Search for Alien Life

Our Solar System

When looking for alien life, we might want to start in our solar system. You might think we would be aware of life prospering on one of the planets or moons in our solar system, but that's not necessarily true. Even if it was, they might have been habitated before we were around.

Since only Earth and Mars have ever layed in the habitable zone of our solar system though, the search is going to be rather simple. As we know almost for sure there was liquid water on our red neighbor planet until 3.5 billion years ago, because we found carbonate globules resembling limestone cave deposits in marsian meteorites that can only for in the presence of liquid water. Although it may not have been habitable long enough to support intelligent life, microbial life may well have existed on Mars.

Microbial life on Mars

Almost 30 years ago already, a team of scientists led by David McKay at NASA's Johnson Space Flight Center found evidence for microscopic life on Mars in a marsian meteorite. What they found in the meteorite's carbonate globules were clear indicators that Mars is very likely to have once been habitated:

• They found so called PAHs, polycyclic aromatic hydrocarbons that are naturally produced only by microorganisms, fungi, plants and animals.
• Within the globules, they also found microscopic grains of magnetite and iron sulfide, two substances rarely found together unless produced by bacterial metabolism
• If that wasn't enough evidence already, when examining the globules with an electron microscope, they found it to be covered with worm-like forms, likely being fossilized bacterial

Because of this and because liquid water on a planet in the habitable zone already is a big enough reason to search for it, the Marsian Perseverance rover went on a journey to search for biosignatures around the Jezero lakebed in 2020. It has yet to find an incontrovertible proof, however many scientists's educated guess is that we will find one over time.

Applying local observations

There is 2 planets falling into the habitable zone of our Solar System and both of them either have literal proof of life (you and me, for that instance) or are very likely to have been a harbor for microbial life before. If all of the planets that could have possibly had life in the past actually had it, it is very likely for life to not be as rare of an occurence as some scientists believe but rather a much more likely process to happen everywhere in the universe.

That however would mean that we should be able to observe a lot of planets being inhabitated. If any intelligent species has emerged just a couple hundreds of years before us - a blink of an eye in cosmic timescales - we should be able to see their Dyson Spheres or Dyson Swarms, Space Habitats and space traffic all around their home planets.

The thing is... we don't. Now, what the f***? What we just discovered is called the Fermi Paradox. Let's look at it in greater detail.

Additional Resources

1.
More information about the meteorite NASA scientists found littered with hints of microbial life can be found in this article from the American Museum of Natural History.