With the December 2021 launch of the James Webb Space Telescope, one of the most expensive and ambitious scientific initiatives ever attempted commenced operations. Now that the telescope has been successfully deployed in its unique position in space, its advanced instruments will be able to gather data on questions that scientists once could only dream of answering. Is there life on other planets? How do supermassive black holes mold the mass in their galaxies? JWST may soon be able to tell us.
In this episode, host Steven Strogatz speaks to two researchers leading JWST’s observations of our universe: Marcia Rieke, the principal investigator of the telescope’s Near-Infrared Camera, and Nikole Lewis, an astrophysicist studying planets outside of our solar system.
(For more on the JWST and the history of its construction and launch, read Natalie Wolchover’s article, “The Webb Space Telescope Will Rewrite Cosmic History. If It Works,” which was recently honored with a 2022 Pulitzer Prize for Explanatory Reporting).
Steven Strogatz (00:03): I’m Steve Strogatz, and this is The Joy of Why, a podcast from Quanta Magazine that takes you into some of the biggest unanswered questions in science and math today.
The James Webb Space Telescope has arrived at its destination — about four times farther away than the moon, almost a million miles away from Earth. At a cost of $10 billion, it’s the largest, most complicated space telescope ever built. It spent much of January 2022 unfolding its mirrors and components so that it can get into position to give us a glimpse of the early universe. The telescope is expected to change astronomy as we know it, and the excitement around it is almost palpable. There’s a whole generation of astronomers who have been waiting for the Webb telescope. Some say it has the potential to open up whole new fields of science.
(00:54) But how exactly will it do that? How will the Webb telescope change astronomy and cosmology? What might it reveal about our universe and its history?
Our first guest today is Marcia Rieke. She’s the principal investigator on the James Webb Space Telescope’s Near-Infrared Camera. Marcia is an astronomer at the University of Arizona in Tucson. She’s one of the pioneers of infrared astronomy, with a decades-long career that includes work on the Hubble Space Telescope and the Spitzer Space Telescope.
(01:26) Later, we’ll be talking with Nikole Lewis, an astrophysicist at Cornell University. Nikole specializes in exoplanets. And, particularly, exoplanet atmospheres. Exoplanets are planets that orbit stars outside our solar system. Nicole will be using data beamed from the James Webb Space Telescope to look for conditions that just might support life on exoplanets. But first, Marcia Rieke. Thank you so much for joining us today.
Marcia Rieke (01:56): I’m very happy to be here.
Strogatz (01:57): I’m really excited to talk to you. I mean, you have done such beautiful work over the years, such important work. And actually, on this theme of the Webb Space Telescope — it’s unusual that there is so much public excitement. I mean, so much media coverage about it. Help us understand why is this so exciting? Why is this new telescope so important for astronomy and cosmology?
Rieke (02:20): I think the excitement has derived from being the successor to Hubble, and people have gotten used to lovely Hubble pictures, great discoveries from Hubble. And the whole structure of the Webb telescope project is to go beyond Hubble but to continue the excitement and even increase it. Because we’re opening up a new part of the electromagnetic spectrum. That is, opening up a new part of the set of wavelengths that light can have, to study. And we’re doing it with a telescope bigger than Hubble, and that works at much, much longer wavelengths. And so, we’re going to be seeing quite a new view on the universe that will tell us many, many new things, including some we probably aren’t guessing right now.
Strogatz (03:08): The things that we can’t even imagine maybe, just yet. Many people imagining a telescope might be thinking of one of those giant telescopes sitting in an observatory on a mountaintop somewhere. But this is a space telescope, the Webb. Just remind us, why is it so advantageous to — and really, why would you go to the bother of putting a telescope in outer space?
Rieke (03:28): Well, we go to the bother of putting a telescope in outer space because the ones on the ground have to look through the Earth’s atmosphere, and that limits what we can do. And in the case of Webb, we want to observe infrared wavelengths, so the telescope needs to be cooled. And if you tried to cool a telescope on a mountaintop on Earth sufficiently to see these dim, faint, distant things, the atmosphere, water, etc., would freeze out on the telescope, and it wouldn’t be usable anymore. So we need to go to the vacuum of space. And that lets us cool the telescope and it also removes any absorption by the atmosphere.
Strogatz (04:10): Let me just think about the Hubble for one second before we dive in on Webb. Many people would remember, I think, that there was this absolutely gorgeous image taken by the Hubble, really breathtaking image that’s called the Hubble Deep Field, that lets us look at thousands of galaxies in one snapshot. But, I have to admit, when I saw that picture, it just, it looked very pretty. Lots of amazing-looking galaxies tilted in all sorts of different angles and different colors and shapes. But I think I might have missed the point of what we really learned from that Deep Field image. What was so exciting about that image to you and your colleagues?
Rieke (04:48): Oh, what was exciting was that we found galaxies much further away than we’d ever found before. You have to understand that astronomy, when looking at very distant objects, becomes a kind of time machine, because it takes light so long to travel to us. So, what Hubble did in that Deep Field image is reveal galaxies where the light took, hmm, 12 billion years to reach us. And if you know the age of the universe, you know that that means that we were looking at these galaxies when they were less than 2 billion years old. The fact that galaxies that we could identify as having stars existed at that time was very surprising. No one knew that that would be the case.
Strogatz (05:36): It’s really a beautiful, vivid use of language you’ve got there, this time machine analogy, and it really is literally true, isn’t it? That we are, when we look at these very distant galaxies, we truly are looking back in time.
Rieke (05:49): Exactly. And that’s one of the goals behind Webb, is to go back even further in time, go back closer to when the Big Bang happened and fill in the last remaining steps of how galaxies form and evolve to be what we see as galaxies today.
Strogatz (06:07): How far back do you estimate you might be able to go with the new telescope?
Rieke (06:11): Well, we’re hoping to get within a couple hundred million years of the Big Bang. And when you remember the universe is 13.7 billion years old, that’s getting back just a tiny fraction from the start of everything.
Strogatz (06:27): Assuming it all works, these would be unprecedented baby pictures of the universe.
Rieke (06:32): That’s exactly the idea. We want to see the first steps of galaxy formation after the Big Bang.
Strogatz (06:39): So what about infrared astronomy — and your own specialty? What’s the contrast there between Hubble and Webb?
Rieke (06:46): Oh, there’s a huge difference. The Hubble telescope mirror is kept at 76 degrees Fahrenheit, which was the temperature in the lab where it was made. And that means that that, that temperature, there’s quite a bit of heat radiation given off by the telescope itself. And that heat radiation would swamp the signals from the very most distant galaxies. Because Hubble is warm, there’s never been any reason to put an infrared instrument on that works at longer wavelengths than about three, four times visible light’s wavelength, because it is so warm.
And so the Webb is designed to be both cold, and to operate at much longer wavelengths. And so it can observe things at wavelengths that Hubble can’t touch. And that’s important because the universe is expanding, and so there’s a redshift, the same kind of frequency shift that you sense when an ambulance goes by and the pitch of the siren changes. So these very distant galaxies that we’d like to detect have quite a substantial shift in their wavelengths so that what left the galaxy as visible light, shows up to us now as infrared light, and Hubble simply can’t detect them.
Strogatz (08:13): So tell us a little bit about some of the instruments that are on the Webb, and what they’re going to be looking at?
Rieke (08:18): There are four instruments in total. I’m the PI [primary investigator] of the Near-Infrared Camera, NIRCam, which will take images analogous to the Hubble Deep Field. It will do a lot of other things as well.
(08:34) Another instrument is called NIRSpec, which as you might guess, is the Near-Infrared Spectrometer. And that’s set up so that if we position one of these very distant galaxies on its input, the light will get spread into its constituent wavelengths, or colors, if you will. And so then we can do more detailed studies, such as understanding the relative composition of the stars in the galaxy, how the stars in the galaxy might be moving, a lot of interesting things like that.
(09:09) Another instrument is called MIRI, which stands for Mid-Infrared Instrument, and it works at yet longer wavelengths still. And so, for objects that might have some dust obscuring them or have really, really, really high redshifts, very distant, it might be able to detect them. It will also be great for studying exoplanets and stars that are just forming in our own galaxy and hidden by clouds of dust.
(09:43) The fourth instrument is called NIRISS, Near-Infrared Slitless Spectrometer, and that has several functions. Its most interesting, to me, is that it can take spectra of transits. That’s when an exoplanet passes in front of its parent star so that the starlight gets absorbed by the exoplanet atmosphere. And if you take a spectrum, you can then determine what is that exoplanet’s atmospheric composition. That’s exactly the kind of data that Nikole Lewis will be very interested in.
Strogatz (10:20): Let’s talk a little bit about you personally and your group. You have, how much time with the Webb telescope yourself for the observations you would like to make?
Rieke (10:29): My team has a total of 900 hours.
Strogatz (10:32): Is that a little or a lot?
Rieke (10:33): Well, the telescope can be observing for about 8,000 hours per year, so I would say it’s quite a lot.
Strogatz (10:42): Yeah, it sounds like it. It’s a big fraction of the whole year? Okay. And what are you going to be looking at?
Rieke (10:48): We have a quite diverse program. We are going to do a very deep survey. We’re going to do two fields, the original Hubble Deep Field, and another very well-studied field called the Ultra Deep Field that was done with Hubble, and also Chandra X-ray Telescope. And then the next biggest chunk of time will go to observing transits of exoplanets, to start understanding their atmospheres.
Strogatz (11:14): How do you do a deep field image?
Rieke (11:17): What you do is lay out a pattern where you want to point the camera so that you cover enough area. So you take an image, step, take an image, take another image, and so on. Actually, when we’re taking an image, we’re pointing at one place for maybe 20 hours. We’ll change the wavelength we’re observing, observe another 20 hours, and go through all nine filters that we’re using. And then we’ll go to the next position. And for the biggest part of the Deep Survey we’re doing, we’re going to take over 7,000 separate images that will get combined together, to make the kind of images that you saw from Hubble, of a deep field.
Strogatz (12:02): I think I understand what you’re saying. I mean, I’m remembering, as a child, the idea of like, just sort of opening up an old-fashioned camera, with a very long exposure, just pointing at the sky. Is it something like that? You spoke of pointing for 20 hours in one direction?
Rieke (12:17): Right, but we don’t take a single 20-hour exposure, because there’s an effect in space called cosmic rays, which are actually protons boiling off the surface of the sun, for the most part. And when they fly through an infrared detector, they make a little bright light trace. So what we do, to avoid having those ruin the picture completely, is, we’ll expose for about a third of an hour. We’ll then, metaphorically speaking, close the shutter. And then we’ll open it again, after having read out the image, and take another one and do that repeatedly. And we’ll probably change the pointing just a little bit in between each exposure so that we can average over some bad spots on the detector and so forth. Our main deep survey is comprised of over 7,000 images.
Strogatz (13:13): And so this will then, the hope is, let us see back to the earliest days ever. Could you give us a little background on what’s the standard picture for the origin of galaxies, stars? Like, is there something analogous to the standard model for particle physics? Do you have a standard model for galaxy evolution or star formation or all that?
Rieke (13:36): Well, there’s certainly a kind of commonly accepted framework right now. So there’s the Big Bang, the universe starts expanding. Initially, all the material in the universe, the gas, which is mostly hydrogen, helium, and a few other little bits, is expanding and cooling. And about 400,000 years after the Big Bang, it cools enough that the separate electrons and protons combined to form hydrogen atoms. The electrons basically go in orbit around the hydrogen. That’s when the cosmic microwave background was emitted, is when that happens.
(14:15) As that gas cools further, we know that there are some — what we might think of as lumps or clumps in it. It’s not perfectly smooth. There’s a little bit of structure. And where there’s a little bit more hydrogen gas, that gas will have a slightly higher gravitational field and will cause the gas to clump together. And as more and more clumps together, eventually it gets to the point where there’s enough material that it can start forming stars. And then, those stars will be the constituents, eventually, of galaxies. But there’s some parts in this that we’re not real sure exactly how it works, partly because we know there’s dark matter in the universe, and how the dark matter clumps might work to channel the hydrogen into clumps that would form galaxies is — there are a lot of missing parts in that theory.
(15:12) But something makes everything clump together, stars form, and then they become galaxies that we can detect. Once we get to about 500 million years after the Big Bang, we have a pretty good idea of what the galaxies look like from Hubble. And we know that there’s a kind of galaxy evolution sequence where, there’s, some of these small galaxies that form initially merged together to form bigger ones. Others stay more isolated, and the stars just evolve. Some of them keep enough gas and are rotating or whatever else might influence the star formation conditions, so that they don’t necessarily form all their stars at once.
(15:58) And there are still even gaps in this part of the picture. Because we know around us today, there are sort of two main classes of galaxies. There are spirals, like the Milky Way, which typically still are forming stars and have a lot of gas and dust around. And then there are galaxies that we call ellipticals, they look a little bit like fuzzy footballs in pictures. And for some reason, those galaxies take all of their gas and form the stars very early on, and we say that their star formation is quenched. But why there’s this difference between these two categories of galaxies is still not at all completely understood. We have some ideas, but, again, observations from Webb will be very helpful in filling in the blanks in these models.
Strogatz (16:51): What about black holes? Or the so-called supermassive black holes? Are we going to be getting some information about their role in this story?
Rieke (17:00): Oh, indeed. In fact, there are a number of guest observer proposals to study that very question. And why it is that we see today this very tight relationship between the size of a black hole at the center of the galaxy, and the total mass of the galaxy, even though that mass is distributed over areas far bigger than the black hole. Why this relationship exists is one of the outstanding problems in black hole and galaxy astronomy right now.
So when we do these deep surveys, one of the virtues of doing this in the infrared and so on is that we will be able to have a better idea of whether or not there might be a black hole in the centers of some of these galaxies, and then we’ll be able to do more studies relating what we see as a potential black hole, probably best revealed using NIRSpec, with the other properties of the galaxy best revealed by NIRCam and MIRI. And so we’ll be able to see if this relationship between the size of the black hole and the size of the galaxy holds all the way back in time. Or if it takes a while to get established.
Strogatz (18:18): Let’s go over that. A lot of interesting science in there. First of all, you’re referring to a relationship between the size of the black hole and the size of the galaxy itself.
Rieke (18:28): Right, and when I say size here, what I’m actually referring to is the mass. So, a black hole, a massive — supermassive black hole might have a mass of, say, 100 million times the mass of the sun. It’s in a galaxy that is probably more like 10 billion times the mass of the sun. And regardless of whether it’s a 1 million solar mass black hole, or a 10 million one, or any place along the spectrum of sizes, if you then go and measure the mass of the galaxy surrounding that black hole, it’s almost always exactly the same ratio. So if you take the mass of the black hole and multiply by a number, like — I forget the exact value, it’s between 100 and 1,000. Then you know the mass of the galaxy that it’s residing in, and that says automatically that there’s some process that keeps those two in lockstep, and what that is, is not obvious.
Strogatz (19:35): What is the point, that its gravitational influence is mainly focused right near itself?
Rieke (19:40): That’s right. Yeah, so if you, for example, look at the black hole in our own galaxy, which has a mass of about 4 million times the sun. It dominates the gravitational field in the galaxy only for a distance of maybe 10 light-years from the center, and yet the galaxy itself is 50,000 light-years across. So how does something in this tiny region influence the whole rest of it?
Strogatz (20:13): You’re saying maybe we would find that that relationship could hold even back to the earliest galaxies.
Rieke (20:19): Yeah, that’s the question. Because at what point does this relationship start getting established? And that would tell you some interesting things about galaxy evolution. And there’s a chicken-and-egg problem. Which came first, the black hole or the galaxy?
Strogatz (20:36): If I can ask you now to reflect on your career — I mean, you got started in the ’70s. Did you ever expect to see what the James Webb Space Telescope is expected to see?
Rieke (20:48): No, because when I started out, infrared astronomy was quite primitive, by our standards now. We didn’t have a light sensor that could take a picture. We had a single pixel, so to speak. And to make an image, you had to measure a spot, move the telescope, measure the spot, and you had to kind of step your way around the sky. And that tended to mean that, unless it was a very interesting source, we mostly studied just one spot on an object. So for most galaxies, we’d study the center of the galaxy and not much else, because it was too hard to do otherwise. And so the idea that I can take an image where there’ll be thousands of galaxies in that one picture, it’s like, I can’t believe it.
Strogatz (21:36): I’m sure it is very personally gratifying for you, it must be, to be still going strong in your career, and have this kind of ability now to see things. You must be dying to wait for those first images to come back.
Rieke (21:49): Oh, I hope I don’t — the very first images that will be of us lining up the telescope or we see one star 18 times, because the segments aren’t lined up. That’s okay, I’m going to be really happy because it means starlight has gone through the telescope, and has registered in the camera, and I can’t wait for that to happen. Once that happens, I will really start to relax.
Strogatz (22:15): Is that sort of your personal nature, like you don’t want to get ahead of yourself?
Rieke (22:18): Yeah, there’s a certain element of that.
Strogatz (22:20): If it’s not too ridiculous, do you mind if I ask you about the next generation of telescopes, space telescopes? You have to think ahead, right? It takes decades to get one of these built.
Rieke (22:30): Right, and there was just what was called Astro2020, a National Academy of Sciences decadal survey that looked at that question. It turns out that I’m old enough that I probably am not going to participate in another one. But astronomers do have ambitions and they are looking at ones that are bigger and more complicated than Webb even. But they won’t get started until the 2030s.
Strogatz (22:35): Can you give us a rough description of what they would be like, and what they would look for?
Rieke (22:59): There were two different kinds proposed to the decadal survey. One is kind of like Webb on steroids. It’s called LUVOIR, L-U-V-O-I-R. And its goal is to prove definitively that we found an Earth-like planet capable of hosting life. And the telescope mirror would be more than twice the diameter of Webb’s, and its sunshield would be enormously larger. That’s one place where I have some skepticism because it was already hard enough to test Webb’s, but anyway.
(23:43) Its rationale would be to study exoplanets and find evidence of life. Another strategy uses a smaller telescope but puts, not a sunshield, but a starshield out in front of the telescope at a distance maybe 20,000 miles away, to block out the light from a star to be able to let the telescope take images of the planets orbiting it. That’s an interesting idea as well, and so you can kind of see that both of these telescope projects are aimed at studying exoplanets in greater detail than you can with Webb.
Strogatz (24:23): Let me just thank you for this really delightful conversation, Marcia. Thank you so much.
Rieke (24:28): Thanks for having me.
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Strogatz (24:53): My next guest is Nikole Lewis, an exoplanetary scientist and assistant professor at Cornell University. Nikole and I actually work together and know each other. We co-advise a Ph.D. student and a postdoc together. But this is actually going to be my first time talking to her about her science, aside from our advising of students, and I’m really looking forward to it.
(25:16) Nikole looks at exoplanet atmospheres in particular, and atmospheric processes such as cloud and haze formation. Nikole co-led a spectrographic survey of the TRAPPIST-1 system in 2018, using data from the Hubble Space Telescope. This was the first such survey for Earth-sized exoplanets. And, just to give you an idea of how new all of this is, the first exoplanet was only discovered in 1995. With the James Webb Space Telescope, Nikole hopes to get a closer look at exoplanets and their atmospheres. Thanks for joining us on The Joy of Why podcast, Nikole.
Nikole Lewis (25:54) Great to be here, Steve.
Strogatz (25:55): I’m pumped. I’m really excited to hear about planets, and — I mean, you would describe yourself as a planetary scientist, I called you an exoplanetary scientist, but —
Lewis (26:04): Yeah, I mean, my training is as a planetary scientist, that’s what I have a Ph.D. in, and again, as you mentioned, the field of exoplanet science is so new, that people couldn’t decide where I lived. My planetary science brethren that study solar system objects are like, “You’re an astronomer!” And my astronomer brethren are like, ”No, you’re a planetary scientist!” So, I just made up a new category of exoplanetary scientist.
Strogatz (26:25): Just for a second, since I never took a course in planetary science, or certainly not exoplanetary science, as kind of a layperson in your area, it seems to me that this must be a golden age for you. Because for a long time, like ancient people could only see something like five planets with the naked eye. And then, throughout my childhood, I was taught there were nine planets. But, I mean, now you have something like 4,500 plus planets to think about and look at.
Lewis (26:54): Yeah, it certainly is the golden age, and I like to tell people, I really got lucky. I entered into my Ph.D. program in 2007, and there were probably about, maybe, 20 exoplanets known to date, at that point. And then I’ve just sort of rode the wave since then, I mean, we’ve been sort of exponentially increasing the number of known exoplanets ever since I got into the field. So it’s a great time to be doing this.
Strogatz (27:17): It’s astonishing. I mean, it’s like the era of discovery, like the analogies are kind of silly, like the whole universe has opened up. Well, yes. What a time to be a planetary scientist. So, let’s start at the beginning, then. We’ve been saying “exoplanets,” but what are they?
Lewis (27:32): Yeah, so, an exoplanet is actually any planetary body outside of the solar system. And actually, as a fun story, by the IAU, which is the International Astronomical Union definition of a planet, exoplanets are not in fact planets. Much like Pluto’s not a planet. But they are objects that are roughly the size of planets in our own solar system, that just happened to orbit other stars.
Strogatz (27:53): So they’re not — that seems kind of strange to say they’re not planets. But all right. Why are they so interesting to people like you?
Lewis (28:00): They’re interesting because they provide us different data points, basically, for understanding, actually the planets in our own system. And, in particular, Earth, right? If you look at our own solar system, we have one planet where we know that life came to be. We have a neighboring planet, Venus, that’s roughly the same size. You’ve got two smaller, rocky planets that are almost airless. Mars has a very tenuous atmosphere. And then you’ve got these big giant planets out there.
(28:25) And so if we’re trying to put ourselves in context, and try and understand, like, how did we get here, and are we alone? We really don’t have a large enough sample, and so the really exciting thing for me is to have this sample of 4,500 planets to play with, that are in all different regimes and actually a much broader range of sizes, compared to our own solar system planets. So it’s a great way to get out there and study physics and chemistry in ways that aren’t possible in the solar system.
Strogatz (28:52): A sort of analogy occurred to me as I was walking over today. That in the old days, if like, if you were a biologist, and you only had nine animals to study, nine types, compare what that subject would be like compared to having 4,500 types. The animals standing in for the different kinds of planets that are out there now.
Lewis (29:10): Yeah, exactly, and, you know, the first exoplanet that was discovered was a Jupiter-sized planet that orbited quite close to its host star, called a hot Jupiter. And people for a long time just wouldn’t believe it. They’re like, “That’s not where gas giant planets live. They live far away from the star.” So, it’s been a real revolution in our understanding of what planetary systems look like.
Strogatz (29:28): Let’s just underline something that you said there, when you mentioned rocky planets and giant gas planets. What do you mean by a rocky planet? What do you mean by a gas planet?
Lewis (29:38): Yeah, so, when I think about them I actually think a little bit about their atmospheres, and that’s primarily how I study them. In our own solar system, all the planets, the ones close and far away, formed from the same nebula that, actually, our sun formed from. So we’re all birthed from the same cloud of gas and dust. And what happened over time is, the planets that were farther away from the sun, were able to gobble up a bunch of gas and ices. And that’s how they became big. And the planets close in didn’t have access to this icy material. So they couldn’t get very big. And in the end, their initial atmospheres were in fact blown away, and they actually grew atmospheres out of stuff that was sort of outgassed from their planetary surfaces.
And so that’s how you end up with this, sort of, small planets, that are close in. And we consider them rocky because they weren’t able to incorporate ices into their planetary material. And then things that, in our solar system happen to be farther away, that were able to gobble up a bunch of hydrogen and helium during the formation of the solar system.
Strogatz (30:39): You mentioned that there was this astonishing discovery of Jupiter-sized, or kind of roughly that size, enormous planets close to their star — when you’re talking about exoplanets — which seems insane, that they could be that big and that close.
Lewis (30:54): Yeah. And so, the detection techniques that were used to find the very first exoplanets around, sort of sun-like stars, were the same types of detection techniques that have been used to study binary star systems for years. So over half the stars in the sky are actually in binary or multiple star systems. We’re kind of weird in the fact that we don’t have another star that shares an orbit with our own. And so those techniques were used to just look for ever-increasing smaller and smaller things.
And so here, what people saw was a signature of a star that clearly had something roughly the size of Jupiter tugging on it. And when they did the calculations, they said, oh, well, then it has to be in this, basically a four-day orbit. And that just blew everybody’s mind. Again, in our solar system, Mercury is on an 88-day orbit. This planet, not only just for a gas giant, just for any planet, is exceptionally close to the star.
Strogatz (31:49): So you’re blowing my mind in all kinds of ways here. You say, typically, a star will have a companion star, that binary is more the rule than the exception? Did I hear that right?
Lewis (31:58): That’s right. More than half the stars in the sky are in a binary or multiple star system setup. And that’s because you know, the clouds of dust and gas from which stars and planets form often will actually create more than one star.
Strogatz (32:11) So, it’s really giving me this impression, that I suppose I should know from the history of science, which is that we have a very parochial view. You know, like, we think it’s gonna be the way it is here, when we look elsewhere. And it isn’t. I mean time and time again, in the history of science, that’s turned out to be wrong.
Lewis (32:29): Yeah, very much so. I’ll talk about one of the biggest discoveries that happened, probably in the last 10 years, with the Kepler Mission, is that we found out that most of the exoplanets that we’ve discovered to date actually are in between the size of Earth and Neptune. And those size planets don’t actually exist in our solar system. Most solar system formation models didn’t predict something of that size forming. They’re just like, something the size of Earth or smaller should form, and then something, basically, the size of Neptune or larger should form. And so that sort of blew open, very wide, our understanding of how the solar system formed and the fact that we are actually not the normal thing that we see out there in our galaxy.
Strogatz (32:41): Here, I can’t resist trying to make this vivid for anyone who hasn’t thought about these things before. The place where we both live, Ithaca, New York, has this fantastic scale model of our solar system that you can walk through. So everything is 5 billion times smaller than it really is, in this scale model that we call the Sagan Walk. And on that scale, the Sun looks about the size of a dinner plate. Mercury looks about the size of, like a tiny grain of couscous. And the Earth and Venus are sort of like a little pea, very little. And Jupiter is something like about a brussels sprout, maybe. And so when you say that there’s something in between Neptune and Earth, I’m picturing like, a good size pea. Maybe.
Lewis (33:51): I don’t know, maybe a lima — I guess it’s not round.
Strogatz (33:53): A lima bean? Okay.
Lewis (33:54): A lima bean. In between a brussels sprout and a pea. Yeah, so, you know, most of the planets in our galaxy, and we assume in other galaxies, are actually somewhere in between the size of a brussels sprout and a pea on this scale.
Strogatz (34:05): If you’re okay with this. I don’t want to force you into my vegetable analogies here.
Lewis (34:10): No, no, it’s fine. I’m just trying to think of a good vegetable that’s in between the size of a brussels sprout and a pea.
Strogatz (34:16): Like one of the smaller grapes on the bunch.
Lewis (34:18): Yeah, maybe. And we call them, actually, mini-Neptunes and super-Earths. Basically, we just adopted, they’re somewhere in between these two things. So they’re either super-Earths, or they’re mini-Neptunes and so, sure, super-Earths and mini-Neptunes are the size of a grape on the scale of the Sagan Walk.
Strogatz (34:33): Yeah, just in case anyone’s trying to picture this and you haven’t experienced the Sagan Walk. So, it could take you, like, five minutes to walk from one planet to the next, once you’re in the outer part of the solar system. Do we know anything about the exoplanetary systems’ orbital distances from their stars?
Lewis (34:47): Yeah, that’s the other interesting thing is that we’ve actually found — most of the planets we’ve found to date orbit a lot closer to their stars than the planets in our solar system. And part of that is an observational bias, just based on the techniques that we have, but nearly all of the planets that we’ve discovered to date orbit within a year, or what’s called one astronomical unit of their host star, and that means you could have anywhere between one to seven planets actually crammed into the same distance between the Earth and the sun.
Strogatz (35:15): Okay, so if I could ask you for a kind of blanket statement about what do we think we already know about exoplanets? And what do we not know?
Lewis (35:23): That’s a big question.
Strogatz (35:24): Yes, I realize it’s a big question.
Lewis (35:25): Yeah, I mean, I think what we do know about exoplanets is that they’re abundant. So I will use that. For every star in the sky, there’s certainly at least one planet. And so, we should expect that most stars that we look at have planets orbiting them. And you know, part of what the Kepler Mission was actually trying to do was to measure something called eta-Earth, which is the frequency of Earth-size planets in Earth-like orbits around sun-like stars. And unfortunately, we weren’t able to run the Kepler Mission as long as we would have liked to, but we started to get a handle on, sort of that frequency. And it does still seem that Earth-size planets in Earth-like orbits around sun-like stars are not as frequent as we had hoped. But there are literally planets of all shapes and sizes everywhere in our galaxy and also in our universe.
Strogatz (36:11): What about this system that you have taken a special shine to, the TRAPPIST-1 system? Tell us a little about that, and why you find it so interesting.
Lewis (36:19): The TRAPPIST-1 system, it’s been quite a ride. Again, it’s one of these systems that no one expected to be there. And in fact, the team that discovered it had decided that they were going to look for planets around these very, very cool stars. And a lot of planet formation theory was like, there shouldn’t be planets around these cool stars, they’re too small, they’re almost — are so small and cool, they’re actually the size of Jupiter, that they’re kind of on this edge where they might not be able to fuse hydrogen and helium in their core, so therefore, they wouldn’t really be a, you know, a stellar object.
(36:50) Most of the surveys out there had focused more on sun-like stars, stars that are like our own, or, you know, just little variations thereof. And so this team decided, “We’re going to look for planets around these very ultra-cool stars.” And when they went out there with, actually, some fairly small telescopes and started their survey, they kind of hit a gold mine right away. They found this system; they actually were able to see two distinct indications of planets around this star. And then what they thought was a third.
(37:19) This was amazing. Like, first of all, a lot of people were like, “Oh, planets shouldn’t form around these small, low-mass stars.” And they already knew there was two, probably three. And then they were able to use what’s — the Spitzer Space Telescope, which I started my career on, and was retired in 2020, to go and look at the system for a really long duration. And when they did that, they were able to uncover there were in fact, seven Earth-size planets orbiting this star that’s basically the size of Jupiter.
(37:46) These planets are so close to their stars. So again, the furthest one out is basically in a 20-day orbit. And again, remember that Mercury is an 88-day orbit. So it’s basically this whole, like, scaled-down, size of the solar system. But here, you actually have seven planets that are size of Earth, right? Where in our own solar system, we have two planets that are size of Earth, Earth and Venus. And then, we have big gas giants, and then some smaller, rocky ones. So it was just mind-blowing again. That’s what the data were saying, but everyone’s like, how the heck did this happen? But what it did is that nature gave us an opportunity to study seven Earth-size planets, all in one system. Just imagine if there were seven Earths in our solar system, how much better we’d understand, you know, how we came to be.
Strogatz (38:32): So, let’s start talking about what the Webb Telescope can do for you and for humanity and science. Should we start with the TRAPPIST system? Are you going to try to point the telescope at it?
Lewis (38:43): Yeah, and I’m not alone in pointing the telescope at the TRAPPIST system. All of those planets in the TRAPPIST system, so all seven of them, will be observed. The team that I’m leading will be focused on the planet TRAPPIST-1e, which happens to be the one that’s sort of smack in the middle of what’s called the habitable zone of the system. It’s where we would expect, if liquid water could exist on the surface, then it probably can exist on the surface of TRAPPIST-1e.
And so, what we’re gonna do is actually just try to get in there and figure out if this planet has an atmosphere, and if it does, what’s that atmosphere made of. Because there’s still a lot of people who, again, feel that these small stars, which actually are quite long-lived, but tend to be very active, so they send out lots of flares. And we know from, you know, our sun that those flares can be very powerful, and it actually can strip planets of their atmospheres. And so, that’s what we’re doing. All of us are going in and trying to find signatures of atmospheres on these planets, and that will help us to understand their ability to not only have liquid water on their surface, but potentially to support life.
Strogatz (39:46): When you try to look at atmospheres of these seven Earths, or whatever we’re going to call them, Earth-like planets. What is the technique that lets you make inferences about their atmospheres?
Lewis (39:56): What’s interesting is we do not directly image the TRAPPIST planets. By directly imaging, I mean, for a long time, and thinking about the solar system, we thought that the only way we’d be able to study exoplanets is if we were somehow able to suppress the light of the star such that we could see the faint planet next to it. And it turned out, because we found so many planets that orbit so closely to their host stars, we could take advantage of a technique called the transit method. And this is where the planet actually passes in front of the host star, as seen from Earth. And we, of course, in our own solar system, from Earth, see Venus transit the sun, and Mercury on occasion.
(40:32) And in this configuration, what happens is that as that planet is passing in front of the star, that starlight gets filtered through any atmosphere that the planet may have. And so what we actually end up seeing is these signatures of different chemical species absorbing the starlight, as that light passes through the planetary atmosphere. We’re also able to study light emitted directly from the planet, in the same way. Basically, we’re not looking for the planet in front of the host star, but we’re actually looking for when the planet ducks behind the host star. So when the planet disappears, it allows us to then measure how much of the light from the system was coming from that planet.
(41:08) People, planets, we all emit predominantly infrared light. We reflect a lot of sunlight. That’s how we sense the world around us. But, and the same reason why you would get, you know, night vision goggles to find people out in the forest at night. You can use that same type of technology, but here, we’re going to put it into the James Webb Space Telescope.
Strogatz (41:27): What kinds of things can you learn from these infrared measurements? Or do you hope to learn from the Webb’s infrared measurements?
Lewis (41:33): Infrared wavelengths is actually where most molecules absorb. That’s just, kind of how they bend and wiggle, is they interact strongly with infrared light. And so if you want to study molecules, you want to go to the infrared. Also, if you want to study light emitted from a body that’s, you know, basically room temperature, again, you want to go into the infrared. And so that’s why Webb is very, very powerful. And we can’t access those wavelengths of light from the ground, because in fact our own atmosphere absorbs them. So we can’t look through the atmosphere and see them. So that means we need to put something up in space. And we had the Spitzer Space Telescope, which is really in many ways the precursor to the James Webb Space Telescope, up until 2020. But it was designed during a time when we didn’t even know exoplanets existed. And so it was not optimized to do these types of studies.
Strogatz (42:22): And would you say that the Webb is, I mean, it was built — well, I suppose it was started — was it even started before we knew about exoplanets?
Lewis (42:29): I mean, it was started after we knew about exoplanets. But one of the nice things is — and I will say this as a positive — because of all of the delays that actually happened with the James Webb Space Telescope, there were able to be retrofits put into the facility to make sure that it could do extraordinary exoplanet science. One instrument in particular has an entire, what we call mode, devoted to doing these transiting exoplanet type observations. And all the instruments on the James Webb Space Telescope now have operational modes that are specifically designed for observing exoplanets.
Strogatz (43:03): I’ve heard the phrase “follow the water” used in this connection. What does this mean?
Lewis (43:09): Yeah, I mean, when we think — and again, it’s a very Earth-centric view, right? When we think about life, and life needs a solvent, and water is a great solvent. We do the same thing in the solar system, we follow the water when searching for life, and this is why we’re looking for life in the oceans that exist, say, below the icy surface of Europa or Enceladus. And so we know that where there is water, we have this nice place where you can start to make the soup that’s necessary for life to emerge. And so that’s largely what we’ve been doing in terms of the search for life outside of the solar system as well. We’re looking for planets that are at just the right distance from their stars, such that we think they might have temperatures, surface temperatures, that could sustain liquid water.
Strogatz (43:53): Okay, like in the most optimistic scenario, if we were observing life, we wouldn’t be observing life directly, it sounds like. We’re observing life’s impact on the atmosphere. Is that the idea?
Lewis (44:02): Yeah. The fact that life exists on Earth — we cause havoc in our own atmosphere, our atmosphere is actually out of equilibrium compared to what it would look like if, you know, humans and trees and everything else didn’t exist. And so what we want to do is measure the amount of key atmospheric molecules like water and methane, oxygen, and ozone, and maybe carbon dioxide. And by looking at how much of each of those molecules are present in a planet’s atmosphere, we’re able to then say, is this system in equilibrium or out of equilibrium? And if it’s out of equilibrium, could it be that life is causing this disequilibrium?
Strogatz (44:02): Of course, that would be the most amazing thing ever, probably, found in the history of science, right? If we could find convincing evidence of life somewhere else. I can’t think what would be a bigger discovery than that.
Lewis (44:52): Yeah, I mean, it would be huge, but I think we’re all cautious about this, because there are also lots of other processes where light can interact with molecules and cause disequilibrium. And so, lots of people have been trying to run these false positive scenarios. How often if we see disequilibrium can we be 100% sure that it is in fact from life? And so there’s a lot of ambiguity out there. And so, there will be a lot of discussion in the scientific community, when we start to see these signs of disequilibrium, whether or not it’s because of life, or if it’s because of some process on the planetary surface or even in the atmosphere.
Strogatz (45:26): So, personally, though, what is it that you find most exciting about the possible things that you might see, once the Webb Telescope’s making its observations?
Lewis (45:35): So, one of the exciting things for me is, and it’s actually how I started my graduate career, is studying weather on exoplanets. And the weather on these other worlds is extreme. You have winds that are on the order of a kilometer per second. You have temperature differences between the day and the night sides of these planets on the order of 1,000 Kelvin. So, quite a huge difference between the day and the night side. And for me, trying to understand what these planets look like, you have to understand sort of what their weather looks like, right? So, you can make measurements and be like, yes, there’s water in this atmosphere, and there’s carbon dioxide, and maybe there’s some methane, but that doesn’t really paint a clear picture of what that planet looks like.
(46:14) And so, I’m very much focused on observations that are going to help us to be able to visualize planets in three dimensions. And so we’re not going to be able to take pretty pictures of these planets, that’s just not within the capability of Webb or any facility on the horizon. But what we will be able to do is use data that’s of a quality we’ve never known before to discern what these planets actually look like and be able to paint a realistic picture for people to enjoy.
Strogatz (46:41): The Webb Telescope is so exciting, actually, in so many different parts of astronomy and space science. Leaving exoplanets aside for a minute, are there things, like, about the early universe or some other aspect of astrophysics that you find especially thrilling to think about, that the Webb might reveal?
Lewis (47:00): Yeah, I mean, the Webb was designed to look at some of the earliest galaxies that formed in our universe. And again, the way that we do that is we have to look at light in the infrared, because light from those galaxies has been what we call redshifted quite substantially over time. And so we have to keep looking farther and farther into redder and redder wavelengths to see those objects. And so Webb will be able to basically give us pictures. And, you know, I don’t get pictures of my exoplanets, but there are going to be awesome pictures of galaxies that are going to come out of Webb. And some of those pictures of galaxies will be some of the earliest galaxies that ever formed in our universe, which is exciting. I mean, I, you know, I can look at the Hubble Deep Field and I can appreciate it, I actually have an image of the Hubble Deep Field hanging in the workroom in my building. And so, you know, I’m looking forward to seeing all the types of awesome imagery of galaxies and stars and nebulae that we’ve come to love from the Hubble Space Telescope, and see those types of images from the James Webb Space Telescope. Not the same wavelengths of light, but it will still be just as awesome.
Strogatz (48:05): Well, that feels like a great place to wrap up this fascinating conversation about exoplanets and Webb Space Telescope. Thanks so much for joining us today, Nikole.
Lewis (48:14): My pleasure.
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Strogatz (41:05): The Joy of Why is a podcast from Quanta Magazine, an editorially independent publication supported by the Simons Foundation. Funding decisions by the Simons Foundation have no influence on the selection of topics, guests, or other editorial decisions in this podcast or in Quanta Magazine. The Joy of Why is produced by Susan Valot and Polly Stryker. Our editors are John Rennie and Thomas Lin, with support by Matt Carlstrom, Annie Melchor, and Leila Sloman. Our theme music was composed by Richie Johnson. Our logo is by Jackie King, and artwork for the episodes is by Michael Driver and Samuel Velasco. I’m your host, Steve Strogatz. If you have any questions or comments for us, please email us at [email protected]. Thanks for listening.