Original subtitles

If you’re a fan of Science Fiction,

or just really keen for humans to find alien life,

you might have heard of the Drake equation.

It was coined in 1961 by astronomer Frank Drake,

and it provides an estimate

for how many advanced alien civilizations

there could be in our galaxy.

But I hate to break it to you.

1961 was over 60 years ago.

And three decades before we discovered

any planets beyond our solar system.

In all that time, astronomers have been able to learn

a lot more about how the universe ticks.

How abundant the ingredients of life are.

And what conditions may or may not be

necessary to give rise to life.

So, the question we have here at SciShow is this:

Is there any value in going back to that equation?

Or should we come up with something entirely brand new?

[intro jingle] For everyone who doesn’t have

the original Drake equation

etched into their memory…

including me…it looks like this.

In some versions,

the multiplication signs aren’t there,

but remember that’s really the only kind of math

this formula really makes you do.

And I know that’s a lot of letters.

But I am here to walk you through them.

N is the number of alien civilizations

that live in a given galaxy.

And that given galaxy is almost always ours,

the Milky Way.

For the terms on the right side of that equals sign,

you can think of each

as belonging to one of three groups.

Astronomical, biological, and social.

R* is the formation rate of stars in a galaxy.

How many stars form every year, on average.

Fp is the fraction of stars in that galaxy that have planets.

Ne is the average number of planets per solar system

that are potentially habitable.

Fl is the fraction of habitable planets that actually develop life.

Fi is the fraction of those inhabited planets

that manage to develop life where that life is intelligent.

And then Fc is the fraction of those planets

where intelligent life has developed technology

we can detect from Earth...

And finally, L is the average time that technology

is going to be broadcasting into space.

Not how long the civilization has been around…

just how long they’ve been able to “talk” with the universe.

Like, even though anatomical humans

have been around for a couple hundred thousand years,

give or take,

our radio signals have been leaking

into space for a lot closer

to like a single century.

And now you might be screaming,

why does this even matter?

It’s not like it shows us

where the rest of those N civilizations are,

or puts us into contact with them or anything.

Well, technically,

the Drake Equation was always

more of a conversation starter

than a truly helpful equation.

Which I say with all the love in the world.

Frank Drake created it to help organize a radio astronomy conference.

It was a way to get astronomers thinking about

what things they’d need to know to answer the question

“How many aliens are out there?”

And at the time, only R* had any kind of estimated value.

But Now we’ve got a little more to go on.

And over the decades,

astronomers have learned enough to start debating

whether the Drake equation needs a makeover,

or just needs to retire.

One revision from 2013 proposed simplifying the entire Drake equation to just this:

Well, simplifying in terms of aesthetics, at least.

This version basically recognizes those three groups

I mentioned before,

but also smooshes

most of the biological and social stuff together.

It’s not really less complex.

In fact, when you peer under the hood,

it’s probably even more complex because of how complicated the universe turned out to be.

First, we’ll take a look at what’s hiding

in that deceptively simple Rastro term.

Over the past six decades, astronomers have learned

just how not-constant the original R* term is.

Cause It turns out a galaxy’s star formation rate

evolves over time.

And when it comes to hosting planets,

not all stars are equal.

Different kinds of stars are better or worse

at forming different kinds of planets…

especially habitable planets.

For example, red dwarfs can go through

a stellar version of the Terrible Twos

that strips planets of their atmospheres.

Meanwhile, stars that are similar in mass to our Sun evolve pretty fast.

And shorter life spans mean shorter amounts of time

that a planet can host life.

So according to many astronomers,

it’s actually the stars in-between red dwarfs and our Sun,

called K stars,

that are our best bet for habitable planets.

But even three decades in, we’re still in the early days for exoplanet research.

Astronomers have identified…

like… 30 rocky worlds that could maybe,

hypothetically, host life as we know it.

And our search for these worlds is still very biased.

But not towards our own setup,

like trying to find Earth 2.0 around Sun 2.0.

It’s actually biased toward finding planetary systems

where the planets orbit really close to their stars,

just because it’s a lot easier to see those planets.

So we’re still working out how many planets

we’re really dealing with out there

even before considering the lack of nuance

brought to you by the Habitable Zone.

What’s that, you ask?

A Habitable Zone attempts to predict

where a planet could orbit a given star

and maintain liquid water on its surface.

Because liquid water is a critical ingredient

for life as we know it.

But a planet just being in the Habitable Zone

doesn’t mean it’s actually habitable.

For example, red dwarfs are small and dim,

so their habitable zones are super close.

Not only close enough for a planet

to get its atmosphere stripped away,

but close enough for the star’s gravity

to sometimes lock the planet into place…

one half in perpetual day, and the other half in perpetual night.

In contrast,

just because a planet isn’t in its star’s Habitable Zone

doesn’t mean it’s uninhabitable.

For one thing, a star’s Habitable Zone

actually migrates as it ages.

So when life first arose on Earth 3.8-ish billion years ago,

our planet wasn’t inside the Sun’s habitable zone.

Supposedly,

it was too far out,

and any liquid surface water should have been a bunch of ice.

But Earth had a thick atmosphere

full of greenhouse gasses that kept it warm.

Then, there’s the fact that planets

are not the only worlds that could potentially host life.

They may not have appeared in any part of Drake’s original equation,

but any good conversation about habitable worlds these days

is eventually gonna turn to moons.

Moons like Europa,

which thanks to its gravitational interaction

with Jupiter and its lunar siblings,

has a salty ocean buried underneath kilometers of ice.

And get this: the Europa Clipper mission,

which is set to launch in October 2024,

has a little silver plate engraved with,

among other things…

the original Drake Equation.

So it’s clear that some NASA nerds still hold it in their hearts,

even if other astronomers want to re-work it.

And with that rework have come attempts to actually calculate Rastro,

or similar terms.

For example, that 2013 paper estimates

that one habitable planet forms in our galaxy every 10 years.

But that really is just a starting point

for whatever the rest of the equation looks like.

So let’s move onto Fbiotec,

which smooshes together all the terms

from the original Drake equation

that deal with life actually

evolving on potentially habitable worlds,

and also evolving enough to produce signals we can detect.

Unfortunately, here’s where we run into a bit of a wall.

Astronomers can try to use Earth as a proxy,

but we don’t know how easy

it is for a planet to produce life,

let alone intelligent life,

because we only know of one place where it happened.

And everywhere else, we’ve only found the ingredients for life,

like water, carbon, and nitrogen.

Although to be fair, certain ingredients

seem to be, like, everywhere.

We’ve found them on moons, on asteroids,

there were even some complex organic molecules

found floating loose in protoplanetary disks.

But it’s all a far cry from life itself.

That being said, the Drake equation

and its re-worked successor

don’t seem to be accounting for everything they might need to.

Like… what about aliens colonizing uninhabited planets?

Or what if some aliens are populating planets

with robotic probes that can send signals

implying there’s life on that planet,

even though there isn’t?

Or what if an alien civilization got wiped out,

but the technology it left behind kept transmitting? .

Are we going to have to argue about alien AI

before we figure out what constitutes AI down here?

Well… maybe.

One paper from 2020

used the Drake equation to

estimate the number of potential artificial civilizations

in the galaxy,

and compare it to the number of potential biological civilizations

the original Drake equation was focused on.

AI civilizations often wound up outnumbering the biological ones.

So our search for life may actually be

more likely to turn up super incredibly intelligent computers.

Which is a… really wild concept.

And also a super creative use of the Drake equation.

But again… not really what Drake

had in mind with the original.

All those non-straightforward ‘civilization’-establishing

considerations are pretty new.

And those are some pretty big holes to fill in order

to really feel like we’ve covered all our extraterrestrial bases.

But maybe that’s more than you really need.

If you think the 2013 version is as simple as it gets,

Drake himself is about to rock your world.

The most important term in this equation is time,

which Drake believed so much

that he had a license plate

that read NEQLSL.

And here’s the thing.

We don’t have to limit our search

to the kind of proverbial yelling

that Drake originally had.

The original formula was focused

on aliens sending radio signals

out into the universe,

but we can try to hunt for other signs of alien civilizations.

Or, we can focus on biosignatures

that may tell us whether there’s any life on a planet,

like, at all.

Now that we have more flexible and sensitive tech,

we’re not limited to only listening for the loudest signals of extraterrestrial life.

For example, we can see if a planet

has a significant amount of oxygen gas in its atmosphere,

perhaps maybe that’s an alien form of photosynthesis.

And compared to a century of radio waves,

life has been pumping oxygen into Earth’s atmosphere

for like two billion years.

In other words, we’re not just chopping off

some of those later terms

based on intelligent life evolving and developing.

We’re jacking up the values we can plug in for L.

and ultimately, a biosignature-based Drake equation

requires a different kind of rewrite

than a mere smooshing together of terms.

We have to reconceptualize what’s important.

Will we really only be satisfied once we find an alien

that can tell us to live long and prosper?

Or given how much our technology

has progressed since Star Trek went on the air,

maybe we lean in to finding any definitive signs that life exists

beyond this tiny blue dot floating through the cosmos.

We can still etch the original Drake equation

into our spacecraft plates, though.

It’d be a solid conversation starter for any aliens that want

to hear the story of how we eventually found them.

And if I had my way,

I’d sneak into whatever factory

they’re making those plates

and carve a thank you to our patrons.

Because I want the aliens to know

how much your support means to us as well.

You’re awesome.

Thanks for watching.

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