- Hey, smart people.

Joe here.

So recently, this really cool telescope actually took a picture of an exoplanet, like an actual photo of a planet orbiting a star outside of our solar system.

That's awesome, but then I saw the image.

(Joe imitates farting) And I have to be honest, I was like, "Really, that's the best we can do?"

And then I remembered that this little planet's more than 350 light-years from Earth, okay?

And 30 years ago we didn't even know exoplanets existed, and now we've found more than 5,000 of them?

And every star in our galaxy probably has at least one planet, so, yeah, it's pretty cool that we have this.

But what if we could do better?

I mean, how big of a telescope would we need to actually take a picture of an exoplanet, one that looks like our satellite pictures of Earth and Mars?

And could we ever actually build it?

In this video, we're gonna find out by learning about the weird way that telescopes bend light, and how we can improve them, and what's the limit for how big a telescope can be.

Well, it turns out there's one crazy idea that might actually be possible.

(light whimsical music) Studying actual exoplanets is a pretty new thing, but the idea of spotting signs of alien life on other planets isn't a new thing.

In the late 1870s, an Italian astronomer, Giovanni Schiaparelli, pointed his telescope at Mars, and he started drawing maps of its surface.

You see these lines crisscrossing this map?

They were about to set off more than a century of Martian fever.

Now, he didn't quite say the word alien or extraterrestrial, but he labeled these lines with this ambiguous word, canali, meaning channels.

And when his work got translated into English canali was translated to canals, and suddenly people assumed that he discovered the handiwork of an advanced Martian civilization.

Other astronomers started reporting sightings of these canals too.

A few years after Schiaparelli, the American astronomer Percival Lowell even built a state-of-the-art observatory in Arizona so that he could look for himself.

He published books, claiming the canals were Martian irrigation systems built by a dying civilization to transport water from the poles.

He estimated that the largest ones were dozens of kilometers wide.

But in the 1960s, when the Mariner probes flew by Mars, they took some closeup images that revealed absolutely zero canals.

Mars is the barren, red, dusty planet that we know today.

And spoiler, no Martians.

Now, looking back, it seems like this whole Martian craze all just came down to bad hardware.

Schiaparelli was looking at Mars through a telescope that had a 22-centimeter lens, pretty high-tech for its time but that's only about 20 times bigger than the lens in our eye.

And the size of that lens determined its resolution, or how much detail it could make out, because physics puts some hard limits on that.

- [Narrator] Hit it.

(bright upbeat music) - When you look at an object that's really far away, like Mars, or even just a train on the horizon, it looks really tiny, right?

Not because it's actually small but because it's only taking up a small part of your whole field of view.

In other words, its angular size is tiny.

But say that train's coming closer, its angular size grows and suddenly you can make out details like windows and headlights that were impossible to see before.

The reason you can't just see faraway things in infinite detail is because of the fact that light behaves like a wave.

See, whenever light passes through an opening, like your eye or a telescope, it bends and spreads out just like a wave in water.

This is called diffraction.

Because of this diffraction, a telescope can't focus light from a distant point into a perfect, small point.

Instead, it creates a tiny fuzzy spot like this.

The amount of spreading due to diffraction determines the size of that fuzzy spot.

A narrower opening causes more spreading and a larger, fuzzier spot.

A wider opening causes less spreading, resulting in a smaller, sharper spot.

Now imagine two stars separated by a minuscule angle in the sky.

If their light enters a telescope with a narrow opening, thanks to diffraction, the two fuzzy spots will overlap and blur together into a single point, hiding what's actually there.

But if their light enters a wide opening, the diffracted spots are smaller and tighter, and we can easily see them as two distinct points.

Now, our eyes don't really have pixels, at least not in the same sense as cameras or screens do, but you could kind of think of it in the same way.

If your camera has low resolution, a bunch of details will land on the same pixel so you won't be able to tell them apart in your picture.

But with denser pixels or a larger image, more of those details will land on separate pixels and you'll be able to see them more clearly.

It's the same with telescopes.

If you have low resolution, thanks to angular size and diffraction, you simply won't be able to make out finer details.

Now, one way to fix this problem is just get closer.

Then whatever you're looking at will have a bigger angular size, and its details will be easier to make out.

That's why you naturally pull your hand closer to your face when you're looking for a splinter or a paper cut.

But that's not always possible, especially when you're trying to observe something like Mars, right?

We can't exactly just pull it closer.

That's where telescopes come in, 'cause telescopes do two key things.

First, they magnify the light coming in so that something with a really small angular size looks bigger to your eye.

And, second, they improve the resolution.

Because telescopes have a wider opening than your eye, that gives them more resolving power.

The smallest angle a telescope can resolve is roughly the wavelength of light that you're observing divided by the diameter of your telescope's opening, or the aperture.

Schiaparelli was just looking at visible light, so let's stick with the average visible wavelength, about 550 nanometers.

If you plug that in, divide it by the diameter of Schiaparelli's lens, you get a resolution of around two 10,000ths of a degree.

So his telescope was enough to basically read a car's license plate from three or four blocks away.

But if you're looking at Mars, even when it's at its very closest to Earth, that minuscule angle means that the smallest thing you can make out is several 100 kilometers wide, at best.

This means that Schiaparelli could make out big features like dark basins and volcanic plains, which he called seas, but there's no way he could have seen canals, because there are none.

What he was seeing were probably just optical illusions, because our brains are really good at finding familiar patterns.

That's why random rock formations on Mars can look like a face, or even a mouse.

To see what he thought he saw, Schiaparelli would've needed a telescope lens over a meter wide.

This means if we ever wanna look for features like cities or actual canals on an exoplanet that's literally trillions of kilometers away, our telescopes are gonna need to get seriously big.

Now, on one hand, we've come a long way since Schiaparelli's day.

We're not using glass lenses much anymore, since they don't even get much bigger than a meter wide before the glass itself starts to sag under its own weight.

Instead, most telescopes now use large mirrors to focus light.

The biggest one ever is now being built in Chile, with a mirror dish 39 meters wide.

We even have some mirror telescopes up in space, where they can see clearly without worrying about the atmosphere or light pollution, or stuff like that.

Super sensitive detectors on telescopes can now pick up light billions of times fainter than what our eyes can detect.

And all these advances have let us see unbelievably far.

This picture from the Hubble Space Telescope reveals some of the oldest and farthest galaxies in the observable universe.

We're talking more than 13 billion light years away.

That's a view of the universe just 100s of years after it was born, but an exoplanet is like a trillion times smaller than a galaxy, so we still have the same issue as Schiaparelli.

We just can't get the resolution that we need to get a clear, detailed image of exoplanets.

Although there are other reasons this is complicated, mainly the fact that you have to somehow separate the planet's light from the star's light, but we're just gonna put that aside for now.

The nearest exoplanet, an Earth-sized rock orbiting our neighbor Proxima Centauri is over four light years away.

At that distance, you'd need a telescope around two kilometers wide just to have enough resolution to see anything resembling a planety shape.

Telescopes, like James Webb, have actually been able to pick up hints of certain chemicals in exoplanet atmospheres, including ones that might even be fingerprints of life.

We've actually got a great video about that, and you should watch it later.

But that's still not enough to pick up on alien engineering, or cities, or invading fleets of spaceships.

To do that, you'd need a telescope over 700 kilometers wide.

That's bigger than Lake Superior, and that's just for the nearest exoplanet.

To spy on the planets around, say, Trappist-1, a star with three planets in the habitable zone, you'd need an even bigger telescope.

Now these planets are around 39 light years away, it's over 350 trillion kilometers.

At that distance, you need a telescope over half the diameter of the Earth to resolve something 30 kilometers wide on the surface.

But there is one hack that lets us reach really sharp resolutions by using lots of small telescopes instead of one absurdly large one.

Just outside L.A., six one-meter-wide telescope dishes are spread across a mountain, working together as one large telescope.

It's what's called an interferometer.

An interferometer is an array of different telescopes that are all looking at the same object at the same time.

Now, each telescope collects light independently, then a bunch of mirrors guide that light to a central detector where all the light beams meet.

Here is where the magic happens.

Each light beam travels a slightly different distance to each telescope in the array, so when they meet at the central detector they're not quite synced up.

So when they come together, those slightly offset waves interfere with each other just like waves in water.

Some add together and some cancel out.

You end up with a messy interference pattern that really doesn't look anything like the original object, but with powerful computers astronomers can analyze these patterns to reconstruct an image of what they're looking at.

It's kind of like if you threw a bunch of stones into a lake and then analyzed the ripple patterns to figure out exactly where each stone hit and how big it was.

The farther apart the telescopes are in an interferometer, the higher the resolution that you can get.

In fact, an interferometer's resolution is the same as a single telescope with a mirror that covers the entire distance between the telescopes.

It feels like magic, honestly, but it's true.

These six telescopes can be moved up to 330 meters apart.

That's like having the resolution of a telescope with a 330-meter mirror.

With vision like that, you could make out someone's fingernail all the way across the United States, from coast to coast.

Now, that's still not enough to resolve a whole exoplanet, but it's our sharpest optical telescope yet.

And there are even bigger interferometers.

The Event Horizon Telescope is an interferometer that detects radio waves rather than visible light.

It's really a network of detectors all around the world, essentially making a telescope almost as wide as our planet.

Astronomers have used it to image fine details that are on the edge of our galaxy's central supermassive black hole, which is more than 26,000 light years away.

Its resolution is so sharp, it can make out features that look as big in the sky as newspaper text viewed from across the Atlantic Ocean.

And yet, even that is still not enough to see features on an exoplanet's surface.

If we could build an optical interferometer as big as the Event Horizon Telescope, then we could finally get the resolution we're looking for.

But no one's been able to do that yet.

Putting an optical interferometer in space would make that more realistic, and NASA actually considered doing that back in the '90s.

But coordinating different spacecraft to get the extreme precision needed would be its own nightmare.

So, for now, some of our best tools for studying exoplanets are spectrometers, tools that split the light from exoplanets into a rainbow of colors.

And by looking at the colors that appear, or the colors that are missing, we can figure out what kinds of molecules are in the planet's atmosphere, emitting or absorbing those colors.

It's not a picture of a planet maybe the way we're used to seeing pictures, but it can actually tell us a lot.

Long before we can ever capture images of alien engineering on an exoplanet, we may be able to detect molecules like water or methane that could hint at life below.

But there is one more idea.

What if, instead of using gigantic mirrors or arrays of telescopes spread across the solar system, we could use gravity as a lens instead?

Because that kind of already exists.

The Sun itself acts like a massive gravitational lens.

As light from behind the Sun passes through its gravitational well, which is the effect of the Sun's gravity warping space-time, it bends and focuses that light on a specific point downstream, just like in a telescope.

There's just one catch, it just so happens that where the Sun focuses that light is way out in space ,about 14 times as far away as Pluto.

That's pretty inconvenient.

The Voyager probes, the farthest human-made objects from Earth, which have been traveling since 1977, aren't even halfway there yet.

But if we could one day put a detector at that far-off focal point, we would essentially have a lens as big as the Sun.

The resolving power would be absolutely enormous.

We could map the surfaces of exoplanets 1,000s of light years away and resolve whole Earth-sized planets all across our galaxy.

And we know it would work because astronomers already use massive objects as lenses all the time.

The whole reason we can see some galaxies at the edge of the observable universe is because their light passes by enormous galaxy clusters that focus that light like a lens.

That lets us see these objects clearer and brighter from the other side.

So even if this isn't happening tomorrow, or even anytime soon, it's not out of the question that we could one day image an exoplanet in detail.

Back when Schiaparelli was sketching out his maps of Mars, he probably never imagined that one day we'd be snapping pictures of that planet from spacecraft, or driving robots across its surface.

Some of our best ideas were crazy ones too.

Stay curious.

(light instrumental music) For the record, we're talking about Abraham Lincoln.

(Narrator laughing) (indistinct).

One more idea.

That sounds really weird.