A planet where the beer is free - EWTS #027

Joe: Hello and welcome to episode two, season three of Enough with the Science. I'm Joe.

Senan: Don't mind him, it's episode 27. [laughter] He doesn't actually look at our website where the numbers of the episodes are on the website.

Joe: And I hope you've listened to all of them. They just keep getting better and better.

Senan: So Joe, today we're all about XO today.

Joe: XO?

Senan: XO, yeah. Not Oxo.

Joe: Oxo is a sauce, and XO is the name of an officer in the army or in the navy; is it an XO?

Senan: It's also older brandy; XO brandy. But it's none of those things this week; it's exoplanets.

Joe: Very good.

Senan: Right. So exoplanets, we better define them to start off. So they're planets that are not part of our solar system and are very far…

Joe: Far, far away.

Senan: Far, far, far away in fact. [laughter] And the story today is how we discovered that our galaxy is packed with millions of planets. It's a great…

Joe: Do you think we would have bothered if some of the other planets in our own solar system were habitable?

Senan: Yeah, I mean we think Mars might have been once before it lost its atmosphere, because we definitely have seen signs of liquid water on the surface of Mars, so there was seas there at some point.

Joe: Long, long time ago.

Senan: Which means there was an atmosphere and so on. Now we haven't found any evidence of life there yet, but you know, hey.

Joe: While Martians wouldn't let us know that anyway. [laughter]

Senan: But here's the thing, right; from the small bit of our galaxy that we've actually surveyed, we have discovered over 6,000 exoplanets confirmed, and a few thousand more that are in the queue to be confirmed. [laughter]

Joe: They're in the middle of the application process. There's queuing there to see can they get their citizenship of exoplanetship.

Senan: And because we've only looked at like a relatively small part of the galaxy, if we extrapolate those numbers out, it means there are millions of planets in the galaxy. And that's just our galaxy; there are millions of other galaxies.

Joe: Okay. Now, why are we interested in this?

Senan: Surely some of them must have life. Just as a function of the sheer number of them that are there, you would have to think there's more than one planet in the universe with life.

Joe: Here we go. This is when here we go down the X-Files: we are not alone.

Senan: Do you know, the most amazing discovery ever would be if we discovered for sure that we were alone. We never will.

Joe: Well, we or we might actually technically be alone. [laughter] Because let's face it; no listeners.

Senan: Well, I mean, in general humans. In general humans, yeah, in general humans.

Joe: But there is something that seems to push us in the direction of yes, we're special.

Senan: Oh yeah, we like to think we're exceptional, yeah. And I mean, you know, some leaders of some countries like to think they're exceptional, too; it just seems to be a natural part of human nature.

Joe: But mathematically the odds of us being the only life or sentient life in this even in our galaxy is pretty…

Senan: Oh yeah, it would be shocking to discover that there was no other life in our galaxy. The numbers just don't support the idea that there's no other life in our galaxy.

Joe: So that's why we're interested in exoplanets.

Senan: Yeah. [laughter] But there's also some amazing science involved, because we can't actually see these planets directly.

Joe: They're invisible.

Senan: They're not invisible, they're just awfully hard to see. Our equipment isn't good enough at the moment to allow us to see them. And the reason for that of course is that you've got a planet beside a star; the star is super bright, and the planet doesn't have any light of its own.

Senan: They're both very far away, so you could be talking about the furthest away ones that we have discovered are like 100,000 light-years away, in other words light takes 100,000 years to get there.

Joe: It's a long time. That's very far away.

Senan: And that's still within our own galaxy, believe it or not. [laughter] From that distance, it's like trying to look at a speck beside a massive cauldron of fire. The analogy I sometimes use…

Joe: You sometimes use this? Dinner parties where this subject comes up? Or more specifically, when you and I are rehearsing this podcast. [laughter]

Senan: Is the idea that you're in a football stadium, one of those big ones…

Joe: No one is going to believe that. No, no, no one's going to believe that we rehearsed this podcast. [laughter]

Senan: The analogy is that you're at one end of a football stadium, it's night-time, and there's big floodlights on at the top of the stand at the other end of the football stadium. And somewhere flying around just behind those lights is a fly. There's no way you'll see that fly with your naked eye, because the brightness of the floodlights just completely washes out your vision.

Senan: Trying to see an exoplanet in space is a far worse problem than that, but that's the kind of problem it is. So we can't image them directly, so we've got to do some really innovative science to derive the fact that they exist.

Senan: We'll come to that in a minute, but first of all let's talk about the distance problem. It's unlikely any human being is going to ever set foot on one of these planets, or certainly in the foreseeable future.

Joe: Well, cover all your bases there.

Senan: I'm going to stick my neck out here and say should I ever have any grandchildren, or should they ever have any grandchildren, none of those descendants will ever set foot on one of these planets either.

Joe: And that's not just because they're your grandchildren. [laughter]

Senan: Well, clearly the fact that they're my grandchildren would give them top of the queue to get on a spaceship. And don't say why. [laughter]

Joe: I'll buy them the ticket. [laughter]

Senan: Anyway, let's just digress into this issue for a minute. So our nearest exoplanet neighbour is four light-years away. It's a star called Proxima Centauri; it's the nearest star to our Sun other than the Sun itself, which obviously is a star. And even that one, which has two confirmed exoplanets around it, is out of reach.

Senan: And that's using current technology. So the first thing is, to actually get there, current combustion rockets, the type of engines that are currently used in our rockets, would need to carry literally an ocean of fuel; literally the amount of fuel that is equivalent to the amount of water that's in the oceans here on Earth.

Joe: Right, a lot.

Senan: Because not only do you need to accelerate to get there, you need to decelerate; in other words, turn your spaceship around and face the engine towards your destination and turn on your engine again to slow you down. There's no point in accelerating up to this ridiculously fast speed and then just buzzing through the place in half a second because you forgot to slow down on the way.

Joe: I can't understand brakes; do they not have brakes?

Senan: No brakes in space! [laughter]

Joe: That's a great tagline. I don't know why it's great, it just sounds good: there's no brakes in space. We'll work that in somewhere.

Senan: Good title for a book. [laughter] So you need the fuel not only to accelerate, but also to decelerate, so that when you arrive there you can stop. And the weight of that fuel would be absolutely enormous; like you're talking about carrying tanks half the size of the planet. And that weight, which you have to carry with you, would severely limit your acceleration. Because the more weight you have…

Joe: I'm seeing the problems. I'm understanding the problems.

Senan: And the result is, even to get to the one nearest to us using our current technology would probably take in the region of 3,000 years. And we would be somehow have to acquire the amount of fuel that's equivalent to the size of our oceans.

Senan: So we need new engine technology. Maybe someday somebody might invent fusion engines that are able to use the fuel much more efficiently. There's this cool idea from science fiction back 20, 30 years ago called a Bussard ramjet.

Joe: Oh, this is science fiction.

Senan: Getting there any faster than 3,000 years is science fiction at the moment. [laughter]

Joe: Okay, 3,000 years is ETA.

Senan: And that's to the nearest one, remember. [laughter] Interstellar space, which is the space in between that you would have to travel through in between our star and theirs, is not actually totally empty. There's a handful of hydrogen molecules just floating around, drifting around there; there might be one every 10 feet or something.

Senan: So there's this idea that if you could accelerate your spaceship up to a fairly fast speed using a conventional engine, then you could extend this massive cone-shaped magnetic field out in front of you that would guide those hydrogen drifting around hydrogen atoms into your ship so you could collect them and use them as fuel. But you'd need to be going fast enough to get enough of them to actually run your engines on in the first place. And that's called a Bussard ramjet, and at least it means that you wouldn't have to carry your fuel with you, right?

Joe: Well, surely that's after wormholes and like jumping into black holes, that's somewhere along those lines in terms of dealing with the distance problem, no?

Senan: It's something like that, but at least it sounds like it might be plausible, like wormholes is very theoretical. [laughter]

Joe: But the hydrogen thing is true?

Senan: Oh, the hydrogen thing is true. Basically after the Big Bang, almost all matter was hydrogen, and so there's a lot of it left over out there in the gulf of nothingness that's in between the stars. But it's well spread out. [laughter]

Senan: And then finally, there's the spectre, the scary spectre of something called rogue planets that we've discovered. So these are a particular type of exoplanet that could be dangerous, but we'll come back to that. [laughter]

Joe: There's always doom. There's always doom somewhere.

Senan: So how do we actually detect these? Well, the first one was detected in 1995, confirmed in 1995 by two French astronomers, Michel Mayor and Didier Queloz, and they actually got the Nobel Prize for that. Part of the reason they got the Nobel Prize was detecting for sure the first exoplanet, but the other part was the type of planet it was basically upended our understanding of how planets form.

Senan: How did they actually detect it? This planet was around a star which we call 51 Pegasi. The books say the star is about 50 light-years away; I'm just wondering if the 51 and the 50 are related somehow. [laughter]

Joe: They got it wrong. They got the name wrong. [laughter]

Senan: Anyway…

Joe: So they discovered an exoplanet that's not around our nearest star?

Senan: No, it's around a different star, 50 light-years away.

Joe: And our nearest star is four light-years away.

Senan: Yes.

Joe: You'd think they'd start looking at our star, the local star first, would you?

Senan: I don't know what the thought process was, but the method they used is only suited, or at least it was at the time because of the sensitivity of the equipment they had, was only capable of detecting quite large gas giant planets, because I'll explain how it works in a minute. But our near neighbour, Proxima Centauri, the one that's only four light-years away, the two planets that have been discovered there are not gas planets; they're smaller rocky planets.

Senan: Anyway, how they discovered it, it's a thing known as Doppler wobble or radial velocity; that's the technique they used. So we need to delve into the science a little bit here. Last week we mentioned something called Doppler in relation to sound, and when an ambulance goes past you the sound of the siren changes a little bit. Well, the same effect happens with light, because light is a wave just like sound is.

Senan: Picture this: you've got a star in the middle, and you've got a fairly big planet orbiting around it. That big planet, although the star has the most gravity in that relationship, that big planet still has a reasonable amount of gravity of its own because it's a big lump of stuff together in one place.

Senan: So as it rotates around the star, it's actually tugging that star one way or the other. So when it's on the left-hand side it tugs it a bit to the left, and so on; when it's on the right-hand side it tugs it to the right. When it goes behind it from our perspective, it tugs it away from us a little bit, and when it goes in front of it from our perspective, it tugs it towards us a little bit. And that has the Doppler effect on the colour of the light coming from that star.

Senan: So we see one way we see the light shifting slightly towards the blue side of the spectrum, and the other way we see the light shifting slightly towards the red side of the spectrum.

Joe: Right, that's because…

Senan: That's because the light waves are being as the star moves away from us very slightly, the light waves get stretched, and as the star moves towards us very slightly, the light waves get compressed. So that changes the wavelength, and wavelength determines colour.

Senan: So it's a very brief, very small effect, but these guys managed to create equipment that could detect it. And then they could detect that there was a regular pattern, that switch from slightly red-shift to slightly blue-shift and back again happened on a regular, like a metronome, every so often. In the case of this particular planet it was every 4.2 days, because that's how long it takes this planet to go around its star.

Joe: That's what they say. [laughter]

Senan: That's what they say, and I believe them; they got a Nobel Prize. If it's good enough for the Nobel committee, it's good enough for me.

Joe: Someone else checked their homework. [laughter]

Senan: Imagine that, it's so close to its parent star that it's closer than Mercury is to our Sun. Like our planet Earth takes 365 and a quarter days to go round; this thing takes 4.2 days to go round.

Joe: It's moving.

Senan: It really is moving. And the other thing is, they are able to work out based on how often the wobble happens and how long the wobble takes to happen each time it happens, they're able to derive what size the planet is. And the planet is approximately 60% the size of our planet Jupiter, its weight. So Jupiter by the way is by far the biggest planet in our solar system.

Joe: So this one, 51 Pegasi b, would be the largest planet in our solar system, other than Jupiter?

Senan: Is Saturn within the 60 to 100? I'm not sure if Saturn is greater than 60% of the size of Jupiter. Anyway, the point is that a planet that size, 60% of the mass of Jupiter, could not be a rocky planet. We know that from our understanding of planet formation, so it has to be a gas giant planet.

Senan: And that was part of the reason that they won the Nobel Prize, because it upended our understanding. At the time, our understanding of how planets get formed said that gas giants can only form way out in the edge of the solar system; they can't form close to the star. So this was a revelation, that here you had a gas giant closer to its star than Mercury is to our Sun.

Joe: They must have had a choice though, at one stage, to say that this is just a really, really big rocky planet?

Senan: No, because the physics of rocky planets, the problem is that once you get enough rock in one small place, it compresses down, the gravity gets so large, because the more stuff you put into the bundle the more gravity it has. The gravity compresses it down to a really dense ball of solid rock.

Senan: That means that in the immediate area surrounding it, its gravity in the space immediately surrounding it is exceptionally strong. And that means that as it's travelling through space, it's going to collect the gas that's floating around in space and hold onto it. So what starts off as a really, really dense rocky planet soon collects its own massive cloud of gas and turns itself into a gas giant.

Senan: So that's a known understanding of how planets form, that in other words rocky planets can only be up to a certain size.

Joe: And gas giants can only form in the distant reaches of the solar system?

Senan: It's true, well, look… [laughter] I suggest you look up these guys' phone numbers and present your questions to them. [laughter]

Joe: They had to kick out something; they had to throw something out and they just decided, right, let's get rid of the fact that rocky planets can only be this size, and… Okay. No, sorry, I'm being facetious; we'll continue.

Senan: That's fine, that's why you're here. [laughter]

Joe: Absolutely, I'm doing my job. [laughter]

Senan: Right, so that's the radial velocity or Doppler wobble method. But that's not the only method. There's another one that has actually proved to be even more triumphant in terms of how many planets it has found, and that is the transit method.

Senan: So the idea is that if a planet rather passes between us here on Earth and its parent star, this is a very far away planet, that we will see a very slight dimming of the light of that star for the brief period where the planet is in front of it. Even though the planet is much, much, much smaller than the star, it still will dim the light a little bit as it passes in front of it.

Joe: So it'll change the colour and dim it?

Senan: The two can be combined, although this transit method is separate from the Doppler method we spoke about a moment ago, but yes, it is possible in some cases. The Doppler method you need unbelievably sensitive equipment to detect anything smaller than a gas giant, so it's a bit harder. But yeah, the two can be combined.

Senan: Now, this might seem like a fairly straightforward idea, until you think about the distant star, the planet that is going around it, and us have all got to be in a straight line for this to work.

Joe: Yeah.

Senan: So that means that the orbital plane of that system that we're looking at, that far-off star that we're looking at, happens to be by pure fluke lined up with us.

Joe: For no other reason than fluke?

Senan: Oh, no other reason than fluke, because the different stars, the different planetary systems that are in our galaxy, the many millions of them, the orbital plane, in other words the slice if you like through space that all the planets around that star are orbiting around, could be in any direction. They're not lined up with each other in the galaxy.

Senan: So yeah, Kepler space telescope from 2009 to 2018 looked at about 150,000 stars, and it found more than two and a half thousand confirmed exoplanets based on this transit method. Which is pretty cool. That was kind of what opened the floodgates, because when you think about it, there's a relatively small percentage of the stars in the galaxy where their orbital plane just happens to line up with us.

Joe: Yes.

Senan: And yet, even on those ones, and even on the bit of the sky that the telescope was looking at, we still found over two and a half thousand.

Joe: Right, so then to extrapolate out from that…

Senan: If you extrapolate out from that, there's millions of planets in our galaxy; there's no doubt about it. So it was a pretty cool finding what Kepler found.

Senan: There was another one called the TESS satellite which was launched in 2018, I think, and it has found a lot more candidates for exoplanets, many of which have subsequently been proved. So what tends to happen is one of these telescopes or these space satellites finds a candidate that it thinks might be an exoplanet, based on observations of what its parent star is doing. But in order for that to be confirmed, an astronomer has to actually go and analyze the data and see if it agrees with what the computer thought it found.

Senan: This one has found lots of candidates, many of which have since been confirmed as actually exoplanets, and there's a lot more candidates in the queue, as it were, waiting to be confirmed. This transit method is interesting because it also allows you to derive information about the size of the planet and how far away it is from its star and all that stuff, based on how long the dimming happens for and how much dimming happens and all that stuff; they can plug those numbers into equations and figure out roughly what size these planets are or how far away they are from their stars, and so on.

Senan: And there's another interesting wrinkle in it: in systems where they have detected more than one exoplanet, the individual planets interfere with each other's gravitational pull on the star depending on where they are in their respective orbits. Although it yields some really complicated maths trying to figure out the relationship between these different planets when all you're doing is looking at how much the star is dimming by or how much the star is wobbling by, it does allow them to derive a lot more detailed information about the size of those planets, how far they are from their star, and so on.

Joe: And it could be more than two?

Senan: Oh yeah, there could be several. We'll talk about a system later on that has seven.

Joe: Right.

Senan: As I said, we've discovered over six and a half thousand now, and there's loads of different types. So there's hot Jupiters like the one that was discovered first in 1995; there are super-Earths, as they're called, which are planets which are slightly bigger than Earth or up to maybe twice as big as Earth; there's something called mini-Neptunes, which are like small gas giants…

Joe: Okay, just now, let's go back to the super-hot Jupiter, is it? Hot super-Jupiter? How did they know it was a gas giant?

Senan: Because they could detect the mass, in other words how heavy it is, based on how often it was making the star wobble and how much it was making the star wobble. Once they could measure the mass of it, the weight of it, they knew it was too heavy to be a rocky planet. So that's kind of how they know it was a hot Jupiter.

Senan: Let's talk about the Goldilocks zone. We've tended to focus our search in recent years on planets which are in the so-called habitable zone or the Goldilocks zone, so that is the…

Joe: Where Elon Musk wants to live? [laughter]

Senan: I'm not going to comment on that, because his lawyers are better than my lawyers. [laughter]

Joe: What could possibly be wrong with that? [laughter]

Senan: Anyway, for every star there's a zone, a certain distance from the star where if planets are orbiting in that zone, they are at the right temperature to have liquid water. And based on our understanding of how life gets started, we believe liquid water is necessary for our type of life to be invented, anyway.

Senan: So those are the ones that are in the Goldilocks zone, and we have tended to focus on that because wouldn't it be wonderful to find another planet with life on it, or somewhere we could live after we wreck this one. So there's a focus on that, and in fact there's a specific satellite that's launching in a few years' time that will look at those more closely, the ones that we think are in the Goldilocks zone.

Senan: Let's get back to these rogue planets. So these are ones that don't have a star of their own; they're just floating around in the darkness between the stars.

Joe: Do you think rogue is a bit strong, isn't it? Nomad; nomad planets, or… [laughter]

Senan: I think they call rogue because the potential damage they could cause. [laughter] Remember, they've no light of their own; they're a planet, like any planet they don't have any light of their own, and they don't have a star near them to illuminate them. So it's very, very hard to see them.

Joe: Have they any idea how these things like, I mean obviously they were around a star at some point, or they were part of…

Senan: We can't say that with absolute certainty, but a lot of them probably were. In the early days of a solar system when its planets are initially forming, you get all kinds of collisions happening, and you get situations where one really large planet passes quite close to a smaller planet, gives it a real strong tug from the gravity of passing nearby, and might be enough to accelerate it out of away from its star altogether. So there are different ways that it can happen.

Senan: But it's not certain that; I mean, maybe rogue planets can form completely separate from star systems, you know? Planets generally are formed from clouds of gas and dust that start to clump together, and gravity then makes the clumps bigger until eventually a planet gets formed.

Senan: But the thing about these, you can't see them, except for how we detected them, which I'll come to in a moment. [laughter]

Joe: We actually don't know they're there. Now wait a minute! [laughter]

Senan: But the risk from them is, if one of them came barreling through our solar system, obviously the number one risk you'd be worried about is that it doesn't actually hit us. [laughter]

Joe: Yeah, that would be a that would not be good. [laughter]

Senan: But the other risk is that even if it doesn't hit us, if it's a planet it probably has a fair amount of gravity of its own. So if it passes anywhere near some of the other existing planets in our solar system, it could tug them out of their existing orbit into some new orbit, which a new orbit that maybe one day will collide with us, you know what I mean?

Joe: Okay.

Senan: Or might tug us out of our Goldilocks zone further out into our solar system where everything goes frozen, you know what I mean?

Joe: Yeah, or might just like kind of change the axis of our planet slightly so we've an endless summer in Ireland.

Senan: That would be fantastic, yeah.

Joe: Well, let's be optimistic.

Senan: Let's be optimistic, yeah. How have they been detected? Well, we've Mr. Einstein to thank for that. He predicted that gravity can bend light. Normally light travels in straight lines, but he predicted that gravity can bend light, but the effect is only really noticeable when you get an object with a lot of gravity, like a planet.

Senan: What happens is, it's called microlensing. So if you have a star that's like a far, long distance away, and then you have one of these rogue planets that's not near the star, but let's say it's halfway between us and this really far away star, but it passes directly in front of it from our perspective, the gravity of that planet is going to…

Joe: Like an eclipse?

Senan: Yeah, kind of. But an eclipse as we experience it here on Earth doesn't really involve any gravity bending light. This passes in front of the star, and the light from the star gets bent around the planet while it passes in front of it, by the gravity of the planet itself.

Senan: So when we see that happening, effectively it looks like the star has briefly gotten bigger and brighter, and maybe the shape of it has been a bit distorted as well.

Joe: But so we don't actually see the thing in the middle of the light; we see the light that's been distorted?

Senan: We see the distortion and magnification of the light of that star for the brief period while the planet is passing in front of it. And we can, based on how long that happens for and how much the light of the star is distorted, we can do some calculations about the likely size of that planet.

Senan: And using that so to spot something like that, you've got to be it's flukey lucky to be looking in the right place at the right time to spot that brief moment where one of these rogue planets passes in front of a star that's so far away.

Joe: Kind of just flashes, yeah. What was that? [laughter]

Senan: We have still found thousands of them, even though it's so flukey to find them.

Joe: Okay, maybe it's not that flukey. [laughter]

Senan: So again, if we extrapolate the numbers, we could be up in millions of these things floating around in our galaxy.

Joe: Another thing to worry about. [laughter] Rogue planets; doom, doom surrounds us. [laughter]

Senan: That's a fun one to help you get to sleep at night.

Joe: And they're travelling at phenomenal speeds, I imagine.

Senan: Oh, I would think so, yeah. I mean, if they were planets which were originally part of some solar system, and some interaction they had with another planet caused them to be accelerated fast enough to fling them out of their parent system, yes, they're going fast.

Senan: And because they're not illuminated by their own light, you know, we might only if one was coming this way, we might only have a week or two of notice. Like we might only see it coming a week or two out before it hits us or whatever.

Joe: Would they tell us?

Senan: Would you want to know if it was definitely going to…

Joe: Would they tell us? Would they, the panic that would ensue?

Senan: No, I think they should just tell us, all right, everybody's taking a week off work, and here's…

Joe: Paid.

Senan: All bars are free. [laughter]

Joe: We've a very simplistic outlook on what makes people happy. [laughter]

Senan: Right, we need to talk about the TRAPPIST-1 system.

Joe: Trappist is a beer.

Senan: That's right. [laughter] Trust you to know about beers. [laughter]

Joe: Chosen specialist subject. [laughter]

Senan: I'm telling you, you'd kill it on Mastermind. Anyway, this TRAPPIST-1 system is only 40 light-years away from us, so it's a near neighbour; we'll never set foot on it, but it is a near neighbour. [laughter] But this one has seven Earth-sized planets that we have confirmed, and they're all bunched up close to each other.

Senan: So if you were standing on the surface of one of them and you looked up in the sky, you would see one of your neighbour planets bigger than we see the Moon when we look up in the sky.

Joe: Right.

Senan: So it'd be pretty weird.

Joe: This is like an M-dwarf star with seven rocky planets. Now, what are the planets called? Come on, come on, imagination. [laughter]

Senan: There's a competition at the moment to name them, so if you can come up with seven original names…

Joe: M-dwarf planet, seven rocky planets. [laughter] Somebody has got to name them. [laughter]

Senan: The scientists were feeling particularly creative on the day they named these ones, because they were b, c, d, e, f, g, and h. [laughter]

Joe: Right, now I have another question then. [laughter] There's something missing there. Did I miss one? What happened to a?

Senan: A, I don't know. [laughter]

Joe: Right, we've got to get in contact with these guys. We need these guys on the podcast to answer some questions.

Senan: Is 'a' the letter they assigned to the star itself? I don't know.

Joe: What was the name of the star?

Senan: TRAPPIST-1.

Joe: No then. [laughter] TRAPPIST-1a? [laughter]

Senan: But it's an interesting one, because at least three of them are in the habitable zone of the star. Imagine if we in our solar system had three Earths.

Joe: Yeah. How long would it take us to wreck them? [laughter]

Senan: Well, the first thing would happen of course would be a war between the three planets. [laughter]

Senan: Anyway, the James Webb Space Telescope, that super space telescope with all its fancy instruments, most of which work in the infrared zone, has been analyzing some of these planets in TRAPPIST-1, and it thinks the ones near the star probably have no atmosphere. I'll come to how we look at atmospheres now in a moment.

Senan: But chances are, it looks like the star is fairly active for solar flares, and that those solar flares are stripping away, blasting away the atmosphere into space of the planets that are in near the star. Which is kind of what happened to Mars a long time ago. Mars did have a decent atmosphere with liquid water at one point in the past, but it doesn't have a magnetic field to protect it, so our Sun's…

Joe: As we dealt with in a previous…

Senan: Well done, I'm glad you remembered that. [laughter] But that doesn't rule out all of them; some of them might have atmospheres. So it's a pretty tantalising system if we do ever develop this fanciful, fictional, super cool, super fast, super efficient spacecraft engine, I'd say TRAPPIST-1 will be on top of the list for where we visit first.

Senan: Atmospheres: I mentioned a minute ago James Webb Space Telescope is looking at the atmospheres of some of these exoplanets. How in God's name do we analyze the atmosphere of something that's so far away and is almost blasted out of the sky in terms of brightness by its star?

Senan: And actually that's part of the solution to the problem. So it uses something called transit spectroscopy. Spectroscopy is a technique that's used widely in science for all kinds of reasons, but essentially if you take a light of a certain wavelength and you shine it through a gas, the way it interacts with the molecules of that gas, effectively some of it is going to be absorbed and might be re-emitted, so the light is changed by the interactions it has when it goes through that gas.

Joe: So it's like a stick in water?

Senan: No, that's refraction, that's different. This is a different situation.

Joe: That's the level I'm working at. [laughter]

Senan: Well, at least you're no, I am impressed, you are thinking about it. You are thinking about it.

Joe: That's it, yeah. You're the bright star, I am the invisible planet. Got it. [laughter]

Senan: What happens, right; when a planet passes, this only applies to planets which are in the transit situation where they're passing in between us and their parent star, so we can see them going in front, or at least we can see the changes to the star as the planet passes in front of it. A very tiny fraction of that starlight will pass through the planet's atmosphere and continue on towards us after it has passed through the planet's atmosphere.

Senan: And that passage through the atmosphere changes the light slightly. So every star has its own frequency signature; in other words, the collection of colours that are in the light from that star.

Joe: Oh, is that unique? Unique, are unique?

Senan: Fairly unique, yeah, fairly unique.

Joe: Well, they all look the same at night-time.

Senan: They do, yeah. The changes in colour are probably not subtle enough. But for example, if you look at there's a star in the Orion constellation, it's noticeably red, even to the human eye. But yeah, there are differences in colours.

Senan: Anyway, the point is each star, its light is made up of a whole bunch of different frequencies, like a signature for the colour of that star. So we can measure that when there's no planet in front of it, and then we can measure the very slight change in those frequencies when there is a planet in front of it. And we can determine what chemicals were in the gases of its atmosphere.

Senan: What chemicals did that light pass through, because some frequencies will be absorbed by those chemicals, some frequencies will be absorbed and re-emitted at a different frequency by those chemicals, and so on. So we see slight changes to the fingerprint while the planet is in front of the star, and we actually have instruments now that are sensitive enough to figure out what all those chemicals, or a lot of the chemicals in that atmosphere are.

Joe: Right, so the light passes through something and is changed, and because of the change we can tell what it passed through?

Senan: Yes, pretty much. I wish you had explained that, we would have saved about five minutes. [laughter]

Senan: We're looking for obviously we're looking for things like oxygen and nitrogen, which are the main components of our atmosphere…

Joe: Chocolate.

Senan: I don't know if there's a whole lot of chocolate in our atmosphere. [laughter] We're also looking for like biomolecules. So those would be chemicals like methane that we know are often created by life, by living things.

Joe: Yeah, maybe we don't want to find those. [laughter] Imagine if somebody found those.

Senan: Well, you know, at least we'll know which direction to look for the invasion then. [laughter]

Joe: Move our planet away in the opposite direction quickly.

Senan: It's good information. We're running on in time-wise, so I'm just going to quickly rattle through what's going to happen in the future. So the James Webb Space Telescope has exquisite infrared equipment, so that's going to do a lot of analysis of the atmospheres of some of these discovered exoplanets.

Joe: I think that may be the first time those three words have ever been used together: exquisite infrared equipment. Exquisite. [laughter]

Senan: Super good, super good. [laughter] PLATO is a European satellite that's going to go up in March '27 next year, and it is going to focus on looking for Earth-like planets in the habitable zones around stars. During its lifetime, if it stays alive long enough, it may cover up to a million stars that it's going to analyze looking for Earth-like planets.

Senan: ARIEL is another European satellite, 2029, and that one is got special equipment to allow it survey the atmosphere of these exoplanets in great detail. The Nancy Grace Roman Space Telescope launched just a couple of weeks ago, and it's going to do a microlensing survey of the Milky Way looking for more rogue planets.

Joe: Because you need more of them.

Senan: Yeah, well, we'd really like to know if any of them are coming towards us. [laughter]

Joe: Yeah, would we? We probably couldn't do anything about it. I mean, let's face it, you're not going to shove a planet out of the way.

Senan: This is very Hitchhiker's Guide to the Galaxy stuff, you know, with yeah, sorry Don't panic! Don't panic! [laughter]

Senan: The Nancy Grace one also has a cool coronagraph tech demo. If this works, it'll be the first time we'll actually be able to directly take a picture of one of these planets; it'll probably be a tiny speck, but [laughter] this coronagraph thing that it's like an experimental camera that's built into this satellite, and it has this really fancy set of mirrors and masks that hopefully will allow it to blot out the light of the star without the blotting out the planet beside it.

Senan: Because the star is so bright at the moment, like millions of times brighter than the planet, it's impossible for us to photograph the planet, but if we can somehow mask out the brightness of the star…

Joe: Just put the photo into ChatGPT: [laughter] mask out this star.

Senan: So yeah, it's a really exciting field. I mean, it's mind-blowing for me to think that there are so many planets out there, and there's got to be life; just the numbers, there's got to be life out there somewhere.

Joe: There's two oddly-shaped entities having a podcast, talking about it. [laughter] Right here. [laughter]

Senan: Blarg and Blorg are talking about the possibility of of other podcasters around the galaxy. [laughter]

Senan: Right, that's it for this week, and hope you enjoyed our exploration of exoplanets.

Joe: Very good, and do check out our website.

Senan: Yes, it is enoughwiththescience.com.

Joe: Okay, catch you next time.

Senan: Full transcripts available there, yes. Good to see you next time.