ScienceMatters - Season 3, Episode 2: The Search for Earth 2.0
How long has oxygen been in our planet’s atmosphere, and what could the answer mean for life on other planets? School of Earth and Atmospheric Sciences Professor Chris Reinhard researches the early Earth, and potential Earths outside our solar system.
Renay San Miguel: Hello and welcome to ScienceMatters, the podcast of the Georgia Tech College of Sciences. I’m Renay San Miguel.
Take a deep breath. Fill your lungs with oxygen, that gas which guarantees life as we know it on planet Earth.
Go ahead, breathe it in – unless there’s a code orange air quality alert in effect. If not, feel free to inhale while I tell you that oxygen is a relatively new addition to Earth’s atmosphere.
So says Chris Reinhard, assistant professor in the School of Earth and Atmospheric Sciences. He researches the early Earth, and potential Earths outside our solar system.
Chris Reinhard: So the Earth has only had the amount of oxygen, or sort of roughly the amount of oxygen that it has now, for something like 10 percent of its history, maybe less.
And there’s some evidence to suggest that only for the last 10 percent or so of Earth’s four and a half billion year history would you even be able to see this oxygen, even though it was being produced by the biosphere maybe going back three billion years.”
So that’s a little scary to me, and to others as well.
Renay San Miguel: Scary? Reinhard later says “sobering” might be a better word, but you may still wonder: Why is it sobering that oxygen on Earth is a recent phenomenon?
Because it suggests there’s much more to learn about the planet’s ability to maintain that oxygen.
Also, when Reinhard talks about being able to see oxygen, he’s talking about detecting it. So why is the fact that it may not have been detectable a concern?
Because it may mean we could zoom right past another potential Earth with our fancy space probes...and not know if life is there.
(Satellite sounds from space)
Renay San Miguel: To find out more, Reinhard looks to the heavens, specifically recently discovered exoplanets, which are planets like ours outside the solar system. What he’s searching for are atmospheric biosignatures – molecules detectable in an atmosphere that may indicate life on the surface.
Reinhard and his research team at his Earth System Science Lab conduct the interstellar part of this search using ground-based and space telescopes. That’s why he’s excited about the 2021 launch of the James Webb Space Telescope, representing the next generation of orbiting observatories.
Reinhard also casts his research net within our own atmosphere. He applies theoretical methods, relying on elaborate modeling software in powerful computers, as well as advances in analytical geochemistry, to discover how Earth evolved as a planet that sustains life.
Speaking of exoplanets that might harbor life, one of the most recent, and biggest, exoplanet discoveries, was announced in September 2017. Here’s NASA’s Thomas Zurbuchen, associate director of NASA’s Science Mission Directorate.
Thomas Zurbuchen: There are actually seven earth size planets orbiting the nearby Trappist I star about 40 light years away. What’s more, as you can see in this illustration, is that three of these planets marked in green are in the habitable zone, where liquid water can pool on the surface. In fact, with the right atmospheric conditions, there could be water on any of these planets.
Renay San Miguel: First of all, don’t you love how scientists think 40 light years from Earth, which is trillions and trillions in actual miles, classifies as “nearby?” But in the vastness of space, it is kind of in the neighborhood.
Reinhard argues that those planets may indeed have promising atmospheric conditions, but we might not know about it.
That’s why it is sobering that for 90 percent of Earth's history as an inhabited planet, oxygen may have been undetectable. If this turns out to be true elsewhere in the cosmos, then it's possible that exoplanets that have all of the right stuff to produce and accumulate oxygen in theory only do so for small portions of their evolutionary history -- or never manage to do it at all.
All this is because oxygen basically just showed up in Earth’s atmosphere only recently, relatively speaking.
And if that’s the case, given the evidence of climate change...could it leave just as quickly?
Renay San Miguel:
Reinhard and his team won a NASA grant in 2018 to come up with a model for the kind of atmosphere Earth had four billion years ago. That will help the space agency determine what kinds of instruments they should put in future deep-space probes.
In the process, Reinhard is looking at so-called anoxic worlds, where oxygen would be extremely limited.
Chris Reinhard: There are a couple of questions embedded here, and one of them is, you know, the Earth gives us an example of a planet on which you can have oxygen and photosynthesis for billions of years but not accumulate oxygen in the atmosphere to levels that would be detectable. So this is what we sort of refer to as a false negative. And so the question is: how does one deal with that? There's a somewhat deeper question which is, how likely is oxygen and photosynthesis to evolve at all, right? So how common is this metabolism throughout the galaxy and the universe? And this is an area that's under a lot of argument. I mean there are some folks who think that it's probably pretty common because it uses an electron donor for its metabolism, water, that on habitable planets is going to be pretty ubiquitous. So it's, in some ways, it's a very competitive strategy. On the other hand, the biochemistry of it is exceptionally complex; it's not an easy thing to do. In fact, we've been trying to sort of engineer it in the lab for decades and have not been so great at it in many ways. So, you know, there's a question as to how widespread this would be. I think one of the things we have to be thinking very hard about is robust signatures of life on planets that either have not evolved oxygen and photosynthesis at all, or it has evolved, but it hasn't oxygenated the atmosphere. And so the question is, you know, what are good atmospheric biosignatures on planets that don't have oxygen in the atmosphere.
And also just sort of, you know, are there other biosignature gases, you know, for example, methane, that we might be able to use that are just as compelling. Are there other gases that on anoxic worlds would be just as convincing as oxygen is on the Earth? That's sort of the motivating question behind the grant.
Renay San Miguel: So any luck so far in finding a planet that has more similarities than differences with Earth?
Chris Reinhard: We keep getting closer and closer to finding that sort of Earth 2.0. in recent years we've come up—I say by “we” I mean a sort of large community of people—have found, you know, something on the order of a dozen or so planets that are getting closer and closer to sort of Earth-like. But one needs to be very careful when they use the term Earth-like because that could mean a lot of different things.
Even amongst these Earth-like planets, there's really only one that I know of that's really like sort of Earth 2.0 as far as we can tell.
Science Channel documentary: Kepler-452b sits right in the habitable zone, what we call the Goldilocks zone; not too warm, not too cold, not too bright, not too dark; just right for life.
And when scientists factor in the light from Kepler-452 it becomes clear that this is the most Earth-like world humanity has ever discovered.
Renay San Miguel: That is from a Science Channel documentary on Kepler 452-b, discovered in 2015. It’s 14-hundred light years away, so it would take 26 million years to get there using the fastest space probes.
Chris Reinhard: It's something like 40 percent larger than the Earth is, but it's, you know, it's pretty close in terms of size. There are indications that it's made of similar stuff—so it's rocky and potentially has an atmosphere and oceans and that kind of thing.
Renay San Miguel: A certain distance between a planet and a star may be in the so-called Goldilocks zone, but Goldilocks left a mess in the bears’ dining room and the bedroom. She was essentially the AirBNB guest from Hell.
Humanity is leaving its own mess when it comes to Earth’s climate, but that distance between here and the Sun plays a role as well – as it does on exoplanets.
Chris Reinhard: Climate is a key characteristic of an overall planetary system—so it's something that controls other aspects of how the planet works and it interacts with other aspects of how the planet works and the star that the planet is around and all these things.
You can take it a step further back in some respects and think about things like, how was the star evolving over time? So we get a particular amount of energy from our star right now, and that's a fundamental boundary condition on our climate. And sort of in some ways our climate system is the surface of our planet trying to sort of dissipate the energy that we get from our star and like how does that propagate through the system—that's what climate is.
Renay San Miguel: So the age of the star plays into—is a factor?
Chris Reinhard: Yeah, it's tremendously important, right. So if you go back to the sort of beginning of our solar system, our star was giving off something like 30 percent less energy than it is now. And all other things being equal, that makes a huge difference in climate. And so, you know, similarly, as we sort of project into the future, our star as it is it burns more and more of its fuel, is going to progressively give off more and more energy, and so temperatures at the surface of the Earth, all other things being equal, are going to start to increase. I mean at a certain point this can become a kind of catastrophic runaway within the climate.
Broadcast news report: This past July was the hottest month on record, according to a report released Thursday by the NOAA. Above average temperatures blasted 17 contiguous U.S. states.
Chris Reinhard: So you know, what is the Earth's biosphere going to look like 500 million years from now? A billion years from now? And in particular, we're very interested in the question, what are some of the other features of Earth's atmosphere that provide hints that there's life on Earth? So if you were to observe Earth with a telescope from light years away and ask the question Is there life on this planet? You'd only have fragments of information to work from. And so one of the things you would see, for example, is that our atmosphere is 20 percent oxygen. And you know, for most folks, that's a really strong indicator that there's life at the surface of the planet. And so one of the questions we’re really interested is, you know, how much longer is this going to be the case because our current atmospheric chemistry is actually a pretty recent phenomenon. And so the question is, you know, is this going to last for another couple hundred million years or is it going to be a couple billion years? And this starts to feed back on sort of how long we can expect planets that are inhabited like the Earth to have these signatures for long periods of time, and how long can we really expect them to have these biosignatures that we might be able to detect because we're only going to be looking at these things for the blink of an eye, right? And so we want to try and increase the statistical likelihood that we're actually going to detect something, and so we want to try and look for signals that are—can persist on planets for very long periods of time.
Renay San Miguel: What about planets that aren’t in the habitable zones, or give the impression they’re frozen?
Once again, something that may have happened in the Earth’s infancy, may be going on among exoplanets. That possibility was first explored by one of our planet’s best-known scientists.
Carl Sagan documentary: We’re going to explore the cosmos in a ship of our imagination.
Renay San Miguel: Famed astronomer and astrobiologist Carl Sagan, who helped bring science to a mass audience with his successful 1980 PBS series “Cosmos,” came up with what he called the Faint Young Sun Paradox.
Remember what Reinhard said earlier about climate and the energy given off by our sun during Earth’s infancy? In the early 1970s, Sagan and a colleague at Cornell University suggested that the Earth should have been frozen during a given time in its development. But it wasn’t, as suggested by sedimentary and other data.
So what happened?
Chris Reinhard: This idea of a faint young sun paradox comes from the notion that if you were to take the modern Earth and the way that its climate system works—so the amount of CO2 in the atmosphere, the amount of methane in the atmosphere—and you were to sort of dial the sun back, you know, 3 billion years or so, the amount of energy that it’s giving off drops something like 20, 25, 30 percent depending on how far back you go. If you go all the way back to beginning, you're talking about almost a 30 percent drop in the initial amount of energy that the planet's getting.
So if you take the modern Earth's greenhouse effect and you dial it back in time, you freeze the Earth over. But we know from the rock record that the Earth wasn't frozen over back then. And so this is actually a really basic argument for the facts that the Earth's atmospheric chemistry has changed really dramatically over time because there's no other way you can reconcile this. And so Carl Sagan and a colleague in a paper, Sagan and Mullen, published I think in 1971 suggested that, well, perhaps there are other greenhouse gases that might be important here. So if you don't have a lot of oxygen in the atmosphere, for example, you might build up ammonia or methane or, perhaps, even hydrogen. So these gases that are very unstable in the modern atmosphere but are exceptionally powerful greenhouse gases, you might use these to sort of warm the early Earth.
Renay San Miguel: OK. And if those don't happen, the early Earth doesn't—I mean maybe it stays frozen?
Chris Reinhard: Perhaps.
Renay San Miguel: Just the idea that that kind of, I don't know, a quirk in the history in the development of the Earth could result in life -- could’ve have helped to get life going on the planet. And maybe that's repeating itself on other planets?
Chris Reinhard: Sure. Absolutely.
Renay San Miguel: Reinhard says he has a lot to look forward to over the next five years in his research.
More scientists from different disciplines will join forces, he says, to share more data on the search for life elsewhere, and more insight into life’s beginnings on Earth.
Chris Reinhard: One of the things that I'm really excited for in the near term is better quantitative models for what regulates biosignature gases in the atmospheres of planets that are like the Earth—or even not so much like the Earth—and so developments in models that actually couple together planetary oceans and planetary atmospheres explicitly and in ways that are much more robust. I mean I think we're going to—just in the next few years, I think as a community we're going to make really significant strides in that area. There has been in very recent years, and I think is building—and in the next four or five years is going to be really exciting—much more crosstalk between people who are trying to constrain planetary interiors and how they evolve over time, and how do things like volcanic outgassing work, and people like me who think a lot about how oceans work and how ocean biospheres modulate these signals, and then people who work on atmospheres and, above that, people who work on observations, and all of us sort of piping together to come up with a kind of integrated view for how planets work and what sort of signals we might want to look for. I think in the next few years the advances are going to be really, really exciting.
Renay San Miguel: My thanks to Chris Reinhard, assistant professor in the School of Earth and Atmospheric Sciences, for the tour of Earth’s early years, and the possible sister planets outside the solar system.
His lab’s website is at R-E-I-N-H-A-R-D-dot-gatech-dot-edu.
Also our thanks to NASA, Discovery Channel, A&E Networks, and PBS for the use of TV clips.
Siyan Zhou, a former research associate in the School of Psychology, composed our theme music.
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This is ScienceMatters, the podcast of the Georgia Tech College of Sciences. I’m RSM. Thanks for listening.