The diversity of Rocky Worlds with Dr Oliver Herbort

Welcome to Growing Worlds, the podcast when we explore the science of exoplanets through conversation with early career researchers on the front line of discovery. I'm Lorenzo.

I'm Jenny. In each episode we meet a guest who takes us behind the science of planets, stars and the search for life in the universe.

Today we turn our gaze toward rocky worlds, Earths and hurt likes that sparks our curiosity about habitability.

And help us understand where our beautiful home world, the only known place in the universe to host life, sits in the vast sea of small rocky worlds out there. We're joined by Dr. Oliver Herbert from the University of Vienna. Oliver, welcome to Growing Worlds.

Thank you very much, Jenny and Lorenzo. It's a pleasure to be here.

Oliver is an Ariel Postdoctoral Fellow at the University of Vienna, where he studies the atmosphere of rocky exoplanets, particularly how gases that we can observe tie back to what's going on below, so on the surface and the inside of the planet. He earned his PhD in 2022 at the University of St Andrews in Scotland, His recent work includes developing models that link chemistry of an exoplanet atmosphere to its surface mineralology, which is going on using spectral data to infer whether the planet's crust is rich in sulfide, silicates, and iron oxides. I hope I haven't mispronounced anything here.

because it's quite a complicated job that we are doing, Oliver.

So, know, all the easy things really. So I think before we really dive into the topic, Oliver, it would be wonderful to learn a little bit more about you. So to begin, could you tell us like what sparked your fascination with rocky planets? Was it our home world or was it something else entirely?

back in my youth, like, well before I started studying physics in Göttingen, central Germany, I was always curious about the world itself, some science in general, physics in general. But particularly, I loved astronomy. I loved getting to know, like, different planets, what planets are out there.

If we look back in time of like 20 years or so, there were not that many planets known yet, but quite some already because the field of exoplanets is a really young field that

Mm-hmm.

is 30 years old basically. So I kind of grew up in an age where, we have new planets, we detect more planets, we have even more planets coming in.

Hello.

There's a lot of things that weren't already learned in the newspapers or so.

or on some of the German TV shows that kind of sparked the interest in understanding some science, understanding planets, understanding their atmospheres. And it was quite curious that quite some time ago, I actually saying, it would be super cool to investigate rocky planets and their atmospheres, how diverse they could be, how different they are.

And then at some point later, I started to study physics and like always had in my head, I want to do astrophysics. I want to do planet.

Mmm.

But it was like, yeah, keep an open mind to other fields of physics. Also interesting fields like aerodynamics, example, fascinating field. But I basically came back to, to do astrophysics, did some other stuff in the meantime, like for my bachelor thesis, working on globular clusters.

and the question of whether there are intermediate mass black holes in the center of these, because they're kind of scaled down versions of cores of galaxies. Still an ongoing

Mmm.

field of research right now. It's also fascinating, but I then went to more radial velocity measurements of stars and detections of exoplanets for my master thesis, and then, as Lorenzo said earlier. to rocky planets. So it was always there in my head, got to

Yeah.

do some other stuff and then went to rocky planets. And now I'm actually there where I thought I would like to be like 15 years ago, which is quite fascinating.

Yeah, so it was more kind of the influence of researchers, their stories hit in the media rather than kind of practical astronomy because of course you can't really see any small rocky worlds outside of our solar system from your backyard.

that kind of encompasses it quite nicely. And yeah, it's just a question of like TV shows, like documentaries about the universe, documentaries about

Mmm.

Titan, for example, like just a really, really cool world to explore.

Yeah, and were you curious at that time about, you know, how different they could be to our Earth, how similar they might be? Were you already kind of wondering around those lines?

Absolutely. So this the question of like how are they how are they different to what we know is kind of one of the driving questions of like what can be out there to really have an agnostic view on so it to just look at it and just say Not just we in our solar system. We have like these kind of three types of atmospheres Or four maybe that doesn't matter how many there are but like these are the ones we have and that's it No, no The universe is much more diverse. The planets in our galaxy are just much more diverse than what we have here. And quite some of those planets that we found as scientists over the last decades have revealed exactly that. Things that we haven't expected before. Hot Jupiters, planets that are gas giants but really close into the host star with really high temperatures.

We don't have those in our solar system, but they exist. They're actually relatively easy to detect and relatively easy to investigate, but they were not expected.

No, because we, as you say, we don't have one. So, and you, don't know, you know, what to look for unless you know what to look for kind of thing. So it yeah, very unexpected.

Yeah, exactly. Expect the unexpected, kind of.

Mmm.

And so you always suspected that there was some diversity out there, that it wasn't as simple as the solar system was showing. It didn't come to you as a surprise that the variety that we are observing now.

That is a good question, that is an excellent question. I don't know, I probably haven't thought about some of the intricate questions of, for example, like carbon planets. didn't imagine that use ever to exist because that just never came up in my mind. But wouldn't it just be boring if everything in the universe would be like our solar system?

yeah, for sure.

Yeah, now.

Before we carry on, do want to take a little side track into carbon planets. What are they? I've not heard of these. What is a carbon planet?

Let's come back to that in, I don't know, five years or something. No, joke aside, planets that are very rich in carbon, that carbon is like one of their main components by mass for the whole entire planet. Do these really exist? That's a really excellent question. Some of the planets that we know so far would be kind of.

compatible with those carbon rich planets, but do they really exist or is it just a model scenario that remains to be seen because we really have to investigate them further and to actually look at

So can you take us through a little bit more about the sheer diversity we do see in small rocky worlds out there?

Yeah, because we always talk about atmospheres, right? But you also work on mineralogy. So what happens on the ground? So can we imagine things like mountains of diamonds or caves

Yeah!

of gold and liquid oxygen? I don't know.

Yeah, because we do see these big media stories, right? Where they're like, there's this planet where it's like raining diamonds and, you know, all these plants are bits and plants are that. it's like, is that actually true? I you are our expert here. Or is that all just wishful thinking pie in the sky?

There are about six questions you've just been asked. I try to get all of them. If I forget one of them, if I forget

Yes. I said,

You

one of them, just come back to me. starting from the first question there, like what kind of diversity to expect So in our solar system, we do have like Earth, nitrogen and oxygen dominated, a bit of CO2 and some like methane. If we look back in our history or our own Earth, there was not that much oxygen before the great oxidization event. So there's already a time change there. If we look on Venus, CO2 dominated. Mars?

No.

not a thick atmosphere, but CO2 dominated atmosphere. Titan, methane dominated, actually, nitrogen methane atmosphere at different pressures that we have. We have the 90 bars of Venus, the one bar of Earth, the couple of millibars of Venus, of Mars. And that kind of pressure regime, the pressure changes, I would expect for other rocky planets to...

to be there as well. Some planets will maybe actually have even higher pressure. Then things go really wild because at some point the assumption of an ideal gas, for example, doesn't really hold at all anymore if you have a thousand bar atmospheric pressure. just chemically a completely different subset.

Wait second, let me call you here in a moment. So we are talking about atmosphere of different molecules and atmospheres of different

Yes.

pressures. Can you compare this to the Earth and to what does it mean for atmosphere to have different pressures of different molecules? What are the implications of this? Help us.

What are the implications of those? if we assume our planet right now, which has a small fraction of carbon dioxide in it, if we just switch that fraction of carbon dioxide, just randomly selection, no reason at all, if we just crank that up a bit, what would happen to the planet? It would heat up. Nudge, nudge, global warming, don't do that. But just

Ha ha ha ha.

Okay.

understanding that, understanding these other planets, how they are different, how they react differently to the incoming radiation from the host star. What does that have for effects? That's, think, one of the things that we can learn from exoplanets and apply to our own solar system planets. If we don't have oxygen in our Earth's atmospheres, life as we currently have it on Earth, most of it, or at least the land dwellers, will have difficulties or just cannot.

exist because they need oxygen to breathe.

Mm.

So our Earth, we are oxygen breathing animals because we evolved from on Earth at this stage what we are. How would that be in a different planet where it doesn't evolve to oxygen rich in an atmosphere? I don't know. Would be very different, would be very interesting to look at.

maybe some other planets that don't have that much nitrogen. Actually, doesn't really do much in our atmosphere. There's the N2 dominating the atmosphere. There's some nitrous oxides, but that's basically it. Nitrogen is fundamental for life to be incorporated in the life forms. If the planet has, or the atmosphere of the planet doesn't have that much nitrogen available because it was lost early on, for example, that would evolve totally different.

Mmm.

they are just by changing the certain parameters of these small, maybe someone would say insignificant things, they would change vastly the track of the planets, the evolution of the planet. We are here because we have the track we had. There are so many points where the track could have just diverted and There are planets out there which are just similar to Earth, but just evolved slightly different for some by chance or just by some tiny reasons. Some other planets may have a lot of sulfur or may just be a bit hotter, may have more carbon in the atmosphere, more carbon dioxide, more hydrogen in the atmosphere. So actually being not the oxidized forms like carbon dioxide, like NO2.

or sulfur dioxide if you have quite a lot of sulfur, but rather the reducing forms, something like methane or ammonium in the atmosphere. Totally different planets, but could also exist.

I think that is probably the big question, isn't it? Is whether our atmosphere, the evolution of our atmosphere, is that particularly critical for the evolution of advanced life? Right. Because, I mean, I'm of the opinion that it's simple life, single cell, know, microbes, things like that. We will probably find that even in our solar system somewhere else, let alone around other planets like that.

Probably, this is my opinion, right? We haven't found anything yet, but this is what I think. And then, you know, complex life and by that, that's not even, you know, humans which are going out into space. That is just, you know, multi-sangular, you know, small creatures, plant life, things like that. does that require specific circumstances to emerge? I guess.

looking at all of these different atmospheres and trying to understand all the different possibilities out there and where Earth sits in that is probably a big question in planetary atmosphere research.

Yes, it's the entire field of astrobiology, which isn't just planetary atmospheres. It's drawing from the geologists, from the biologists, from the physicists, from the chemists. And if all of them work together, we can try to tackle the question of where does life form? That's one question. The other question is how does life actually evolve on a planet?

that is linked to the stellar evolution, to the planet evolution, the interplanetary interior

Move.

evolution, whether you have enough volcanism, but not too much, because too much volcanism also is not good again. So you have to be

Yeah, hello Venus.

really in that sweet spot.

Mmm.

Why are we in the sweet spot? Well, we are here because we are in the sweet spot, I would say. So

Mm. Mm.

it's difficult to ask the question of like, what, why?

like why wasn't it different for us? Well, we are here because that's the track we had. But what would change if we have more volcanic eruptions? Here we probably have a higher CO2 composition, partial pressure in the atmosphere, more greenhouse gases, hotter planets. The life that we have currently wouldn't really like it. Maybe some other life forms just really love that slightly warmer planet so it's not us humans, it's some other creatures roaming Earth. So the question there, how we actually come to the place where we are, how actually we would evolve differently, really important questions, really, really cool questions.

I'm totally

So.

on your side for there will be small life forms, small microbes or something.

Okay.

It's already really difficult to get there, to form those first forms. We know on all comets or so, there are the amino acids, which are kind of the building forms of life. So we have amino acids basically everywhere. Even in the intestinal medium, we have like

Yeah.

Amino acids, are all over the place. But you have to do stuff with it. You have to kind of

Boom.

exploit some energy source with it. Some hydrothermal vents or so like some hot interior water going out to a water ocean could be on Enceladus for example. There is some energy source there. If you have amino acids there, some energy source, they can

you

accumulate, they could do something. So there could be some microbial life there. Will it grow larger? Well, they have to have enough nutrients. They have to have enough energy to support themselves.

Yeah, and that oxygen production on Earth was key for our planet was, know, when micro-obstacles creating oxygen, dumping it into our atmosphere, it then could be used as an energy source for more advanced, well, to kind of unlock the energy rather than an energy source, but in order to unlock energy for more advanced forms of life. And that oxygen production was critical.

Absolutely, that's a critical part of doing the evolution of life. But also don't forget it's not just a positive effect. It's also for a lot of life forms the great oxidization event is actually a very very detrimental thing

You

Mm-hmm.

because Why did they produce the oxygen? It was their waste product

Mm.

Yep.

Basically we humans breathe out carbon dioxide if we're in a closed room we have too much carbon dioxide, we're suffocate. The same thing happens for those small microbes. A lot of them did not survive the great oxidization event. So in a slightly alternative world, just life died out.

Hmm.

There's no humans on that planet, just by chance. But some survived, and those surviving ones brought us here at some point. making us able to actually wonder about these questions.

Yeah, and of course, like we can't go back in time, right, and actually look at the old Earth and we can't go out to all of these rocky planets and actually sample them and things like that. So how do you go about actually studying all of these different atmospheres, understanding them, theorizing new atmospheres? Can you kind of take us through what you actually do to understand?

the diversity of small rocky planets out there.

So there are two approaches here. One approach is to go from the bottom to the top basically. To go from, we start with the surface and then we build an atmosphere and then we look at like how they would look in with spectral observations. That's the one side, that's the modeling side.

When you say we start from a surface, what kind of surface are you talking about?

That surface, at least in my models, it's a relatively simple model. We just, we assume that everything is nice and easy. Everything is in chemical equilibrium. So the most stable state, which is wrong. But all models,

Hahaha!

Okay, good point to start with,

all models are wrong. Some are useful. And I'd argue

I like that!

that the models that we are producing are quite useful. We know there are some limitations.

You always should state your limitations, what your assumptions are, and then you're fine. So we assume that everything is neat and order, everything is in chemical equilibrium, and we have a certain rock phase in contact with an atmosphere in chemical and phase equilibrium. So basically, a condensate species is saturated, you can imagine water as an example. If you have 100 % humidity of water in your in the gas, will condense the water. If you have less than that, the water will evaporate. So a planet that globally is under saturated in the atmosphere with water will at some point just boil it off and doesn't have any liquid water anymore. Also no water or ice. However, if we are actually super saturated, so we have more water present, that would like to be in the atmosphere but can't because it's condensing, then you have to condensate there. You can do the exact same calculation with other minerals and basically look at what minerals should be there and what kind leftover atmosphere is there. And there's some work that

And this is Botanar.

you can have to characterize different atmospheres, so like the oxygen-rich atmospheres, some methane-rich atmospheres, and some atmospheres where you have the coexistence of methane and carbon dioxide, water gas you have in all of these basically, nitrogen you always expect in all of these. So you have like these kind of three groups of different atmospheres and some of the minerals only appear in just one of these groups or two of these groups. So if you have a certain atmosphere and you see some methane, you very most likely do not have any calcium sulfate, so CaSO4, as a condensate. Those are just not compatible.

Okay, so you kind of in the computer you build all these different versions of small rocky planets with all slightly tweaked levels of things in their atmosphere and then depending on what's in the atmosphere that can tell you then about what the surface is sort of made of, what can be present, what minerals are there, what chemicals are there, what can't be. So then when you have a look and you sort of steadying atmosphere, can be like, okay, we've detected hydrogen and nitrogen and oxygen at these ratios. What does that tell us about the surface? Is that what you're doing? very cool.

Exactly, exactly, that's exactly what I do, or at least what I try to do. The difficulty there

true.

is that it's not a, I have these four molecules in the atmosphere, and that's a direct link to exactly this composition. There are

Okay, yeah.

chair.

a lot of degeneracies in there. And

Okay.

with the current state of observations, well, we cannot really just observe it and just say, yeah, we have. 78 % nitrogen and 22.5 % something

Yeah

or something like that. No, that's just not possible. Like, yeah,

Okay.

there is oxygen in there. There is some CO2. At

Okay.

most like a percent. That's the kind of what we can kind of observe in the near future.

Okay.

But with that, just the choice of molecules that we find.

we can constrain some parts of the surface. And it would be so cool if we can just, if we look at a planet and say that planet has this kind of gases. So most likely it's going to have these kind of molecules at the start, these kind of minerals on the surface.

No.

Finding that out would be absolutely fantastic. It's not a one

Yeah, I guess that's.

to one link. but it's a way towards it.

Yeah, and that's the thing is because if these planets are covered by these atmospheres, you can't see the surface. It's like

Exactly.

the issue we have with Venus. OK, we can use sort of ground penetrating radar. We do not have radar that can reach exoplanets, right? So our only way then to kind of try and figure out what is going on beneath the clouds, as it were, is by doing all of this modeling and trying to make the connections are hidden.

Exactly, that's the point. And

Mmm.

if we look on the Venus example, actually some of our models suggest that if you, for example, have a relatively temperate part of the atmosphere, like around one bar, you have like some somewhat like a few, there are 300 Kelvin, roughly Earth conditions at one bar, which actually is at the Venus cloud layer also roughly the temperature and pressure.

and you do have some cloud species of like H2S or H2SO4. What does that tell us? Or, yes, sulfuric acids, so acid

What's that? What's that in words? thank you.

a lot of letters.

clouds, Venus has. And if we have those, you have to have the sulfur there. And if your surface is just underneath that, you're just not warm enough to have the sulfur in the atmosphere. You may have just had a recent volcanic eruption that gives you some acid rain like it does on Earth, but that's not a long-term stable thing. On Venus, the sulfuric acid clouds are a long-term stable thing. So you have to have a deep atmosphere with a really high surface temperature to sustain the sulfur in the atmosphere. So finding

I see.

as an example, H2SO4, H2S clouds on an exoplanet at a relatively low temperature, that would mean this planet either had just massive volcanic eruptions just recently, or we actually have a thick atmosphere underneath. And that's more Venus surface.

Yes, and that's the thing is because you know, in the solar system, like Venus and Earth, nearly the same size. So you can't just go on sort of size alone. They are so completely different.

Absolutely, absolutely. And every nitty-gritty detail that we can kind of squeeze out of the limited amount of data that we have or that we can obtain in the near future is going to be really interesting for us. And it's going to be just super fascinating if we could say that planet actually, because we see this one thing or these two things here, we can constrain some parts of it. Some constraints of that.

to have like, we have to have this kind of mineralogy there means, we can actually probably say something about the interior at some point.

Bye.

Because the interior, even on Earth, well, we can't really go there. We can dig a few kilometers into our crust, but that's it. We can have seismology. But we can't really go there. For exoplanets, it's even worse. We can't even go into the surface. Actually, As you mentioned earlier, the planets that have an atmosphere, the atmosphere is covering a lot of the potential observability of the surfaces.

Mm-hmm.

If we look at those rocky planets around dwarfs, so really small stars, they're really active and they probably just blow away the atmospheres of those rocky planets that are relatively close in.

You it's just so frustrating because like dwarfs, these red dwarf stars like, like Proxima Centauri, they're everywhere, absolutely everywhere in our galaxy. And it's like, but they're

and

probably horrendous for life.

And they're not just horrendous for life, they are horrendous for the atmospheres. So

Yeah.

Good.

at least life as we know it, you kind of have to have an atmosphere for it. Can there be other life forms that kind of evolve just on a bare rock? I'm not a biologist. Maybe something could work. I don't know. But

Mm.

we know that under the conditions of liquid water and an atmosphere, you can evolve. That's what we know. So let's just stick for the argument right now for that. So we have a rocky planet, we lost our atmosphere,

Yeah, that's the thing is, unless...

what can we do? We can do a lot then. We can actually look at reflected light and the reflected light of that planet, maybe actually tell us something about kind of the mineralogy. And then we can, if we do that not just for one planet, because all of the rocky planets that we have so far observed, the small rocky planets.

are consistent with no atmosphere. maybe we can look at them and just look at, let's look at the reflected light and which kind of rocks at some point are kind of mimicking the reflectance spectra of these. It will again not be a one-to-one thing. It's like this one rock covering the entirety of the planet because the planetary surface in itself is diverse. There are multiple.

different rocks there. But maybe we can exclude some things and saying that is not this is also really fascinating.

Yes.

Oliver, I'm glad that you mention rocks again, because I have this question with me for a while now. What should

He

I look for if I want to go somewhere and collect diamonds with a bucket?

Yeah.

Just going around with your bucket finding diamonds. Good luck. If you find

I'm

a good place, don't tell anyone. You collect the diamonds first. Probably have to go underground. Since I did, for example, my...

my undergrad, my bachelor and master's in physics, not in geology at all, I only went into the geology part doing my PhD. I learned so much about all the different minerals and

Mmm.

fancy names of the minerals. And whenever I go somewhere and I just see some fancy rocks and I'm like, I use this in my models. This is like kind of basaltic rock. This is so

I love that.

cool. I have that in my models.

Yeah.

or you go to a and they're like, oh, this is, don't know, Lysidite or so. It's just like, cool. I know this. This comes out as a result in my models. This is like in there and it looks really cool. Absolutely fantastic.

Yeah, be like,

rocks.

I play with that and I play with that and I use that and that's there and yeah.

Pyrite, actually a really fun thing, like FES2, actually sparkles like gold, like fools gold. It's

Full of goals, yeah.

Yeah, food's cold.

really cool to find and then you walk around and you just see them because you can actually find it in quite a lot of stones that bridges are made out of, for example. And you just look closely to those bridges and you're just like, there's some Pyrite here. Most likely, mean, it could technically be also be gold, but most likely it's Pyrite.

Yeah, I don't think if it was real gold they would have left it there.

Mmm.

I have some of my planets have that as like cloud condensates in there. And

Mmm. Wow.

that you're just imagining planets with sparkling clouds. It's just really,

Wow.

really cool.

Okay, so...

What other kind of wild, cool planets are there that are in your models? You know, we've got pirate in the clouds and what other things?

One other absolutely fantastic thing is lava walls.

Love a wolf.

Lava worlds, yes. So planets, like on our earth we have like occasionally there's a volcanic eruption, there's lava there.

Mmm.

But now imagine an entire planet covered in lava, in magma. It's just so hot. It's just,

And as in this is not just a planet that has just formed.

no, a planet that's just so close in that the radiation just melts the rocks on the day side at least.

So an entire planet which is sunburned.

Yes.

I'm melting!

Well, actually Lorenzo, your point of the entire planet, good question. Are those tidally locked? do they show the same size to the planet? Do they actually have a molten day side and a solid night side? Or are they actually molten throughout? It's a question that is still unsolved, or at least for some of the planets like Road 7B.

We do actually have some data suggesting that the night side is much, much, much, colder. The day

Hmm

side is thousand Kelvin hots, so molten rock. The night side not. Some of the young ones that you mentioned, Jenny, they probably are doing their formation, have a global magma ocean, just magma all over. Can you kind of retain that for not just a hundred years or a hundred million years, but like over giga year time scales?

Yeah.

Can you retain that if you're close enough in? Could you have a molten-day night set? Do those planets have atmospheres?

Yeah.

And if so, what are those atmospheres?

Mmm.

That's also really interesting. And currently, there are observations being taken for 55 concrete, for example, that suggest that there should have been a CO2 atmosphere. There some other data sets that say, not. But that's what...

drives the fascination forward. Maybe there are clouds forming and then dissipating against it. Sometimes you see it, sometimes you don't. There lots

Yeah.

of explanations for it, and that's what drives the interest in there. We don't have that in our solar system, but that's what makes them interesting.

Hmm.

Wow, that's amazing, that's a lot. A lot going on up there. Oliver, I think we are close to the end of our time together here. So our closing question, what planet, exoplanet or celestial body represents you and why?

That's a tough question. I think one of the really fascinating ones that is really, really interesting and has a lot going on and is just super active and does a lot of things at the same time is actually one of them that I just mentioned. 55k Cree would be a planet

nice.

that is doing so many things. There's the magma site.

maybe the colder night side, outbursts potentially. So much is going on there, clouds, whether the atmosphere is rich in metals or not. So many things happening there. And I think as I am quite an active person that does a lot of things and has difficulty

Mmm.

saying no to activities. So I think that's kind of encompassing quite some of my nature.

I love that. And I also feel your pain because I too am someone who has trouble like saying no to things. And it leads to a very exciting and busy life, but also a very exhausting one.

Absolutely, and that's also why I was here. Lorenzo just asked me, like, do you want to be part

Say no.

of this? Absolutely. Absolutely I want to be. And it was a fantastic chat with you. So I didn't

you

really give it a second thought of whether or not I was going to be here. Just like, absolutely, yes, of course I'll be here, Lorenzo.

Mmm.

Thank you, thank you Oliver and thank you for guiding us through all the diversity of rocky planets and the atmosphere and the surfaces and the rocks that are up there. Thank you a lot.

Thank you so much for having me.

to everyone listening. Thank you for tuning in to Growing Worlds. Next time we'll explore