Where in the universe do we expect to find life?

There are about 200 billion trillion stars in the universe. That’s a ‘2’ followed by ‘23’ zeros.

Difficult to imagine, right?

Because we have evolved to grasp things at a very moderate scale. For example, we can easily imagine how heavy a bag of a kilogram of rice would be, or how high a building of ten storeys would be. But as we increase this scale to tonnes and kilometres and so on, our perception starts to fade away. But our universe has a magnitude on cosmic scales, much more than what our human minds can imagine.

Neil deGrasse Tyson, an astrophysicist, helps us understand these extreme cosmic scales easily in his book “Astrophysics for People in a Hurry” as he explains “there are more stars in the universe than the grains of sand on any beach, more stars than the seconds that have passed since the formation of Earth, more stars than the words and sounds ever uttered by all human beings that have ever lived.” Such is the vastness of our universe and we have always looked for extraterrestrial life in this limitless cosmos.

Life has several characteristics; it grows and develops, uses energy, reproduces, has a complex chemistry, maintains homeostasis, and responds to its environment. Any life that prospers tends to modify its environment and inevitably leaves some evidence behind. When we look out for life outside our home planet, we cannot do it blindly. Instead, it should be based on our knowledge of science that makes us equipped to look out for this evidence of life and narrow down our search in the right direction.

One way to narrow down the search is by looking at the right place, the ‘Goldilocks zone’. Also known as the habitable zone, it is the region around a star that has the right temperature for water to remain in liquid form. We know that liquid water is essential for life as we know it, hence a planet in this zone has a higher probability of supporting life than anywhere else. To narrow our search further, we then have to look at the places which have other necessary conditions for life within this zone.

Goldilock zone where we should look for life — Credit: Ian J. O’Neill (@astroengine on Twitter)

The most important indicator of life is water. If you are an alien who happens to look at Earth through a telescope in search of life, what would you see? You would see a blue planet, which indicates the presence of liquid water, giving you an idea of Earth’s temperature and atmospheric pressure. You would also see polar ice caps; melting and freezing in a cycle that will enable you to understand Earth’s seasons. We have to use the same approach in our search.

We have to find planets that are of the right mass. As massive planets have huge gravity, it would be difficult for larger life forms to survive there because the immense gravity would crush them under their own weight. For example, a horse weighing about 300 kg on Earth, would weigh just 50 kg on Moon as it has weaker gravity due to its smaller mass. But the same horse would be more than 750 kg on a massive planet such as Jupiter and its bones would crush under its own weight.

We should also consider the size of the stars. Larger stars have shorter lives because they rapidly consume all of their hydrogen, giving less time to life on planets around them to evolve and sustain.

We also have to look out for chemical indicators using spectroscopy which is basically the analysis of light. Every element or molecule emits, reflects, absorbs, or uniquely scatters light. We have to find these chemical fingerprints of molecules such as ammonia, carbon dioxide, methane, oxygen, etc. which are indicators of life. For instance, oxygen is a chemically active element and easily bonds with hydrogen, carbon, nitrogen, etc. to make a wide range of molecules. If we find a planet with abundant oxygen in its atmosphere, something is certainly maintaining this constant supply of oxygen, which would otherwise react with other elements to form molecules and cease to exist in the atmosphere. On Earth, life in the form of plants and phytoplankton maintains this supply by photosynthesis. Other molecules such as methane also thrive in presence of life.

Other evidence could suggest the presence of intelligent life such as the presence of radio and microwaves. A technologically advanced life cannot hide these signals as they are emitted from almost all kinds of communication systems, including RADAR and so on. If we happen to detect an unusually large amount of these wavelengths from a distant planet, there is a high possibility that the planet shelters intelligent life. Another such example here on Earth is Sodium, which is emitted from sodium street lamps. Alien life would certainly notice this unusual emission of sodium and expect a technologically advanced civilization on Earth.

Moreover, if we see the presence of markers such as nitric acid, sulfuric acid, and other pollutants on a planet, we can deduce that those life forms have also polluted their planet like us and that’s not a sign of ‘intelligent’ life.

Since the discovery of the first exoplanet in 1995, we have discovered over 4000 such planets till now and this number has doubled approximately every 27 months. This rapid rate of discovery means that there are still a large number of such planets yet to be discovered, which increases the possibilities of life in the universe.

There is a popular saying that if you scoop a cup of water from the ocean and say the ocean has no whales, that is just a bad way to explore. Life can be anywhere; we just have to keep exploring. Our universe is vast in terms of space and time. Looking at this complexity of the universe, we all can resonate with astronomer Carl Sagan who said “these are the things the hydrogen atoms do given 15 billion years of cosmic evolution”.

Check out our article on weird crop patterns: “The Mowing Devil: Story Behind the Strange Geometrical Fractals”

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