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] 17 00:00:38,480 --> 00:00:42,890 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.