Is There Life on Earth?
Hey Smart People, Joe here.
Yes.
The answer to the question in the title of this video… is YES.
There is life on Earth.
We know that because, well we live here.
But what would we think if we were looking at Earth from 6 million km away?
That's the distance from which Voyager 1 captured this image on February 14, 1990;
all the complexity of our living planet summed up in a single pixel of bluish light.
Now, if one day some extraterrestrials download that image off of Voyager, how would they
be able to tell there's life on Earth, based on… that?
This is the question we face as we get ready to aim the most powerful telescopes ever built
at distant worlds outside our solar system.
If we're gonna search for signs of life… what exactly are we searching for?
Since the discovery of the first exoplanet–a planet orbiting a star outside our own solar
system–in 1992, we've confirmed the existence of almost 4,000 distant worlds.
Scientists think every star in the sky may host at least one planet of its own.
More than 2,000 exoplanets were discovered by the Kepler Space Telescope–moment of
silence.
Never going to forget you.
Looking at artistic renditions of these alien worlds, if you didn't know better, you might
think we can just point a big telescope at an exoplanet and snap an image of it.
But Kepler's raw data looks less like this, and more like this.
Kepler would stare at one spot in the sky, looking for stars that dimmed as an exoplanet
crossed in front of them, blocking some of their light.
By putting together a bunch of data like the size of the star, how much light is blocked,
how often the planet passes in front, then we can estimate the size and mass of the exoplanet.
And if you know how big something is and you know its mass, you know its density, like
if it's a gassy planet or a rocky one.
And because we've studied how orbits work in our own solar system, that same data can
tell us how far an exoplanet orbits from its star.
Finally, if we measure how hot a star is (by looking at the color of its light), we can
tell if a planet has the right conditions where liquid water, or as I call it… “life
juice”… *could* exist on its surface.
Based on all this, we've learned some exoplanets are tiny ice-Earths, some are these big warm
Neptunes, even hot Jupiters… and only some are potentially habitable.
But there's a big difference between could have life and does have life… to tell the
difference, we need to see something that could only be made by life.
I'm not talking intelligent life, or even complex life.
The tiniest puddle of replicating pond scum on an exoplanet would still be the biggest
discovery we've ever made, ever, about anything.
We need to find… biosignatures.
So, a “biosignature” is like a chemical fossil.
Something we can see that must be produced by life, and–this is important–it can't
be made by some natural process.
So what the heck counts as a biosignature?
Voyager 1's “Pale Blue Dot” is the Earth-selfie Carl Sagan is famous for, but he had a different
one taken a few years later that not many people know about.
In 1993, as the Galileo spacecraft passed by Earth on its way to Jupiter, it turned
its sensors towards our home planet, to ask “if we had no previous knowledge of whether
Earth was home to life, would we actually be able to detect any of our own biosignatures?”
So, life on Earth has been around for at least three and a half billion years, and biology
has changed the atmosphere in some pretttty BIG ways during that time.
Take these chemicals.
Here's what their levels would be on a dead Earth, versus what they actually are.
Sagan was looking for a kind of “chemical disequilibrium”– basically you look for
chemicals that shouldn't be there, and if you find them, there must be something on
the planet consistently making them.
And when he looked at Earth… he found… water.
Which actually wasn't very hard.
H2O turns out to be one of the most abundant molecules in the universe.
Liquid water is a necessary ingredient for life, but it's not a sign of life.
Galileo also found methane.
Methane breaks down really fast in a planet's atmosphere, so if you find it that means something
is making it.
Here on Earth it's made by microbes and by burping cows–we have a lot of both.
But, there are natural processes that can make methane too, so it doesn't necessarily
mean “life”.
We've also detected methane on Mars, and Saturn's moon Titan has lakes of the stuff.
No sign of life on either of those.
Well how about carbon dioxide?
I'm alive, and I make it… but so do volcanoes.
Not a perfect biosignature.
Ok, what about oxygen?
O2 was incredibly toxic to Earth's earliest life forms, and for the first billion years
of life there wasn't much of it around… until photosynthesis showed up and started
just throwing it away.
Today this previously poisonous photosynthetic trash gives us life.
But not so fast.
You guessed it, there are natural processes that can make O2… too.
Like on super-hot planets, ultraviolet light can break down water, kick out the hydrogen,
and leave oxygen behind.
But the levels of oxygen and methane that the Galileo measured on Earth were way higher
than those natural processes would predict.
This was the “chemical disequilibrium” Sagan was looking for.
But it still wasn't a smoking gun, just suggested life as a possibility.
A maybe.
Sagan did find one other biosignature on Earth that was especially weird.
On lighter areas of the planet's surface–dry land–there were massive areas that absorbed
red light, and just beyond that, into the infrared part of the spectrum, a whole bunch
of light that wasn't absorbed.
Since no rock or mineral that we know of absorbs red light quite like this, the best explanation
was a pigment covering the planet's surface, one that absorbs red light, and hates near-infrared
light.
That pigment?
Well, we know it as chlorophyll.
It absorbs red and blue light, but not other colors, and that's why so much of Earth
is green.
This biosignature is known as the “vegetation red edge”
Since Sagan's little Galileo experiment, scientists have added to the list of possible
biosignatures and they've learned a lot about how we might tell them apart from natural
processes.
In general, we know different chemicals absorb different colors of light.
So if we can somehow measure how an exoplanet's atmosphere filters light from its star, we
can get a fingerprint of all the chemicals in that atmosphere.
Now that we know what to look for, how do we detect these signatures from light years
away?
The best study of Earth-like planets will come from looking at light from the host star
reflected off the planet and filtered by the atmosphere.
Basically the same way we take pictures of Earth today, only much, much, MUCH farther
away.
Thing is, Earth-sized planets are about ten billion times dimmer than their stars, and
exoplanets are separated from their star by *extremely* small angles.
Directly imaging an exoplanet is like trying to see a moth buzzing around a searchlight,
on top of the Eiffel Tower, from New York City.
To do this, astronomers have designed starshades, which can be placed tens of thousands of kilometers
in front of orbiting space telescopes, to precisely block out the star's light and
make the exoplanets visible, the way blocking out a car's headlights with our hand helps
us see at night.
Of course we only know the signs of life as it exists here, the only place we've found
it.
Somewhere else, life may use completely different chemistry, giving off completely different
biosignatures.
And even here on Earth, life hasn't always looked the same.
Back in the ancient Archaean Era, early life forms lived under a cloudy methane haze.
And the first photosynthesizers may have been purple microbes, not green.
This would all be SO much easier if we could just sense some convenient radio signal coming
from an exoplanet, sent by an intelligent technological life form.
But when you consider human technological civilization only covers 0.000002% of our
planet's history, our chances of listening in the right place at the right time are not
great.
We're looking at the very edge of what is technologically possible, so best not to hold
your breath on finding alien life just yet.
Stay curious.
Notice anything different?
Oh yeah, I got a haircut.
