Andres Almeida (Host): On September 17, 2025, NASA announced that the number of exoplanets, planets outside our solar system, tracked by NASA has reached 6,000. In the three decades since the groundbreaking detection of exoplanet 51 Pegasi b, the first confirmed planet orbiting a Sun-like star, astronomers have concluded that exotic worlds are everywhere.
These worlds come in all shapes and sizes, from hot Jupiters to rocky Earth-like planets. And they’re expanding our understanding of what’s possible in the universe.
So, how do exoplanets get discovered and what does it mean for astronomy? And how do researchers determine if a planet might be in the habitable zone of a star, an area called the Goldilocks zone. Here, the conditions could be just right for liquid water to exist on an exoplanet’s surface. Life on Earth started with water.
Let’s talk about it on this episode of Small Steps, Giant Leaps.
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Welcome to Small Steps, Giant Leaps, the podcast from NASA’s Academy of Program, Project and Engineering Leadership, or APPEL. I’m your host, Andres Almeida.
Today we’re joined by Dr. Eric Mamajek, deputy chief scientist of the Exoplanet Exploration Program at NASA’s Jet Propulsion Laboratory in California.
Andres Almeida (Host): Hey, Eric, thanks for being here.
Dr. Eric Mamajek: Thanks for having me.
Host: So, we’re now at 6000 confirmed detections of exoplanets and counting. How has our perspective of the universe changed since 51 Pegasi b in 1995?
Dr. Mamajek: So, it’s been a remarkable 30 years. I’d say, you know, there’s a few things we know now that we didn’t know 30 years ago, back when we were just finding the first planets. I think the first big overall discovery is planets are basically ubiquitous. Star Trek was right: There’s planets everywhere.
[Laughter]
And it’s hard to believe, a generation ago, we didn’t know that. I think that was one of the key findings from the NASA Kepler mission. Statistically, most stars have, have planets, and there’s probably, you know, perhaps, trillions of planets in our galaxy.
The other thing is that Earth-like planets, and I have to be careful with the term “Earth-like.” I’ll say “Earth-size” planets receiving similar amounts of light from their star seem to be common, and something between perhaps 10 to 50% of stars seem to have planets that are similar in size and mass as the Earth and orbiting where the surface temperatures would be temperate. And given a proper atmosphere, they could have liquid water, and who knows? Perhaps the conditions for life.
But that, that is another finding from Kepler, that statistically, the planets, at least similar in gross characteristics to Earth, seem to be common.
And I’ll say, just the diversity of worlds has been incredible. I don’t think people foresaw…I’d say hot Jupiters were a surprise except to a few early theorists who were thinking about migration of planets and disks (they were right).
You know, we’re finding planets on extremely close orbits. We’re finding planets on extremely distant orbits. We’re finding planets, you know, small rocky planets orbiting, you know, in less than a day, around their star, and their surfaces are probably completely molten. Kepler also showed us that planets intermediate in size between the Earth and Neptune appear to be extremely common, and perhaps the most common type of planet in our galaxy, and we don’t have any in our solar system. I’d say that’s a surprise.
You know, the field is taking off where we’ve gone from, you know, very little knowledge about planets around other stars to all these great findings just in one generation. And the nice thing is this field is continuing to surprise. So, you know it’s ripe for more discovery.
Host: It’s fascinating you said in our own galaxy, there are trillions of potentially planets. And knowing how many galaxies there probably are, that’s, that number is exponential, huh?
Dr. Mamajek: Yeah, the latest numbers I’ve seen, our galaxy probably has something like 100 to 200 billion stars. And so, we’re probably talking of order a trillion or trillions of planets, just, just extrapolating from the statistics for exoplanets that we know from surveys like Kepler and from transit work and microlensing.
Host: So what methods were first used to discover the first exoplanets, and what’s advanced since then?
Dr. Mamajek: I’ll start with the very first one, which produced the planets that have survived with irrefutable evidence. And that was pulsar timing.
Back in the early ‘90s, astronomers were using the Arecibo radio telescope doing timing of pulsars, and they found that, you know, a pulsar was moving back and forth in a sinusoidal fashion, but actually a complicated fashion. It was suggesting multiple planets. So, a pulsar is a neutron star, so something about 1.4 times the mass of the Sun, down compressed to the size of a city and comprised of neutrons.
And they rotate very fast, and they have strong magnetic fields, and they beam radio radiation at us, and they basically can act like clocks. And you can find periodic signals, if other periodic signals on top of the pulsing and the pulsar.
So, that was, that was actually, I’d say, a big shock in the early ‘90s. You know, that was a discovery that not only there are planets, but there’s planets around very odd objects. We thought the planetary system would have been destroyed around a neutron star that had formed from a supernova.
Shortly thereafter, the first radial velocity detections were taking place, and that was 51 Pegasi in October ’95. I remember because I was an undergraduate. It was, it was a big deal. And, and that one was a Jupiter-size planet orbiting a Sun-like star on a period of four days.
And that was remarkable, because the, the common wisdom at the time was other solar systems are probably going to look like our solar system, and Jupiter and Saturn, they formed far out where there was more ices. And we were expecting the large planets to be, you know, far out, and perhaps rocky planets close in. And I think 51 Pegasi, the first discoveries, right, the pulsar planets, and 51 Pegasi b, that was already telling us that we were going to start seeing some really weird cases that we were not suspecting. It wasn’t clear that our solar system was a typical outcome of the star and planet formation process.
So, radial velocity was, was very, took off in the late ‘90s. There was a lot of large planets found orbiting nearby Sun-like stars. And then the first case of a hot Jupiter that passed in front of a star was found around 2000 and that was HD 20945 8b. That was a big deal, because now we knew for sure these really were planets. There was some controversy. Could these things be, you know, pull-on stars that were tugging on other stars, or brown dwarfs or something? And with 20945 8b, it was clear that, no, it was something the size of our Jupiter that was passing in front of a star. And then you could do things like measure densities of planets, right? You’ve got the size and you’ve got the mass. So now we’re starting to get constraints on the composition of planets.
And so that opened up the world for transit observations. And now transits have found thousands of planets. We found there was a lot more hot Jupiters found from the ground using the transit technique, finding these 1% dips in the light of, of sun-like stars.
And then if you go to space, you can do even better photometry, even better measurements of the stars’ brightness, and so that allowed the detection of smaller planets around many more stars. Hundreds of thousands of stars could be surveyed, and they have yielded thousands of planets.
So, the NASA Kepler Mission found thousands of planet candidates, many of which have been validated or were detected through other methods. And the TESS mission that’s, that’s up there now, is continuing to find hundreds of new planets. So, the transit technique remains, remains active, and they’re finding, you know, many cases of small planets.
And the, the other one that was took off in the 2000s was microlensing. If you stare at a wide field containing, you know, many hundreds of thousands of stars, perhaps like towards the galactic center, you will get cases where the stars and their planetary systems will gravitationally lens another star, and you’ll see a magnification in light. And so there’s been many micro lensing detections found from ground-based surveys. And the NASA Roman telescope is going to do this on a on a wide scale in the, towards the galactic center, in the years ahead.
And if you were living in the 1980s and you were going to say, what technique do you think will yield the first planets around other stars? I think most people would have put their chips on astrometry…
Host: Huh!
Dr. Mamajek: …detecting, you know, the perturbations and the position of stars. And that was not to be, astrometry. There was attempts in the 20th century to find planets through astrometry. It simply wasn’t doable from the ground. It’s just, it’s, it’s, you don’t have the needed accuracy.
If you go to space now, if you get to the micro arcsecond level, or 10s of micro arcseconds level, hundreds of micro arcseconds level, you could start to detect giant planets around stars. And so the European Space Agency Gaia mission is, is starting to help with finding perturbations of stars due to giant planets. So, space astrometry is just starting to take off with the Gaia mission and hopefully other other missions in the future.
Host: Astrometry, that’s similar to how Neptune was discovered. Is that correct? Where they found a tug?
Dr. Mamajek: Yes, that’s, that’s one version of it. Yeah, from the perturbations on Uranus’ orbit in our system. Yeah, you’re looking for gravitational perturbation.
Host: In this case, it would be looking at stars.
Dr. Mamajek: Yeah, if you look at the position of a star in the sky, if a star is all by itself, it’s on its, its own happy orbit around the galaxy, you’ll see a wobble back and forth due to the parallax, due to, due to the Earth’s motion around the Sun, you’ll see a reflex motion. And then you’ll see a net motion across the field, a proper motion, just due to the differences in the galactic orbit of that star and our Sun.
But on top of that, you can see little bumps and wiggles if there are gravitational perturbations due to, to, due to orbiting bodies. People have done this detecting stellar companions for a long time, but now, now the technique is getting accurately enough, accurate enough you can measure planets.
Host: So, what are some of the challenges in separating false positives in the data, all that noise?
Dr. Mamajek: So, I just mentioned some of the methods. For example, with radial velocity, you know, both with radial velocity and astrometry, you’re looking for perturbations on a star. And radial velocity, you’re looking for the radial perturbations on the radial motion. For astrometry, you’re looking for perturbations on the position of the star in the sky.
And you know, you can get false positives from stars tugging on other stars. You know, if their orbits are aligned just right, they could produce a perturbation that looked similar to what you’d expect from a planet.
And that was, that was one of the reasons there was, there was some skepticism on the first radio velocity planets in the early ‘90s. And in fact, one of the, one of the famous early discoveries, HD 11476 2b, that was thought to be an 11 Jupiter mass companion discovered in the late ‘80s, that ended up being a star orbiting another star in a nearly pull-on orbit. So there’s some famous cases of those false positives.
With transits, the big contamination is you’re looking for dips in the light of a star. One of the weird cases you can have is an eclipsing binary star, so one star passing in front of another star, and if that system is much fainter than another bright star next to it, it could produce a signal that, without lights all smeared out, the whole thing could look like one star that was having a little, a little dip in its light due to a planet. And instead it was, it was actually a faint binary, binary star next to a much brighter star.
So there’s active observing campaigns that were vetting the Kepler targets and now vetting test targets, searching for these contaminant stars. So, all the discoveries you’re hearing though, from the TESS mission, they’ve been imaged with high-contrast imaging like speckle imaging or adaptive optics on large telescopes. And they can say, “Yes, the light appears to be from that star,” or there’s some faint stars around, but they don’t appear to be bright enough to be contaminating the light.
And there are some weird cases where we don’t know which star is responsible for the transit. The test pixels are very large. They’re about 20-some arcseconds. So, there are some cases where it’s a little bit ambiguous which star actually has the transiting planet.
Host: When you were talking about Earth-size planets, that makes me think of the Goldilocks zone. Are most of these Earth-size planets in that zone?
Dr. Mamajek: So far, very few of them are in that zone. And we’ll talk about a few different ways. One is thinking about it in terms of how much light the planet receives from its star.
So, for stars like the Sun, if you have an atmosphere somewhat similar to Earth, you know, some mix, perhaps, of nitrogen, carbon dioxide, oxygen. You know that basically the, the amount of light would be something like 95% to about 170% of the amount of light we get from the Sun, which, right now is about a kilowatt per, per square meter.
So that’s been, that’s been called the quote, unquote, habitable zone, or Goldilocks zone.
And very few of the planets actually are in that zone. And then a lot of the ones that are found are large. You know, they’re Jupiters or Neptunes. They may have large envelopes of gas, envelopes, atmospheres of hydrogen, helium, and so they probably don’t have a proper, you know, surface or oceans, as we think of them. They’re probably under very high pressures, more analogous to, you know, the Jupiter and Neptunes in our system.
So, I actually, I’ve been tracking the small habitable zone planets. And, you know, as we’re hitting 6,000 known exoplanets. The count of small, rocky planets orbiting within the habitable zone of stars stands at about, I think it’s around 46 now (ballpark).
So, it’s, it’s a, you know, it’s a pretty small number of the ones we found so far, and most of those are around red dwarf stars. So, they’re not similar to our Earth. In fact, very few of them, maybe one or two of them, are around stars more similar to our Sun. It’s just easier to detect the signals of a small planet around a small star than it is to detect a small planet around a big star.
So, there’s a lot of cases, like, for example, the nearest star, Proxima Centauri, has a Earth-size planet in its habitable zone. But that’s planet has probably undergone a very, very different evolution than, than our, our Earth has. It goes around its star every 15 days. It’s received a lot more stellar winds and flares from this active red dwarf star. It’s tidally locked. It, I mean, for any for some basic assumptions about the composition of the planet, the age of the star, the system is almost certainly tidally locked. You know, one face of the planet is facing the star all the time.
So, we know of dozens of, of Earth-size planets orbiting stars and where their temperatures might be temperate, but they’re mostly around red dwarfs.
Host: What characteristics do researchers look for for habitability?
Dr. Mamajek: The starting point, I would say, is, you know, how far from the star does it, does it orbit? You know, we can measure the luminosity of the star, and we can calculate how far it orbits the star and how much radiation it receives from its star. So that’s probably a starting point.
The techniques are usually sensitive to either usually mass or for the transit technique, radius, so we can, we can get an idea of the size of the planet. Is it able to is it able to retain an atmosphere? Or it’s not too big, if it retains too much atmosphere? The planets bigger than about one and a half times the radius of the Earth seem to be a different type. They seem to be the so-called sub-Neptunes that probably have thick gaseous envelopes. They’re more analogous, probably to Neptune than the Earth. So, that’s a starting point.
You know, in the future, for example, one of the, one of the projects NASA is just getting underway now, is the Habitable Worlds Observatory. You know, we’re hoping to actually launch a space telescope, perhaps by 2040 or so, that will be able to image Earth-size, temperate Earth-size planets around nearby stars, and measure their spectrum. And once you have the spectrum, then you can say something about the chemistry of the gasses in its atmosphere.
And so, if you’re looking at the Earth, you’ll see gasses like oxygen and water, carbon dioxide. If you go to the UV [ultraviolet] you start to see ozone.
And the Earth, as we know it, has changed over time. The Earth has had very different atmospheres in the past as evidenced from the geological record, and so people have been trying to construct, well, what would an Earth, even a planet that had an evolution similar to ours, what might it have looked like when it was only a billion years old, a couple billion years old, as opposed to 4 billion years old now?
So I, I’m really looking forward to the, you know, when we’re starting to get more detailed spectra. And also, you know, for example, James Webb Space Telescope, that is a NASA and ESA effort. We got to remember, there are, there’s opportunities for, for international cooperation on these very large telescopes.
So, you know, there’s certainly, people were eager to publish their results, even, even when the results are, you know, you might consider boring, right? Where some of the recent results with James Webb Space Telescope of close-in rocky planets, they’re looking for atmospheres. And so far, some of them have been consistent. The spectra have been more or less consistent with a line as if it was just a rock, which they could be, or they could just have very, very, you know, dense atmospheres of heavy gas.
Host: I think that’s fascinating, though, because even 10 years later, you might look at the data again, and…
Dr. Mamajek: Yeah, I mean, that’s, there’s always opportunity for doing, you know, research on archival data. There’s, there’s a lot of great stuff that can be found in these, these old data sets. Once, you know, once the missions launched, it’s taking data. People think the results just come out right away. And there’s a lot of, there’s a lot of data that come from these missions, a lot of opportunities for analysis and discovery.
For example, you know, the WISE mission. NASA WISE mission was imaging the sky in the the infrared. There’s, you know, there’s efforts that are still finding, you know, faint, warm sub stellar objects just free-floating in the galaxy.
Host: So, there are ways for the public to get involved in citizen science. Can you explain how?
Dr. Mamajek: There’s Backyard Worlds finding basically there’s so much data, that it actually helps to get citizen scientists involved and looking for faint, moving infrared blips on the sky. And some of these have turned out to be very interesting objects, sort of intermediate in scale between planets and stars.
Exoplanet Watch is another one. One of the interesting cases is that when you have a big, expensive telescope like James Webb Space Telescope, you want to make sure that you’re pointing at the planet passing in front of its star, passing behind its star, and you want to know the exact time so that you can plan the observations you want. You’ll want data before the, the transit and after the transit.
The ephemerities for these orbits can go stale, right? They’re discovered, somebody you know will quote an orbit, and that orbit has uncertainties, and the longer time goes on, those uncertainties can add up.
And you know, we’re certainly getting, you know, additional epochs from TESS, but TESS doesn’t cover the whole sky, and there’s opportunities for measuring some of these transits from the ground. If you know which star it is, and you know roughly the right time, you know you can do a ground-based observing campaign to detect additional transits, and then that tightens up the, the orbit and cuts down on the uncertainty. You don’t want JWST looking at a star and realize that it was, you were actually hours off the time of the transit.
Host: Ugh, heartbreak! Eric, what was your giant leap?
Dr. Mamajek: I don’t know if there was so much giant leaps as, as, you know, there’s multiple steps and opportunities, and I’m thankful for those. I was an undergraduate at Penn State, and I was able to work on radio astronomy projects and spectrograph lab projects with Eric Feigelson and Larry Ramsey, professors at Penn State. Those were great opportunities. I was able to do a master’s in Australia and a Ph.D. in Arizona.
In Australia, one of the things that really shook me, that made me realize there’s just a lot of discoveries out there to be made was shortly after I arrived in Australia for my masters, the data that I was analyzing revealed a there was a new star cluster within 100 parsecs, which I found amazing. Like, how was this missed? This is the Eta Chamaeleontis cluster, a new group of about five-or-10-million-year-old stars in the constellation Chamaeleon.
And I found that amazing that new objects that nearby could be found in the data sets. And the trick was combining X-ray data with astrometry, and all of a sudden, you see this group of X-ray emitting stars close together on the sky co-moving. So, that turned into several papers. I started working on, you know, young stars and protoplanetary discs and debris discs, and then brown dwarfs. I went to University of Arizona, and started collaborating with some of my colleagues. Matt Kenworthy has been a longtime collaborator of mine. It’s turned into projects and now involving imaging planets around star, young stars and, and transits, finding interesting objects transiting stars.
So, I’d say the other one was J1407 b, it was this ringed object that we published around 2010. That was a really weird object, something about an astronomical unit in size, but had very complex ring structure, similar to Saturn’s rings. We still don’t know what that object is. It’s some type of sub-stellar object that has a really complicated ring structure around it. It’s probably a rogue planet. It doesn’t appear to be orbiting its star right now, otherwise we would have detected it.
So, there’s been some, there’s been some interesting discoveries along the way, and they’ve turned into interesting directions for future projects.
Host: This has been fascinating. Thanks for sharing everything about exoplanets and all your work. Thank you, Eric.
Dr. Mamajek Hey, thank you!
Host: That wraps up another episode of Small Steps, Giant Leaps. For more on Dr. Mamajek and NASA’s exoplanet research, visit nasa.gov/podcast and click on Small Steps, Giant Leaps. There’ll you can find a full transcript of all our episodes including links to the topics mentioned today. You can also check out our other podcasts like Houston, We Have a Podcast, Curious Universe, and Universo Curioso de la NASA. As always, thanks for listening.
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