The hunt for habitable exoplanets, and how a warming world could intensify urban air pollution

This episode dives into the ongoing exploration of potentially habitable exoplanets and examines the link between rising temperatures and worsening air pollution in urban environments.

1. The James Webb Space Telescope offers unprecedented views into the atmospheres of distant exoplanets, though direct observation remains elusive due to the planets' proximity to their stars and the overwhelming glare.
2. Research on exoplanets is hampered by stellar activities such as flares and sunspots, which can obscure or distort the data needed to confirm the presence of atmospheres.
3. In urban environments, rising temperatures accelerate chemical reactions that worsen air pollution, a factor not fully accounted for in current emission models.
4. Natural emissions from plants, particularly terpenes, play a significant role in forming ozone and aerosols, overshadowing declines in anthropogenic VOC emissions from vehicles.
5. Strategies to mitigate urban air pollution should include reducing fossil fuel use and considering the emissions characteristics of plants when planning urban green spaces.

1. The Exoplanet Exploration

Explores advancements in space telescopes that enhance our understanding of exoplanets. Researchers focus on the Trappist one system, hoping to find signs of habitability. Dan Cleary: "The capabilities of JWST have opened new possibilities for examining distant worlds."

2. Challenges in Observing Exoplanets

Discusses the technical and observational challenges in studying exoplanets due to stellar variability and the limitations of current telescopic technology. Ava Fannerstill: "Stellar variability complicates data analysis, pushing researchers to develop new techniques."

3. Urban Air Pollution and Climate Change

Analyzes how increased temperatures affect chemical reactions that lead to more intense air pollution in urban areas like Los Angeles. Ava Fannerstill: "Our research indicates that plant-emitted VOCs significantly contribute to air pollution, especially on hot days."

1. Support advancements in space exploration technology to gain deeper insights into potentially habitable exoplanets.
2. Advocate for and participate in urban greening projects that consider low VOC-emitting plants to enhance air quality.
3. Reduce personal vehicle use and support policies aimed at decreasing urban dependency on fossil fuels.
4. Stay informed about local air quality and participate in community efforts to monitor and improve it.
5. Educate others about the impact of climate change on urban environments and promote sustainable living practices.

On this week’s show: Scientists are expanding the hunt for habitable exoplanets to bigger worlds, and why improvements in air quality have stagnated in Los Angeles, especially during summer, despite cleaner cars and increased regulations

Staff Writer Daniel Clery joins producer Meagan Cantwell to talk through the major contenders for habitable exoplanets—from Earth-like rocky planets to water worlds. Preliminary results from two rocky exoplanets have some researchers concerned about whether they will be able to detect atmospheres around planets orbiting turbulent stars.

Next, producer Ariana Remmel talks with Eva Pfannerstill, an atmospheric chemist at the Jülich Research Center, about how volatile organic compounds, mostly from plants, are causing an increase in air pollution during hot days in Los Angeles.

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Megan Cantwell
This is a science podcast for June 21, 2024. Im Megan Cantwell, filling in for Sarah Crespi.

Dan Cleary
First up this week, staff writer Dan.

Megan Cantwell
Cleary joins me to talk about the hunt for habitable exoplanets. What started as a search focused on Earth like rocky planets is expanding to planets with thicker atmospheres, or even planets wrapped entirely in water. Next, producer Ariana Remel talks to researcher Ava Fannerstill about an unexpected source of volatile organic compounds causing air quality to worsen on hot days in Los Angeles.

Dan Cleary
If you were to be on one of the planets in the Trappist one system, what would that be like? What would that feel like?

Ava Fannerstill
Living on one of these planets around TRappis one would be very bizarre to us. You'd be living in this sort of strip around the border of nighttime and daytime, in perpetual sunset. The other thing that would be very peculiar would be the other planets, because they're all very bunched close together. When one came close, it would be larger than the moon. If more than one of those planets were inhabited, you would be able to see the lights of towns and buildings and so on on a neighboring planet as it drifted by. It would be a strange, very science fiction like existence living on one of these planets.

Dan Cleary
There's no doubt that living on a planet in the Trappist one system 41 light years away from Earth would be nothing like existence as we know it. But that doesn't rule out the possibility that there could be some sort of life present. This week in science, staff reporter Dan Cleary wrote about the hunt for other earth like planets and the hurdles that researchers have faced so far in this pursuit. Thank you so much for joining me on the podcast, Dan.

Ava Fannerstill
Yeah, my pleasure.

Dan Cleary
This is definitely not the first sort of story where you've covered, potentially life on other planets, whether other planets are habitable. But now, within the past few years, JwST finally launched, giving us new insight into these planets. What can we probe into these planets that we couldn't before?

Ava Fannerstill
What JWST gives them, first of all, is it's a space telescope. So it's looking at objects a long way away, but without the interference from the atmosphere in between.

JWST is not only a lot bigger than Hubble, but it covers a very different wavelength range. So it's mostly in the infrared. It can look at cooler objects. It can hopefully detect molecules that are useful to life in the atmospheres of these sort of planets that we've been talking about. They already are discovering amazing things, but mostly, thus far, about much bigger and hotter planets to look at the smaller, cooler ones that could possibly host life. It takes more time.

Dan Cleary
Researchers have had such a long time to dream up their ideal list of exoplanets to look at that may be habitable. What are the main ones that researchers are hoping to look at with JWSt?

Ava Fannerstill
Well, apart from Trappist one, which is the sort of ideal because it's so close, it's only 41 light years away. It has these seven earth sized planets. And as such, it makes a sort of ideal laboratory for astronomers to look at habitability, because they've got a whole collection of planets of different temperatures, because they're all different distances from the star, so they can see how temperature and their different orbits affect their possible habitability.

You could look at different sorts of planets other than small and rocky. The slightly larger planets, you could have water worlds. There's one particular one that is, again, quite close to Earth, which is called LHS 1140 b. It's about 1.7 times larger than Earth. There's an even larger planet, which is called K 218 b, which is more like Neptune than it is like Earth. It's got a thick atmosphere, which is going to be principally hydrogen. Planets are formed with hydrogen, but if they're small, hydrogen just boils away because it's very volatile. But a bigger planet can hold onto the hydrogen. Observing a large planet like that with a thick atmosphere is easier for JWst to do, because the signal of the light coming through the atmosphere is going to be stronger. So some people say we should be looking at large planets like that, because your chances of getting a positive signal are greater.

Dan Cleary
The planets that are in the Trappist one system, they're similar to us in size.

Ariana Remel
Right.

Dan Cleary
What are kind of the factors that are making not only an exoplanet interesting to observe, but what make them good specific, specifically for JWST to get interesting results from.

Ava Fannerstill
Despite the great power of JWST, if they look at a system like Trappist one, they can't actually see the planets themselves. And the glare of a star is still too great, and the distance between the planet and the star is too small for even a big telescope like GWST to separate them. So what you have to do is look at how the light from the whole system changes. And that works particularly well if youre looking at the system edge on. So youre seeing the planets move around the star, and they pass in front of the star, so theyre blocking a little bit of its light. And some of that starlight is coming through their atmosphere. And also when they move behind the star, suddenly the light from the planet is disappearing. And so by comparing the light from the whole system with the light, when the planet is suddenly gone, you can subtract one from the other and get the signal that is just coming from the planet. So those are the two techniques that astronomers are using. A transit when it goes across the front, and the other technique is watching it disappear behind the star and seeing what disappears. Edge on planetary systems are key to studying exoplanets, but also closeness to earth. If you go very much further away, it gets much, much more difficult, and you have to use a lot of telescope time to get the same information. Telescope time is very precious.

Dan Cleary
It seems like, okay, there's seven planets. Maybe they would have been able to take a look at everything by now, but that's definitely not the case. They have not looked at every single planet in the Trappist one system. So the two that they've looked at so far, have the results been promising there, or were they expecting to see an atmosphere or potentially anything habitable?

Ava Fannerstill
Yeah, well, they looked at the two nearest to the star, which are called b and c. They didn't find any evidence of an atmosphere, or at least not a thick atmosphere. It's possible that Trappist one c might have something very much thinner than is on Earth, and we can't detect it yet. But those results were a bit disappointing. They have looked at most of the others very briefly, and again, they couldn't detect an atmosphere. But what they did discover is that the star is making their job much harder. Ideally, you would have a star that would just give a nice, steady backlight for your observations. But actually, the star has a lot of sunspots and flares, and that complicates the data you're trying to get about the planet, because you've got to disentangle this sort of variability of the star from the variability of the planet. And that's forcing astronomers to come up with new techniques to try and separate those two signals.

Dan Cleary
So do they think it's going to be possible to separate it, or could it potentially not be possible? In which case, we aren't going to get any sort of clear answer on anything in the Trappist one system?

Ava Fannerstill
Well, they're clever people. They're coming up with new things in the third year of observing, which is just about to start. They're testing in one of these techniques where they're looking at two of the planets around Trappis one, planet b and planet e. Since they now know that b has no atmosphere, they're looking at these two planets which transit the front of the star in quick succession. So they know b is going to produce a spectrum with no atmosphere, and then they can compare that to when either transits across which when the star won't have changed very much, and e, they hope, has an atmosphere. So any differences between those two spectra, hopefully will be signs coming from the atmosphere of Trappist one e. That's the plan, but it's going to take a lot of transits over the next two years to get enough information to see that signal.

Dan Cleary
And how many times is it transiting in a view that we can see per year? It's a lot more than our Earth, right.

Ava Fannerstill
All of those planets, the longest orbit is less than three weeks. So they're all making quite quick orbits around the star, but just because of the orientation of the telescope and so on, you can't look at them all the time. This little experiment of comparing planet e and planet b, they're going to do it 15 times over the next two years, and hopefully that'll give them enough signal. They'll be able to discern something of an atmosphere above the noise of the star.

Dan Cleary
This is going to be done over the course of two years. When it comes to looking at those planets in the Trappis one system that are a little further from the m dwarf, how much longer is it going to take to observe that? To one get an answer of does it have an atmosphere? But then additionally to that, what's actually in the atmosphere?

Ava Fannerstill
So looking at lots of transits, I've seen estimates of over 100 hours of observing, so many, many, many transits to get a detection of carbon dioxide. So that's a molecule they think probably exists around these sorts of planets, and it has a very strong signal that JWST in particular can pick up.

Dan Cleary
Could you put into context 100 hours in a year? How much time is that from all of the time that JWST has to.

Ava Fannerstill
Observe in a whole year, it has 10,000 hours. A lot of that goes to completely unrelated sorts of science. So of those 10,000 hours, about 30% is typically spent on exoplanets, and thats all different sorts of exoplanets. So the amount of time spent actually looking at TRappist one is maybe a couple or 3% of the total time, but still that is dozens or hundreds of hours.

Its a small chunk of what JWST does, but everyone wants to use JWST, so there's a lot of competition of.

Dan Cleary
The people you talk to. What are kind of the main ideas that people think we should be investing in right now to find these habitable zones?

Ava Fannerstill
A lot of people think trappist one is what we should be focusing on, because it's so close, it has so many planets. The space Telescope Science Institute, which operates JWSt, they're donating 500 hours over the next year to sort of widen this search. And so astronomers have suggested what they should do is just look at 15 to 20 other systems where there's a known earth sized planet and just see whether there's an atmosphere around it.

Dan Cleary
These other planets, are they also similarly orbiting around these more turbulent m dwarfs, or are they in very sort of different systems?

Ava Fannerstill
Typically, they would be around m dwarfs, because you have this issue of not having a star that's too bright, which would just overwhelm the signal from the planet. When this type of star, m dwarfs are young, they're particularly turbulent, and they blast out a lot of x rays and extreme ultraviolet light. It could just boil the atmospheres away. And so these earth like planets very close to that star could have nothing. They could just be bare rocks. We don't know. And that's why the search is so intense at the moment, just to find out whether some of these atmospheres survive.

Dan Cleary
If it turns out that it's unlikely that there will be an atmosphere around an m dwarf, then what other types of planets could potentially be habitable?

Ava Fannerstill
People have done a lot of atmospheric modeling. So taking what they think the ingredients of a planet will be and playing around with the parameters to see what atmosphere it might have, whether it would have oceans.

There have been a lot of suggestions that you might have water worlds, an ocean that just covers the entire planet, and that could be a habitable exoplanet. It might not have anything that looks like us on it, but it could have microorganisms that thrive in global ocean.

Dan Cleary
When we're thinking about biosignatures that would be present in an atmosphere that has life, we're obviously thinking about our own planet. But I guess if it were a water world, could it potentially have different biosignatures, or what could we expect to look for that would indicate that there's life there?

Ava Fannerstill
The whole subject of biosignatures is very tricky, because we only have one planet with life on it. We have life growing in hot springs or hydrothermal vents deep under the ocean, where there's no light, but they're getting heat from hot brine coming up from beneath the seabed. So we know that life can live in all sorts of bizarre situations. But viewing an exoplanet from tens or hundreds of light years away, trying to discern that is very, very difficult. What you're looking for is an atmosphere that's in disequilibrium. So it's in a state that you cant explain how it could maintain itself in that state unless there was a biological mechanism going on in that planet. What JWST can do is find the planets that have the right physical conditions, the right temperature, liquid water, enough light getting to the surface, all of those things that life might need. Thats the sort of thing you couldnt discern with JWSt. But actually spotting whether life is there may have to wait for the next telescope.

Dan Cleary
Are there plans for any other telescopes that could potentially gather this information?

Ava Fannerstill
Yeah, there are. NASA.

It's very, very early stages, but they are looking a larger telescope, which is going to be called the habitable world's observatory.

Dan Cleary
That sounds very targeted to look for this stuff, then.

Ava Fannerstill
Exactly. Exactly. Which would be similar to JWst in that it has a folding mirror, but it would be larger, and it would probably look at different wavelengths. It might be optical light rather than infrared. But other people are also looking at the possibility of having a sort of flotilla of separate telescopes. If you have several telescopes flying in formations hundreds or thousands of kilometers apart, and you combine their light through a process called interferometry, you get a very, very detailed image, as if you had a telescope that was as wide as those telescopes are separated. So say they were 100 km apart, it would be like you had a 100 kilometer telescope.

Dan Cleary
Wow, that's cool.

Ava Fannerstill
Yeah. So it doesn't gather as much light as 100 km, but it has the resolution, the fineness of detail. And with something like that, you might be able to zoom in on an exoplanet and see whether it has areas of ocean and areas of continent or cloud cover or ice caps, things that we would recognize in an image of a planet in our solar system that requires very sophisticated technology we don't yet have. But people are working on it, working on the designs of how you would achieve that sort of telescope. Part of the job of JWST is to beat a path for the next telescope to have an even closer look.

Dan Cleary
Thank you so much, Dan.

Ava Fannerstill
My pleasure.

Dan Cleary
Dan Cleary is a staff reporter at science. You can find a link to the story we talked about and see what your view might look like from a trappist one planet@science.org. podcast Stay tuned for a chat with.

Megan Cantwell
Producer Ariana Remel and Ava Fannerstill about why a warming world could worsen air pollution.

Ariana Remel
Photochemical smog is a notoriously noxious form of air pollution that blankets the landscape in a complex concoction of ozone and aerosols. And it's a health risk that's all too familiar to denizens of megacities like Los Angeles, California, where gas powered motor vehicles generate some of the toxic chemicals that react to form smog, such as volatile organic compounds called vocs. After decades of air quality regulations, VOC emissions from cars and other anthropogenic sources have declined. Yet Angelenos are still plagued by the hazardous haze, especially during the hot summer months. This week in science, Eva fanishtile and colleagues wrote about an overlooked source of volatile organic compounds and why that matters for urban planning in a warming world. Welcome to the Science Podcast, Ava.

E
Hi. Thanks for having me.

Ariana Remel
So urban air pollution is a hot topic in atmospheric chemistry, especially in California. I understand that you did this research while you were a postdoc at UC Berkeley. Did you ever experience air pollution from photochemical smog or other sources while you lived in the Bay Area?

E
Yeah, in the summer months when I was living in Berkeley, there often were spare the air alerts that told us to stay inside and don't drive and things like that because there was dangerous photochemical smog. So, yeah, it happened. Although air pollution is much more of a problem in Los Angeles than in.

Ariana Remel
The Bay Area, certainly there are many things that we know about what causes smog. What was the data gap that you were trying to address in this particular study?

E
We were trying to find out, on the one hand, why air pollution in Los Angeles is temperature dependent. I mean, of course, the chemistry goes faster when temperatures are hotter. So that's one part of the explanation that is already known. But that's the other part of might also the emissions change when it gets hotter? And might the composition of the emissions change? And the other question was, why has air pollution stagnated for at least the last decade or so and not gotten better? Although our cars are getting cleaner with all the regulations that are in place.

Ariana Remel
There'S a lot of chemistry that goes on in the atmosphere. You pointed specifically to volatile organic compounds. Can you tell me a little bit more about what those compounds are and the other chemical players that are contributing to this type of air pollution?

E
Yeah. So you all know volatile organic compounds because many of them are fragrant. So you can smell them. For example, the smell of a pine forest or the smell of your perfume or also bad smells. All those are usually volatile organic compounds, and they are present in the atmosphere in very, very low levels and parts per billion levels and below. Despite that, they are really important for air atmospheric chemistry. Because they are so reactive, they can contribute to the formation of air pollutants. Some examples would be terpenes that come out of plants and that create, for example, this nice pine smell or lemon smell is also a terpene. And then you also have typical traffic emissions, like benzene, that is also by itself already a cancerogenus VOC.

Ariana Remel
So how exactly are these vocs reacting to form ozone and aerosols?

E
Yeah. So for that, you need sunlight. And with the sunlight in the atmosphere, you create radicals that then react very rapidly with the reactive vocs. This reaction forms over a reaction chain, on the one hand, particles, because you oxidize them, they get heavier, and at some point, they can stick together and become really aerosol particles. And when you inhale them, that creates a health problem. On the other hand, if you also have nitrogen oxides from cars present in the atmosphere, then this reaction also forms ozone. So the bad ozone that's close to us at the earth's surface, not the wood ozone that protects us from uv light in the stratosphere. And the bad ozone, if we inhale it, also is really bad for our lungs. It also damages plant tissue, so it reduces harvests and so on. For that to form, you need the combustion of fossil fuel emissions that form nitrogen oxides. And these then, together with the vocs, form the ozone.

Ariana Remel
So we know that vocs contribute to air pollution, and we know that emissions from traffic have declined over the last few decades. So what are we missing here?

E
Yeah, that was exactly the question that we posed in our study, and that is why we equipped a small aircraft with really high tech instrumentation that can measure hundreds of vocs at the same time, and not just measure the VOC concentrations, but also directly measure emissions by a complex method that we call airborne eddy covariance. For that, we also need really high speed wind measurements. And by combining these concentration measurements with the wind measurements, we can then calculate emission fluxes so we can directly quantify how much is emitted per area and time in a certain location that we are flying over. And doing that for hundreds of vocs at the same time then, of course, helps us to find out what is emitted, where, and which might be VOC species that have been overlooked in the past or underestimated in the emission models.

Ariana Remel
Tell me a little bit more about this aircraft and what taking these measurements actually looked like, yeah, so this aircraft.

E
It'S a twin otter propeller aircraft, really small, you can't really stand up in there. There were two pilots, two scientists, and then space for three or four scientific instruments.

And we were flying at 1000ft altitude over Los Angeles. So it's really low, not a usual aircraft altitude that you are allowed to fly at. We got actually a lot of trouble because in Los Angeles there were lots of airports and you fly from one tower airspace into the next one every five minutes. So our pilots had to do a lot of talking with all these towers to explain them what we are doing and that we do not want to be diverted from our route, because it's also important to fly at the same altitude, straight lines, to do these measurements. Since it was summer in Los Angeles and we were flying really close to the surface, it also got quite hot in the aircraft. It didn't have any air conditioning. We had to cool one of our instruments, so my colleague had to lie down on the aircraft floor and put ice packs on the instrument to keep it from shutting down. So yeah, we were lucky it didn't happen because then we would have lost a lot of data.

Ariana Remel
Oh my goodness. That is certainly a demonstration of your dedication to this project.

With all of the effort that went into taking these measurements, what did you actually find?

E
We did these kinds of flights at different temperatures, so we flew patterns over Los Angeles, or let's say the greater Los Angeles basin. That way we were able to map emissions and compare them with emission models, but also to see how these emissions depend on temperature.

And what we found was that temperature dependent emissions comprise the major part of the air pollutant formation from these emissions. So the temperature dependent part of the emissions is the biggest chunk of what contributes to ozone formation and particle formation. And that was kind of surprising because the typical traffic emissions are not temperature dependent. So it's really other sources that are doing this.

And we found that a big part of that is terpenes coming from plants mostly. I mean, there's also terpenes and lots of fragrance products that contribute, but a big part of it must be plants because it's so strongly temperature dependent. And another result was also that some human made emissions. So anthropogenic emissions are also temperature dependent. For example, solvents, which makes sense physically, they evaporate more when it gets hotter. But this was not yet considered in emission models.

Ariana Remel
So I understand that heat helps to speed up chemical reactions. That makes sense that vocs that are already present are going to be on the fast track to making some of these pollutants. But why are the plants making more terpenes in the heat? Help me understand that element of this.

E
Yeah. So it's actually a well known fact that plant emissions depend on temperature. So the hotter it gets, the more they emit. Why they actually do that is not entirely known. There are different theories. So they might be protecting their tissues from reactive oxygen species. They might also be protecting themselves against oxidants like ozone, because then the ozone reacts away with the terpenes that they emit. And they, of course, also sometimes use terpenes to communicate with each other. And under stress, these emissions can even increase more. So then it can be orders of magnitude higher than what you would expect from the normal temperature dependence.

Ariana Remel
And I guess if terpenes are also some of the molecules that are associated with fragrances, it kind of makes sense that summer and spring, when plants are flowering, there might also be more terpenes, is that right?

E
Yeah, that's correct. I mean, we cannot really distinguish where our terpenes came from, but certainly when we were flying, we saw that huge areas in Los Angeles had these flowering jacaranda trees. So many streets were purple, which looked really beautiful from the plane perspective. And flowering can cause orders of magnitude higher emissions compared to normal situations as well. So that could certainly have been a contributor.

Ariana Remel
Okay, so what I'm hearing here is that the terpenes emitted by plants might actually be an overlooked source of volatile organic compounds that are contributing to air pollution. But I don't think the solution is to, like, cut down all the trees.

Ava Fannerstill
Right?

Ariana Remel
I mean, there are some really clear benefits to having green spaces in cities, but climate change projections are telling us that summers are going to get hotter, that extreme weather events, like heat waves, are going to become more frequent. So did your research point to other strategies that we might be able to try to compensate for the fact that these plants naturally emit more terpenes in the heat?

E
Yeah, absolutely. So, yeah, of course, it's not a solution to get rid of the trees in our cities, because they provide shade, they improve local climate. They are really important. Now, what is important to consider is that what you need for ozone formation from these vocs that are emitted by the plants is also nitrogen oxides that are emitted by the burning of fossil fuels. So if you get rid of the fossil fuel used, then you also get rid of the ozone. So reducing our use of fossil fuels for powering our cars and our electricity has many benefits. So it will not only reduce the temperature increase that increases, then the turbine emissions, but it will also locally reduce the air pollution because you need to get rid of the nitrogen oxides and then your plant emissions are no longer a problem for ozone. Another point is also since we cannot really regulate the emissions from trees, what we can regulate is the anthropogenic part of the emissions. As we saw in our study, some anthropogenic parts of the emissions are also temperature dependent, like solvents. And this whole part of solvents and household chemicals has not been very much regulated in the past. So I think there's also quite a lot to gain in looking at how these chemicals impact air pollution.

And last but not least, of course, when you do urban greening in your city and plant trees, you can, apart from other factors of course, like how is your tree adapted to the local climate and how much shade does it need to create? One factor should also be how much blue seeds does it emit. There are some species that emit more vocs than others, so that can certainly be a factor to consider to enhance air quality in our cities.

Ariana Remel
This is such fascinating research. Thank you so much for joining us on the podcast today to share your results.

E
Yeah, thank you so much for having me. It was a pleasure to speak with you.

Ariana Remel
Eva Fanestille is an atmospheric chemist at the research center Zulich. Be sure to read the paper from fanishtile and colleagues in science this week. Its available now, and you can find a link@science.org podcast and that concludes this.

Megan Cantwell
Edition of the science podcast. If you have any comments or suggestions, write to us@sciencepodcast.org to find us on a podcast app, search for Science magazine. Or you can listen on our website, science.org podcast.

This show is edited by me, Megan Cantwell, Ariana Remel, Sarah Crespi, and Kevin McLean. We also had production help from Megan Tuck at Prodigy. Our music is by Jeffrey Cook and Wen Khoi Wen on behalf of science and its publisher, AAA's. Thanks for joining us.