[MUSIC PLAYING] MATT O'DOWD: This episode
is supported by LegalZoom. In 1990, an experiment
conceived by Carl Sagan was performed using
the Galileo spacecraft. The purpose? To detect life on a planet
based on measurements by a space probe. The experiment was
successful, and abundant life was unequivocally confirmed. That planet? The earth. Now, a quarter of
a century later, we're on the verge of conducting
that same experiment on a world orbiting another star. [THEME MUSIC] In December of 1990, during
its first gravity assist flyby, the Jupiter-bound
Galileo spacecraft turned its eye
towards the earth. In a plan devised by,
among others, Carl Sagan, Galileo would measure the
spectrum of Earth's atmosphere, take pictures, and
look for radio emission during this brief flyby window. The aim would be to test if
such a probe could positively detect life on a world
using only data taken from space, and with as few
prior assumptions as possible. The data and conclusions
from these experiments were published in
Nature in 1993. In this paper, Sagan et
al provides a framework for finding life on
other worlds decades before our technology would
allow a similar experiment beyond our solar system. But that technology
has finally arrived. But first, let's talk
about what life on Earth looks like to an
observer in space. The easiest-- and so
probably the first-- way we'll spot alien life
is by its effect on its planet's atmosphere-- in particular, the chemical
content of that atmosphere. So let's look at
what Galileo saw. Looking at the spectrum
taken with its near-infrared spectrometer you can
see these deep dips that result from molecules
in Earth's atmosphere absorbing specific
wavelengths of light from what would otherwise
be the smooth heat glow of the atmosphere. Those dips, absorption features,
are undeniable signatures of certain molecules. We see strong absorption
from H2O and O2. Apparently, there's water
and oxygen in our atmosphere. That's a relief. Water is essential
to life, and oxygen is essential to us, but also
an indication of the presence of photosynthesis. Going to longer wavelengths
we see carbon dioxide, nitrous oxide, methane,
ozone, and, well, more water. While some of
these molecules are known to arise
from life on Earth, their presence isn't enough to
confirm life on another planet. See, an important and
necessary condition for claiming a detection of
life is a clear departure from thermodynamic equilibrium-- that is, the natural
chemical balance that an atmosphere should
fall into without something weird like life messing it up. It's definitely not
enough to find molecules associated with life-- like water, carbon
dioxide, or methane. These can arise from
non-biotic processes. Geological processes
alone can produce lots of interesting
chemical reactions. A set of non-equilibrium
chemical abundances must be observed
that can't possibly be explained without life. As Sagan puts it, life is the
hypothesis of last resort. It's never aliens
until it's aliens. As an example, methane
in the presence of oxygen is a big giveaway. Given how quickly methane
oxidizes into water and carbon dioxide, there should barely
be a single methane molecule in an oxygen-rich atmosphere. Yet there is, impossibly
more than the thermodynamic equilibrium abundance. So where is all of that
methane coming from? Well, know that
around half of it is from natural biological
systems, like methane bacteria. Anthropogenic sources account
for the other half, including burning fossil fuels,
and, as Sagan puts it, flatulence from
domesticated ruminants. Yep, Carl Sagan
published a nature paper about how to detect
alien cow farts. Nitrous oxide also appears
in high disequilibrium. It's destroyed by
solar ultraviolet light in the atmosphere with a
half-life of around 50 years. But that means it has to
be continuously produced in order to be seen. While there are non-biological
ways of making nitrous oxide, like lightning, these aren't
nearly enough to account for the amount observed. On Earth, we know it comes
from nitrogen-fixing bacteria and algae. Of course, these are
just the disequilibria that we find on earth. Alien life might have
very different chemistry and so result in
other disequilibria. However, given a decent
understanding of chemistry, geophysics, and
exometeorology, we can be pretty sure
when something we see is out of whack. There is one
molecule that we want to find in high abundance
regardless of alien chemistry or even equilibrium values-- we want to find liquid water. While water is
basically everywhere we look in the
universe, liquid water is a little harder
to come by, yet it's incredibly important for
evolutionary chemistry. Even if we ignore the fact that
all Earth life requires it, water is by far
the best substance in the universe for brewing
up and supporting life. Water has a high
dielectric constant, which means it's good at
storing electrical energy. This allows it to
easily break apart ionic bonds, such as
those found in salts. This makes it a
powerful solvent, so it's great at facilitating
chemical reactions. It also has a high
heat capacity. It takes a lot of energy to
cause it to change temperature. That means it can exist as a
liquid over a large temperature range and grant's
temperature stability for the delicate organisms
living in it or made of it. If you see water in the
atmosphere and the temperature is right, it potentially exists
as a liquid on the surface. Galileo also observed the
spectrum of Earth's surface and took color photographs. Of course, these revealed
strangely green land masses due to chlorophyll
absorbing red light and reflecting green light. Now these wouldn't
be considered proof of life on an alien
world, but maybe a strong indicator,
especially in the presence of atmospheric signatures. Galileo even captured radio
signatures from Earth. It "discovered" the
artificial-looking modulated narrowband transmissions
of an emerging technological civilization. The Galileo flyby
was a major success. It proved the existence
of life on Earth. That's of questionable
scientific value on its own, however this experiment
gives us a roadmap for what to look for in other
star systems, decades before it became
possible to do so. We are still a long way from
sending a probe like Galileo to another solar system,
but we're working on it. Programs like
Breakthrough Starshot, as discussed in this video,
promise the first close-up observations of a new world
in several decades time. While we need to travel
far to get high res pics of an exoplanet,
we finally succeeded in taking spectra
of their atmospheres without leaving home. We do it by enjoying
lovely exoplanet sunsets. To be more precise, we analyze
the light of a distant star as it passes through
the atmosphere of one of its planets. This only happens for
transiting exoplanets, those that happen to
be aligned so that they pass in front of their parent
star from our point of view. Only a tiny fraction
of the star's light passes through the planet's
atmosphere when this happens, but by carefully subtracting
most of the star's light, we're left with a set
of absorption features from the planetary
atmosphere itself. Take HD 189733b for example-- this is a so-called hot
Jupiter, a gas giant, even larger than Jupiter, that
orbits its star closer than the orbit of Mercury. The parent star was observed
using the Hubble and Spitzer Space Telescopes
during a transit. The spectra revealed the
presence of water, methane, and carbon dioxide. However, given the planet's
blazing 700 degree Celsius surface, it's unlikely
that extraterrestrial life would be found. It's too hot for liquid
water, and the methane is likely from other
non-biological sources. We've looked at
Neptune-sized objects like HAT-P-11b, detecting
clear skies and water vapor. We've even started to look
at super-Earths, like in 55 Cancri e, detecting
hydrogen and helium. We don't quite
have the technology to analyze an
Earth-like atmosphere around an Earth-like planet. Those planets are just too
small and their atmospheres too thin for any
current telescope. But that will all
change next year-- in 2018, the James Webb
Space Telescope will launch. Its gigantic 6.5 meter diameter
mirror and incredibly sensitive infrared spectrograph, coupled
with the clarity granted by being in space, will
enable us for the first time to perform Sagan's
1990 experiment on an Earth-like alien world. For example, we should
be able to detect the atmospheric compositions
of the seven exoplanets around TRAPPIST-1,
assuming they actually have decent atmospheres. This is facilitated by the
fact that the star itself is very dim, making subtraction
of its light easier. Three of these
worlds-- e, f, and g-- lie in the habitable zone,
the distance from the star where liquid water is possible. Now, there are other
reasons to think that the TRAPPIST-1 planets
are not ideal for life, but who knows. Perhaps our first detection
of alien life is only a couple of years away. There are billions and billions
of potentially water-bearing Earth-like planets
in our galaxy alone. The prospect of there
being life on other worlds seems very, very good. Our first evidence
of it is likely to be found in the disequilibria
of alien atmospheres. And if life is
common, that evidence will probably be found
within our own lifetimes. In fact, we may even
discover alien life in the next few years. This would answer one
of the oldest questions in science and philosophy-- are we alone in the universe? Perhaps that answer is
already traveling to us in the light of a distant
planet's atmosphere calling to us from
across spacetime. Thanks to LegalZoom for
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