The astronomer Chris Impey’s latest book, Worlds Without End: Exoplanets, Habitability, and the Future of Humanity, explores the implications of the fact that there are, by his rough count, a mind-boggling four billion Earth-like planets in the cosmos. It’s a staggering, reality-bending figure that leads one to imagine that, one day, humankind may very well expand our species to another habitable planet. The only major hitch? The nearest Earth-like exoplanet is around a hundred light-years—or thousands of trillions of miles—away. “There is no Planet B,” Impey says matter-of-factly on the latest episode of our At a Distance podcast, adding that, “We’re not gonna go there; the energy cost is insane. We have to look after this planet.”

In our conversation, condensed and edited below, we speak about how truly vast the universe is, the importance of science fiction and the imagination when it comes to studying space, humanity’s footprint on Earth as “modest” when considered from above, and why he won’t likely be getting into a deep-sea submersible anytime soon.

Click here to listen to the full interview on our At a Distance podcast.

Hi, Chris, welcome to At a Distance. It’s so great to have you.

It’s good to be with you.

To start, let’s pull back—way back—and look at a distance at not only earth, but our solar system. Could you explain the research into exoplanets you unpack in your new book, and how truly vast this network is? What scale are we talking about here?

Well, exoplanets have been hard to find, historically. The reason we didn’t detect them until 1995 is because the distance to the stars is huge. It’s trillions of miles, tens of trillions of miles just to the nearest star. A planet like the Earth reflects a billionth of the sun’s light. So you’re trying to see something that’s hundreds or thousands of trillions of miles away, that’s a billion times fainter than the star that it sits very close to in the sky. And that, in a nutshell, is why finding exoplanets was so hard, even though we expected them to exist—most people thought it would be very unusual, bizarre, if the star formation process that led to the sun and gave us eight planets didn’t happen elsewhere.

I found it fascinating to learn that there are four-hundred billion stars in the galaxy. That’s pretty mind-boggling.

Right, and the research over the last few decades has pretty much said that there’s at least one planet per star. Four-hundred billion is also, in round numbers, the number of planets in the galaxy. That’s a lot of real estate.

And there are probably, as you write, four billion Earth-like planets.

Yes. If you’re even being quite strict with your definition and ask for something that’s habitable and roughly Earth size, yes, it’s a sort of rough number, but four or five or six billion. It’s a huge number.

I found it particularly interesting when you write, “Earth is not the best of all possible worlds.” I think we tend to live in a rather Earth-centric approach to thinking about things—humans tend to, anyway—and I was wondering, are you saying that there are better planets for human habitation out there?

Or just for life in general. There are two sides of it. But one is just that the Earth is a nice place for us to live right now. But over four billion years, it’s had a very violent climatic and geological history. It was uninhabitable for big chunks of that time. In fact, there were several occasions when the Earth went through a snowball episode, where it completely froze and it might not have come out of that, and life could have been extinguished. Life was on the hairy edge; the whole biosphere could have been eradicated. So yes, the Earth is nice now. But you have to take a long view.

Then, as far as other planets, yes, we can find Earth-like planets. But it turns out that super-Earths, which is the most common category, there are more super-Earths than there are Earths out there, because they are big, they hold a thick atmosphere, and they have a lot of geology and heating, which is useful for sustaining life. By some arguments, they’re more habitable than Earth-like planets.

You write about, at various points, how tenacious certain planets or environments can be, and yet, life adapts. Could you talk a bit about that ability for life to adapt to these different environmental niches?

Sure. It’s a tricky situation with astrobiology because we have a sample of one way of life on one planet—one biological and genetic experiment that started four billion years ago. Then we’re trying to extrapolate to what might be out there. Actually, we can’t do that. Formally, we can’t do that. A philosopher or a logician would say, “No, you can’t extrapolate from a sample of one.” But what we do know about life on Earth is that it radiated very quickly into almost every conceivable ecological niche, including some pretty rough ones. It went into the deep oceans. It’s in deep rocks, spaces of pores within rocks deep underground. It’s not just painted on the surface the way we like to think of it. We have these extremophiles, as they’re called, sort of microbes for the most part, that can tolerate above the boiling point of water, below the freezing point of water, incredible battery acid–level conditions or toxic metals that would kill us, microbes [that] can survive extremely low density regions or super high density at the bottom of the ocean. So microbes are persistent, and they are very adaptable.

Then the question is, what might they have done in another physical environment on an exoplanet? So we imagine, from the example of one that we have, that they’re going to be pretty sturdy there, too. They may have inhabited places that we might think, on the face of it, are pretty uninhabitable—to us, anyway.

Touching on this—and it’s a subject that obviously connects—is this idea of fantasy or science fiction. So I wanted to ask, how much of this is fantasy, is science fiction, and how much is real? How do you see the line between the real and the imaginary?

It’s an important line because, at the moment, we don’t know of life anywhere else. So we’re entitled to speculate. At what point does speculation turn into fantasy? I’m not sure. Most astronomers cut their teeth on science fiction of the fifties, sixties, seventies. As did I. Science fiction has been very prescient, in some cases, about anticipating—not just people like Arthur C. Clarke, who are famous for it—but other science fiction writers have anticipated things that turned out to be true in the real universe.

We should be guided by our imaginations, I think, in science, because we don’t know everything that’s out there. Now, the universe is a pretty amazing place, so I think it’s totally valid to [consider] science fiction or fantasy or the imagination. Eventually, the reality will settle in as we do our observations and make measurements and find out what’s really out there. But before that, we’re allowed to speculate.

Could you talk a bit about the advancement of our understanding of the cosmos, particularly during your time that you’ve spent in the field, when it comes to adaptive optics, or just the pace of discovery?

Astronomy is a discovery science. A lot of things have been found in my working lifetime. We’ve found massive black holes, and we’ve learned how to detect gravitational waves. And we’ve discovered exoplanets, and we’ve found dark energy and dark matter that— We don’t know what they are.

Astronomy was driven by technology. Astronomers have to use extremely sensitive instruments to measure deep space. And the technologies for that have improved. They have to use very large telescopes, and the size of telescopes has grown dramatically in my lifetime, from four or five meters up to twenty or thirty meters coming up soon, as space astronomy has improved. And these technologies are driving discovery. That’s part of why exoplanets were found—it was based on the technology of making a spectrograph extremely accurate, that could tease out the very subtle signature of a planet orbiting a star that was technology-driven, too. That continues; we continue to innovate in detectors and telescopes and space astronomy. We can expect discoveries to keep continuing.

On this program—it’s not a throughline, necessarily, but it’s certainly been something that’s come up quite a bit—is the overview effect. You touch on this in your book. From your vantage, what do you think about it? Do you think we could one day shift the overview effect to another planet? It does seem to be an Earth-centric view.

It’s an Earth-centric view. But the overview effect, as you know, framed by astronauts for decades, is salutary, because it’s reminding us, first of all, however impressive we think we are, our footprint on the planet, as seen from space, is pretty modest. You can barely see something like the Great Wall of China, you can see the lights from our cities, but our imprint on the planet is quite modest. Some of it’s negative, of course.

So the overview effect is the fact that you see the slender sheath of the atmosphere when you’re in Earth orbit, and you realize that it’s fragile. You see no boundaries, no countries with lines between them, and no polarization based on ideology is written on the globe. That’s the overview effect, in part. Now, if we ever move somewhere else, we get to rewrite the history. We get to write a new history starting from page one. Maybe we’ll write a different history. It will be a very interesting experiment.

We’re obviously in what you call in the book a “space boom.” Many others have also referred to it as that, with Bezos, and Musk, and Branson. I was curious, how do you think about this moment we’re in right now, when it comes to the long view of space travel and exploration?

I think we are within sight of leaving the Earth in a permanent way—small numbers of us, of course. The whole issue of us finding another planet—this is back to the overview effect, again—the difficulty of space travel, the enormous expense means we’re never going to [fully] leave the Earth. I mean, there is no planet B. The nearest Earth-like exoplanet is probably about a hundred light-years away. That’s thousands of trillions of miles. We’re not gonna go there; the energy cost is insane. We have to look after this planet.

But I do think we will get our first toehold on other worlds. We will have self-sustaining colonies on the Moon and Mars, maybe not in ten years, and maybe not even in twenty years, but certainly in thirty, forty, fifty years. That will be as momentous as when we first left Africa fifty thousand years ago and spread around the world. It will be a new branch of humanity. It really will be a new branch, because if it’s not continually cycling the population back to the home planet, that small genetic outpost will diverge from the human tree and eventually become a new species.

You sort of alluded to the next question I wanted to ask: Where do you see this fifty years from now? Do you think we’ll be in a universe where there will be space cleanup crews the way we have trash trucks out on the street? Will we actually be mining asteroids? What sort of scenarios do you see playing out?

I think some of the beginnings of a sort of industrial and economic activity will develop, in the first instance, and we can see it with the private space program. These are lost leaders. These companies do not have a significant revenue stream. Elon Musk has a partnership with NASA that generates a few billion dollars for contracts to supply the space station and so on, but there’s no revenue stream from tourism. These billionaires are sinking a few billion dollars a year from their private wealth just to keep everything going and afloat. So it’s a fair way from a self-sustaining activity. When the first companies set up on Mars, or the Moon, or try to mine an asteroid, people are going to lose their shirts, just as people did when they went out West and tried to become rich from gold, or whatever it was, a hundred and fifty years ago. So it’s going to be a Wild West in a literal and a metaphorical sense, and it’ll take a while to stabilize.

The concern for a lot of people is that it’s a lawless place. It’s literally a Wild West, because space law does not really apply beyond the Earth. There is no space law, there’s no enforcement, and there are not even any treaties from the U.N. that dictate who owns what or what happens when there are territorial disputes. Or how do you safeguard against slave labor? There’s some pretty ugly things that could happen, because they are not ruled out by law, and there’s probably no enforcement against them.

It’s gonna be interesting to see. I would like to be optimistic about how it plays out. But our worst tendencies are on display, always.

At the end of the day, what are the advantages, as you see it, to stepping up space exploration? What are your hopes for how things develop?

I don’t even pretend that it’s a completely rational thing to do. There’s not a simple economic argument to be made. The space companies—the private space companies—have business models. They imagine that, eventually, tourism in space will be cheap enough that they can make money off it. But the arguments for the difficult and very expensive things like setting up a colony on the Moon or Mars, I don’t think they’re economic. I think people will do it out of that sense of exploration or adventure. Or if it’s countries, of course, it will be for power in space, as China and America are engaged in that new space race to replace the original one with Russia. This is a projection of national power into a new arena. That’s going to be a motivation, too.

It won’t be self-sustaining in that sense for quite a while, but people will be willing to invest in it, for the glamor, the excitement, the power, the glory. There’s a lot of things all mixed up into the space race.

To close, looking back here on planet Earth, are there places that excite you that haven’t been explored here? An astronomer is often looking away from the Earth, but I’m curious—you’ve spent time in Chile’s Atacama Desert, you’ve traveled around quite a bit. Do you see any sort of sense of “space exploration,” put differently, here on Earth—just quite literally, space exploration? [Laughs]

Well, I think it’s pretty clear that the deep ocean is very poorly understood. It’s an extremely difficult environment to operate in. I mean, going back to the hydrothermal vents, which have life forms that are alien to the stuff we see on the surface, and possibly life started there in the oceans. So it’s telling us something about how life operates on a planet. This is life that doesn’t need a star. The whole idea that life in the universe needs a star sitting there, it’s not true, because we have life on Earth that doesn’t need a star.

I think learning more about the deep ocean—by the deep ocean, I mean kilometers down in total darkness, super high pressure, very difficult environment. We know so little about that. That’s definitely a frontier that I’m not interested in personally—I don’t really get claustrophobia easily, but I’m sure I would if I was in a little bathysphere kilometers under the ocean in total darkness at a hundred atmospheres of pressure. That might freak me out. I hope somebody will do it, and there are people doing it, but that to me is a frontier.