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The best books on Exoplanets

recommended by Chris Impey

Worlds Without End: Exoplanets, Habitability, and the Future of Humanity


Worlds Without End: Exoplanets, Habitability, and the Future of Humanity


With 10 billion potentially habitable worlds in our galaxy and 100 billion galaxies in the universe, the probability there is life beyond Earth is high. We're also likely to find out more in the next five to seven years, says Chris Impey, University Distinguished Professor of Astronomy at the University of Arizona and author of Worlds Without End: Exoplanets, Habitability, and the Future of Humanity. Here, he recommends four brilliant books about exoplanets as well as one about life on Earth, our only example of biology to date.

Interview by Sophie Roell, Editor

Worlds Without End: Exoplanets, Habitability, and the Future of Humanity


Worlds Without End: Exoplanets, Habitability, and the Future of Humanity


Before we get to the books you’ve recommended, could you briefly say what an exoplanet is? I thought it was a potentially habitable planet, but that’s just a small subset.

Yes, it’s much broader. An exoplanet is just any planet orbiting a star other than the Sun. It could be a Jupiter-type planet, a Mars-type planet or an Earth-type planet, and anything in between.

I should preface all this by saying I’m a cosmologist by training. I came to astrobiology and exoplanets a little later in my career, just because the field was so exciting. Mostly in my research, if it’s less than a billion light years away, I don’t care. What most people don’t realize is that all the exoplanets we’ve found are in our backyard; they’re really quite close in our galaxy.

Also, I’m not sure everybody appreciates how recently it was that we saw the first one. “Exoplanet hunting was a field of broken dreams for much of the twentieth century,” you write in your book.

Yes, the first discovery was in 1995, by a Swiss pair, Michel Mayor and Didier Queloz. That led to a Nobel Prize in Physics in 2019. There were many people who made claims in the 70s and 80s or earlier that proved to be wrong. There is also a hard luck story, involving a claim that was correct but was retracted. Later, it was reinstated but by then Didier Queloz and Michel Mayor had made their discovery of 51 Pegasi b.

Even worse, the very first exoplanets were found by radio astronomers in 1992, three years before 51 Pegasi b. They were pulsar planets. They’re legitimate planets, they’re just in a very strange place. Nobody understood how there could be planets around a dead star. That was the very first discovery. It was just such an oddball that people said, ‘We don’t understand that. Let’s just set it aside.’

Your book is a really interesting introduction to exoplanets, but it’s also a nice way of finding out about what’s going on in astronomy in general, what people are researching, what we know, what the technology we have can do.

Yes, the field has been propelled by advances in astronomical optics and technology. Exoplanet work has pushed the envelope in terms of the precision with which we can measure a spectrum or blank out a bright object to see a faint object near it and all these kinds of techniques. That of course benefits everything astronomers do. It’s a good story about the technology aspect of astronomy, which people don’t really think about. They just think, ‘Oh, they’re big telescopes’ and that’s the end of it. The instruments behind the telescopes are important. They’re impressive. And they’re pretty expensive, too.

Let’s turn to the books you’re recommending. The first one is The Little Book of Exoplanets by Joshua Winn, which is out in July. Is that a good introduction to the topic and where we’re at?

Yes, it’s a good primer. It’s not that short a book, actually, but it’s written in a very nice, concise way. Joshua Winn, who I’ve met a few times at Princeton, is a top-tier exoplanet researcher. He’s on the TESS science team. TESS is the space telescope in orbit that succeeded the Kepler mission. Kepler was the workhorse for finding exoplanets for seven or eight years, but it’s finished now and TESS has taken over the mantle, especially of looking for nearby, Earth-like planets. Joshua Winn is part of the TESS team and it’s always interesting to hear about a subject from someone in the field who is at the top of their game.

“Everyone’s just waiting for the next step, which is to find biology”

The book is also organized extremely well. He just leads you through the subject in what seems like a logical, sensible way. He’s not always historical. A purely historical approach doesn’t always work, because science progresses in chaotic ways sometimes. He just steps back from the subject and says, ‘Okay, what would someone need to know first, and then second, and then third, to understand what’s going on?’ I liked it for that reason as well.

It’s a rival book, of course, but that’s not a problem. The field is big enough that there can be multiple books on exoplanets!

Your book is very accessible. You take science education seriously and that showed through. I was able to follow everything, even though I’m not a scientist by background. Do you feel that you’re covering different ground from him?

My book is aimed at a broad audience where you don’t need to know anything about astronomy or exoplanets. The Little Book of Exoplanets is for a general audience as well, but I would say it’s at a slightly higher level. I enjoyed it for that reason—because I learned some things too.

In my book, I was also trying to make a connection to the state of the Earth because, otherwise, exoplanets are just out there, far away. Who cares? If you’re excited by exoplanets fine, but what’s that got to do with anything that’s happening on Earth? It may have been a stretch when I was conceiving it, but I tried to make the connection between how we define habitable planets far away in the universe, and how we think of the Earth—because the habitability of the Earth is, of course, in jeopardy.

I found that interesting, also from the point of view of contact with life on other planets. If Earth hasn’t been habitable that long in the scheme of things, how do we know it’s going to correspond time-wise with sentient life on another planet?

Yes, and when we talk about Earth, we get these warm, soft fuzzy feelings. It’s our planet, the pale blue dot and so on. But the exoplanet game has told us that it’s not that special, even as a water world. There are planets that have 100 times as much water as the Earth does in its oceans, these incredible water worlds. Then there are planets that are more habitable. Earth is not the “best of all possible worlds” as Leibniz might have said. It’s a good planet, there’s nothing wrong with it, but it’s on the edge of chaos. There was at least one snowball episode, and possibly a second one, where the planet might have tilted into a deep freeze that it would never have escaped from, and life would have been extinguished. There was also heavy bombardment early on that possibly obliterated life on Earth. We’ve seen historical climate change back in the geological record that dwarfs anything we’re doing to the planet ourselves. Planets can have crazy histories.

We’ve found planets out there that are more tranquil than Earth and probably a little calmer for long-lived life. That matters because we care about biology elsewhere. There could be Earth clones, or better, more-habitable-than-Earth planets, that got a head start on the Earth by eight billion years, because the Earth formed four-and-a-half billion years ago, and the universe is 14 billion years old. There was plenty of carbon and water within half a billion years of the Big Bang. There are potential biological worlds out there that have had an awfully long time for evolution to produce interesting things. That’s a thought experiment at the moment, because we haven’t found them, but it’s a possibility.

Let’s go on to the next book you’ve chosen, which looks beautiful. It’s called Envisioning Exoplanets and it’s by Michael Carroll. 

This is a coffee table book, though it’s currently on my shelf because my coffee table has a rotating set of books. It’s a beautiful book. Michael Carroll is an internationally reputed space artist, and he’s very knowledgeable. There are maybe 200 beautiful, full-color illustrations in the book.

The important prefatory comment is that even though we have 5,300 exoplanets that we know of, we don’t have images of more than 100 of them. Most of them are found by these indirect methods of Doppler radial velocities, and the transit of eclipses. Only a very small fraction has had a picture taken of them and even those are just a few pixels or a dot. These illustrations are all visualization, imagination and projection from the basic data. Here’s the mass, here’s the size, we think it might have this in the atmosphere, what would it look like? He follows that through and says, ‘Here’s what I think it might look like.’

“We have 5,300 exoplanets”

It’s nice to see something that, unfortunately, scientists can’t. We’re not going to be able to see images of exoplanets—maybe ever. It’s very hard. We’re never going to go there, they’re too far away. We don’t have instruments or telescopes that could ever see them in any kind of detail. For now, space artists are the people who are going to have to represent these worlds and imagine what they look like.

I’ve met Michael Carroll a couple of times at a meeting called Spacefest. It’s a wonderful aggregation of astronauts and space artists and engineers who worked at NASA and astronomers and people like me. It’s every year and it moves around the western US.

Are his renditions convincing? Could it be that exoplanets do look like his illustrations?

Yes, they’re convincing because he knows the science. In very general terms, we can say that if a planet has a certain mass, it’s likely to have plate tectonics, so there will be volcanoes. If it has a certain mass, it’s going to have either a thick or a thin atmosphere. We can even speculate what the atmosphere might contain and therefore imagine what the sunlight diffusing through it will look like. Also, they orbit different stars. Some of them orbit red dwarfs, so there will be a dull red light that suffuses through the atmosphere. He does as much of the scientific guesswork as he can and then, of course, the details are completely imagined—because we don’t know any.

Let’s move on to the next book, which is called Imagined Life: A Speculative Scientific Journey Among the Exoplanets in Search of Intelligent Aliens, Ice Creatures, and Supergravity Animals. This is by James Trefil and Michael Summers.

Compared to Carroll’s book, this book is even more speculative, because it’s speculating about alternative biologies. The problem is that we have just one example of biology, the one that happened on Earth. Everything on Earth—from a fungal spore to a blue whale, to a redwood tree—is the same biological experiment. We don’t know what a different biological experiment would lead to, or what the creatures and life forms that emerge would look like. If you vary from the one example we know, you’re immediately in the realm of pure speculation. It’s a tricky business because we simply don’t know.

I enjoyed the book. I did have a little asterisk in my head as I read it saying, ‘Wait a minute! We don’t know that that could happen or that it would be like that!’ But they’re imagining the possibilities of biology if it wasn’t the way it is here—which is fair game, given that we don’t know.

You could call it science fiction at some level, though they do have a scientific core to what they’re writing about. For example, they’ll say, ‘Well, if gravity was stronger, you wouldn’t get tall creatures, you’d have squat creatures. If the atmosphere had a different composition, you’d have this or if you had deep oceans, you might have that.’ They can speculate by a small extrapolation from things on Earth. But when it gets really weird, we basically have no idea.

And are they coming at this from a scientific background?

Yes, I’ve met James Trefil a couple of times. He’s written dozens of books, many more than I have. He’s a physicist. Michael Summers is a planetary scientist. They’re combining their knowledge because they have slightly different expertise. I’m sure it was a lot of fun for them to write. I know Trefil’s other books and they’re straight down the line about some scientific topic. This is the book where they got to go way outside the box.

You mention science fiction quite a bit in your book. Is there a work of sci-fi that’s good on exoplanets?

Science fiction has been rife with imagined worlds for decades. It’s been played out in so many different ways that it’s hard to think of any particular example. It’s part of the wallpaper to have strange planets. Sometimes the science fiction anticipates fact, like the Tatooine planet in Star Wars. It’s an Earth-like planet that orbits a double star. We found one of those, long after Star Wars was filmed and written. That was kind of cool.

I didn’t realize, before I read your book, that there was ancient Roman science fiction.

Yes, the Romans and the Greeks speculated about all manner of things. There is Johannes Kepler’s (1571-1630) book too, which I read in translation. It’s about going to the moon and the creatures living on it. These were prescient things to write about. People think science fiction has been around a century or two, but it’s quite a lot older than that.

Is Somnium (1634) a good book?

Kepler is not a very readable author. Or maybe it comes down to the translation. He was a mystic. I haven’t read all of his books, but I’ve read major chunks of Harmony of the World and it’s quite dense. It’s pretty rough going, actually. Somnium is his work of fantasy and it’s still a little dense. As a writer, he’s not a great stylist.

In the coffee table book you recommended, there’s also a picture of the trial of Giordano Bruno (1548-1600).

The image I always have of Giordano Bruno is that statue in the Campo de’ Fiori in Rome—this hooded figure in his cowl. It’s such a happy piazza with markets and people drinking coffee and talking. Then there’s this looming figure of someone who was burned at the stake right there 400 years ago. It’s a little somber.

I knew Bruno was burned at the stake for heresy, but I didn’t realize he was relevant to exoplanets!

He speculated amazingly and widely about the stars being other suns. That was very far ahead of his time. And then why not have planets around them? And why not have creatures on the planets? And why not have creatures that are as smart or as advanced as we are? He went the whole way.

“If intelligent life is thinly scattered through the galaxy, then operationally we’re alone”

Now, a misleading thing that has happened is he’s been characterized as a martyr for science. He was burned for religious heresy. His rap sheet has this wonderful phrase that he was “an impenitent, pertinacious, and obstinate heretic.” He held extremely unconventional views within the umbrella of the Catholic church and that’s what got him into trouble. I don’t think the science speculation is why he was martyred. Scientists took that narrative and ran with it, but I don’t think it’s quite right. Galileo almost suffered the same fate and that was because of his scientific views.

Let’s go on to the next book you’ve recommended which is The Planet Factory by Elizabeth Tasker, who is an astrophysicist. Some of the Amazon reviews were very enthusiastic about her engaging style of writing.

Yes, she has what I would call a jaunty style. I like it. Her writing has a very approachable flavor. She uses analogies well, she nicknames things. She’s British and works at the Japan Aerospace Exploration Agency now. It was a fun book. I enjoyed it.

And how does it differ from some of the other books we’ve been talking about? Is it also a general introduction to exoplanets?

No, it’s not general in the way that Winn’s book lays it out and is almost pedagogically encompassing. She’s more interested in the quirks of the planets. I wouldn’t call it The Guinness Book of Records approach, but there’s a bit of that. She says, ‘Wow, there are oddball planets!’ Then she takes it beyond that, and asks, ‘How do they happen? How does nature make these strange planets? Why are they strange compared to the planets in our solar system? Is our solar system dull? Is it typical?’

These are important scientific questions, but she does have a lot of fun with the oddballs—the planet where it’s raining molten lava or diamonds or the planets that have been stretched into a pear shape because they’re so close to their star. She writes about all sorts of extreme planets that we’ve found.

Are our planets typical?

It’s not clear that we’re typical. The Copernican principle would say we should be average, but the single most common exoplanet type is a super-Earth that’s a few times larger, and five or eight times heavier than the Earth. Our solar system doesn’t have any of those, so we don’t have the most common type of exoplanet among the eight in our system. We’ve got Venus as a twin of the Earth and then you jump up by almost a factor of ten to Uranus and Neptune. There are other things about our solar system that may not be typical. But the diversity is so large now that you stop knowing what’s normal.

Does she cover whether there might be life on some of these exoplanets?

She’s not as interested in the biology per se. She’s spending a lot of time talking about the planets that are so weird they probably won’t be habitable. I think she has more of a geological planetary science perspective, where she just is amazed at how these things form and that their properties are so bizarre. She’s laying that all out.

I can’t remember which side you came down on: do you think there’s life out there in the universe?

I do. It’s a probability argument, which is dangerous, but if you take some notion of habitability—where the planet’s temperature is okay for liquid water, it probably has an atmosphere and geological activity, which we think facilitates the evolution of life—then your round number is 10 billion habitable worlds in the galaxy. Then multiply that by 100 billion galaxies. On statistical grounds, I think it’s very, very likely there’s life and even quite likely there’s intelligent life.

But that’s no guarantee. When you have the example of one, you don’t know if there were fluky things that happened in the history of life on Earth. Also, for 90% of the history of life on Earth, you couldn’t see it without a microscope. Getting advanced life—plants and animals—took billions and billions of years. Maybe that’s hard, maybe sometimes it takes 10 or 20 billion years or never happens. Maybe it’s easy to get microbes and hard to get complex life. Or maybe just getting the first cell is harder than we think. You can’t draw any conclusion from the fact that it happened here. That’s the trouble.

That’s why the motivation for finding those first examples elsewhere is so strong, because once we find another version of biology the game will be on. Then we can ask, ‘Is it the same biology as us or is it different?’ If it’s the same, then that’s very interesting. If it’s different, the box will be open, and biologists are going to have to develop a general theory of biology they don’t have yet.

Setting aside exoplanets for a moment, what about the possibility of life in our solar system? Can I bring in David Bowie and ask if there’s Life on Mars? (1971)

To a planetary scientist, Mars is probably on the edge of habitability. It doesn’t have a lot of geological activity, it’s dead now. It doesn’t hold much of an atmosphere, so it’s torched by cosmic rays and UV radiation. If there is life there now it would have to be quite far underground and farther than any of these rovers, which just scratch around on the surface, can find. There are almost certainly subsurface water aquifers, so there could be microbial life there now. Then there’s quite strong indirect evidence that 3 billion years ago, there was a thicker atmosphere and standing bodies of water. There definitely could have been life in the past.

But we’re going to have rocks back from Mars in about eight years. There are rovers collecting samples as we speak, in a place called the Gale Crater which was almost certainly under shallow water a few billion years ago. There could be fossilized life forms in those rocks that are going to come back to Earth in the early 2030s. That will be very exciting. That’s probably the first way we’ll be able to answer the question.

We do have 130 Mars rocks that came to us through meteorites. It’s just bad luck that almost none of them are sedimentary rocks. And then the problem with meteorites is you don’t know where they came from. They just got blasted off Mars somewhere and then came to Earth many years later. They landed in our laps. You can’t draw the conclusions that you could when you go to a place that you think was habitable, drill, make a sample and bring it back and study it in the lab.

Then there’s Titan, where we’re sending a quadcopter, which is this drone technology. That’ll arrive in the mid-2030s. Titan could have Life 2.0, as we don’t know it, because it has ethane and methane lakes rather than water lakes. There could be hydrocarbons, and a weird form of biochemistry there. That will be amazing.

There’s also the Europa Clipper, the ice mission due to be launched in 2024. It’s heading towards the watery moons of Jupiter. We’re not going to get down onto the surface and go under the ice pack—that’s a harder thing and that will take a while—but we may also get hints of life on Europa also in the next decade.

So yes, our solar system has possibilities.

Isn’t one of the reasons for caution and skepticism about other life—which you cover in a chapter of your book—the fact we haven’t received any messages yet?

Right. There’s the failure of SETI, the Search for Extraterrestrial Intelligence. It’s been 70 years now and nothing has been received. Then there is the issue of whether there is microbial life. Advanced technological life could be very, very rare compared to microbial life. I think if you have the ingredients for biology, you probably get biology in a large fraction of cases or at least some fraction of cases. There are billions of habitable worlds around the galaxy and they probably have pond scum and various forms of microbial life.

And maybe Earth—where it took a very long time to get advanced, complex life—doesn’t happen very often. If intelligent life is thinly scattered through the galaxy, then operationally we’re alone. The signals take too long to reach us or they become too weak with distance. It could exist and if it’s rare, it might be hard to know.

Let’s get to your last book which is Life on a Young Planet by Andrew Knoll who is a professor at Harvard. Here we’re turning back to the Earth and the first three billion years of evolution on our planet. Why have you included this book?

I’m not an astrobiologist and because we don’t have any other examples of life yet, I like to learn about the Earth. One of the most impressive popular science books I’ve ever read was Stephen Jay Gould’s Wonderful Life. That was a lovely book about the development of complex life, the Cambrian explosion.

Knoll’s book is in the same category. It’s very well written and has a broad sweep of 3 billion years of history. He’s a very preeminent geologist and paleontologist and his expertise covers a number of fields. He’s at the top of his game and he lays out the subject beautifully. It’s just a nice book setting out a subject that I’m not trained in, in a way that I can understand it.

It also addresses the issue of how inevitable life is, if you have all the ingredients. How does it progress? Again, with a sample of one, you can’t generalize. You can’t inductively draw a conclusion. But he raises all the issues.

He’s a good writer, and I like the fact he’s not hyperbolic. He avoids dramatic statements. He doesn’t draw a conclusion when there’s no conclusion to be drawn. He’s quite cautious. I appreciate that, because in his field we’re dealing with slender evidence. Sometimes it’s hard to find an old rock. If you just go anywhere out on the surface of the Earth, an average rock you might pick up is very unlikely to be more than 100 million years old. Finding a three billion-year-old rock is hard. The evidence we have of life that far back is very sparse, and the rocks have all been altered by geology, heating, pressure, and so on. Knoll is suitably cautious about drawing too firm conclusions about this long geological record that we have.

Is he looking back at how life started? In your book you mention that it looks like it may not have started in the “warm little pond” Charles Darwin speculated about in a letter.

Knoll is not spending a lot of time back there because then the speculation level gets extreme. There are argumentative academic debates about when the first evidence for life is. It’s quite solid at 3.7 billion years ago, but there are people who are pushing 3.9 or 4 billion years old. He’s smart enough to stay away from that debate, because it’s hard to be sure when you go that far back.

But what is true from the sequencing of RNA is that the Archaean branch of the tree of life—that extra branch that was discovered 50 years ago by Carl Woese—does seem to indicate a warmer-than-tepid environment. That’s what led people to the deep-sea hydrothermal vents. It’s not Darwin’s warm little pond, it could be quite an intense high pressure, high temperature, chemically toxic environment where life started. That’s quite possible.

The interesting thing about those environments is that you don’t have any sunlight. When people say, ‘for biology, you need a star,’ you don’t really. You can have life without one. I covered that a bit in my book on the issue of rogue planets—exoplanets that have been booted from their star systems early on, when it was all chaotic. Some of those planets are heavy enough to hold atmosphere and have geological heating. They are just little self-contained ecosystems. You don’t need a star to have biology and a planet can also just float through interstellar space without one.

You’ve written quite a few books before, how does Worlds Without End: Exoplanets, Habitability, and the Future of Humanity fit in? Is this a field where you have to be constantly writing books because it’s changing fast and what’s been discovered is changing so rapidly?

I’ve dipped into this episodically. I wrote a book with Random House called The Living Cosmos 12 years ago which was about the broad sweep of astrobiology (not just exoplanets). That was my first approach. Then I had a couple of slightly more academic books, including Talking About Life: Conversations on Astrobiology, published by Cambridge University Press. That’s how I learned a lot.

Exoplanets were just booming so much, I felt I had to come back into the subject. It is moving fast, but we’re at a tricky phase now. We have 5,300 exoplanets. I mean, you feel sorry for a young research student in the field. It used to be if you found a cool planet or an Earth-ish planet, you could get a paper and a headline. Now you can find 100 planets, and nobody cares that much because there are so many. Just increasing the body count won’t move the field forward much—except in an esoteric research-y sense. Everyone’s just waiting for the next step, which is to find biology. That’s what people, including scientists, want to happen.

It’s just a lot harder than finding a planet in the first place. We don’t know how long it’ll take. The people I know who are working on this say five to seven years. But they’re also cautious. They point out that the evidence is not necessarily going to be compelling. It’s going to be ambiguous. If you find oxygen in the atmosphere of a planet, you think, ‘Oh, there must be microbes, because that’s where the oxygen we breathe came from!’ Then geologists will say, ‘No. Wait a minute. We have very simple geological ways to get 10% of oxygen in an atmosphere.’ It’s not 18%, like we have, but it’s quite high. Oxygen could be a false positive. Methane is a false positive. All the things that we think might be traces of biology could be just traces of geology.

So proving you’ve found something is hard. And then the null is even harder. Proving there’s no life on these planets is hard too. We think these are going to be nice binary experiments, we just inspect these planets up close, and we say, ‘That one’s dead. That one’s living. That one’s dead.’ It’s not going to be like that. It’s going to be an arduous process without a lot of screaming front page headlines.

So what kind of announcement do you expect to wake up to in five or seven years? What could it be?

I’ll be very excited when the Mars samples come back, because they can be scrutinized molecule by molecule in a lab. If there’s stuff in there that represents fossilized microbes, we’ll find it. That’s a good experiment.

The exoplanet atmosphere experiment is hard. It’s a very tough field because it’s very competitive. Just like with the history we started by talking about—where careers were destroyed by claiming exoplanets and being wrong—there’s going to be people claiming life and being wrong. It’s going to be tricky. I don’t know how it’s going to go.

Those claims are going to start happening in four, five, seven years, because the technology to do the experiment is maturing. James Webb will do some of it, but it was never designed for this experiment. Its design predates the discovery of exoplanets, so it was not optimized for that. It’ll do it in a very painful way, taking a lot of time and not looking at that many targets. Three huge, ground-based telescopes that are under construction will all take their first light in the next five to six years. They are all looking to do this experiment. They’re the killer app of a big telescope that blots out the star that’s a billion times brighter than the planet, smears the planet light into a spectrum, and looks for biosignatures or traces of life. These are 20- to 30-meter telescopes, so will be very good at doing it. Everyone’s racing to do that experiment, so I think they’re going to be pushing the envelope quite soon.

Interview by Sophie Roell, Editor

May 3, 2023

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Chris Impey

Chris Impey

Chris Impey is a University Distinguished Professor of Astronomy at the University of Arizona. He has won numerous teaching awards and authored textbooks and nine popular science books.

Chris Impey

Chris Impey

Chris Impey is a University Distinguished Professor of Astronomy at the University of Arizona. He has won numerous teaching awards and authored textbooks and nine popular science books.