Pod Insights In Our Time The Habitability of Planets About this episode Melvyn Bragg and guests discuss some of the great unanswered questions in science: how and where did life on Earth begin, what did it need to thrive and could it be found elsewhere? Charles Darwin speculated that we might look for the cradle of life here in 'some warm little pond'; more recently the focus moved to ocean depths, while new observations in outer space and in laboratories raise fresh questions about the potential for lifeforms to develop and thrive, or 'habitability' as it is termed. What was the chemistry needed for life to begin and is it different from the chemistry we have now? With that in mind, what signs of life should we be looking for in the universe to learn if we are alone?
With
Jayne Birkby
Associate Professor of Exoplanetary Sciences at the University of Oxford and Tutorial Fellow in Physics at Brasenose College
Saidul Islam
Assistant Professor of Chemistry at Kings College, London
And
Oliver Shorttle
Professor of Natural Philosophy at the University of Cambridge and Fellow of Clare College
Producer: Simon Tillotson
Reading list:
David Grinspoon, Venus Revealed: A New Look Below the Clouds of Our Mysterious Twin Planet (Basic Books, 1998)
Lisa Kaltenegger, Alien Earths: Planet Hunting in the Cosmos (Allen Lane, 2024)
Andrew H. Knoll, Life on a Young Planet: The First Three Billion Years of Evolution on Earth (Princeton University Press, 2004)
Charles H. Langmuir and Wallace Broecker, How to Build a Habitable Planet: The Story of Earth from the Big Bang to Humankind (Princeton University Press, 2012)
Joshua Winn, The Little Book of Exoplanets (Princeton University Press, 2023)
In Our Time is a BBC Studios Audio Production
Transcript 10,666 words · 8 speakers
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2:20 Speaker 05 This is In Our Time from BBC Radio 4 and this is one of more than a thousand episodes you can find on BBC Sounds and on our website. If you scroll down the page for this edition, you'll find a reading list to go with it. I hope you enjoy the programme. Hello. How and where did life on Earth begin? And what did it need to thrive? And could it be found elsewhere? These are some of the great unanswered questions in science. Darwin suggested we look for the cradle of life here, in some warm little pond. More recently, the focus moved to ocean depths. Yet new observations in outer
3:00 Speaker 05 space and in labs raise fresh questions about what's known as habitability, the potential to develop life. So what was the chemistry needed for life to begin? And is it different from the chemistry we have now? And what signs of life should we be looking for in the universe to learn if we are alone? With me to discuss the habitability of planets are Jane Birkby, Associate Professor of Exoplanetary Sciences at the University of Oxford and Tutorial Fellow in Physics at Brasenose's College, Sidhu Islam, Assistant Professor of Chemistry at King's College, London, and Oliver Shorttle, Professor of Natural Philosophy at the University of Cambridge and Fellow of Clare College. Oliver, there's a range of ideas about where life began on Earth. Can you give us an overview?
3:45 Speaker 06 Yeah, absolutely. So this is a really fundamental question for our field. And what we're really asking here is what environment could chemistry take place in that ultimately transforms simple building blocks through biochemistry and ultimately producing biology? And what that's asking of an environment is those simple molecular building blocks to be there to begin with. It needs to be an environment where the pieces are present, which we can start building up into more complex molecules. There needs to be an energy source, maybe to drive that chemistry and possibly also that's there to be exploited by the nascent life when it eventually emerged.
4:27 Speaker 06 The environment needs to be bounded in its pressure and its temperature conditions so that those more complex molecules can form. High temperatures themselves tend to break apart molecules into simpler, simpler parts and we're looking here to build complexity. So that places limits on the temperature range of our environment. And possibly what we might also want is some variability. It's not really necessarily one environment that we're looking for. It's perhaps a diversity, a chain of linked environments that can ultimately, through a series of steps, take these simple starting blocks and build life for us. So that's what we're asking for of the where. There's every reason to think conditions like those might have been supplied on Earth.
5:12 Speaker 06 So you might think that life on Earth began on Earth. We don't necessarily need to think about looking elsewhere. And so two sort of major schools of thought here have put life either in kind of deep sea hydrothermal vents and its origins there. And I think part of the motivation for that has been the sort of chemical energy and the thermal energy in those environments available maybe to drive chemistry and maybe also available for any life that forms. Another environment, type environment that looks very promising is the surface of the Earth and warm little ponds, Darwin's warm little pond at the surface of the Earth where similar chemistry can take place, but also there's the diversity of environments.
5:55 Speaker 05 What can you say about the timing? When did life begin here on Earth?
5:59 Speaker 06 So we can come at this from two, two directions. We can put a hard limit on how far back life might have started and that, that limit is defined by the age of the planet at about four and a half billion years old. And a hundred million years after its birth, pretty much, it will have experienced a giant collision with another planet that ultimately produced the moon. And so that would have sterilized the surface of the planet and is about as early as we could think life might have formed. Now, if we come in the other direction and try and look back through the geological record and try and find evidence of, of early life in the rocks, then we reach back three and a half billion years. And that's about as far back as we can go and see direct evidence of life in the
6:41 Speaker 06 forms of stromatolites, these kind of layered fossils that are evidence of algal mounds having existed, mounds of microbial life existing on the early Earth. And to reach back any further than that, we have to look not for direct fossil evidence but for the sort of fried remains of life, so graphite. And there's some graphite reaching back to about four billion years ago, which has an isotopic fingerprint suggestive of perhaps being life. But that leaves a huge gap of maybe three, four hundred million years in which we have almost no records to access to even ask the question of whether life was on Earth. So that's a huge window in which maybe life might have arisen.
7:21 Speaker 05 Thank you, Jane, Jane Birkby. We are one planet among billions. There seem to be more billions every time I read more about this. When looking for signs of life elsewhere, what do you hope those signs to be? Looking for life elsewhere beyond the Earth, there's two camps. We can look for life in our solar system, so perhaps looking at sites like Mars or some of the icy moons. These are places where we could potentially send landers, and we've done that for Mars. So we can actually go and dig in the soil and analyse in situ what we're doing. But for most other things that we're trying to do, particularly if we're trying to find an Earth twin, so something the same mass and size as the Earth, at
8:03 Speaker 05 the same distance from the Sun, so that the temperature is such that we could have liquid water on the Earth. We think liquid water is necessary for life. And that really limits the temperature range that we can look for that in, so between sort of zero and a hundred degrees Celsius. So beyond the solar system, we want to look at planets orbiting other stars. So we need to use telescopes to do that. The chances of sending a mission or a probe There is very far off. So for example, if I was an alien looking at the solar system, I would have several different techniques that I could use to do that. One of them being that I would maybe want to take a direct image of a planet. You talked about the influence of the sun. Is it just by chance that Earth is at the precise position it is?
8:48 Speaker 05 Because it gives it all that it needs. So it is quite interesting to ask, is this precisely by chance? If life needs the conditions that occurred on Earth, then perhaps it's unsurprising that life occurred here. It could be that life is just a natural byproduct of the star formation process. So we know that planets form in discs around stars as they collapse down from giant molecular clouds to form a star. And then you have a disk of material and your planet forms in that disk. And all the processes, the chemistry, the physics that follows from that may just very naturally result in biology. And it just happens to be that our rock, the distance from the sun was correct for that environment. If that's true, then potentially that happens elsewhere.
9:31 Speaker 05 What do we have to search for to find these signs of life? So looking for signs of life beyond our solar system, so planets around other stars. The thing that we can see is the atmosphere. We can't really get to the surfaces yet. And we can't get to the interiors. We're very much looking at the atmosphere. So this sort of thin shell of gas around the planet. And what we do- What are you looking for? You say the atmosphere, but what messages are coming back? Yes, we take the light from the planet. We split it into all of its different colors, so its different wavelengths. And what we're trying to understand is what is in that atmosphere. So life on Earth has changed our atmosphere. So if you go back to very early times, there was very little oxygen in our atmosphere. When?
10:17 Speaker 05 Even if we go back three and a half billion years, where we first have some evidence of life, oxygen was still a very, very low amount. It was only around two and a half billion years ago that we had what we call the Great Oxidation Event. And this is when cyanobacteria started photosynthesizing and releasing oxygen into our atmosphere, that we had this huge spike in oxygen. Oxygen, I think it went up to almost thirty percent. And that allowed the development of the Cambrian explosions, so lots of different life forms that evolved. So we want to, oxygen is one of these key signatures that we can look for as evidence of life. Without life, oxygen in our atmosphere would just disappear over the course of a few tens of thousands of years. So it doesn't stick around, something has to be constantly replenishing it.
11:01 Speaker 05 And so our atmosphere is out of equilibrium with the standard chemistry and geology that happens on Earth, right? It's life that makes the difference. So if I can find other planets that have this signature that looks like it's out of equilibrium, that is what I'm looking for as an initial sign of life. So we can do that using spectroscopy. And that's where we we split the light into the colours and then we look for the missing colours. So for example, oxygen has a very, very specific set of colours that it absorbs. So those colours don't make it to the Earth. And that pattern that they absorb, those specific colours are unique to oxygen. There's no other molecule in the universe that can make that pattern. So if you can detect that, then it's a robust signature of that molecule existing.
11:45 Speaker 05 Can you tell us a little bit more about that? Yeah, so if we can very finely resolve those colours, then other things that we might look for, for example, on Earth, methane is something that is produced by life. So we'd be looking for the combined set of colours missing from oxygen and from methane. That this would give us another signature that we could look for in the atmosphere that would perhaps indicate a disequilibrium that might be caused by life. Thank you. Saidul, Saidul Islam, when it comes to the chemistry of life, what are the most basic changes we should be looking at? And how the lifeless molecules become more interesting, shall we say?
12:27 Speaker 01 Biology is made up of a restricted set of molecules. Even though there's a huge diversity of organisms on our planet, at the basic level, we're quite similar. So we use nucleic acids, which is RNA and DNA. We use proteins, which are made up of amino-
12:44 Speaker 05 What do you mean by use?
12:45 Speaker 01 It's part of our machinery. The genetic information is embedded into the nucleic acids. The proteins-
12:53 Speaker 05 Are the ones that carry out the functions, so they're the catalysts. And then you have to have a compartment, a cell membrane, and that's usually made of fatty acids and things like that. So these molecules, they're all comprised of a set of atoms, carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur. So we need to know how we got those elements into organic molecules. We can assume that the sulphur came from hydrogen sulphide, which could have been belched out from volcanic exhalation. The oxygen could have come from water, H two O. And then we have to start thinking about the carbon and nitrogen that's necessary.
13:36 Speaker 05 The early Earth atmosphere had a lot of carbon dioxide, much more than we've got now, and it would have had nitrogen as well. The issue with those two molecules is getting them to react in such a way that you can start processing those gases into organic matter. And the early Earth would have suffered from bombardments that Ollie was talking about. And these bombardments were violent, they were shocking. And what had happened in the atmosphere is everything would have been atomized and then you'd have had some recombination chemistry taking place. And some of that chemistry would have generated molecules that would have been easier to provide the carbon and nitrogen. And that could have been something like hydrogen cyanide.
14:18 Speaker 05 And so that's what we are trying to sort of find out, how these highly reactive molecules are self, I'm not going to say spontaneous. But they somehow self-assembled to form the molecules that we recognize now that are essential for the way biology works. Why are you not saying spontaneous? Spontaneous can mean different things to different people. So when I think of spontaneous, I think you go from zero to something highly complex. I think the chemistry that took place happened in a sequence, so it was stepwise. And it progressively became more and more complex. In that sense, I don't use the word. Spontaneous, nor do I say spontaneous emergence of life. I think it was a gradual transition
15:02 Speaker 05 from inanimate matter to biological entities. Why does hydrogen cyanide fit into all this? Hydrogen cyanide and hydrogen sulfide are two very toxic gases, but it turns out that if you have hydrogen cyanide or hydrogen sulfide and you have sunlight bathing these aqueous solutions, you start getting easier organic chemistry. So hydrogen cyanide is what we call reduced, and then it becomes organic matter, and then carbon-carbon bond-forming reactions can start to take place. And the chemistry that emanates from it starts producing the building blocks of the nucleic acids, RNA, the building blocks of proteins, and also
15:47 Speaker 05 the cell membrane. Thank you. Ollie, when we talk about life in this context, what do we mean?
15:53 Speaker 06 This is almost an unanswerable question, but I I think actually what we can do is maybe define some sort of operating principles, which for different communities lets them get somewhere with that question of what life is. So starting maybe with the astronomers, what what life is for an astronomer searching for life beyond Earth and around another star. As James mentioned, what we're looking for are changes in the whole planet's atmosphere, changes in the chemistry of the atmosphere of a whole planet, driven by life. And so life there is a planetary scale phenomenon, changing the properties of the whole planet. It's like a it is a life as a biosphere. For a geologist, looking back to try and understand when life of them might have emerged on Earth, trying to reach back in
16:38 Speaker 06 time in the rock record, life is sort of a footprint in the rocks. You know, maybe it's the actual the fossil of a of an, you know, an extinct organism, or maybe it's the isotopic fingerprint created by the action of life. And for a planetary scientist searching for life on Mars in the soils like we've heard about, it could be a similar thing we're looking for. For a biologist or for an organic chemist looking to create life in the laboratory, then life Probably is something which has separated itself from its environment. It's formed a compartment, a cell of some sort. It's got an internal machinery to exploit and extract energy from its environment. And it's undergoing replication.
17:23 Speaker 06 And in that sense is capable of evolution by natural selection.
17:27 Speaker 05 Thank you. Jane, what might you be able to tell us about the stage of life on any planet? Whether it's in step with us, with this, with Earth, for example?
17:37 Speaker 07 In terms of being able to tell the actual stage of evolution on that planet is very much dependent on the environment. So we can age a planet by looking at its host star. We can age the stars to varying degrees of certainty, depending on the type of star that it is. And then from that, I mean, at the moment, the only comparison we have is we have one planet that we know has life. So we can try and make comparisons. One of the things we're looking at at the moment are there are these planets, we call them lava planets. And they're planets that are very, very close to their host stars. One year for them takes less than a day, so they orbit their star in less than a day. And it means that they're extremely hot, 2,000 degrees. And their surfaces are essentially a magma ocean.
18:21 Speaker 07 And that's very representative of very early Earth, when it was being bombarded by all the impacts that were building up the mass to create the Earth. And so we kind of have this laboratory where we can maybe study what was going on in very early Earth environments. And so it's not that we would maybe expect to find life there, but we could start to understand the conditions that maybe gave rise to that. Other than that, yeah, it's thinking about the different ages of the stars and being able to tie that to what we see in the atmosphere.
18:50 Speaker 05 Holly.
18:51 Speaker 06 I wonder whether it's worth also noting that, you know, earlier in the 20th century, so before the exoplanet revolution, the search for life proceeded by listening for life and its electromagnetic, you know, its broadcast into the galaxy, listening out for, you know, its television signals. And, you know, that was obviously a very limited search for life, listening out for that life when it had reached technological competence. Which, you know, life on Earth has done, you know, only in a century of its multi-billion year existence. And that's a real contrast to what we're able to do now by looking at the atmosphere.
19:26 Speaker 07 You do have two ends of this. So what I've talked about is very. Very early stages of life, but we could look for what we call techno-signatures. So we could imagine that there.
19:36 Speaker 05 What's that?
19:37 Speaker 07 A techno-signature is anything that suggests that there is an advanced intelligent civilization that has industrialized in some way and is able to create technology for very different kinds of processes as we do on Earth. For example, on Earth, we're not looking for very long-term trends. So trying to study, for example, the amount of carbon dioxide over the course of time. We're very human-centric, so we can study this for, at best, we've been doing maybe a hundred years with telescopes. So our timeline for looking at changes in CO2 is not long enough. So we're looking for things that have stuck around. A good example of this is CFCs.
20:20 Speaker 07 These used to be used in old refrigerant coolers. We released CFCs into the atmosphere. This is what causes the hole in the ozone layer. And over time, that's been repairing itself as we've used it, CFCs less and less. But if we release CFCs into the atmosphere, they create a spectroscopic signature. So again, they absorb different colors of light. And so we could potentially go and look for something like that in another planet. And it would be very unlikely that there would be some geological, non-biological scenario that would actually cause that.
20:51 Speaker 05 Perhaps it's a loose cannon, but irresistible to ask what function does alcohol play in all this?
20:57 Speaker 07 Alcohol. It'd be unlikely, I think, that we would see alcohol in the atmosphere, or sort of in a biosphere of a planet. But we know that alcohols and also sugars are part of this, as Sidhu was saying about this prebiotic chemistry, this organic material that we need to make in order for life to emerge. And actually, so Sidhu, you were talking about making this on the Earth and creating conditions on the Earth. But there's another part of this where in space we do actually find sugars and alcohol. And it's created by what we call cosmic ray chemistry. So this is perhaps a high-energy photon, a gamma ray or something, striking on a dust grain or an ice grain. And actually igniting that chemistry that would actually form those different sugars and alcohols.
21:42 Speaker 07 And then during the bombardment of our atmosphere by meteorites, comets, et cetera, in Earth's early formation. Is that that material may have rained down as we were bombarded. So it could be that some of it was delivered to us rather than being formed in situ on Earth.
21:58 Speaker 05 Thank you very much. I'm getting a high-speed education here. Seidel, if somehow life does begin, is there a pattern?
22:07 Speaker 06 I think the chemistry that would have taken place at the beginning would have followed a pattern depending on the geochemical environment. The way I see the the chemistry that led to biology or the building blocks, I think it was an inevitable consequence of the early Earth environment. What happened after that?
22:27 Speaker 05 But how did the biology get in place?
22:29 Speaker 06 Well, that is what we're trying to do at the moment. So this is what, you know, trying to recreate life in the laboratory. We're not anywhere near that yet because we're still struggling with trying to make the building blocks of the biological molecules. But molecules by themselves aren't life. They have to come together. They have to work in a coordinated fashion. And then a really complex way to develop into a network of reactions, which we now recognise as a metabolism. Once that's in full swing, then you can start imagining that there were some environmental pressures that would have led to natural selection. So there might have been a situation where building blocks were dwindling. So
23:11 Speaker 06 that organism, that very primitive organism would have had to start to adapt and start using other building blocks or nutrients to start recreating those things to replenish their genetic material and their proteins. So at the beginning, I think for the chemistry, I think there was there was a pattern. Once the biology was in full swing, those organisms could have spread. They could have moved into different environments. It could have been a hot environment, cold environment. It could have been under the sun or could have been shaded away. And so we would have had to adapt to those environments.
23:47 Speaker 05 In the same way that we continue to adapt to our environments now, and we know that because there's lots of different organisms living different lifestyles on our planet now. You've got organisms at the bottom of the ocean that are feeding off of hydrogen gas. There's organisms like ourselves who are feeding on plants, who generate their own food. And all these organisms are using different types of energy sources. Thank you.
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27:02 Speaker 05 Only back to you. Should life begin? What are the contingencies on which it depends?
27:09 Speaker 06 I think the story of life is probably ridden with contingencies at all scales and possibly across all times as well. So it kind of the biggest scale I think about some of these contingencies is the scale of the whole galaxy. So Earth and the solar system sit in a sort of somewhat suburban location in our galaxy, 26,000 light years from the galactic center. But if we move towards that galactic center, the density of stars in space increases by maybe a factor of 10 and the rate of which stars are being born also increases dramatically. And that fundamentally alters the environment at a kind of astrophysical scale in which planets are existing
27:51 Speaker 06 and creates a contingency where maybe a system is subject to gravitational interactions that could throw planets around, ejecting them from systems, causing massive bombardment, and also sterilizing radiation as massive stars explode and die. So there's a contingency at that largest scale. Just to note on that is that we're not talking about stars coming right through our solar system to create those kind of interactions. They can be very far away. And
28:18 Speaker 07 It's just the smallest perturbations can actually set off a chain of events in that planetary system to cause a destabilisation. So even on a galactic scale, things are happening.
28:28 Speaker 06 Yeah, absolutely. So maybe we're fortunate to be where we are with the real estate we've got. If we move kind of into the solar system itself, then we've already discussed the distance from the star, you know, it really matters how much light, how much heat you're receiving from your host star. And we've got the perfect example of this in the solar system with Venus and Earth. Venus with a surface temperature of over 400 degrees with a massive atmosphere of carbon dioxide is the sort of poster child of inhospitable kind of planetary conditions. But it's just that fraction closer to our sun. And so maybe it was it was too close to ultimately sustain liquid water at its surface that ever existed. And then the planet itself and its own history and its contingency. Well, we have another
29:11 Speaker 06 brilliant example in Mars versus Earth. Mars is that bit smaller than Earth. It's about one tenth the mass of the Earth. And it had a rich geological history, but one which largely shut down billions of years ago. And so if life ever emerged on Mars, you know, the geological processes weren't there for four and a half billion years to perhaps keep it going like they were on Earth.
29:33 Speaker 05 Jane, can I come back to you? What makes exoplanets a good place? What are exoplanets? And what makes what makes them a good place in this discussion?
29:41 Speaker 07 Yes. So an exoplanet, the word is short for extrasolar planet. And that just means that it's a planet outside of our solar system. For the cases that we're talking about, let's say they're also planets that orbit other stars. There are potentially rogue planets, free floating planets out there that have no host star. But for the sake of this conversation, let's assume that they do orbit another star. And so the reason that they're good sites is that they're analogous to our own solar system. So you have a surface that you can form on. Perhaps you have liquid oceans and an atmosphere. So we're sort of, it's very, very basic properties that we're trying to create the conditions similar to Earth.
30:20 Speaker 05 And what do they bring to the table?
30:22 Speaker 07 We're able to study the conditions that can arise on a rocky body orbiting a star. And it turns out that those conditions are vast and diverse. If you can think of any kind of scenario, it probably exists. We found the wildest systems, we found planets that have multiple suns in their skies. We have these planets that, as I mentioned, the lava planets that are super hot. So it really opens up our laboratory as such to actually study other systems that may be able to host life. So astronomy is an observational science. We can't go into a lab and poke something. So what we do is we look out into the universe to look for other laboratories where we can
31:04 Speaker 07 say, okay, that isn't Earth, but it's it's younger or older than our system, or that's an Earth, but it has a different type of host star. Maybe the host star is colder. That means that habitable zone region where liquid water might be able to exist at the right temperature around the star is much closer in. So our very nearest star system, Proxima Centauri, has a planet in the habitable zone. And that, and one orbit takes about 12 days. So one year on that planet is 12 days. So they're very different environments and it allows us to put life in context of all of these different scenarios and try to understand whether it can thrive in those scenarios or whether Earth really is truly special.
31:45 Speaker 05 Can I develop that with you, Sadal? We've heard about the chemistry needed for life to start to survive. What's needed for it to thrive? For life to thrive, it needs an energy source. It needs a continual supply of nutrients. And it needs an environment that is conducive for it to replicate, to grow and to expand its population. But the the the main thing is that that we should remember is the we don't want to be making exact copies of the same thing. So we want an environment where copies are slightly mutated. So then if there is a selective advantage for them, then it can continue to pass on that advantage on to successive generations.
32:26 Speaker 05 What you don't want is these sharp transitions in what the environment was like. So if there suddenly you went from a really hot environment to a cold environment, then you're probably Really going to cause some kind of catastrophic event where a population of cells or organisms will die. And that's precisely what happened at the great oxidation event. And and we think of oxygen when we're, you know, we're breathing it in right now is a really important gas that we're using to sustain ourselves and stay alive. But actually, at that point, the organisms that were around found oxygen completely toxic to them. Besides just a few organisms, the ones that were able to adapt to use that oxygen as a source of energy.
33:09 Speaker 05 And that's when the complexity of life started to really take off. We started to go from single celled organisms to multicellular species. Ollie, with that in mind, how likely is it that conditions exist for life as we know it beyond this planet?
33:27 Speaker 06 I think the really interesting thing here is just how resilient and adaptable life on Earth has shown itself to be. Um. So, you know, the limits of the biosphere, life on Earth extends, you know, almost halfway to space through our atmosphere, tens of kilometers up into the sky. It extends kilometers beneath our feet into the crust with, you know, single celled organisms living off the chemical energy in rocks. It exists in, you know, at temperatures around hot springs, over a hundred degrees C, in ice, down to minus twenty degrees C, in extraordinarily acidic conditions in the the water that's running off acid mine waste, and also extremely alkaline conditions in vents.
34:08 Speaker 06 So life, and this comes back, I think, to Sidle's point, life, once it's established itself on a planet, once it's got a foothold and evolution can operate, has shown itself enormously adaptable to a really wide range of environmental conditions. And in that sense, I think, looking out to planets, both in the solar system, whether it be early Mars or even perhaps Mars today, and the icy moons, the satellites, the icy satellites around Jupiter and Saturn. It seems plausible to think that if if you could put life there that could survive initially, it would adapt and radiate to fill that that kind of environmental space. But what's a much harder question to really put a probability on? Is how prevalent are the conditions for life to start?
34:51 Speaker 06 And so I think the conditions for life, if you sort of dropped it there, are probably, you know, fairly common throughout the, throughout the galaxy. We found planets almost around almost every star, many of them, you know, possibly with the right temperature that wouldn't immediately cook your life if you dropped it on the planet. But how contingent, how, how particular are those conditions for inorganic chemistry through a whole series of steps to lead to organic chemistry, biochemistry and biology? Well, that's where there's this really interesting conversation between astronomy, the organic chemistry in the lab and the geology to think about, well, really, how likely is that?
35:27 Speaker 05 You think the potential for it happening elsewhere is high?
35:29 Speaker 07 It's a very debated question. If we're looking for very, very simple life, things of sort of single cell, that is perhaps more likely than the complex evolved intelligent life that we have on Earth.
35:43 Speaker 05 That takes us, I think, Jane, to the equipment in the pipeline, the equipment to, to scan the skies for these signs. But which signs are you really scanning the sky?
35:52 Speaker 07 Yes, so we live in a very special time in that there are multiple space missions and extremely large ground-based observatories that are being built right now to enable us to do this work.
36:05 Speaker 05 What do you mean by extremely large?
36:06 Speaker 07 The European Southern Observatory, ESO, are building the Extremely Large Telescope, called the ELT. Astronomers are not good at naming things. And it will be 39 metres in diameter, which is enormous. The largest telescopes we have on the ground right now are 10 metres across, so it's a factor of four larger. And what this means is it does two, two special things. One is it has the ability to collect more photons from, from the systems that we're looking at. So it's a big light bucket, but its large size also gives it superior spatial resolution. So by that, I mean, how finely can I resolve small things in the image that I'm trying to take? So we potentially, from this, will be taking a direct image of our very nearest habitable
36:51 Speaker 07 zone exoplanet. So this is around Proxima Centauri. And to give you an idea of the, the complexity of the technology and what it needs to achieve, you're trying to find a very small planet that's Quite faint next to an extremely bright star. It's the equivalent of trying to take a photograph of a firefly next to the beam of a lighthouse. Uh, standing on a ship twenty kilometres away. Okay, that's the enormity of the challenge that we're trying to do, but this is what the E L T will do for us. So we will be able to take one of those spectra, so look for the missing colours in the atmosphere of Proxima B, which is the name of the planet. Um, that we'll be looking at in the, in the habitable zone there. So that comes online in twenty twenty nine.
37:34 Speaker 07 It's really quite soon. And if this planet has an atmosphere that we can look at, then those observations will take of order ten to thirty hours. So it's quite possible that by twenty thirty that we actually have the spectrum of the nearest exoplanet. And if, I think if life is very common, then perhaps we will see signatures of that disequilibrium in the atmosphere that maybe indicates that something interesting is going on there.
38:01 Speaker 05 So this is the nearest you're gonna, so far, the nearest you're gonna get seeing life on other planets is through this telescope?
38:07 Speaker 07 Yeah, so we have uh, the E L T. At the same time, or a few years before that, I think in twenty twenty six, the Plato mission will launch. Uh, and this is really to scan the whole sky to actually detect planets around the stars, uh, the brightest stars. That are the easiest ones for us to then go and follow up with the E L Ts. Uh, we also have uh, J W S T, which is a six metre telescope in space right now. That is looking at some of the very, very brightest and special cases. An example of this being the Trappist One system. This is a system that has seven earth-sized planets. And they sort of create this very interesting laboratory of taking a rocky planet and moving it slowly outwards.
38:50 Speaker 07 Uh, you have seven instances where you can study them and they're all at different distances. And so some of them are in the just right temperature to have liquid water and others are maybe too cold and others too hot. Um, but they actually orbit a very tiny star. Uh, and so their orbital periods are a much shorter sort of.
39:05 Speaker 05 Between sort of a few and maybe twenty days. Uh Sidle, an extra dimension about looking as life on earth as it is here now.
39:14 Speaker 06 We, um, in terms of chemistry in the laboratory, it's less sophisticated to the telescopes that Jane is using. In the laboratory, it's about getting all hands on deck. So it's people like myself doing the actual physical experiments. One of the things that limits us is our ability to analyse mixtures. The more advanced that technology becomes, the easier it becomes for us to start looking at complex chemical systems. And I think that is going to be the the greatest advance that's going to take place in the next twenty or thirty years to allow us to start developing chemistries that could then progress to something that looks like life.
39:56 Speaker 06 We don't actually know how far we are into that journey yet. There's lots of groups working on this problem. You solve one problem and then you open up another can of worms. And lots of times, that's one of the reasons why it's difficult to sort of get to the bottom of that is because of our analytical techniques. I, for example, use superconducting magnets to analyse complex mixtures and that's the the more powerful they become, they're easier it becomes to resolve, to see exactly what's going on. And so if that technology advances faster, then it'll make our life much easier.
40:30 Speaker 05 So there are so many, Ollie, there are so many uh different areas of expertise. Is there any way that these are being brought together? And mutually enriching each other.
40:39 Speaker 06 Yeah, so there's a really rich conversation here between the astronomy, the organic chemistry and the planetary science and and the geology. And, you know, in a way, at the heart of this is the environment in which the chemistry needs to happen. Because we need to take it out of Sidle's lab. Yeah. And we need to somehow let nature be the experimentalist here, which is both frightening but also an exciting opportunity.
41:02 Speaker 05 The experiment, what what does that mean, nature be the experimentalist?
41:05 Speaker 06 Well, we need to get rid of the experimentalist altogether. We need to just let it it happen. Um.
41:09 Speaker 05 Which being what?
41:10 Speaker 06 Uh, the chemistry that leads from the simple building blocks to the organic molecules that eventually. Essentially assemble themselves into the first cell and and life. And that needs to happen in an environment that can support that and the multiple steps along that journey. And that's an astrophysical environment. It probably requires the light of the star to to be participate in the chemistry. There's the local geological environment, whether it be a a warm Darwin's warm little pond on the surface fed by by hydrothermal springs, giving just the right chemicals, the hydrogen sulfide we've heard about. And also the, you know, the the actual organic chemical reactions themselves. And their sensitivity to things like how acidic the the waters are.
41:54 Speaker 06 So, you know, that initiates a conversation where we link the astronomical context to the planetary context to the chemical context and move backwards and forwards. The chemistry pushes us in one direction when the chemistry looks like it's successful. And the the geology pushes us in another direction when we ask the question, what what else does that environment look like? What else is there that the chemistry's got to work with when it suggests a hot spring at the surface of a young planet on a volcanic island? And that kind of dialogue is a really rich part of this field that's emerged in the last two decades. I I would agree. I think there's a lot more communications between all the all the sub disciplines and and the experts within within the natural sciences, because the origin of life isn't just a chemical problem or a biological problem.
42:39 Speaker 06 It's a problem for all of us. And I think that over the last two decades, there's been a lot more. I think one of the problems is the language that all the different sciences use. So you're kind of talking past each other.
42:52 Speaker 05 Now what we have is, and and it may be because of the internet or, you know, communications are far better now. But but you have this situation where we all see it as a as a common problem and a common goal, and we're working towards that. And I think that's is something that has only come about in the last 20 years. I wouldn't, I'm I'm not really sure why that it took so long for that to happen. But I think it's something that's highly beneficial for for for the problem that we're working on right now. Jane, from what you've seen, does life as we know it seem unique here? Life on Earth, the very, very specific life on Earth that we have, particularly humanoid life, perhaps.
43:32 Speaker 07 That is unique. It depends if the evolution that happened on Earth is is common elsewhere, if if evolution proceeds in in similar ways. Um, so one of the key things when we look for life elsewhere is I think we have to be very open minded about what we might expect to find. Um, and so that's why it's it's so great to talk with chemists and and sort of earth sciences, geochemists, like understand all of the different different conditions that can arise and what we should look for. Um, and should we be looking for other things? So that's part of the fun part of my job is when I go to gatherings um with people talking about prebiotic chemistry. I get the fun job of saying, oh, what if I change the light source? I can change the star.
44:14 Speaker 07 And it's like, oh, but then the um the the radiation is different. Oh, how does that affect the environment? Um, so I I take a lot of joy out of doing that. It's making problems for you. Yeah, no, I think I think what what we're doing is we're always adapting our our ideas. And and that adaptation comes from information that experts in other fields are telling us because if we stick with one pathway, we could be completely and utterly wrong. And that is a that is a big problem. So I think it's really important that there is dialogue and cross collaboration between all the sub disciplines. For this, for a really cohesive cohesive and strong model to be built about what exactly happened on the early Earth.
44:57 Speaker 05 What you see is the greatest leap forward that you could have. I mean, practically, in your sense.
45:02 Speaker 07 Uh, generating life in lab, in a test tube, from scratch, de novo, from simple chemicals. That is the the greatest leap.
45:10 Speaker 05 Olu?
45:11 Speaker 06 For me, it's the current exploration of the surface of Mars by the rovers there, which are collecting samples, the intent of which is to ultimately bring back to Earth for study in the lab. And so here we've got a planet which we know was transiently wet early in its history. And with some of those samples, we could perhaps ask the question, was being wet, having liquid water at the surface of planet enough to allow some chemistry to run through to to forming life?
45:36 Speaker 07 A point about Mars, the fact that Mars is quite similar to planet Earth and how we don't really know what happened beyond four billion years. The rock records have been completely chewed up, if you will, by the tectonic movements of our planet. And so I think it's really important that we try and bring back samples from other planets, particularly Mars, because it's the closest one to us.
46:00 Speaker 05 To determine exactly the historical events that took place there. Because some of those rock records, they're from about four billion years ago, they still exist. Whereas we don't have that here anymore. So I think it'll be really important for us to bring those back. Thank you, Jane.
46:17 Speaker 07 For me, it would be finding that disequilibrium signature in an atmosphere. So we have the ELTs that are coming online very soon that can do the very nearest, very brightest things. But in the 2040s, 2050s, we're also now making the design for the Habitable Worlds Observatory, which will be a large space-based telescope that will be the successor of the Hubble and JOST, that really will look for signs of life on other planets.
46:44 Speaker 05 Well, thank you. Thank you very much to Jane Bergby, Sadiq Islam and Oliver Shorttle. Next week, the stories from Greece and Rome, the Shakespeare raiding for Julius Caesar, or Elenus and Antony and Cleopatra and more. That's Plutarch, parallel lives. Thanks for listening.
47:02 Speaker 07 And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvyn and his guests.
47:09 Speaker 05 Signing with you, Jay. What would you like to have said that you didn't have time to say?
47:14 Speaker 07 So I'd like to talk about the host stars. I mentioned two systems, the Proxima Centauri system and the Trappist One system. Both of these systems, their host star is not like the sun at all. They're around what we call M-dwarf stars or red dwarf stars. So they're only about 10% of the size of the sun and they're only about half as bright. So the amount of radiation that they can put out and the temperatures that they give off really means that it's a very different environment for the planet. So I talked about the habitable zone around these stars being much closer in, only about a 12-day orbit. And at that point, very interesting things start to happen. We think in the Proxima Centauri system that it's possible that the planet is so close to the host star that it's tidally locked.
48:01 Speaker 07 I mean, it has a permanent day side and a permanent night side, much in the same way that we only ever see the same face of the moon. The star, Proxima Centauri, only ever sees the same face of the planet that orbits it. And so we don't necessarily get that diurnal rhythm of a planet rotating round and having a night and day. You could maybe perhaps imagine that life, if it lives there, exists on the terminator region between night and day. That if it did need to go back and forth, that it could do. And so the conditions are very, very different. But because the planet is so close to the star, these small stars are actually far more active than our own sun. So our sun has flares, what we call coronal mass ejections.
48:44 Speaker 07 And these are actually much more common on these M dwarf stars. And in the Proxima Centauri system, the star flares every six days. And that has the effect of potentially stripping the atmosphere. So it's entirely possible that in these systems, there are no atmospheres on the planets. And the frustrating thing about that is that these systems are actually the easiest ones for us to observe. Because of the size ratios involved, it's easier to do our observations. So although we've made it easy for ourselves, we don't yet know if those planetary atmospheres survive in that stellar environment. So the host star is key to understanding the environment that the planet and potentially that life could evolve in.
49:26 Speaker 05 Why?
49:26 Speaker 06 Yeah, I think the kind of search for habitability in exotic settings, which we're forced, you know, in a sense, for exoplanets, we're forced into by virtue of what we can look at. You know, the obvious thing would be to go and look for an Earth-like planet around a Sun-like star. And that's technically enormously challenging. And so, like Jane said, we've got a crop of planets around stars that are different. And some of the planets themselves are different and with no analogue, no equivalent in the solar system. And one type of object that's particularly enigmatic and exciting right now is a sub Neptunes. These planets, which are intermediate in mass between Earth and Neptune, so maybe, you know, a few Earth masses. And what, at a basic level, what is their structure? What are they made of?
50:10 Speaker 06 And there's the possibility that some of these planets perhaps have vast liquid water oceans. So is a vast liquid water ocean on a planet maybe three to five times the mass of the Earth, is that? Is that enough for life? Or is that actually missing some key ingredient that actually? Just a little bit of water, like the amount of water we have, where you've got water and rock. Is that what you need? So it raises this question, exoplanets and sub Neptunes raise this question of habitability in these kind of exotic alien conditions.
50:42 Speaker 07 And to add to that, so the sub Neptunes and what we also call super Earths to sort of bridge this gap between Earths and Neptunes, they're the most common type of planet that we have found. In all of our searches, these things crop up over and over again. So if they are potential sites of life, there's lots of them out there. So it would be nice to know whether or not they are worth pursuing for habitability.
51:08 Speaker 06 And there's a sort of, there's an interesting solar system exploration angle here as well. Because just like we're searching the surface of Mars for signs of life in its, in its early history, missions are now heading out to the icy moons in the solar system to explore their physical and chemical properties. And so here, although on a completely different scale to these vast sub Neptune exoplanets, we have little mini ocean worlds, you know, hidden beneath this thick layer of ice, presumably long-lived ocean in which, you know, maybe in some sense, the conditions for life are there, but do those little moons have the conditions for the chemistry to create the life? Well, we've got a chance to, to potentially answer that through solar system exploration.
51:48 Speaker 05 Do any of you have any idea what life might look like on these other planets or be like?
51:53 Speaker 07 If you would like a science fiction depiction of that, I highly recommend watching The Expanse. This is very interesting. It's set in a time when humans have, they also live on Mars and in the asteroid belt. And particularly those that have lived or been born in the asteroid belt or sort of, you know, many generations over time, that their bodies change. So the gravity is less strong than on Earth and so they become very tall. And it means that they can't come back to Earth because they can't withstand the force that they would have if they were standing on the surface of the Earth. So you can think about how physiology is affected simply by changing the gravitational field. I'm sure maybe Sidell has many more comments on, on.
52:34 Speaker 07 I, yeah, so I, I feel like I'm gonna make a, I'm gonna be a bit more mundane and I'd like to think that there is life.
52:41 Speaker 01 Out there, then it's probably similar to us. So it's gonna be carbon. I think it's be a carbon based life. Some people advocate for silicon based life, but there's all sorts of problems. For chemical reasons, they just wouldn't be able to survive in a in a in a watery environment. So we're talking about water. Water is really important, but you don't want too much water.
53:03 Speaker 01 So it's just gonna it's gonna be just right, you know, the Goldilocks zone. So, you know, if there's a vast expanse of an ocean, for example, and you're trying to do chemistry in it. The analogy I would use is if you drop a dye into water, it just completely dissipates. You know, to the point where you can't see it anymore. So it becomes almost like a homeopathic dilution. So you want these molecules to be intimate with each other to undergo chemical reactions. And if there's too much water, then the chances of that happening start to diminish.
53:32 Speaker 06 And in a way, that's the beauty of Darwin's Warm Little Pond is it's it's little, it's finite. The chemistry can happen in a restricted environment. You can keep hold of what's been made and do something else with it and not lose it to an infinite vast ocean, which is otherwise.
53:47 Speaker 05 So his little pond is still a ballet laboratory.
53:50 Speaker 06 I think in some respects, it's, you know, it's it's gained more popularity or more relevance in a way, given, you know, the interest we have in surface chemistry happening at planetary surfaces in the presence of light. And in the presence of changing environmental conditions, whether it be day, night, freeze, thaw, wet, dry, all of which become possible and chemically useful at a planetary surface compared with at the bottom of its ocean.
54:14 Speaker 01 Darwin wrote about this to his friend, John Dalton Hooker, and that was in eighteen sixty three. Well before, you know, he had any insight into the chemistry and the biochemistry that we have the privilege to be able to to understand.
54:31 Speaker 06 Seen that. And so I think he was well ahead of his time. He talked about proteins and phosphoric acids and ammonia in his warm little pond. You know, there's a slight change from it, but I think the sentiment of what he meant was there. And yeah, and I think I think he should be applauded for that. And like all good scientists, he was writing that in response to criticism of his own work. It was frustration with people's criticisms of his theory. Being deficient because of not having an origin of life explanation. He was writing those letters.
55:01 Speaker 05 Sure.
55:02 Speaker 07 So something else I want to add into this, it doesn't necessarily apply in the same way for the icy moons where they have this protective outer ice crust. But something else that we think is potentially very important is to have a magnetic field. So our magnetic field on Earth protects us from a lot of the charged particles that come from the sun. So when we do have flares, our planetary defence shield activates. You see it as the northern lights or the southern lights, depending on where you are. But that magnetic field really protects us and it protects our atmosphere from being stripped away. So this is another question for these planets around small M dwarf stars, is if they don't have a magnetic field, they don't stand a chance of resisting those flares from other stars. But it's incredibly difficult to measure
55:44 Speaker 07 the magnetic field of a planet. And so that's one of the key areas of research at the moment for the exoplanet science. But even in the solar system, there's only, not every body in the solar system has a magnetic field. So yeah, it's finding out whether that is important for habitability.
56:00 Speaker 05 You've been talking, I mean, you throw away words like, well, the word billions all over the place. How many billions of planets do you think there might be out there?
56:09 Speaker 07 Well, we know of just over five and a half thousand that we've actually confirmed. But as Ollie said earlier, the statistics show us that there probably is a planet around, at least one planet around every star. And so when you start counting up billions of stars in our galaxy and then billions of galaxies. The numbers grow and grow. It's hard to really put a number on that.
56:31 Speaker 05 Yes, you lose the imagination to curb, don't you?
56:35 Speaker 07 Yeah.
56:35 Speaker 06 And it really is a revolution. I mean, an incredibly recent one, in a way. We've gone in the last thirty years from, you know, essentially no evidence for planets existing around any other star in the universe to now, in the space of thirty years, knowing that they're ubiquitous and, you know, essentially an inevitable outcome of of a star being born, which is a remarkable transformation, our understanding of the natural world.
57:01 Speaker 05 Well, thank you very much. That was terrific. I think Sam is on his way. Who'd like tea or coffee? Melvin, do you want tea? Tea, please, yes. Tea would be wonderful.
57:12 Speaker 03 Tea, thank you very much. In Our Time with Melvyn Bragg is produced by Simon Tillotson and it's a BBC Studios audio production.
57:21 Speaker 07 The Wraith Lectures 2024, hosted by me, Anita Arndt. The series is about the complexity of human violence. All violence is not the same and all violence perpetrators are not the same. Four questions about violence, explored by the forensic psychiatrist Dr Gwen Adzhead. By listening to perpetrators, we can learn more about the genesis of violence and perhaps particularly where we might be able to intervene to reduce the risk of violence happening in the future. The Wraith Lectures from BBC Radio 4. Listen on BBC Sounds.
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