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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.
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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.
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