Just how hard are exoplanets observations? What kind of telescope will it take to find life? And how much information can we get out of a single pixel of light? I discuss these questions and more in today’s Ask a Spaceman!

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Hosted by Paul M. Sutter.

 

EPISODE TRANSCRIPT (AUTO-GENERATED)

In 1990, the Voyager 1 spacecraft, then 6 billion kilometers away from home and destined to become humanity's most far-flung emissary, turned around and took a picture of home. The resulting image is now legendary, a haunting portrait of our tiny, fragile world, just a pale blue dot suspended in a sunbeam. Imagine you're an alien species, and you're searching for evidence of life on other worlds. you've found a promising candidate, a rocky planet orbiting a medium-sized star in its habitable zone. Let's say that all you have, with the most advanced technology at your hands, is that pale blue dot. Or worse, what if all you could get was a single pixel, an image not even large enough to discern a sphere? Everything you'll ever know about us has to come out of that one pixel. oceans, continents, weather, whatever life crawls or breathes or builds, all collapsed into one tiny smudge of light. What could you figure out? Well, it turns out, an absurd amount, but only if you're clever, patient, and willing to build the most ridiculous telescopes imaginable.

Let's start with what we can do today with the machines right at our fingertips. Well, I mean, they're sitting millions of kilometers from Earth, but they have dedicated channels for turning data back to us, so it's... Kind of sort of like fingertips, right? A trick we use to figuring out what's on another exoplanet, and especially for hunting for life on said exoplanets, is called transit spectroscopy. When a planet crosses in front of the face of its parent star, aka a transit, a sliver of starlight passes through the planet's atmosphere on its way to us. And that atmosphere is full of stuff, mainly molecules, and different molecules absorb different wavelengths of light. Water absorbs certain wavelengths, methane others, bits of cheese, still others. Every molecule has a unique fingerprint of colors it likes to eat and not emit. So all you have to do, and I am waving away all the technical difficulties of actually achieving this, is compare the star's light with the planet in front of it to the star's light without the planet in front of it.

And boom, you get the fingerprint of the planet's atmosphere written right there in what wavelengths of light are blocked. Of course, the reason that there's an enormous amount of technical difficulties actually achieving this is that the signal is fantastically tiny. For an Earth-sized planet around a sun-like star, the atmosphere modifies the starlight by about one part in 10,000. And that's on a good day with ideal conditions with strong signals. And that, my friends, is why we build monster telescopes like the James Webb. Launched in 2021, it sports a 6.5-meter mirror, which is big enough to measure these kinds of tiny differences. It's an incredible machine with one caveat. It wasn't built specifically to find life. It was built to do a hundred other jobs as well. Early galaxies, star formation, black holes, and planetary atmospheres. Biosignatures, aka signs of life, are just one item on a very long menu. And that means the James Webb is not going to hit a home run for us. We're hunting for four molecules in particular.

Oxygen, ozone, methane, and water, which combined are called the classic biosignature cocktail. Why these four and not, you know, literally anything else? Because on Earth, they are the atmospheric evidence for biology. Photosynthetic life exhales oxygen. Sunlight in the upper atmosphere converts some of that oxygen into ozone, which then blocks UV, one of the reasons Earth is habitable in the first place. Microbial life, especially the anaerobic kind which hates oxygen, exhales methane. Think cow guts, swamps, termites, deep ocean vents. Water is not a biosignature by itself, but it's the solvent every kind of life we know about requires, so it needs to be there. These four molecules, oxygen, ozone, methane, and water, are what life lives and breathes on our planet. And given that the Earth is the only example we have of life existing anywhere, it's as good as a starting point as we're going to get. And crucially, all of these four molecules need to come together as a package. It's not enough to just have a lot of oxygen in an atmosphere because we know of chemical, and purely in the context of the present episode, boring, I'm sorry, not sorry, reactions, like sunlight breaking up water vapor that can make oxygen in abundance.

Same for methane. Burping volcanoes can produce more than enough methane to be detectable in a planetary atmosphere. So what we're especially looking for is these four elements in disequilibrium. An abundance of these four that shouldn't exist at the same time unless something is replenishing them. Oxygen and methane in the same atmosphere at the same time would ordinarily react and cancel each other out in just a few thousand years. If we see them together, something is topping off the supplies. And that something is life. One of the defining features of living systems is that they throw planets out of equilibrium. A planet with life will have a fundamentally different mixture of elements that should not normally go together going together. So that's what we look for. And the James Webb? Well, it's got a chance, okay? I'll say that, but not a very good one. The problem is that the smallest signal it can reliably detect for anything, any molecule in any atmosphere is around 10 parts per million, which meaning it can't confidently measure any atmospheric signal smaller than that.

For an Earth-like planet around a Sun-like star, The biosignature signal of our classic cocktail is smaller than that. The James Webb could be staring right at a living planet and to us it just looks like noise. The only shot it has of detecting life of this biosignature cocktail around another planet is if it's a rocky planet around a small red dwarf star. Because those stars are so dim, the planet's atmosphere blocks a bigger fraction of its total light, boosting the signal. But that's a real lucky break to find a system like that and close enough that this plan works. Best case scenario, we're talking two, maybe three planets over the entire lifetime of the James Webb. And that's just to have the chance to even see biosignatures. Those two or three worlds could be dead. And statistically, they probably are because most worlds are dead. Now, is it hopeless? No, and it's not hopeless because we have Patreon. That's patreon.com slash pmsutter is what gives this show astronomy, science in general, and humanity at large hope.

So if you want us to have hope, you know what to do. Patreon.com slash pmsutter. I really appreciate it. No, it's not hopeless because we're not done. The James Webb isn't the last telescope we'll ever build. At least I hope so. Coming up next on the docket of space launches is the Nancy Grace Roman Space Telescope, which will largely be concerned with deep universe cosmology surveys, but also incidentally be able to pick out millions of exoplanets using a trick called microlensing, which only reveals the existence of a planet, not whether it can support life, which is cool and all, but not the subject of today's episode. The subject of today's episode is what comes next. The Habitable Worlds Observatory. Surely to be renamed after someone famous, but it's right now little more than a sketch in a notebook. The HWO is like a Super James Webb, which itself was a Super Hubble, so this can be classified as a Super Duper Hubble. And despite a planned mirror size that's actually slightly smaller than the James Webb, it features an instrument that makes it worthy of the title of Biosignature Hunter.

That instrument is... is the coronagraph. A coronagraph is a very, very simple device that has to be made exceedingly well. It's just, well, it's a bit of metal designed to cover the light of a parent star when looking at a distant system. You ever cover up the sun with your thumb to look at something near it on the sky? It's like that, except to achieve the kind of precision it needs. It actually needs a series of precisely shaped masks that exploit the wave nature of light that the starlight destructively interferes with itself and cancels out at exactly the position where the planet's light is passing through untouched. The point of this, of deploying a coronagraph with absurd levels of precision, is to stop relying on filtered sunlight to do our dirty work for us and allow us to get us a picture of the planet itself. in all of its reflected light glory. But this is a bit of an undertaking to say the least. To directly image an Earth-like planet around a Sun-like star, you need to see something roughly 10 billion times fainter than the star it's sitting next to.

That's how dim our planet is. Check this out. I ran the numbers for an analogy. Stand on a beach in California and try to see a match being struck next to a lighthouse in Japan. A typical star is dumping 10 billion times more photons at us at every wavelength we want to look at than the planet is. And all the work, the design of the habitable world's observatory is not in its mirror, not in its communications package, not in its spectrometer. It's going into designing a coronagraph that can achieve just that. And for all this work, the HWO will not be a survey telescope. It's not a scanner. It's a hunter. Its stated goal is to find and directly image at least 25 potentially habitable worlds and take pictures of them. Hubble was a general purpose observatory. James Webb was designed for early galaxies. First, exoplanets. Second, every choice about the habitable worlds observatory, the mirror size, the coronagraph, the wavelength range, the stability requirements was made with find life on another world at the top of the priority list.

But to hit that kind of precision and reach the 10 to the minus 10 contrast so we can actually pick out the fine details of the planet's light, the mirror has to hold its shape to within picometers. That means the entire telescope floating in space exposed to temperature swings and vibrations from its own machinery has to stay calmer than the jiggling of individual atoms. If the habitable world's observatory mirror was expanded to the size of the continental United States, keeping it stable to picometers would be like keeping its entire surface flat to within the width of a human hair. I don't know how to say this politely, but we don't know how to do this. That's why even though it's the same size as the James Webb, it's still a generation-scale engineering problem to solve and why the observatory won't even launch until the 2040s at best. Which is fine, because hunting biosignatures is more than just a tech and engineering problem. It's also a theory problem. Even if we had perfect data, we'd still have an issue.

To translate a spectrum into how much of each gas is in this atmosphere, we need reference tables built from laboratory measurements and theoretical calculations. How much light does methane absorb at 3.3 microns, at what temperature, at what pressure, mixed with what other gases? These are called opacity models, but you know, that won't be on the quiz later. Imagine you're a bartender trying to identify a mystery cocktail by taste alone. But your recipe book was written by someone who tasted the ingredients one at a time at room temperature and never actually mixed anything. Now you're handed a drink that's been chilled, shaken, and blended with five other flavors. Your recipe book is technically correct, which is the best kind of correct. But the actual taste in your mouth is doing things the book never planned for. That's where we are with exoplanet atmospheres. We know how methane absorbs light at room temperature in a lab. We know how water does the same thing. We don't perfectly know what happens when they're both at, I don't know, 700 Kelvin at 10 atmospheres of pressure mixed with hydrogen and traces of a dozen other gases over billions of molecules deep.

This means that there's so much uncertainty in the models of how these mixed-up gases and exotic environments actually absorb light that 10 astronomers might see the exact same spectrum and come up with 11 different interpretations, some of which excitingly point to life and some of which don't. That's why we tend to get a lot of headlines about big biosignature discoveries. And you know what I say about headlines. If it's interesting, it's probably wrong. Because... Just a few months later, those studies will be quietly withdrawn in the middle of the night when nobody's looking. The devil is in the details here and with biosignatures, it's all details. But we've got time, a couple decades, to sort it out on the theory side before the habitable world's observatory starts pointing at planets and telling us what they're made of. And yeah, you heard me right. About two dozen candidate planets. We're putting a lot of eggs into one basket because honestly, it's the only chance we got. with current technology.

And even that is stretching the limits of current technology. And all that, all that, all the precision, all the stabilization, all the engineering, all the theory work, all that for our first pictures of potentially habitable worlds to be all of a single pixel of light. Is that enough? And oh yeah, it's enough. I mean, it's never enough, but it's enough to power the motivation for this monster planet-hunting telescope. Because one single pixel gives you way more than you might imagine. That's because that dot has a color. Maybe even pale blue. It has a spectrum. It changes brightness as the planet rotates and moves through its orbit. The Habitable Worlds Observatory is our first purpose-built biosignature machine. If life is common, meaning present on some meaningful fraction of habitable zone rocky planets, The observatory will find it, or it will tell us that life is much rarer than we hoped. Either way, we learn something. Something that will help is that to gather enough light, the observatory will have to stare at each planet for days to weeks, so that each planet will be its own project, months of prep and planning, then weeks of study, then a lifetime of analysis.

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