Science Friday - Largest Animal Crossing, First Complete Human Genome, Exoplanet Discoveries. November 25, 2022, Part 2

Episode Date: November 25, 2022

Building The World’s Largest Animal Crossing Outside of LA There’s a spot on Highway 101 in Agoura Hills, it’s pretty inconspicuous. There’s brown and green rolling hills on either side of the highway. Homes are sprinkled here and there. And then a small metal gate that leads off on a hiking trail. You probably wouldn’t know it, but soon this spot will be the location of the world’s largest animal crossing. This crossing will reconnect habitats that have been cut off from each other for three quarters of a century and it’ll do it over a highway that is constantly buzzing with cars — 300,000 pass by this spot every single day. In this piece we’re going on a geography voyage — from the north side of the highway to the south, and up the hills, above the highway, to get the real view. We’ll start here — there’s a big open space on the northern side of the highway. It’s at the entrance to Liberty Canyon and where I meet Beth Pratt. To continue reading, go to sciencefriday.com.   Over 5,000 Exoplanets Have Now Been Discovered In March, the NASA Exoplanet Archive logged the 5,000th confirmed planet outside of our solar system. This marks a huge advance since the first exoplanet discovery in 1992, when astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12. Now, the Archive contains confirmed sightings of planets in a wide range of shapes and sizes—from “hot Jupiters” to “super Earths”—but they still haven’t found any solar systems just like our own. In many cases, all astronomers know about these distant planets is their size and how far away from their stars they orbit. The TESS (Transiting Exoplanet Survey Satellite) mission currently in orbit may eventually add ten thousand more candidates to the lists of possible planets. The Nancy Grace Roman Space telescope and ESA’s ARIEL mission, both planned for launch later this decade, could add thousands more. And the James Webb Space Telescope, currently undergoing commissioning, will attempt to characterize the atmospheres of some of the planets astronomers have already discovered. Astronomer Jessie Christiansen, the NASA Exoplanet Archive Project science lead, joins John Dankosky to talk about what we know about planets around distant suns, and how researchers are working to learn more about these far-off worlds.   Scientists Release The First Fully Complete Human Genome Two decades ago, scientists announced they had sequenced the human genome. What you might not know is that there were gaps in that original sequence—about 8% was completely blank. Now, after a years-long global collaboration, scientists have finally released the first fully complete assembly of the human genome. Researchers believe these missing pieces might be the key to understanding how DNA varies between people. Six scientific papers on the topic were published in a special edition of the academic journal Science. Ira talks with Karen Miga and Adam Phillippy, co-founders of the Telomere to Telomere Consortium, an international effort that led to the assembly of this new fully complete human genome.   Transcripts for each segment will be available the week after the show airs on sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

This is Science Friday. I'm Ira Flato. I hope you're having a wonderful Thanksgiving holiday, and you've taken some time to think back on some things you're grateful for this year. As we look back, we've covered so many stories where we can say we're grateful for science, like how carty cells are being used to treat cancer and maybe other diseases. And of course, those new bivalent boosters aimed at protecting us from COVID variants. And like the rest of you, we couldn't help but marvel at the new images of space from the Webb Telescope, or root on the Artemis mission to the moon. So this hour, stories that make us thankful for science.

Later in the hour, we'll revisit our conversation about mapping the entire human genome, but first, let's check in on the state of science. This is KER News. New York News, Iowa Public Radio News. Local science stories of national significance. Southern California, including Los Angeles,

is home to a diverse ecosystem that includes big predators like Mountain Lions, But the 101, a busy multi-lane highway, slices right through the area, dividing that ecosystem and making it hard for animals that live there. So engineers, conservationists, and animal enthusiasts came together to build a giant animal crossing, a grassy path from one side of the freeway to the other. Back in April, on the day that they broke ground on this project after years of planning, I talked to Michelle Lockstone, a podcast host and producer for K.

ACLU Public Radio in Thousand Oaks, California. Welcome to Science Friday, Michelle. Thank you, Ira. Okay, for those of us who aren't based on the West Coast, tell us a little bit about the 101. Where is it? How long does it go? So the 101 Highway is a north-south highway that stretches along almost the entire west coast of the United States. It starts essentially at the U.S.-Canada border and goes through Washington State, Oregon, and California. The Wildlife Crossing will be be built over Highway 101 in a city called Agora Hills, which is just north of Los Angeles. And transportation experts have actually measured how many vehicles pass through this spot in Agora Hills on the 101 Highway. It's 300,000 vehicles every single day.

Wow. Wow. Can you set the scene for what the ecosystem is like around where the crossing will go? Sure. So if you're standing at the location of the future wildlife crossing, you'll see a lot of lot of rolling hills on either side of the highway. You'll see sage scrub, chaparral and patches of oak woodlands. Right now, these rolling hills are a beautiful mix of green and yellow and brown because we've had a little bit of rain recently and the wildflowers have been sprouting up. But these rolling hills, they extend for miles on either side of Highway 101, and there are homes and neighborhoods that have been established amongst the hills and the wild space. To the south of the highway, you have this massive open wild protected space called the Santa Monica Mountains National Recreation Area.

It extends all the way to the Pacific Ocean and it's the biggest urban national park in the country in fact. North of the highway 101 is a similar mix of wild space and neighborhood. And it's amongst this wild space in these neighborhoods on either side of the highway you'll find mountain lions, deer, bobcats, coyotes, badgers, rabbits, mice, wood rats, horn lizards, horn lizards, tree, frogs, snakes, ants and quail, and all sorts of birds. Amazing. So how does the 101 impact all these creatures that live there? It has a big impact on them. I'm going to bring in the voice of Beth Pratt now, who I think describes the situation really well. So Pratt is from the National Wildlife Federation.

She's led the campaign to get this wildlife crossing built. So the first thing to know about where the wildlife crossing will be built is that this is the last. 600 feet of protected wild space on both sides of the highway in our region. Pratt describes this location like an hourglass where you have these big open spaces on either side of the highway that are funneled into the center point of the hourglass, the center point of the hourglass being the highway itself. But unlike with an hourglass where sand would flow from one side to the other, in this case the animals are the sand and they've been blocked for decades from moving through the area because of the impenetrable wall that is Highway 101.

Here's Beth Pratt. The wildlife were already sort of coming to this area. And what the National Park Service study has shown for at least the wildlife, they have collared or tracking like the mountain lions or coyotes, they get here and they're like, I'm not crossing this and they turn around. If they do try and cross, they could be hit by cars. But like Pratt said there, a lot of them don't need. even try and essentially are boxed into the region where they were born. And this has had a disastrous

effect on genetic diversity. The National Park Service has been studying carnivores in the region for 20 years and they found that mountain lions and the son of monikas have very low genetic diversity, basically the lowest that anyone's seen, and that's because of inbreeding. Because mountain lions can't disperse, because of the highway, male mountain lions are mating. with daughters, granddaughters, and even great-granddaughters. One study found that male mountain lions have more than 90% abnormal sperm. For the wildlife experts, the wildlife crossing is a solution to this problem. It would allow the animals to safely cross the highway, firstly, and then it would allow them to mix with other populations in other regions. You know, we've buried the lead here a bit.

There's a celebrity in this story. P-22, a Puma, who is local Los Angeles legend. Can you tell us about P-22? Yes. So P-22 really is our lead character in this story. P-22 is a mountain lion that was born in the Sanamanica Mountains over 10 years ago. P is for Puma, and 22 is because he was the 22nd Puma or mountain lion to get a tracking collar. And when he was really young, he went on this incredible journey. He left the Santa Monica Mountains and managed to cross two of the busiest highways in our region, unhurt and undetected, and ended up in Griffith Park in Hollywood, where there are no other known mountain lines. Because his journey was such a fluke, he hasn't been able to leave Griffith Park and has essentially been stuck there for a decade on his own. But in this time, he's become a bit of a celebrity.

Beth Pratt is kind of obsessed with P-22. She has a tattoo of him on her arm and carries around this life-size cardboard cut out of him so people can take pictures with P-22. She's called him the Brad Pitt of the Mountain Lion world, and the public loved that. For me, what he did was get the public engaged, which is really important. The Park Service and others have been talking about the need for sort of connectivity for a while. But it wasn't something that resonated with people outside of the environmental or scientific world. But all of a sudden, boom, you get this lonely, dateless, handsome bachelor show up in Griffith Park. And it worked. People loved that idea of P-22 and got invested in this idea of connecting wild spaces.

And they started raising money for a wildlife crossing. Now, unfortunately, this particular wildlife crossing that we're talking about today won't benefit P22. He's just too cut off at the moment, but it will benefit many other mountain lines. Oh, that's unfortunate. Let's talk more about the Animal Crossing. How long has this been in the works? Well, the National Wildlife Federation started talking about this crossing behind the scenes in 2012, and then with P22 as the face of the campaign,

They went to the public and started raising money two years later. It was a pretty steep fundraising hill to climb. The project was priced at $90 million, but they've raised the money through state funds, 40% and private philanthropy, 60%. The biggest single private donation was $26 million. Wow. This seems like it would be a major feat of engineering. Okay, can you walk us through what's it going to look like?

Absolutely. So there's so many wonderful details about the crossing itself. In terms of size, the crossing will be about the width of an American football field going over 10 lanes of highway. The people designing it have described it to me like a green roof on steroids or a green to pay. They're taking special care to make sure the crossing matches its environment. So they've taken the very biggest things into account, like how the crossing will fit in with the watershed in the region. all the way down to the microscopic level with the building of soil ecology. Here's Robert Rock, the C-O-O-O-of-Living Habitats and the lead architect,

talking about all the different details that they've considered. Nine out of ten people are not going to even know that we spent all this time thinking about the microbial biomass in the soil and the degree to which that links to carbon sequestration or the minutia of how we design spaces to accommodate the California Kingsnake. you know, we're creating a project nursery for this, where we are going to be growing all the plants that are going to be a part of this construction. And part of that is leveraging, you know, seed bank that the National Park Service has and that we'll be collecting from the site and from adjacent areas. Another big part of the design is it has to be inviting to animals. That's the whole point, right?

They need to get these animals to use it. So here are a few things that they're doing. They're putting massive sound barriers on the crossing itself and along the highway to dampen the noise of traffic. The height and thickness of the bridge has also been considered to avoid the noise of the cars below. They've also thought about the light, so they're looking into lowering streetlights on the nearby off-ramps without affecting safety. The color palette has been taken from the Santa Monica Mountains, as I mentioned before, and this will help darken the structure at night so they don't have this refraps. reflective glow that you sometimes see on concrete bridges. They want it to work for all animals, from a quail to a snake, to the mountain lines. That is so cool. Okay, so how long will it take to

complete? Okay, with all big projects, there's always an approximate date, but Beth Pratt told me that she hopes it will open for business by late 2024, early 2025. And they all have bets in place on what they think will be the first animal to use it. Pratt thinks maybe a lizard. I don't know if you want to hazard a guess, Ira, but I think maybe a bird or perhaps a very brave coyote. I'll go with the coyote. I think I'm with the coyote on this one. So this is going to be the first of its kind in terms of how big this crossing is. But could we expect to see more of these in California or perhaps even across other parts of the country. This is a big hope for everyone involved in the project.

Yes, animal crossings in different forms exist all over the world. We've seen ones for crabs going over roads. We've seen ones for bears going underroads. But one, this big and intricate, going over such a busy highway is a first. And those involved don't want it to be cutting edge for long. Michelle Lockstone is podcast host and producer for KCLU, Public Radio. in Thousand Oaks, California, she reported this story for KCLU's podcast,

the 101. Thank you, Ira. Coming up right after the break, some more science we're thankful for. After decades, we now have a full map of the human genome. This is Science Friday. I'm Ira Flato. We're looking back this hour

at some of the conversations that made us thankful for science. This next story was an easy one to put on that list. Two decades ago, scientists announced a monumental scientific achievement. They had sequenced the human genome, but there were gaps in that original sequence. In fact, about 8% of the sequence was completely blank, and a lot of that used to be dismissively called junk DNA. Well, in late March of this year,

scientists finally released the first fully complete assembly of the human genome, and research published in a special edition of the journal's science. We talked to Karen Miga, Assistant Professor of Biomolecular Engineering, and Associate Director of the UC Santa Cruz Genomics Institute, and Adam Philippi, a senior investigator at the National Human Genome Research Institute. That's at NIH in Bethesda, Maryland. Welcome to Science Friday. Yeah, thanks so much for having us.

Thanks, Ira. Pleasure to be here. Nice to have you. Let me begin with this question of this telemere to Telemere Consortium that you have founded, an international effort that led to the assembly of this new fully complete human genome. Dr. Miga, tell us the significance of that name. Your listeners may recognize that the telomere is at the end of our chromosomes.

And so we chose telomere to telomere to really illustrate that we were trying to complete an entire chromosome in one assembly, end-to-end. Not just broken pizza pieces. Exactly. Yeah, and it's been really wonderful because it really does. does create a full view of a human chromosome for the first time, which is exciting. So let's get into this. What's on these newly sequenced parts of the human genome, Dr. Miga? Right. So the new sequences represent essentially regions of our genome that are known to be

important for fundamental cellular processes. When we talk about regions like the Centromere, which is pretty exciting for my own research, we know they're responsible for how our chromosomes are transmitted every single time our cells divide. So change. ranges in these sites in our genome could actually cause errors that could lead to all kinds of health outcomes. In total, we're talking about 200 million bases, so it's a lot. That is a lot. That's a large percentage. We said 8%. Could you make a whole new genome out of that kind of material? Well, I think when we talk about a chromosome's worth, 200 million bases is about the size of one of our largest chromosomes. We look at the information that's their third largest, so it's

slightly bigger than Chromysm 3. Now, are these parts of the genome that scientists used to refer to as junk DNA? Is that what you have actually identified? I think it would be hard to consider the sequences in these regions to be junk. I think that word, I will probably agree with me, is probably outdated and it's just used as a way to explain processes we don't yet understand. How dumb was that? I really think that these regions are misunderstood. They don't fall in our standard textbook definition of how genomes are organized. They do have genes in these regions. We do have these standard organizations, but they're really enriched with a unique kind of structure where you have a sequence

that's found in a head detail, head detail orientation for millions of bases. And why our genome is arranged in this way in the corners of our genome, I think remains an unknown. And this has been your life's work, hasn't it? It has. I've been passionate about satellite DNAs since graduate school, and so I was really lucky to be able to pair up with such an amazing scientist like Adam Philippi to make this dream come true because he's really kind of the other side of this, where I've been focusing so much on the satellite DNAs and the biology of these unique genetic elements, having that type of mastery

over assembly really brought us to where we are now. Well, let me talk to Adam. Dr. Philippi, why did it take so long to sequence this final 8% of the genome? After the human genome project finished in 2004, the holes that were left were the most repetitive bits of the genome. So, you know, imagine you have a puzzle, and there's a bowl of skittles over in the corner of that puzzle, and that's the hardest bit to put together because all of the skittles look the same.

Those types of repeats make jigsaw puzzles hard, just like they make putting a genome back together again, difficult. And so it was those repeats that had us interested from a computational perspective and gave us a big challenge in putting this back together again. So the reason that they weren't done originally is the technology just wasn't up to the task back in the early 2000s. We could only read small bits of the genome at a time. And when the puzzle is made of small pieces, it's a lot harder to put together than when they're made of big pieces. And so for this project, we came in with new sequencing technologies that have been developed over the last decades. can read up to a million bases of sequence at a time compared to in the early 2000s where we

were limited to a few hundred bases. Now, knowing what we know now, if you have the total sequence, how does this move us forward in learning about DNA? Well, now that we've figured out how to do it, and we can reconstruct these repetitive regions for the first time, it allows us to do that again now for many more human genomes. Or if we have a patient coming to the clinic, we can sequence their complete genome, line it up against this new complete reference sequence, and we're able to get a more comprehensive picture of all of the potential variants that they have within their genome. And then over time, we'll be able to link those newly discovered variants to potential disease associations, for instance.

Is there one disease out there or one treatment that was waiting for this total sequence to be unraveled, do you think? And now it's in the crosshairs. The one I would probably point to first are the so-called Robertsonian translocations. These occur in one in a thousand births, and it's a fusion, essentially, of two different chromosomes. And we've revealed for the first time five entirely new chromosome arms, and they are directly related to this type of chromosomal anomaly. And a lot of our collaborators that are interested in these translocations now have the base precise sequence that they can look into and try to understand how these form and what the potential repercussions could be. Will it also tell us how we're different from other animals, other primates close to us? Yeah, absolutely. In fact, these repetitive regions are some of the most dynamic,

the most variable regions of the genome compared to our nearest primate relatives, the most variable between individual humans. So we have some hope that there'll be very exciting discoveries within these regions that might hold the key to what makes our genome uniquely human. It's kind of a contrast to how we think about function with everything having, to be deeply conserved in these regions, which we know are functional or are placed in these critically functional regions. They're, as Adam mentioned, extremely dynamic and in many cases, human specific. So it's kind of in contrast to what we're used to thinking about in terms of how we think about evolution and conservation. Now that we have these new tools you're talking about,

how far out are we from each one of us getting our own individual genomes mapped? That's definitely the goal of this consortium is to help develop. the technology to a point that a project like this to get a complete human genome can be replicated and become routine. And I think within 10 years, it will be routine to have your complete diploid genome as just part of your medical record. At a cost of what? So for the original human genome project, just to put things in perspective in today's dollars, I think it was around $5 billion and, you know, a 10-year plus effort. This project we estimate, maybe a couple million dollars from start to end. But the technologies that we developed along the way and the technologies that have come from industry and elsewhere have driven this number down so that if we were to redo this project today, we could probably get it done in a month for tens of thousands of dollars.

But the trajectory of technology advances just continued on this exponential pace for 30 years. And I think within the next 10 years, we can easily get it to under a day and very likely, you know, this mystical $1,000 genome. In addition to this economic benefit of making it more affordable and more scalable, but I think that in the process of moving in that direction, we're giving the research community time to study these sequences and balance it with the benefit, going back against that statement that this is junk DNA. Now providing the genomic community and the research community with these sequences for the first time, hopefully they'll see why it's so useful to have this type of comprehensive variant scan. Interesting. Dr. Philippi, I know that part of this research is that your team has mapped some missing pieces in the Y chromosome.

What was missing and why is this such a big achievement? So in the papers that are coming out this week, we actually didn't describe the Y chromosome. The particular cell line that we chose to sequence initially has two copies of the X chromosome. But in the year that followed since we completed that genome, we moved on and got a different cell line that had a Y chromosome and replicated the same effort for this particular one. There's 50% missing in the current Y chromosome reference to date from the Human Genome Project in 2004. A lot of that is this highly repetitive DNA that Karen was mentioning earlier. And it's important for the same reasons that we've completed the rest of the genome.

This is filling in all of the missing pieces in the puzzle. And now we can look at those regions and identify the variance and understand the functional consequences of the sequence in those regions. And the Y chromosome has passed paternally, right? So that's quite important to know about. Yeah, that's correct. It's commonly used in genealogical studies because of that fact, used to build family trees, and anybody that's used 23 and me and any of those other services will have benefited from that. Karen, why is this part so important? Or the Y chromosome. I think that's a a huge question. I think we haven't quite figured out what it means to lose a giant amount of tandem repeats that exist on the Q arm, half of the chromosome. I do know that as we age, you know, these parts of

our genome that are typically repetitive, they change in the way our cells regulate, how they're organized. And over time, the Y chromosome is sometimes lost. So it does offer some new insight. Something about these particular sequences and all the proteins that are binding to them present a huge unknown for people to start thinking about what it's doing in the cell and how it could be influenced with the gain or loss of a Y. On these very large tandem repeats on the Y, it's not just those tandem repeats that we've added, but also added a number of genes nearby and around those tandem repeats, increasing not only the sequence content, but the genic content of the Y as well. Adam, you started this project to complete the human genome back in 2018, but the second half of the project, the computational end, took place during the pandemic. And in fact, a big breakthrough happened right in the middle in the spring of 2020. Can you tell us about that?

Yeah, we were in many ways fortunate that all of the sequencing and lab work that generated the data for this project happened before the pandemic. And so we were sitting on that data spring of 2020 when COVID-operated. break hit. And postdoc in my lab, Sergey Nirk, who was leading the computational analysis, came to me with kind of the early look at that data, trying to assemble it for the first time. And when we looked at it, you put it up on his screen and showed me, and kind of everybody in the room saw that and thought, wow, we actually have a chance at succeeding here. It gave us the clearest picture we had seen to date. And so we kind of rounded up all of our colleagues that were experts in this genome assembly process and worked over the course of that summer for about three months and didn't really expect to complete every chromosome. I would have been happy

if we just got five done. But come August, all of it had snapped together. We had all of the chromosomes complete and ready to validate. And it was just a tremendously exciting summer and gave us something positive to focus on during those difficult times. Do you think that everyone working from home and focused on this project, because it was a worldwide project, right? Did that perhaps get you to where you could get to the endpoint faster? Yeah, it's always tricky to speculate, but I usually have a busy travel schedule, and I was home in my basement every day working,

and so I definitely could focus a lot more on the work at hand and not be diverted with all of these other administrative tasks, travel tasks. And also just all of the collaborative tools, you know, we were on Slack and Zoom all day long, talking to each other kind of constantly. It definitely helped make progress fast. I'm Ira Flato, and this is Science Friday from WNYC Studios. Dr. Miga, how many scientists internationally worked on this project?

Well, when we look over our author list in the main paper, I think we're approaching 100 scientists in total. When we started, it was just more or less Adam and myself asking, is it possible and just starting to sequence and work together? But when we opened the consortium, it was really a grassroots effort. It was an open door. Anyone could join. and we soon had contributors from around the world. Now, I understand you published the Complete Genome

via a preprint server last summer. Are scientists already using it in the lab? For sure. I mean, we've had hundreds of people already download our preprint, siding our preprint. So I think that this is really demonstrating the utility of our work and the fact that there's going to be new discoveries

that will be made and announced in the future. This kind of philosophy of the group to be very open and inviting to everyone has given us those new directions. And it's also given us a lot of confidence that what we're looking at is correct, because we've had, Karen mentioned, hundreds of people looking at every corner of this genome over the past three years. And that gives us a lot of confidence that we've done it correctly. I remember when the project was first announced 20 years ago when they talked about, hey, we've got the human genome figured out. The scientists were saying, well, wait a minute, that was actually the easy part. The difficult, the real work is going

to come into figuring out what the functionality is and how we apply it. Dr. Philippi, do you think this is where we are again? What comes next? Yeah, exactly. We've spent 20 years, you know, digging into what was produced by the Human Genome Project and have just scratched the surface of that. And now we're at this again, where we've got another 8%. And we've been looking at the same parts of the genome for the past 20 years. So this represents a new 200 million bases to be investigated. And so, yeah, we're starting over with this. It's a brand new unknown sequence, and the same excitement will repeat itself now in another 20 years of digging into this new sequence. And Dr. Amiga, what do you hope this new sequence will bring?

I hope that these new sequences will bring some new insight into what these repetitive sequences are contributing to in terms of how our cell functions. how it contributes to cell identity in early development and how it contributes towards human disease. I think that there's so many open questions here. And I think they've just had a roadblock because of the lack of a reference genome. And in fact, many scientists and researchers around the world probably already have data now that they could just map to our reference genome without even doing another experiment and start to find new discoveries and new information, just because they've been ignoring it for so long.

it's so hard to portray for we on this side of the microphone to portray the excitement that must be going on with scientists who have completed this. Would I be correct in assuming that? Well, it's been the most exciting point of my career, for sure. Same. This is really great. This is the most joy I've had in my career. Thank you both for taking time to be with us today. And I hope you'll come back and tell us

about these exciting times when they happen. Happy too. Thanks so much for having us. Yeah, if you're still here in 20 years, Ira, we'll be back. It's a date, okay? We'll meet here. Thank you both for the work you're doing for taking time to be with us today. Karen Meaga, assistant professor of biomolecular engineering, associate director of the UC Santa Cruz Genomics Institute, and Adam Philippi, senior investigator, head of the genome informatics section at the National Human Genome Research Institute that's at NIH in Bethesda, Maryland. When we come back, it wasn't that long ago that we didn't know if there were any social, called exoplanets out there, orbiting distant stars. Well, this year, the number we've found passed

5,000 more on the search for a life and why we're thankful for science right after this break. This is Science Friday. I'm Ira Flato. We're looking back at some of the science stories of this year that we're thankful for, and several of those stories have us looking toward the skies. It was a big year for space exploration, including the search for life elsewhere in the universe. You know, today, it's a given that there are faraway planets orbiting other stars, but as recently as 1992, there were no known exoplanets. In the last 30 years, however, NASA's exoplanet archive has grown, and earlier this year, that number surpassed 5,000.

In March, CyFrise John Dan Koski talked about the hunt for exoplanets with Jesse Christensen. She's the NASA Exoplanet Archive Project Science Lead at IEFRIZE. IPAC Caltech in Pasadena, California. Welcome to Science Friday. Thanks so much for being here. Hi, John. Thank you for having me. So why is this milestone such a big deal for you?

Oh, it's exciting for a number of reasons. One is that a celebration, right? We tried for so long to find planets. And as soon as we started finding them, we started realizing they're everywhere. So reaching a number like 5,000 just feels like real validation of the field. Like we've worked so hard. Hooray.

Another thing that's exciting is what we can do. do with all of those planets, like the questions we can answer now about how planets form and evolve and migrate. And it really is very interesting that we've got this big number now. We can learn so much. So how much do we know about each of these 5,000 at this moment? Yeah, it turns out most of them, we don't know very much. What we know about most of them is their rough size, and we know how long it takes them to orbit their star, so their year. And that tells us how hot the planets are. If you're very close to your star, then you're very hot. If you're far away, then you're very cold. So we know their rough size and their rough temperature.

That's it for most of the 5,000. I want to take a step back and talk about how you actually find a planet that could be dozens of light years away. Just explain it to us what it is you're looking for. So the most successful planet hunting technique that accounts for two-thirds or more of the 5,000 planets is called the transit method. Now, what we're doing with that is we're actually just monitoring the brightness of the star over and over and over again for years at a time sometimes. and we're just waiting for little dips in the brightness because that means a planet could have orbited in front of that star and blocked some of the light.

So if you see these dips, then you look at the star with other telescopes and you confirm that you're actually really seeing a planet. So what are the other techniques? If that accounts for most of the planets that you've seen, what else have you used to find these planets? Yeah, so there's also something called the radial velocity method or the Doppler method. This relies on the fact that planets, as they orbit their stars, are actually tugging on their stars,

the same way the stars are tugging on their planets. So for instance, in our solar system, Jupiter is actually dragging our sun around the middle of the solar system on a roughly 12-year orbit. So when we look at other suns, other stars in the sky, we can actually see them wobbling as well. So then you can measure from the size of the wobble

and the duration of the wobble how big a planet must be to be pulling on the star in that way. We've also found planets using direct imaging. So that's for the very nearby planets. If you're very careful, you can block out the light from the star and actually search around the star for nearby faint glowing blobs, basically, which turn out to be hot young planets. And another successful technique is microlensing,

which relies on relativity, basically, that everything with mass bends space time, planets bends space time. So if you carefully monitor some stars, you can actually see them warped by the planets that orbit between us and the star. Wow, so some very direct methods and some pretty indirect methods in there in terms of how you find planets. Yes, exactly. So you say 5,000 confirmed exoplanets. What exactly does it take to confirm one and how confident are you in all of those 5,000?

Right. So there's two ways to get a planet into NASA's exoplanet archive. One is to measure its mass and show that it's truly a planetary mass object. It's not big enough to be a star. And you can use the radial velocity method for that, for instance, or the microlensing method. Another is to show statistically that it's much more likely to be a planet than anything else. So you look at all of the other possible scenarios that could have created this signal in your data and you rule them all out one by one. You say it couldn't be a background star. It couldn't be an instrument glitch. And then when you have odds of better than a hundred to one that the signal that you see is a planet, then we say, okay, you've statistically shown it's a planet and it can go in the archive. If we found about 5,000, what does this tell us about how many there actually are out there? Is there any way to extrapolate these numbers and say, okay, well, we found 5,000 of them. that must mean we have X number of more planets out there to find. That's the really overwhelming part of reaching this milestone, because we've really only searched our local solar neighborhood. We've only really looked around us in the galaxy.

So if you extrapolate over the hundreds of billions of stars just in our Milky Way, that means there's likely tens of billions of planets. How many places like our own solar system with a range of planet types and sizes have we found? You know, that's really interesting. we might be more unique than we would have expected. When we look at planetary systems around other stars, what we see is that they mostly have similar planets around them. So a star will have a lot of small planets or a lot of big planets.

And it's actually not as common to see a mix like we have in our solar system. We're still extending our observations so that we can test that, but at the moment we really think that our solar system might be an uncommon arrangement of planets. So is there an average planet in your collection? You've said that around many of these stars, you'll see planets that are often of a similar type. Are you finding a similar type of planet amongst these 5,000? The most common kind of planet we've found is actually a surprise because it's not a kind of planet we have in our solar system. We call it a super Earth, and it's up to two times as big and ten times as heavy as our Earth.

And that's really interesting because we don't have one, so we don't know what they're like. We don't know whether they're big rocks. We don't know whether they're little ice giants, like scaled down Neptunes. So they're a big mystery, but they seem like they're the most common kind of planet that we found so far. Is there something about that size range that might make it easier for us to find? Like, is there a minimum viable size of a planet that you could actually see using any of these different techniques that you use? That's a really great question. And it is very difficult to find planets as small as Earth.

NASA had a mission called Kepler, which was trying to do this and still couldn't quite, get there. In the scheme of things, Earth is really small. So it's very difficult to find them. But we know enough to be able to extrapolate how common we think Earth should be, and we still think that super-Earths, which are, as you say, easier to find because they're bigger, are more common. In terms of Earth's size, you say it might be hard to find. If we were standing on one of these other exoplanets, do you think we'd be able to see the Earth? Yes. And actually, there was a really interesting result that just came out last year where a pair of astronomers actually looked at all of the stars that could possibly see us transiting, right? You know, this geometry that I talked about where a

planet has to be lined up just right to block some of the light. We know what stars could look and see us transiting. And those stars have actually been the subject of searches from like the SETI Institute to say like, hey, if you can see us, maybe we could see you. Amongst these planets, is there an average distance from the sun and temperature? Are you finding planets of a certain temperature out there amongst their suns? That one's harder to answer because that really depends on how long we've been searching around a given star for. It's easier to find the close-in hot planets because, for instance, they transit more often. Like if you were looking at our sun, Mercury would transit much more often than Venus and more often than Earth. So we're very sensitive to the close in hot things,

and we have found thousands of close in hot things. We're still incomplete in our searches out here around where Earth is at the cooler temperatures. So it's a bit hard to say yet where, for instance, the peak of planet occurrence is and distance from the star, but that's something we're really trying to answer with our next generation telescopes. Yeah, and this is something we talk about an awful lot. There's this question of the Goldilocks zone, the distance from a star that would allow allow a planet like Earth to support life. How many of those are out there of the 5,000 do you think? That's the million, billion, trillion dollar question. So so far we haven't found any planets like the Earth in the Goldilocks zone of a star like the sun. We have found planets like the

Earth in the Goldilocks zone of much smaller, much cooler stars called M dwarfs. And actually most of the stars in the galaxy are M dwarfs. So it might be that habitable Goldilocks zone real estate is common throughout the galaxy. But there's a big question, which is can planets around M dwarfs, which are a very different kind of star than our sun, actually support life? And we don't know the answer to that yet. Are there certain types of stars around which you find more planets than others? Yeah, these M dwarfs actually seem like they're really, really good at making rocky planets, for instance, which was a surprise, because I think a lot of us expected that bigger stars would make more planets. You know, they're starting

from a bigger amount of gas and dust, the bigger protoplanetary disk with material to form planets. But it seems like maybe those bigger stars, you know, they put out a lot of radiation, maybe they blow a lot of the gas and dust away, and they're not as efficient at turning that into planets. Whereas small stars seem like they're very good at converting their protoplanetary disks into actual planets. So we see many more planets around small stars than around big stars, which is a surprise. Amongst your colleagues and the people who do this work, Are there differences of opinion on how you define a planet or how you should define a planet? That's a very timely question.

Just earlier this week, the International Astronomical Union, famous for demoting Pluto in 2006, decided to put out a proposal for what the definition of an exoplanet should be. So an exoplanet is a planet around another star. And we're already fighting about what the definition of planet is around our own star. So we haven't really settled yet. There is debate. And what I'll say is that the different archives online that try and keep track of these things, we all have our own criteria that we've kind of settled on scientifically and politically,

like this is our box that we're going to fill. So there are definitely different criteria. There is not consensus yet about, for instance, if you have a planetary mass object just free-floating in the galaxy, is it a planet? Because it's not orbiting a star, but it's planetary mass. So that's one of the open questions right now. Do you call that a planet or not? Interesting.

Okay. So what are you most excited about right now in exoplanet research? What's the next big thing? Well, everyone is really excited about James Webb. And one of the most exciting things about the James Webb Space Telescope is that it'll give us the ability to find out so much more about the planets. So remember I said at the start, we really only know their sizes and their temperatures. So James Webb will let us peer into the atmospheres and onto the surfaces of these planets to look for things like clouds and structures and surfaces and surface materials and. compositions. It's super exciting. They'll turn from just like dots on a plot into real 3D world with data associated with them, which I'm really excited about. The NASA Test mission is flying right now. That's NASA's current planet hunting mission. It's doing an all-sky survey for planets transiting the very brightest stars. And those planets will end up being the targets for James Webb for further characterization. So yes, test is a very prolific mission, which is being very successful

right now that I'm also working on. I'm talking with Jesse Christensen. She's the the NASA Exoplanet Archive Project Science Lead at IPAC Caltech in Pasadena, California. I'm John Dankowski, and this is Science Friday from WNYC Studios. Do you have a favorite exoplanet? That's like asking a mother to choose between her children. Between her 5,000 children. Between her 5,000 children. My favorite is always the next one, right?

Like people are like, you know, it's 5,000, don't you have enough? And it's not just a number, right? It's not just 5,000. Every one of these is a whole new planet, a new world. Like, think about the diversity just in our solar system, right? And each world has personality and has features and is different. So of these 5,000, you know, they're all amazing and incredible and rich worlds that I just can't wait to learn more about. And then personally, there's a system called K2-138, which I love. It was found by citizen scientists. So that's people just like your listeners at home on this. their computers looking through NASA data and helping us find planets. It's got six planets around it. And the inner five planets play music. They're in a resonance. And they actually, if you put that to music, it actually plays like twinkle twinkle little star. It's really sweet. That's so amazing. It's just amazing to think about, isn't it? Yeah. It's so much fun. It's so much fun. So if you were to go to one of these planets to take an up close look, though, in do tests and learn a little bit more about. Is there one of these planets that you think about an awful lot?

Yeah, so one of our big holdouts is Kepler 452B. So I mentioned that Kepler was our planet hunting telescope that we were trying to find Earth-like planets with and we just couldn't quite get there. And Kepler 452B is as close as we got. So we think it's about one and a half times the size of the Earth, which is where we start to get worried that it's not going to be predominantly rocky with a thin atmosphere anymore like Earth, but something more like a scaled-down Neptune, so an ice giant sort of thing.

So we're not sure if it's rocky. So that's one mystery I would love to solve just to get there and to know whether it's rocky. We know it's in the habitable zone of a star like the sun, but the other mystery about Kepler 452B is whether it's actually there or not. So the signal we see in the Kepler data is a lot like a type of noise that we also see in the Kepler data. So either it's the very closest thing we found to an Earth-like planet with Kepler, if it's rocky and has a thin atmosphere, or it's not there.

So I would really love to go there and solve that mystery because it has plagued us for a decade at this point. Do you think that we will double this number of exoplanets, triple it? I mean, how many more exoplanets will we find over the course of, I don't know, say your career? Oh, I, you know, hundreds of thousands is the prediction. So for instance, NASA is launching the Nancy Grace Roman Space Telescope in five years or so. And it's going to do a survey of the center of the galaxy where most of the stars are. And it's expected to find 100,000 planets just on its own. Planets are everywhere. That's the big discovery of the last 30 years. It's everywhere we look, we see planets. And so as our technology and our instruments improve, we're just

going to find more and more and more interesting planets. And I'm hoping we find more earth-like planets. Almost all the people I would think who are listening to Science Friday right now get really excited about the idea of a search for new life, a search for new planets. But there are a lot of people who say, look, we've got a lot of problems here on Earth. Why are we spending so much money and time and all these bright scientific minds looking for planets that will never be able to get to? And I'm sure people say that to you too. What do you tell them? Yeah, so usually I have two answers. One is that the question of, are we alone is one of humanity's most fundamental earliest, oldest questions in this whole vast universe and all of its potential

and all of its possibility, could it possibly just be us? Are we alone? That's such a huge question to answer. And the second is, if we can study the planets around us, we'll work out what our fate will be. You hear that in five billion years, the sun will expand to become a red giant and expand out to the orbit near Earth. And then after that, it's going to slough off its outer layers and become a white dwarf, which is just a little cooling ball of carbon and oxygen, and they're just going to cool forever. And there's been a lot of open questions about what does that mean for Earth? Like, what does that mean for us? What's our fate? And one of the discoveries of the last five years is planets around white dwarfs.

It's possible to survive that phase of a star going into a red giant and becoming a white dwarf. And that means that the trillions of years that our sun will spend as a white dwarf, it might be possible that there's a second stage of planet life after this red giant phase. So I think that's a really important question as well, not just like where did we come from and are we alone, but where are we going? That is a good place to leave our conversation. Jesse Christensen is NASA Exoplanet Archive Project Science Lead at IPAC-Caltech in Pasadenaena California. Congratulations on this milestone and thanks so much for joining us. Thank you. It was a pleasure.

SciFri is John Dan Koski in an interview recorded this March. Oh, as of today, that Exoplanet list has reached 5211 to be exact. And that's about all the time we have for this week. If you missed any part of the program or you'd like to hear it again, subscribe to our podcasts or ask your smart speaker to play Science Friday. You can say hi to us on social media, Facebook, Twitter, Instagram, or email us, the classic question. SIFRI at ScienceFri.com. Oh, and if you're looking for a gift idea for a SciFri fan in your family, head to sciencefriety.com slash store to get your holiday shopping done.

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