Thanks for CuriosityStream for supporting this episode!

Go to CuriosityStream.com/Space to learn more.

According to statistics, there's probably life somewhere besides Earth.

There's no iron-clad proof or anything, it's just that the odds of life only evolving on one planet out of trillions are pretty small.

But as true as that may be, it's still not the same thing as knowing we're not alone.

So for thousands of years, people have been watching and listening for some sign of ET.

Except, there's some obvious trouble with that.

Basically all of the planets out there are extremely far away, so far that we might not ever be able to see their surfaces in detail or intercept a clear radio signal.

And if there's microbial life out there, well, we're definitely not going to take soil samples from exoplanets any time soon.

To deal with these limitations, researchers have been investigating new ways that we could look for life, including life that isn't trying to communicate with us.

And they've come up with some promising options.

Some teams are specifically looking for intelligent life, like by searching for evidence of megascale engineering.

These are structures so big that we would notice them from light-years away.

This idea actually first popped up in early 20th-century science fiction, but it got a big boost in 1960, when physicist Freeman Dyson published a paper describing what's now called a Dyson sphere.

Dyson imagined that a sufficiently-advanced species might need so much energy that it would surround its star with solar panels.

The panels would absorb enough heat to give off an unnaturally high amount of far-infrared radiation.

So spotting one of these weird stars would be indirect evidence of the species that built the sphere.

Of course, building something that big would obviously require an incredible amount of material, which itself would have to come from somewhere.

So some scientists have suggested that it might be easier to find those missing materials rather than the thing they were turned into.

For example, we could search for asteroid belts that seem weirdly depleted in certain minerals, which would be possible evidence of a large-scale mining operation.

And as a bonus, we already have the technology to do this, at least, for common elements.

Then again, we don't know if there are super advanced civilizations out there.

We don't even know if there's intelligent life at all. Like, think about our own planet.

Until about a hundred years ago, no species on Earth could even send a radio message, yet our world is absolutely covered in life.

Thankfully, over the years, scientists have discovered ways they could track down even really simple lifeforms.

Which is great, because I don't know about you, but I'm not holding out for finding a Dyson sphere any time soon.

Many of these methods rely on biosignatures, or natural markers of life, and there are a few different kinds.

One is biofluorescence, or the organic creation of light.

Basically, it's light that comes from life itself.

Biofluorescence is different from the bioluminescence found in creatures like fireflies.

To produce light, biofluorescent organisms first absorb it.

Then, they absorb some of its energy and re-emit that light at a different wavelength.

All kinds of life on Earth, from bacteria to flying squirrels, exhibit this trait, and some researchers believe the conditions around certain types of stars might even promote the evolution of it.

M-dwarf stars, for example, emit a lot of ultraviolet light, a kind that's destructive to DNA here on Earth.

So if life existed around one of these stars, it could potentially have developed a biofluorescent adaptation that would allow it to absorb and then reject those harmful rays.

And from a distance, astronomers might see all that light as a single, unusually bright wavelength.

Another biosignature is chemical disequilibrium in a planet's atmosphere.

This happens when some outside process is keeping the amounts of gases at unnatural levels.

One thing that can maintain an imbalance like this is photosynthesis.

Like, look at Earth's atmosphere. It didn't have much oxygen in it until microbes started photosynthesizing billions of years ago.

And if you took Earth's life away, that gas would slowly be converted into other chemicals.

So the atmosphere's 21% oxygen is a clear sign of life.

Looking for biosignatures is great, because they cast the widest net and allow us to search for life of all intelligence levels.

They're also probably the easiest to seek out, because equipment like NASA's upcoming James Webb Space Telescope will be able to study a planet's atmosphere in just a couple of days.

But on the other hand, they're also the most ambiguous signals.

Almost every complex molecule or strange disequilibrium can be caused by some kind of abiotic process, even if it's not the most common path.

That means it will be hard to know for sure that what we're seeing is really evidence of life.

Truthfully, this is a problem that affects the search for life in general.

We've even seen it play out recently with megascale engineering.

You might remember recent observations of a strange object sometimes called Tabby's star.

Its brightness seemed to flicker unpredictably in a way some scientists suggested could indicate the presence of a partially-built Dyson sphere.

But a more detailed look eventually showed that dust was the source of the interference, not aliens.

This is a story that's likely to play out again and again in the coming years.

Short of a message directly from a nearby world, we probably won't know for sure that what we're seeing is alien life.

That can sometimes feel discouraging, but it's still important to refine our methods and continue the search.

Because if we ever do figure it out, it will be more than worth it.

This episode is brought to you by CuriosityStream, a subscription streaming service that offers over 2000 documentaries and non-fiction titles from some of the world's best filmmakers.

CuriosityStream even includes exclusive originals, like one called “Miniverse” that I've really been enjoying.

In it, astronaut Chris Hadfield takes you on a tour of the solar system, but it has a twist, because the solar system is scaled down to the size of the continental U.S.

The format of the film is really cool, and it's also fun to hear about space from someone who has spent time living there.

You can get unlimited access to films like this, and to all of CuriosityStream, starting at only $2.99 a month.

And as a special thank-you to our amazing audience, the first 30 days are completely free if you sign up at CuriosityStream.com/Space and use the promo code “space” during the sign-up process.

Besides getting access to all this content, you'll also know you're supporting SciShow, so thank you!