This was written for the Astronomy course 124.129 at Massey University. Written 23 August 2019.

# Introduction

Are we alone in the universe? This question has been asked by many people since the last century. Because of the great expanse of the universe and the amazing variety of cosmic objects, it does not make sense to believe that Earth is the only planet that harbours life in the universe.

Exoplanets are excellent candidates for other worlds that harbour life. The number of discovered exoplanets has dramatically increased in the past decades. It is possible that at least one of these planets have environments suited for life. This essay will cover the conditions required for life as we know it and how we can detect exoplanets friendly to life.

# What makes a planet suitable for life?

Life as we know it requires four key components: a source of energy, liquid water, carbon and other elements. Life on earth has only been found where liquid water exists (Christopher & Wanda, 2014, p.  219) and where there is a source of energy; that source of energy can be anything from a deep-sea smoker to the sun. (Christopher, 2014) The basic chemistry of life requires carbon as a backbone. Due to the abundance of other elements such as N, S and P in living organisms, it is likely that life depends on these elements and requires them. (Christopher, 2014) It is expected extraterrestrial life to loosely follow these limiting factors. Therefore, searching for life on exoplanets should filter for these requirements. Discussion of these limiting factors is found below.

## Source of Energy

Life requires energy to operate. Living organisms reduce the amount of entropy they have in order to complete certain tasks, such as movement and metabolism. This causes a flow of energy. (Gale, 2009, p. 4) What makes an exoplanet an candidate for supporting life is a source of energy, either from geological processes or from a star.

Geological activity can provide energy to living organisms. For example, deep-sea vents on earth support entire ecosystems. Such organisms rely on deep-sea vents to provide a source of energy and nutrients in order to survive and thrive. (Nadine & Françoise, 2010) It has been hypothesized that life may have started somewhere near a deep-sea vent. Deep-sea vents get their energy entirely from geological processes, mainly from convection currents in the Earth’s mantle. Exoplanets can get tidally heated from tidal forces, as demonstrated on the Galilean moon Europa. (Christopher & Wanda, 2014, p.  227) Therefore it not entirely necessary for an exoplanet to be in the habitable zone of its star; there are many different sources of energy that life can use to flourish.

A star can provide enough energy if the planet in question is within the habitable zone. The habitable zone of an star is the region of orbits where water can exist as an liquid. Liquid water is required for life and keeping water in a liquid state requires energy. On Earth, energy from the sun is used by plants to generate the chemicals required for life i.e. photosynthesis. (Bloh, Bounama, & Franck, 2010)

## Liquid Water

Almost all life requires liquid water to function. Therefore if we are looking for Earth-like life, it is expected to find life where there is liquid water. For liquid water to exist on a planet, two conditions must be fulfilled:

• Pressure, usually from an atmosphere. Water can boil at room temperature if under low enough pressure. It has been theorized that Mars may had liquid water before. Currently, it cannot support liquid water due to little atmospheric pressure. (Hanslmeier, 2013, p.  91)

• Temperature. Water exists as a liquid on Earth between temperatures greater than $0^\circ C$ and less than $100^\circ C$ at 1 bar. (Gale, 2009, p. 10) Life doesn’t usually exist outside of these temperature ranges.

These conditions can be fulfilled by an exoplanet if it lies within the habitable zone of its host star and has a dense atmosphere.

## Carbon and Other Elements

Carbon is essential to life on Earth. Carbon is the backbone for organic molecules which are required by living organisms. Carbon based molecules make up more complex structures such as proteins and carbohydrates. (Hanslmeier, 2013, p.  15) Life may also require other elements such as nitrogen given how common it is in living organisms here on Earth. (Christopher, 2014) Carbon molecules such as $CH_{4}$ present in the atmosphere of an exoplanet may indicate that life is present. (Ulmschneider, 2006, p.  214)

# Methods for searching extraterrestrial life

Currently, we are limited to one method of detecting exoplanets hospitable to life. This method (transit spectroscopy) measures the absorption of certain wavelengths of light through the exoplanet’s atmosphere. When an exoplanet transits across its host star, the light from said star will drop. Interestingly, some of this light passes through the atmosphere of the exoplanet, causing certain wavelengths to drop in intensity. These absorptions are caused by certain molecules and elements in the atmosphere. The first exoplanet’s atmosphere composition to be analysed this way is HD 209458, which has an significant absorption at 589.3 nm. This wavelength corresponds to Na. (Charbonneau, Brown, Noyes, & Gilliland, 2002)

Obviously, an atmosphere composition high in Na would not be a sign of a habitable planet, let alone life. We have yet to analyse the atmospheric compositions of smaller, rocky planets for "biosignatures". Certain chemicals such as $CH_{4}$ would not be expected to exist in great abundance without living organisms producing it. This is because $CH_{4}$ rapidly oxidises with $O_{2}$. Therefore if an exoplanet has a significant absorption for $CH_{4}$, we can say with some certainty that said planet could be inhabited with life. (Krissansen-Totton, Olson, & Catling, 2018) (Ulmschneider, 2006, pp. 214-216) Future telescopes such as the James Webb Space Telescope may be able to illuminate our current ignorance. (Krissansen-Totton, Olson, & Catling, 2018)

# Conclusion

In conclusion, the search for extraterrestrial life is an exciting field of research. We may be able to answer the question of extraterrestrial for at least the exoplanets we currently know of. It is possible we may not be alone in the universe and may finally know of our cosmic neighbours’ existence.

# References

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8. Nadine, L. B., & Françoise, G. (2010). Microbial Habitats Associated with Deep-Sea Hydrothermal Vent Invertebrates: Insights from Microanalysis and Geochemical Modeling. In Trace metal biogeochemistry and ecology of deep-sea hydrothermal vent systems (pp. 51–71). Dordrecht, New York: Springer.
9. Krissansen-Totton, J., Olson, S., & Catling, D. C. (2018). Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life. Science Advances, 4(1). https://doi.org/10.1126/sciadv.aao5747
10. Ulmschneider, P. (2006). The Search for Extraterrestrial Life. In Intelligent Life in the Universe: Principles and Requirements Behind Its Emergence (pp. 201–218). Berlin, Heidelberg: Springer.