Why haven't we heard from aliens? Scientists don't know, but they have a couple of ideas — and Space.com senior writer Mike Wall tackles that question and many more in his new book "Out There: A Scientific Guide to Alien Life, Antimatter, and Human Space Travel (For the Cosmically Curious)" (Grand Central Publishing, 2018). Space.com caught up with Wall by email to talk about the book, when we'll first spot alien life and how he brought concrete, scientific answers to speculative questions.

Space.com: Why did you decide to write this book?

Mike Wall: These are very interesting times. The search for alien life has moved from the fringes to the scientific mainstream, and the private-spaceflight revolution led by companies such as SpaceX and Blue Origin is making it possible for us to think seriously about settling Mars and other worlds beyond Earth. [Read an excerpt from "Out There"]

I guess I wanted to get this sense of excitement across to readers — to let them know that they're living through an era that future generations may well regard as an inflection point in our understanding of humanity's place in the universe and our quest to get off our home planet and out into the solar system.  

Space.com: Which topics did you find most fascinating to cover?

Wall: I've always been most interested in the search for alien life. It is one of the biggest unanswered questions in science, after all: Are we alone? 

And there are so many secondary questions that spring from that big one. If we're not alone, then, how common is life throughout the Milky Way galaxy and the broader universe? What kinds of life are out there? Just "simple" organisms such as microbes, or technologically smart creatures capable of reaching out to us in some way? How could we find these organisms, if they exist? What kind of evidence would be needed to convince everyone (or nearly everyone; unanimity is hard to imagine) of such of an epochal find, and how would society react to the news?

Space.com: Your book deals with a lot of hypothetical situations. How did you bring the reality of existing research to bear when answering such speculative questions?

Wall: You have to extrapolate based on what happened here on Earth, because that data set is the only one we have. For example, we know that life had taken root here by about 3.8 billion years ago, which suggests that it may not be terribly tough for microbial life to get going. We also know that Earth life remained microbe only for 3 billion years after that, which seems to indicate that the jump to multicellularity may be a serious hurdle for life in general.

And it seems reasonable to suspect that alien life, if it exists, may be carbon-based and use liquid water as a solvent. That's what happened here, and complex carbon compounds and water are both incredibly common throughout the cosmos.

But you don't want to get locked in to Earth's example as the only way that things could happen. That's unjustifiable, given how little we know and the staggering diversity of alien worlds, both in our solar system and beyond. So, it's a fine line to walk — informed speculation, with an open mind. Hopefully, I've managed to stay on that line, more or less, throughout the book.

Space.com: Do you think we'll ever run into alien life? In what circumstance do you think it's most likely that will happen?

Wall: I do, and I think it will happen relatively soon. I suspect that microbial life is common throughout the cosmos. And our own solar system harbors multiple potentially habitable alien environments, from the clouds of Venus to the Martian underground to the buried oceans of the Saturn satellite Enceladus and the Jupiter moon Europa. 

I think we'll find evidence of microbial life on one of these worlds in Earth's backyard in the next few decades. We might also spot biosignature gases in the atmosphere of a nearby exoplanet in roughly the same time frame. This is just a hunch, of course. But I'm optimistic.

I'm more agnostic about the discovery of intelligent alien life. That, to me, is such a crapshoot that it's hard to make any predictions about it.

Space.com: Why might intelligent aliens be keeping quiet? 

Wall: I don't think there's any one answer to Fermi's Paradox; it's probably a combination of factors.

For example, I'd wager that intelligent aliens are rare throughout the cosmos, both because it's tough to make the leap from microbe to multicellular life to technologically intelligent creature, and because supersmart species may well destroy themselves once they reach a certain level of technical aptitude (the ability to build an atomic bomb and/or greatly alter their home planet's climate, for example).

And then there's the immensity of space. If "advanced" aliens are spread thinly throughout the universe, it would take a very long time for their missives or their starships to reach us, and there's no guarantee that the timing would work out. For instance, we may have gotten a ping 3 billion years ago, or 100 million years ago, or 500 years ago, and completely missed it.

And then there's the motivation question: We can't assume that every extraterrestrial civilization would want to reach out to their neighbors. Many may be keeping quiet for safety's sake, afraid of betraying their presence to colonization-minded species that could wipe them out. 

Of course, the most depressing answer is the simplest one: We are alone. 

Space.com: Do you think we should keep quiet as well?

Wall: This is a topic of much debate within the SETI (search for extraterrestrial intelligence) community, and I see merit on both sides. Stephen Hawking was right, of course, when he stressed that we cannot know what advanced aliens may think of us, or want to do with us. So, broadcasting signals out into the galaxy could be inviting our own destruction.

But it's also true that we've been broadcasting such signals, in a passive and diffuse way, for a century now already. So, maybe advanced aliens already know we're here, and they're just waiting for a sign that we want to talk, or that we're worth talking to — that we merit inclusion in the "Galactic Club."

I haven't completely made up my mind, but I probably lean more toward openness. Attempts to message ET have already happened, after all, and they'll doubtless continue. It's hard to keep nearly 8 billion people in line and off the airwaves.

You can buy "Out There" on Amazon.com. Follow us on Twitter @Spacedotcom and on Facebook. Original article on Space.com.