billion miles from the sun.

That dot is Earth.

It makes you remember that we are just so very small.

But you are a journalist.

You deal with facts and you deal with evidence.

So, are you convinced we're not alone?

GARRETT GRAFF: So, I think that the math really strongly suggests we are not alone.

The flip side of that is the math also says that the life and intelligent life out there is probably too far away for us to ever know, and that part of what's fun about diving into the UFO history of this, the pop culture, the science of these sightings is, there's this wonderfully human-centric vision that aliens would care about us, that Sort of aliens would travel, master interstellar, intergalactic travel, come all the way here to make friends or invade us or conquer us, whereas, statistically, what's probably most likely is that there's intelligent life out there.

And it doesn't know about us.

And even if it did, it wouldn't necessarily care at all.

AMNA NAWAZ: I want to be as specific as we can about this, because you have studied this so in depth.

When you say intelligent life out there, what does that mean to you?

What does that even look like?

GARRETT GRAFF: I think we don't really know.

And I think part of this journey and sort of coming to try to understand and write about this subject is, you have to be really humble about what we know and what we don't know about just how weird the universe actually could be and just how new our knowledge and understanding of the universe probably actually is.

AMNA NAWAZ: The book is "UFO: The Inside Story of the U.S. Government's Search for Alien Life Here and Out There."

The author is Garrett Graff.

Garrett, thanks for being here.

GARRETT GRAFF: My pleasure.

Thank you.