Carl Sagan's Guide for How to Detect Alien Life

A big part modern astrophysics science is the search for alien life, more particularly for this article, the search for life using NASA's probes. Carl Sagan in particular set much groundwork for using probes like the Galileo for finding life, as finding life being a very important quest for Carl Sagan. If you think about it, it is a very important question for all of humanity. Space in general, even with all our fancy instruments, seems very quiet. Nearly everything we have detected on those instruments has been explainable by natural processes. If we can't find evidence of life elsewhere, then life must by extension, be extremely rare. If life truly is this uncommon, there is a very real probability that humans themselves will simply wipe themselves out before we ever achieve getting off of our little, wet, rock. Finding life becomes an important quest, not only because of the science it will provide, but because doing so will show that humanities journey thus far does not necessarily have to come to an abrupt and sad end.

So how do you find life on an alien planet? 

Carl Sagan laid the groundwork for answering this question by using the best example of life there is, our own planet, Earth, with the Galileo probe. 

1. First thing is to confirm that the chemistry of an alien world is one that supports life as we know it to be, like unusually large amounts of Methane which can be produced by things like, as Sagan puts it, "flatulence from domesticated ruminants". 
2. Confirm through images that  there are large expanses of ocean, there are quickly changing clouds or large changes in reflectivity, or various different colors. The varied surface colors and reflectiveness of earth don't prove life, but they are unusual for what we would expect a lifeless planet to look like. 
3. Detect electronic signals such as radio signals of any type consistent with signals of the sort humans generate with their technology which originate from below the clouds on the planet.

The Capabilities of the Galileo Probe

The Galileo Probe was a spacecraft launched by NASA which studied Jupiter and it's moons. It was the first spacecraft to orbit an outer planet and was the first to flyby and study many of Jupiter's orbiting bodies. Carl Sagan's guide was specifically constructed for the Galileo to detect life on any of Jupiter's moons. 

However, there was a problem. The Galileo wasn't designed for detecting life on an alien planet and because of this, we needed to answer the question as to what life would look like to the science instruments on the Galileo.

An artists conception of the Galileo Probe flying close to Jupiter's moon Io, with Jupiter in the background. 
Image can be found from NASA's photojournal here.

So what do we do? How do scientists figure out what a positive detection of life would look like to the instruments on the Galileo? Scientists know what life on earth looks like and in fact, even to this very day, life has never been found to exist anywhere but on earth. Since we know that life is extremely abundant on earth, if we knew what earth looked like to the Galileo, then we should be able to say if any of Jupiter's moons look similar. Luckily, Carl Sagan was around and had exactly this plan. 

Using Earth to Perform a Gravity Assist

The Galileo was planned in December 1990 to do what is called a "gravity assist". A gravity assist causes spacecraft to take less time to get to their destination by slingshotting around planets using their gravity, hence gravity assist. A gravity assist also means that the spacecraft must get quite close to whatever object is providing the 'assist'. Probes like the Voyager I and Voyager II used gravity assists to greatly increase their traveling speed.

In this case, it meant that the Galileo spacecraft was to do a 'gravitational assist' using the Earth twice and Venus once,  meaning that during the time the Galileo was flying close to earth, it could closely measure what a life filled planet would look like to the instruments on the Galileo. The data Carl Sagan obtained from this Earth fly by could also serve as a guide and a framework for testing for life on alien planets using space probes akin to the Galileo.

An image of earth taken by the Galileo. It was published in 1996 which you can find it in the NASA photo journal. The clouds, varying colors, and large expanses of ocean can be seen from this image, which Sagan in his paper pointed out was indicative of a human friendly habitat.

The Galileo successfully performed all of it's planned gravitational assists with Earth and Venus, greatly reducing the time it would have taken to travel to Jupiter. 

Was the Galileo able to Detect life on Earth?

The Galileo was able to detect all of these things on earth.  However, the Galileo wasn't able to view any visual evidence of human activity on earth, meaning even with its advanced cameras, it couldn't see any buildings or things humans have built. This takes me back to treating it like a trial. We may not be able to directly 'see' life on earth using Galileo, but we can find many pieces of evidence which lead to the conclusion that Earth is guilty of harboring life.

The Galileo wasn't specifically designed to detect life. It was designed to measure things like what is in the atmosphere and take science photos of planets.  There is also a high bar when it comes to detecting life and is, as Carl Sagan put it, a "Hypothesis of last resort".

I think a good way to describe a "Hypothesis of last resort" is to think of a court case in the USA's legal system. In any good legal system, you don't want to be punishing people who are not guilty of a crime that they are accused of, otherwise you get things happening like the Salem Witch Trials.  The attempt at solving this, in the USA at least, is for a court to show what is called "guilt beyond a reasonable doubt". 

This generally means that many pieces of evidence are needed to show guilt, not just an accusation. It involves many questions being asked and that the answers to those questions have shown, all together, that someone is guilty of what they've been accused of doing. If you have enough pieces of evidence pointing to guilt, eventually you might declare that someone is guilty "beyond reasonable doubt". If the questions asked do not show guilt, there is room for reasonable doubt. We then have to then ask whether a person should be punished with no hard evidence of guilt.

Discovering life on an alien planet would be big news to say the least, and it would be very, very, embarrassing for scientists if they quickly declared "We have found life!" and they turned out to be wrong. In fact, doing something like that might lead people to take scientists less seriously regardless of whether or not they are doing good science, which I think is not good at all.

So did the framework which Carl Sagan used in order to begin to lead one to a conclusion that scientists have found life "Beyond a reasonable doubt" or at the very least that Earth is highly suspect of harboring life using the science instruments on the Galileo? Personally, I would say that leads to a conclusion that Earth is suspect, but that life was not proven beyond any reasonable doubt.

Did the Galileo Find Life on Jupiter's Moons?

The Galileo did some pretty cool stuff. It studied and learned many things we didn't know about Jupiter and it's moons, which you can read about here. It even took some pretty stunning photos of Europa

A picture of Europa taken by Galileo in the 1990's and recently re-released in 2014. Using the data available, care was taken to showcase what Europa would appear like to the human eye. Even though Europa is beautiful in it's own right, it looks somewhat barren. Looks can be deceiving though as due to a bunch of liquid water right under the surface, it turns out Europa is one of the most likely places for life to exist outside of Earth in the Solar System. You can learn more about this image from the NASA photo journal website.
Image Credit: NASA/JPL-Caltech/SETI Institute

Despite the Galileo producing many amazing photos which were the first of its kind, discovering many cool geological features, and do amazing science, it did not find conclusive evidence that life exists outside of earth. To put it in Carl Sagan's words

"Although a great deal more exploration remains to be done... our results are consistent with the hypothesis that widespread biological activity now exists, of all the worlds in this Solar System, only on Earth."

Is There any Chance of Finding Life?

Even to this day, the experiment Carl Sagan led in 'detecting life on Earth' can be and is used as a guide as to what to look for when looking for life on an alien planet. The probes we have today studying what is known as the "Jovian system" or Jupiter, it's moons and asteroids continue to produce amazing results. There are even extremely interesting proposals on life existing on Jupiter's moon Titan. If life did exist on Titan, it wouldn't be the same type of life here on earth. Titan is much colder and instead of having lakes of water like Earth, it has lakes of liquid methane and ethan.

A mosaic of Titan. It's atmosphere has a lot of organic molecules with high levels of methane. If life were to exist on titan, it's chemistry would likely be wildly different than the chemistry life has on Earth. It would be quite exciting to find life existing in a form which is entirly alien, even down to the chemistry it relies on to survive. You can find this picture as well as other picture of Titan from the NASA photojournal.

There are also scientists that are currently working to continue the search for life. They even use (and develop) Carl Sagan's original framework in detecting 'biosignatures', which are similar to what Carl Sagan was looking for when he was detecting life on earth. 

There are many other moons that could harbor life. The moon Enceladus shoots plumes of liquid ocean outside of it's surface, as detected by NASA's space probe Cassini

Image as taken by the Cassini Probe of the moon Enceladus. The sun provides backlighting in such a way so as to clearly show the plumes. Picture can be found on NASA's website here
Credit: NASA/JPL-Caltech

Despite progress since the Galileo, the fact still stands that scientists have not been able to conclusively detect life anywhere but on Earth. This highlights the importance of treating the home we call Earth in a manner consistent with sustaining human life. If you think about it, that is the main reason to treat Earth well! Earth doesn't really care about maintaining an environment which is hospitable to human life. It's humans that care whether or not the Earth has a hospital environment. There is irony in humanity's quest for survival that more often than not, the greatest threat to humans are humans themselves. 

Regardless, there is absolutely still hope for the future. Carl Sagan wouldn't have created this framework for detecting life if he thought it was impossible. It was found in 2019 that nearly 10 to 25 percent of FKG stars (stars that are somewhat like our sun for the most part) have rocky planets in their habitable zones. It may turn out that life doesn't exist in our solar system on any planet but Earth, but given the immense number of possibilities out there, I would say there that there is a place humans could call home, it's just on us to find it.