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Alien Earths: How to find habitable worlds in our galaxy

An interview with Lisa Kaltenegger, the founding director of the Carl Sagan Institute, about the modern quest to answer an age-old question: “Are we alone in the cosmos?”
A digital rendering of a planet partially illuminated by a nearby star, with a galaxy visible in the dark space background.
Credit: DK_2020 / Adobe Stock
Key Takeaways
  • Astrobiologist Lisa Kaltenegger’s 2024 book Alien Earths delves into the current state of planet hunting, exploring the methods and tools used to search for habitable worlds in our galaxy.
  • Alien Earths covers the vast possibilities within our galaxy, where each star potentially hosts new worlds, some akin to Earth in their capability to support life.
  • Drawing on the history and diverse ecology of Earth as a template, Kaltenegger’s work emphasizes a multi-disciplinary approach to finding life on other planets, blending insights from biology, geology, and astrobiology.

This week, I have the pleasure of sharing with you my Q&A with Lisa Kaltenegger about her new book, Alien Earths: The New Science of Planet Hunting in the Cosmos. Lisa is a world-renowned astrobiologist and the Director of the Carl Sagan Institute at Cornell University. Earlier this year, we published a joint paper with Sara Vannah proposing a new strategy for identifying planets that share similar characteristics with Earth at different times across its evolution. Life writes its presence on the atmospheres of planets, and the search for living worlds is in reality a search for the “biosignatures” that we associate with life, at least life as we know it. Apart from being an outstanding scientist, Lisa is one of the most engaging lecturers I know, and her infectious enthusiasm is ever present in her book, a must-read. 

Explain what astrobiology is and why it is so exciting. How did you get into it?

We live in an incredible epoch of exploration. We are discovering not merely new continents, like the explorers of old, but whole new worlds circling other stars. Since the first extrasolar planet was discovered in 1995, astronomers have found more than five thousand others in our cosmic neighborhood. Astonishingly, that means about one new world discovered for every other day since we built the first instrument sensitive enough to detect them. And we have only spotted the ones that are easy to find—the tip of the iceberg.

Planets are so common that they circle most stars. And our galaxy, the Milky Way, hosts about two hundred billion stars. This staggering number indicates that there are billions and billions of new worlds to explore in our galaxy alone.

Solving the puzzle of these new worlds requires using a wide range of tools like cultivating colorful biota in our biology lab, melting and tracing the glow from tiny lava worlds in our geology lab, developing strings of code on my computer, and reaching back into the long history of Earth’s evolution for clues on what to search for. With Earth as our laboratory, we can test new ideas and counter challenges with data, inspired curiosity, and vision. This interaction between radiant photons, swirling gas, clouds, and dynamic surfaces driven by the strings of code within my computer creates a symphony of possible worlds—some vibrant with a vast diversity of life, others desolate and barren.

So far, despite claims to the contrary, we have not found any definitive proof of life on other planets. Until we do, we will continue to improve our toolkit and look for signs of alien life the hard way: searching planet by planet and moon by moon. The most exciting phase is about to begin.

How can we find signs of life (or biological activity) by reading spectra from planetary atmospheres?

Signs of life are written in a planet’s light—if you know how to read it. Light carries energy, and if just the right energy hits a molecule, it swings and rotates. Just like the stamps in a passport tell you what countries a traveler has visited, the chemical makeup of the atmosphere of an alien world is encoded in the light that arrives at my telescope from there – including any signs of life on this other world.  

The JWST is the first telescope capable of capturing just enough light with its 21.3-foot (6.5-meter) mirror to explore the chemical composition of the atmosphere of other rocky worlds. Size is the key to collecting light. Imagine a bucket—the larger it is, the more rainwater it can catch in a downpour. The telescope’s mirror operates the same way—the larger it is, the more light it can collect.

Can you list the three main points of your book?

In Alien Earths, I take you on an exciting and surprising journey as we search for life in the cosmos. 

1. Billions of possibilities: One out of five stars has a planet that could potentially be like ours, so not too hot, not too cold, small enough to be a rock like Earth and thus could allow for liquid water on its surface. Our galaxy, the Milky Way, is home to about 200 billion stars, which means that there are billions and billions of possibilities for worlds like ours in the Cosmos. We live in a golden era of exploration right now, exploring these worlds for the first time with the James Webb Space Telescope.

2. The incredible diversity of worlds circling other stars: Some of the exoplanets we have discovered have shaken our worldview. They have been completely unexpected—some are covered with oceans of magma, others are scorched, puffy balls of gas whizzing close to their parent stars, while others are lonely wanderers in space. Diving into fictional worlds in Alien Earths, you’ll find that some of these exoplanets are even weirder than worlds imagined in Sci-Fi. Still, some of them are starting to look just a little bit like home. 

3. The key to finding life in the cosmos is Earth’s history, varied biota, and creativity. Our planet has changed significantly through its evolution: Could a time traveler even identify our young planet? No Himalayas, no Alps, no landmarks to look for, continents broken apart and reformed again and again through Earth’s history. No more familiar constellations on the night sky, which would look eerily different because stars move on their own journey through the cosmos. Earth changes and its wide variety of diverse biota gives us a first hint of how to search for a wide range of life on other worlds.

Finding those clues requires a range of network of thinkers across disciplines working together, a lot of ideas and creativity. Today, solving the puzzle of these new worlds requires using a wide range of tools from biology, geology, computer simulations, and reaching back into the long history of Earth’s evolution for clues on what to search for. With Earth as our guide, we can test new ideas and counter challenges with data, inspired curiosity, and vision.

I built the Carl Sagan Institute to connect ideas from a variety of thinkers in different disciplines in a new and creative way. That, in combination with using our planet’s history as our Rosetta stone, is critical to give us the best chance to not miss signs of life on other worlds. 

Can you give us a quick survey of the odds of finding life in the solar system?

The cheeky answer is that the chances are 50% — plus or minus 50% percent. That means all of them to none of them because we don’t know how easy it is for life to get started anywhere. That’s what makes the search so fascinating. But we have billions of possible places where it could get started – so wouldn’t the bigger surprise be if we find nothing? 

What do you mean by “Alien Earths” as we see in the title of your book?

“Alien Earths” is a reference to worlds that could be like ours, circling other stars. But it also refers to our planet. Earth has changed so monumentally through its evolution that in its past it would be an alien world to us – it did not even have an atmosphere we could breathe for most of its history. Our planet transformed from a young, hostile world covered in lava to a planet with vast oceans, its medley of white clouds, and blue light scattered in our atmosphere, an astonishingly beautiful, fragile, pale blue living planet. A brilliant speck on the vast black canvas of space.

Earth’s history chronicles its profound transformation and allows a first glimpse of the diversity of one rocky world and the life it shelters, hinting at what to look for in the vastness of space.

I know this is a question that we can’t answer quantitatively, but do you think technological civilizations are common in the galaxy? What’s your favorite answer to Fermi’s paradox? (Note: The Fermi paradox asks why we haven’t been contacted by alien civilizations yet, given that our galaxy is 10 billion years old.)

My personal answer is even if there were many civilizations out there, why would anyone want to contact us? We are not really that interesting yet. The underlying assumption of the Fermi paradox is that other civilizations would want to visit or communicate and that other species, if they are out there, would be curious enough to communicate. Let’s set aside the issue of what atmosphere and environment potential alien visitors would need to survive; how intriguing would Earth appear as a destination? Earth is my favorite planet, but in terms of technology, we are just getting started. True, twelve astronauts have visited the lunar surface, but so far, human beings have not even reached the nearest planet, let alone the nearest neighboring star. Given a choice, would Earth really be the planet to pick—yet? In the optimistic case of a cosmos teeming with friendly worlds, the Earth is not yet at the grown-ups’ table. 

The premise that anyone who could call us would do so immediately, seems flawed, making the Great Silence and the answer to the Fermi paradox less eerie.

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Given current (JWST) and future missions, what upcoming observation(s) would be most exciting for you?

As I type this the James Webb Space Telescope is observing the Trappist-1 system and its seven Earth-size planets, with three in the Habitable Zone of their star. It will take time to collect enough light and analyze the data, but we truly live in an amazing time, where we can for the first time ever collect light from a small, rocky world in the Habitable Zone of another star and analyze it for signs of life. That is why I wrote this book now, because we are on the verge of finding out the answer to the thousand-year-old question: Are there other Alien Earths out there? 

Look up into the stunning sky, find your favorite star, and allow yourself the freedom to wonder. What if we are not alone in the cosmos?


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