Understanding Life on Exoplanets
By: Sanchit Gupta
Tags: Exoplanets, Astrobiology, Life, Projects, Astrology

Understanding Life on Exoplanets

Image depicting the Archean eon on Earth. Taken from here

Around 3 billion years ago in the Archaean eon, Earth was very different than it is today. The planet was predominantly covered with water(only 2–3% land) which was mostly green in color due to a large amount of iron. The atmosphere was free of oxygen and was composed of Nitrogen, Carbon dioxide, and Methane. I know you must be thinking that life is almost impossible without oxygen but at that time, simpler microscopic life thrived using anaerobic respiration.

Respiration that proceeds in the absence of oxygen is anaerobic respiration

Image showing Cyanobacteria, taken from here

It was around 2.8 billion years ago when Cyanobacteria, which uses photosynthesis to convert the energy from the sun, started becoming common on Earth. They took the carbon dioxide from the environment and gave off oxygen as a waste product. As the population of these Cyanobacteria increased, they started to change the chemical balance of the biosphere. The free oxygen they produced started to react with all the iron in the ocean and it is said that around this time the oceans on earth turned red because the iron oxidized to form rust. As the oxygen increased in the atmosphere, the biosphere started to become toxic for the organisms which were used to an oxygen-free environment. These organisms started to die out while the population of Cyanobacteria kept on increasing. Cyanobacteria used Carbon Dioxide for photosynthesis which because of increased consumption started to decrease in the atmosphere. This decrease caused the temperature on the Earth to go down as CO2 is a greenhouse gas that traps the energy from the sun and regulates the temperature on Earth. This was the start of an ice age which caused a lot of species to become extinct.

You will be surprised to know that the Oxygen level when all this was happening was only 10% of what we currently have on planet Earth.

Great Oxidation Event

It is said that the increase of Oxygen in Earth’s atmosphere happened in this big event called the Great Oxidation Event. It is termed as an event because it happened in a very short period of time, Geologically speaking. Around 2.4 –2.2 billion years ago, there was an increase in the oxygen levels on Earth. This event marks the first appearance of appreciable oxygen in Earth’s atmosphere. As discussed before we know that the vast population of Cyanobacteria were responsible for this rise.

A question then comes is, if these photosynthesizing bacteria were common around 2.8 billion years ago, why did it take so many more years for Oxygen to increase?

The answer to this question lies in the chemistry of early Earth. Although the oxygen was increasing in the atmosphere, it was also simultaneously removed by the reducing components produced from the Earth’s crust during volcanic activity. Elements like iron in the water and Nitrogen in the air took this free oxygen and came into their oxidized form before the oxygen started to accumulate. Also, it did take a fair amount of time for the Cyanobacteria to become ubiquitous on Earth.

Geologists have found a lot of evidence to support the GOE and we will go through some of those here.

Fossil Evidence

Preservation of Cyanobacteria in mats called Stromatolites. Image from here

We have found fossilized mats of accumulated Cyanobacteria called Stromatolites from before the GOE which suggests that Cyanobacteria started to become common before the oxidation event occurred. We know that these bacteria were major contributors to the oxygen in the atmosphere due to photosynthesis.

Continental Oxidation Evidence

From the Geological evidence we have found that pyrite (Iron Sulphide) was common on Earth before the GOE, but as the oxygen rose in the atmosphere the iron started to get oxidized and pyrite depositions became less common. Instead, there was an increase in the Red bed deposition which are, red-colored sandstones coated with iron oxide.

This absence of Pyrite and the appearance of Red beds coincides with the GOE period and hence shows that there was a significant increase in the oxygen levels which led to the oxidation of iron.

Banded Iron Formations (BIFs)

Image shows the Banded Iron Formation (BIFs). Image from here

As the Oxygen in the Earth’s atmosphere increased, it started the process of oxygenation of Earth’s water. The iron in the oceans started to get oxidized to form iron oxide (rust) which was deposited to form banded iron formation. The BIFs are alternating layers of iron oxide and silica-rich minerals. However, we have found that the deep oceans were not fully oxidized until much later, but the occurrence of banded iron formation during this period gives very strong evidence of increasing oxygen content in the Earth’s atmosphere.

Isotopic Evidence

In this section, we will talk about carbon and sulphur isotopic evidence for the oxidation event. In the geological evidence for the period which marks the end of the GOE, we find a highly positive carbon isotope in carbonates. The high C-isotope values reflect the high organic burial rate. This is because the increase in the atmospheric oxygen caused an increased oxidative weathering of continental sulphide, which turned into sulphate and hence increased the production of sulphuric acid. Now, because sulphuric acid is acidic in nature it caused an increased chemical weathering of phosphorus-rich sediment which led to an increased flux of phosphate to the oceans. The phosphate is responsible for the high organic burial rate. Read more about it here.

The sulphur isotopic evidence is probably one of the most valuable pieces of evidence we have for the GOE. Mass Independent Fractionation (MIF)of sulphur isotope dominated early Earth whereas after the oxidation event it was the Mass Dependent Fractionation (MDF) that dominated Earth. MIF usually occurs in the upper regions of Earth’s atmosphere, where sulphur dioxide is broken down by the UV radiation from the sun. In the atmosphere which is free of oxygen, the fractionated products remain isolated and get preserved in sediments. In the atmosphere, like we have today, the fractionated products get re-oxidized before being preserved on the Earth’s surface, and hence all signs of MIF are removed from these sediments. Since we have found a shift from MIF to MDF during the GOE, we can safely assume that it was the increase in the oxygen levels in the Earth’s atmosphere that caused this shift. You can read more about MIF and MDF here.

I found most of my information about the evidence of GOE from the Geo Girl on Youtube. Do see her videos if you are interested in GeoScience.

Consequences of GOE on Earth

With the rise in the concentration of oxygen on Earth, other gases like Methane were reduced in quantity. Carbon Dioxide which was used for photosynthesis was also reduced, which later led to the ice age as explained before. For Life on Earth, the great oxidation event was a double edge sword. The organisms which relied on anaerobic respiration were forced to find refuge in areas where oxygen concentration was still low, like the deep sea, while this also marked the growth of aerobic respiration which turns out is 15 times more efficient than its counterpart. The aerobic respiration gave life the opportunity to be more complex and hence evolve to what we see today.

Image showing the different stages of oxygen concentration on Earth. Various processes worked together to increase and decrease the oxygen on Earth to bring it to the level it is at today. Image is taken from here

However, it was not just one great oxidation event that made oxygen levels to what we see today on Earth. The full oxidation of the Earth was a more step-wise process which is why we see the evolution of multicellular organisms happened much later.

Life on Exoplanets

Photo by Ben Wicks on Unsplash

Now that we have some idea about how life evolved on Earth, transforming from organisms that flourished using anaerobic respiration in an environment devoid of oxygen to a present-day Earth which has a high concentration of oxygen and life is almost impossible to imagine without it, we move on to answer another major question, how do we find life on exoplanets?

Exoplanet or extrasolar planet is a planet outside the Solar System

A more intriguing question we should answer before we delve into exoplanets is how we define life. Astrobiologists have long tried to answer this question. Keeping the life on Earth in mind they have come up with a list of characteristics that can define life and are used to locate life on other planets, but they also never leave the assumption that life on an exoplanet might be completely different than what we observe here on Earth. You can read about the characteristics of life here.

To look for life in the universe, we look for favorable conditions like a crusty, rocky planet in the habitable zone of a star, with the presence of elements we find essential for life.

Habitable zone is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.

One of the methods used to find life on exoplanets involves detecting biosignatures emitting from these planets. Biosignatures are signals generated by life in the form of gases that accumulate enough to become detectable. For example, Earth has oxygen as a biosignature. Astrobiologists have to be careful when studying biosignatures from other planets as these signatures are not always biologically produced. For example, Mars has no evidence of life but still has 0.16% of molecular oxygen which is formed due to the reaction of carbon dioxide and sunlight.

We detect biosignatures by the spectrum of light reflected and absorbed by the planet's atmosphere. Each element has a unique spectrum of light that it absorbs and hence helps us to identify what the atmosphere of any planet is made up of.

Oxygen as a Biosignature

We know that oxygen can be produced by both biological and non-biological processes then why do we still consider it as a biosignature and get excited when we find traces of it on other planets.

The answer to this question lies in the concentration of oxygen in the atmosphere. We still don’t know any non-biological way using which a planet’s atmosphere can have appreciable quantities of oxygen. That means that if a planet has high amounts of oxygen in its atmosphere then it must be due to a biological process which is using photosynthesis to accumulate this oxygen.

Another benefit of having a high concentration of oxygen in the atmosphere is the production of ozone due to the reaction with UV radiation from the star. Ozone has a very strong biosignature and we can surely tell that the planet has a high concentration of oxygen and hence a very high possibility of Earth-like life.

You will be surprised to know that high quantities of oxygen in an atmosphere is considered one of the most reliable biosignatures, even before liquid water.

Exciting news from the world of Astrobiology

Venus is a planet that is considered Earth’s twin, it has a rocky crust and is comparable to Earth in size and mass. Life was deemed almost impossible on the surface of Venus because of its dense atmosphere. The pressure on its surface is 93 times that of Earth. It seems it is far outside the extremes of what life can handle.

Recently, Astrobiologists have discovered large quantities of Phosphene gas in the Venus atmosphere which suggests that even though life might not be possible on the surface, it very well might be possible in the midst of its atmosphere.

What is interesting about Phosphene is that it doesn’t form easily without being formed biologically by anaerobic respiration. Even on Earth at places that are low in oxygen concentration, life survives using anaerobic respiration producing phosphene in the process. Below is a link to the video which explains why phosphene is such an important biosignature and why we should be cautiously excited about finding this in the atmosphere of Venus.

Video describing the importance of Phosphene as a biosignature and what it means for Venus

I have been long interested in space and biology, researching about life on exoplanets helped me combine these two interests and compelled me to write this article. I hope you find this informative and it helps you develop some interest in Astrobiology. I will be back with more content soon, till then keep reading and supporting. Cheers!