What are the odds we're not alone in the universe?

There are hundreds of billions of stars in the galaxy we live in. Many of them have their own exoplanets. So why haven’t we found proof of alien life yet? And what’s the probability of us contacting or being contacted anyway? That’s exactly what a famous math formula called the Drake Equation tries to answer. It takes into account a whole variety of factors, including the rate at which stars like the Sun are born, how many of those stars have planets, how many of those planets are hospitable to life and the likelihood of intelligent life on that habitable planet communicating with us. Today on the show, Regina Barber https://www.npr.org/people/1082526815/regina-g-barber and Seven Rasmussen https://www.sevenrasmussen.com/ talk about the equation as it was created back in 1961 – and whether it still matters for astrobiologists today. Interested in more episodes on space or math? Email us your question at [email protected] mailto:[email protected] – we might answer it in a future episode! Support public media with NPR+ and enjoy perks for over 25 podcasts like this one. This show’s perks include sponsor-free listening. Learn more at plus.npr.org http://plus.npr.org. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

• 0:00Speaker 00

  This message comes from Capella University. That spark you feel, that's your drive for more. Capella University's FlexPath learning format lets you earn your degree at your pace without putting life on pause. Learn more at capella.edu.

• 0:15Speaker 01

  Hey, it's Gina, your friendly neighborhood astrophysicist, here with a request. Do you have a friend who loves space? If you do, can you share this episode with them? It'll help us grow the Short Wave community so that we can all space out together. You're listening to Short Wave from NPR.

• 0:35Speaker 01

  The universe is so vast. In our galaxy alone, there are hundreds of millions of stars. And circling around many of those stars are planets. And given all that, some scientists have long suspected life exists somewhere else. So where is everyone? Are we truly alone? To me, the idea that we humans are the only intelligent life in the whole universe just seems self-absorbed. Like, it makes us artificially special in a universe overflowing with possibilities. And I know I'm not all alone in that thought. Lots of scientists today think so. Like Seven Rasmussen, an astrobiologist I called up for this story.

• 1:19Speaker 01

  Reminded me that over half a century ago, scientist Frank Drake thought so, too.

• 1:25Speaker 02

  So there was a conference on the brand-new field of astrobiology. In 1961, at the Green Bank Radio Observatory. And Frank Drake has to come up with a presentation for the conference. And he's got a couple days to do it.

• 1:42Speaker 01

  Drake was doing all of this in 1961, well before anyone knew for sure that planets existed outside of our solar system. And he was still like, statistically, surely, there has to be other forms of life out there. So can I make up some math for that?

• 1:58Speaker 02

  So he miraculously comes up with this incredible, long-lived equation that describes how many civilizations this little conference should expect to be out there and how many they should expect to look for.

• 2:15Speaker 01

  The equation has many parts to calculate based on conditions that let us live here on Earth. And depending on how you answer the many components, you could get wildly different final answers for how much reachable life is out there in the universe. Like, when Seven does this calculation...

• 2:32Speaker 02

  When I take my most optimistic numbers and I plug them into the Drake equation, I get that there's a million intelligent civilizations in our own galaxy, a very Star Trek kind of world. When I plug in my pessimistic numbers, I get that there's one intelligent civilization for every 60 billion galaxies out there.

• 2:53Speaker 01

  That's a huge range. And that's with Seven pulling from way more scientific knowledge than Frank Drake had back in the 1960s.

• 3:01Speaker 02

  In 1961, we were decades away from finding the first exoplanet. He had no idea if our solar system was completely unique, if there was a solar system around every star. But he still had the audacity and the boldness to go out there and say, we can calculate how much life is out there.

• 3:20Speaker 01

  To some scientists, doing this calculation is pure speculation.

• 3:25Speaker 02

  There's been multiple attempts to revise the Drake equation. There's always a new paper out claiming they've solved the Drake equation.

• 3:33Speaker 01

  To others, it's a legitimate tool in our search for extraterrestrial life.

• 3:37Speaker 02

  It does not have an answer. It probably will never have an answer. But the conversations it sparks is the number-one reason why we need to still be talking about the Drake equation.

• 3:51Speaker 01

  Today on the show, what's the truth? Can we ever hope to reach alien life and find out we're not alone in this vast universe? And does undergoing the journey to solve that even help science? I'm Regina Barber, and you're listening to Short Wave, the science podcast from NPR.

• 4:13Speaker 03

  Stay tuned for the following announcements and messages from our sponsors. This message comes from NetSuite. AI isn't just opening new business opportunities, it's changing the way businesses get work done. That's the idea behind NetSuite Next, the next generation of NetSuite with AI woven into the tools your business already relies on. If your revenues are at least in the seven figures, get the free business guide, Aligning for the Agentic Era: How AI is Changing Everyday Work, at netsuite.com/shortwave.

• 4:44Speaker 00

  This message comes from Mint Mobile. While others have been hiking your bill, Mint Mobile has kept their wireless plans starting at $15 a month. Switch to Mint Mobile and get three months of premium wireless service for $15 a month. Go to mintmobile.com/switch. Upfront payment of $45 for three-month plan required. $15 per month equivalent. New customer offer, first three months only. Then full-price plan options available. Taxes and fees extra.

• 5:09Speaker 03

  This message comes from Angie. Angie has been helping homeowners find skilled pros to get their home projects done well for over 30 years. For everything, from minor repairs to major remodels, find reviews, pricing, and booking at angie.com or on the Angie app.

• 5:27Speaker 01

  OK, Seven, as we've been talking about, there's a lot that goes into the Drake equation, including the rate at which stars like the sun are born, how many of those stars have planets, how many of those planets are hospitable to life, and how likely it is that intelligent life is on that planet and are able to communicate with us in the first place.

• 5:47Speaker 02

  Yeah.

• 5:48Speaker 01

  And like we mentioned, Frank didn't know about planets outside of our solar system in 1961, when he first thought up this equation. So since then, we've found a bunch of exoplanets. Those are planets outside of our solar system. NASA's official count is over 6,000?

• 6:04Speaker 02

  Over 6,000.

• 6:05Speaker 01

  In our own galaxy, right? This is in the Milky Way.

• 6:07Speaker 02

  Yep.

• 6:08Speaker 01

  So how do we determine which of those are habitable? Like, how are we defining habitability?

• 6:14Speaker 02

  Habitability is probably the messiest question in the entire field of astrobiology.

• 6:19Speaker 01

  Yes.

• 6:20Speaker 02

  Because it really gets into the bias we have for our own planet.

• 6:23Speaker 01

  Right.

• 6:24Speaker 02

  We came out of a pretty, what we consider, temperate environment, where we have our liquid water, we have continents, we have a pleasant atmosphere. But every day, we learn more about environments that germs on Earth are surviving and thriving in that are not, quote, "Earth-like." Strong acids, extremely salty environments, extremely dry environments, temperatures far above boiling. We are really learning more and more about the habitability of our own planet. And so that topic is an incredibly messy thing to wade into.

• 7:04Speaker 01

  Yeah.

• 7:04Speaker 02

  I would say that the things that you really, truly, 100% need for habitability are the basic six elements required for life: sulfur, phosphorus, oxygen, nitrogen, carbon, hydrogen. You need to have temperatures that are probably below water boiling. And you need to have some kind of liquid medium for those ingredients to come together in.

• 7:30Speaker 01

  Yeah.

• 7:30Speaker 02

  And I think that's all you need for habitability.

• 7:33Speaker 01

  I mean, is there controversy on what that is, then? Are people like, there needs to be much more than that?

• 7:39Speaker 02

  Yes, it's absolutely one of the most controversial, ongoing debates in astrobiology, is life as we know it. And then this topic is often referred to as life as we don't know it. So we know about life on our planet, and we don't know about the other ways life could exist. So it's like a Venn diagram, where you have, here's what we know, but how big is the other circle? Nobody knows. It could be as big as the solar system. It could be tiny. What we don't know, we don't know.

• 8:11Speaker 01

  Yeah. Hearing a mathematical equation is very hard, right? So I'm trying to give people, like, what facets of the Drake equation vary so greatly.

• 8:21Speaker 02

  Yes.

• 8:22Speaker 01

  When you calculated it, it went from millions to 1 in thousands, tens of thousands.

• 8:27Speaker 02

  Yeah.

• 8:27Speaker 01

  And it's because of habitability. And we just talked about that. That's one factor in this equation. But there are, like, seven other ones, right?

• 8:34Speaker 02

  So the habitability question is definitely a big one. Because how many planets out there are truly capable of hosting even single-celled life? And after the habitability question, you have to think about, not only is the planet habitable, but does life form? And I, you know, I'm an optimist. I think if the conditions are right and the materials are there and the liquid medium is there, I think you will, something's going to crawl out of the muck and invent the hot dog roller, as I like to say.

• 9:06Speaker 01

  That's a good invention.

• 9:07Speaker 02

  I agree. I agree.

• 9:09Speaker 01

  I love it.

• 9:10Speaker 02

  And then, you know, intelligence is the factor that I think fascinates me the most. Because I really do believe that we are not the only, quote, intelligent species on our planet. We don't know about the communications and the societies and the social dynamics of things like sperm whales, like cephalopods. And so that one, that's the variable that I think fascinates me the most. Because we still have to find out, where do we draw the line on intelligence?

• 9:42Speaker 01

  Right, right. And that's where we get to this other factor. Because even if there are intelligent species, them communicating with us is a whole different story. Right? Like, they have to be around at the same time we are, which, in the timescale of the universe, is like, nothing, right? It's like a millisecond.

• 10:00Speaker 02

  Yeah, so the last variable of the Drake equation, L, the average lifetime of an intelligent communicating civilization, is one of the most fascinating things I think we can even address in a discussion about the Drake equation. Because that's really, really pointing the camera inwards. And it's saying, how optimistic are we feeling about what's going on with the world? You know, are we looking at a Star Trek future where we overcome all of this and we become spacefaring and we're explorers and we're reaching out to other intelligent civilizations?

• 10:36Speaker 01

  Yeah. Where do scientists get this L value of like, how long an intelligent species is even going to be around to send us a message?

• 10:45Speaker 02

  I would say that's even beyond the capabilities of just science to address.

• 10:49Speaker 01

  Right.

• 10:49Speaker 02

  That is a problem for sociology and for politics and for every facet of human life. Of how long are we going to be here? How long are we going to be at the level of communication needed to send an interstellar message? Personally, I think that L may be, I don't know, 10 to thousands of years. But to me, it's not necessarily consecutive. Because we've had the Roman Empire and then we had the Dark Ages. And civilization doesn't, civilization doesn't grow in a straight line.

• 11:22Speaker 01

  It's not linear.

• 11:23Speaker 02

  Exactly. It's not linear. There may be long periods of human history where we are capable of sending messages to the entire galaxy. And then there may be hundreds of years where we don't have that capability.

• 11:35Speaker 01

  Like a solar storm just wipes out our communications for a few years or 100 years?

• 11:41Speaker 02

  That's just one more thing we have to worry about is outburst from the sun. Even our pretty well behaved sun.

• 11:47Speaker 01

  Like we said earlier, there are other versions of the Drake equation. Maybe things have been modified. A popular version happened in 2013 and it's called the Sieger equation. How is this equation different from the Drake equation?

• 12:01Speaker 02

  So the Sieger equation is actually more focused on the odd that we will find any kind of life out there in the galaxy.

• 12:11Speaker 01

  Intelligent or not?

• 12:12Speaker 02

  Intelligent or not, it's based on data that we have available. There's only a couple variables at the end that we truly don't know. So I would say that it's very focused on life in general, whether it exists or not. And it's a lot more practical and well-known. You know, not quite as expansive. It doesn't have the philosophical resonance and weight of the Drake equation, which forces us to ask, how long are we going to be doing this? But it's very practical and it's very much motivated by our own capabilities and what can happen in astrophysics in the next couple decades.

• 12:52Speaker 01

  Yeah, I mean, to me, this iteration of the Drake equation, the Sieger equation, it makes sense because we as scientists don't even know how life started on Earth. Like, that's a huge debate.

• 13:03Speaker 02

  Yeah, the debate between the, quote, warm little pond, like the tide pools camp.

• 13:09Speaker 01

  Yes, and then the hydrothermal vents.

• 13:12Speaker 02

  Yes.

• 13:12Speaker 01

  I'm in the hydrothermal vent camp.

• 13:14Speaker 02

  Oh, interesting, because I'm in the warm little pond camp.

• 13:17Speaker 01

  No, but then Europa doesn't have any sea shrimp. Why would you do that to me, Seven?

• 13:21Speaker 02

  Look, I want the deep sea vent paradigm to be correct. Because I want there to be life on Europa and Triton and all those cool ice moons. Enceladus. But the warm little pond people really had me convinced with the wet/dry cycle that you need to take molecules apart and put them back together.

• 13:40Speaker 01

  That wet/dry cycle.

• 13:41Speaker 02

  It compels me.

• 13:42Speaker 01

  Anyway, we will move on. Let's continue this thought, right? Like, maybe humans aren't advanced enough, you know, to get these signals or to find these, you know, signals of life. So with that thought, what kind of technological advances are needed to get us closer to like, a more scientific Drake equation or to even help answer that Sieger equation?

• 14:03Speaker 02

  Yeah, well, NASA has this awesome telescope plant called the Habitable Worlds Observatory. And what the Habitable Worlds Observatory will have is an instrument that actually the Roman Space Telescope has as well, called a coronagraph, which is a really fancy scientific little dot that you put over the star so you can look at the planet. You put your scientific instrument right on the planet and you get a much better picture of what is happening in that planet's atmosphere. And what is happening chemically that may indicate that, at the very least, there's continuous production of, say, methane, like we have here on Earth. Continuous production of oxygen and other things that can sometimes recombine into other gases in the atmosphere.

• 14:49Speaker 02

  So I would say that the Habitable Worlds Observatory is what I'm most looking forward to.

• 14:53Speaker 01

  So the Habitable Worlds Telescope is out there. We're, you know, 20 years in the future. We get data back. What do you want to see? What's your dream?

• 15:02Speaker 02

  You know, very, very, very basic, but I would love to see a precisely Earth-like planet. That's what we know about the most. I want to see a planet that we already know has extreme potential for life. There may be other planets that are just as high potential for life, but if they aren't, quote, Earth-like, that debate's going to get messy. It'll never end, you know? Astrobiology is a field that has a lot of data, but it needs 1 million times more data than it has. Because we have so much data about the planet that we live on.

• 15:42Speaker 02

  And we have absolutely no data about any other exoplanetary system.

• 15:50Speaker 01

  I love it. Seven, thank you so much for speaking with us. You know, go Star Trek.

• 15:55Speaker 02

  Amazing, yeah. Live long and prosper.

• 16:00Speaker 01

  If you want to check out Seven Rathmussen's book on the Drake equation, "Cloudy with a Chance of Starships" is out now. This episode was produced by Juan Diego Ramirez and Hannah Chin. It was edited by showrunner Rebecca Ramirez. Rebecca and Hannah checked the facts. The audio engineer was Kwasi Lee. I'm Regina Barber. Thank you for listening to Short Wave from NPR.

• 16:24Speaker 00

  Stay tuned for the following announcements and messages from our sponsors. This message comes from Mint Mobile. While others have been hiking your bill, Mint Mobile has kept their wireless plans starting at $15 a month. Switch to Mint Mobile and get three months of premium wireless service for $15 a month. Go to mintmobile.com/switch. Upfront payment of $45 for three-month plan required. $15 per month equivalent. New customer offer, first three months only, then full-price plan options available. Taxes and fees extra.

• 16:54Speaker 03

  This message comes from Angie. When it comes to home projects, it matters who you hire. Angie has been helping homeowners find skilled pros to get their home projects done well for over 30 years. Angie's nationwide network has experts in everything, from landscaping to roofing and more. Read verified reviews, see photos of past work, and request and compare quotes to find your best price, all in one place. Download the free Angie app today or visit angie.com. That's A-N-G-I dot com.