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.
"Its okay to be smart" is probably the most pretentious title I have ever seen for a series.