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Astronomers have found more than 5,000 planets in the last three decades, but that’s not nearly as exciting as potentially coming across the first extraterrestrial creatures. And we may finally be in a position to make that discovery.

Hosted by: Hank Green (he/him)
Thumbnail Courtesy: TMT Observatory Corporation
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Sources:
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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4323125/
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Images:
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https://svs.gsfc.nasa.gov/13375
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https://commons.wikimedia.org/wiki/File:Top_view_of_tmt_complex.jpg
[♪ INTRO] When are we going to discover aliens?

Astronomers have found more than 5,000 planets in the last three decades, but that’s not anywhere near as exciting as potentially coming across the first extraterrestrial critters, or creatures we could talk to. Well, we may finally be in a position to make that discovery.

The James Webb Space Telescope is up and running, and massive next-gen ground telescopes will capture their first light in the coming decade. We have new high precision tools at our disposal, but we need to know what clues to search for. So, let’s go on a journey and explore how humanity might actually find evidence of life beyond Earth.

The hunt for extraterrestrials has been going on for decades, before we even proved there really were planets beyond our solar system. Humans started shouting our presence toward the heavens with the advent of radio transmissions, so it made sense that other life out there may have done the same. And in 1984, the SETI program was set up to listen for those interstellar messages.

Sadly, without any luck to date. But if you think about just how brief a time humanity has had this power compared to the history of all life on Earth, it’s maybe not that surprising that we might struggle to catch an alien civilization at just the right point in time to hear them. So to improve the odds of finding life, we want to look for something that’s a little more universal.

A signal aliens can send by doing nothing more than living. Astrobiologists call these signals biosignatures. Now, there’s no way of knowing ahead of time what alien life will actually look like, or how it will work.

But by studying all the varieties and capabilities of life on Earth, we can get a pretty good idea of what biology might look like in general. Or at least as far as life that is constrained by the laws of physics and chemistry. Because all these planets are more than a puddle jump away, our hunt for extraterrestrial life relies on telescopes capturing tiny amounts of light that is coming from the planet as it orbits its star.

By breaking that light up into its component wavelengths, known as a spectrum, and seeing exactly what colors are or aren’t there, astronomers can figure out crucial details about these distant worlds. And those details can include specific biosignatures. But getting that information is no easy feat.

The amount of light that our telescopes have to work with is mind-bogglingly small. Like, for every Earth-sized planet, we’re hunting for a speck that can be a million times smaller than the star it orbits. But both specks are often in the exact same pixel of data!

And life is going to leave an even smaller visible footprint. To get the necessary sensitivity, we have to build huge complex instruments like the Webb. By the end of 2022, JWST had already captured a few spectra from exoplanet atmospheres, and it spotted things like water and carbon dioxide.

We’re recording this episode at the start of 2023, and so far, the telescope is targeting hot gas giants that aren’t habitable to life as we know it. But these results are exciting proof that the technique works and that Webb is up to the task. So, when we do turn our attention to habitable rocky worlds, we can be ready for a whole range of different biosignatures.

Atmospheric biosignatures are signs of life that can be found, as the name implies, in a planet’s atmosphere. The idea is that some gasses can only be produced by life, or at least are much more likely to have a biological origin, instead of an abiotic one. But that alone doesn’t automatically make them a good biosignature.

They need to be present in high enough concentrations that their presence can be spotted in a planet’s spectrum. And their position on that spectrum shouldn’t overlap with signals belonging to other abiotic gasses, either. Oxygen and methane are two potential biosignature gasses that get a lot of attention, because both are major byproducts of life’s metabolic activity here on Earth.

And as far as candidate biosignatures go, they’re top of the list because they’re most easily detectable with the technology we have. Oxygen is made by green plants, algae, and bacteria during photosynthesis, and methane is a byproduct of methanogenesis, a process that’s common among bacteria that live in low oxygen environments, like at the bottom of the deep oceans. Now, both of these gases can be produced by abiotic processes on Earth.

Oxygen is made when ultraviolet light from the Sun smacks into water molecules and breaks them apart. And methane can be produced in volcanic environments. So if you hear NASA announce that Webb has detected a planet with oxygen or methane gas in its atmosphere, it isn't necessarily a smoking gun.

But if there’s a bunch of oxygen or methane in that atmosphere, sustained over a long period of time, we might be onto something. That’s because both oxygen and methane are also highly reactive. Oxygen reacts with many substances found in rocks, and methane is quickly broken down by sunlight.

And if a planet’s atmosphere contains both oxygen and methane, they will easily react together to make carbon dioxide and water. So an abundant, sustained presence of either gas must mean something is constantly replacing it. And that something could very well be life.

Of course, these biosignatures come with the assumption that alien life can even do photosynthesis or methanogenesis. You and I and chickens and octopuses are examples of the fact that these processes aren’t universal, even on Earth. But both are highly efficient kinds of metabolism, so it’s reasonable to think that some planet of ETs could have evolved a similar system somewhere.

And we don’t have to rely totally on oxygen and methane. There are more exotic atmospheric biosignatures that we can look for. For example, bromomethane is a gas that’s formed by microbes, algae, plants, and fungi as they try to remove toxic metals and halides like bromine and chlorine from their environment.

And on Earth, the only major abiotic source of it is human industry. That’s a big check in the “Pro” column. Likewise, nitrous oxide gas, aka laughing gas, is formed and farted out by some bacteria and fungi when they break down nitrogen compounds inside their cells.

Abiotic sources of nitrous oxide include lightning strikes and intense periods of solar activity, but astronomers can tell when the source is abiotic because other compounds are made at the same time, and their signal would show up in the spectrum, as well. Another Pro. So what’s the Con with these?

Well, not only are they likely to appear in way lower quantities than oxygen or methane, they’re also pretty easily destroyed by UV light. But that doesn’t mean our telescopes can’t spot them if they try. The signal would be a lot more prominent in the spectrum of a planet that orbits a star that isn’t like our Sun.

One that’s a lot less massive, a lot less bright, and a lot more red in color, meaning it doesn’t produce as much UV light. And who said aliens had to be found on a rock that orbits a star like ours? No one.

No one said that. And that’s why one clear target in the hunt for life is the TRAPPIST-1 system. Not only is this group of planets pretty darn close to us, astronomically speaking, there are a whopping seven rocky planets orbiting close to an old red dwarf.

But even then, Webb’s sensitivity might not be high enough to pick out the weak signal of these trace gases from among the noise. We might need to wait until the next next-gen telescopes come online for even higher precision. Like, for example, the European Space Agency’s PLATO telescope, which is set to launch in 2026 and will stare at planetary systems way longer to get clearer spectroscopy data.

There’s also the LUVOIR-A telescope, which is basically Hubble and Webb’s baby on steroids, with a 15 meter-wide mirror for extra precision. But it’s only in the proposal stage now, and probably wouldn’t be operational until at least the 2040s. In the meantime, the hunt can continue for biosignatures that come not from the atmosphere, but from the planet’s surface.

Looking at Earth from space, humanity’s presence is given away by artificial lights. And species that are a lot less technologically inclined can emit light of their own in the form of bioluminescence. We even talked a bit about finding this kind of alien life, before.

But rather than life acting like little glow sticks, some surface biosignatures arise just because life interacts with starlight. Like, why does the Earth look so green all the time? Well, it’s covered in plants.

And why are plants green? Because of photosynthesis. More specifically, the plant cells that perform photosynthesis are filled with pigment molecules that absorb certain wavelengths of sunlight, and reflect the rest.

They look green to us because they absorb most of the red and blue light that hits them, and reflect a bunch of green light. But the color they really love to reflect isn’t visible to the human eye. It’s infrared.

In fact, they reflect so much more infrared light than visible light, if a bunch of alien astronomers built a telescope of their own and pointed it at the Earth, the spectrum they would see would have this super prominent shape… called the vegetation red edge. And if those aliens are anything like us, they would know that there are few abiotic sources that could replicate that effect. Of course, our hunting for this biosignature on other worlds assumes that photosynthesis can evolve and become a dominant process elsewhere.

But as long as there’s a photosynthetic pigment of some kind, we’d just have to look for the general edge shape in the planet’s spectrum. It doesn’t have to be the exact same color. Gotta watch out for clouds, though.

It’s kind of hard to study a planet’s surface when clouds are blocking a chunk of your data. And cloud cover is quite likely on a warm, water-rich world like the ones our telescopes would probably target. The final kind of biosignature combines the ideas of atmospheric and surface biosignatures, but requires our telescopes to stare at a planet for a really long time.

The hunt for temporal biosignatures would look for signals that fluctuate in a regular pattern that suggest alien life might be going through cycles as its planet zooms around its star. In an alien atmosphere, we could see seasonal changes in gas concentrations that indicate life blooming and dying off at the surface. On Earth, atmospheric carbon dioxide levels decrease during the summer when plants grow and photosynthesise a lot, and then increase in the winter.

Similarly, the intensity of the red edge is going to change with the seasons as well. But if we see any seasonality from a planet far far away, we’ll have to avoid jumping to conclusions. For example, methane is known to fluctuate for entirely abiotic reasons.

The molecules break down more readily when there’s more moisture in the air, and that level of moisture will vary if a planet has things like seasons. And seasonal variation means that summer in one hemisphere happens at the same time as winter in the other, so the changes averaged over the whole globe may actually be quite small. When that planet is so far away it’s reduced to just a few pixels, it’ll be hard for astronomers to pull any true signals out from the noise.

The truth is, if we want to be sure we’ve found alien life, it will require identifying a complimentary suite of biosignature signals that all point us towards one conclusion. That conclusion being: we are not alone in the universe. With the James Webb Space Telescope now fully deployed and scanning the skies with the most powerful instruments we’ve ever made, there are exciting times ahead.

And while even this formidable telescope may be limited in certain ways, plans are already afoot for more powerful and specialized instruments, like the Giant Magellan Telescope in Chile, or a Future Great Observatory like LUVOIR, dedicated to probing exoplanets. Perhaps our greatest limitation in the years to come will be finding time to analyze all of the discoveries! The universe is a big place, and we’re only looking for life as we know it in one teeny corner of it.

So we’re just going to have to sit and impatiently wait for what might be the most important discovery made in all of human history. And we’re glad that you’re here waiting with us. In the meantime, your wait for the perfect calendar to start off 2023 is over!

The people who made this video also made a high quality 2023 calendar full of beautiful images, science related holidays, and mesmerizing moon blurbs. The SciShow Space calendar is here for the space fan in you. And you can get your Moon calendar and all the other Complexly calendars with a 50% discount right now, because, look, the year has started.

So it’s time… you gotta… we’re encouraging you to get one! You can head to ComplexlyCalendars.com or the link in the description down below. Thanks for watching this episode of SciShow Space, and stay tuned for even more. [♪ OUTRO]