Essence of Life - What Is the Most Important Factor in Determining If a Planet Is Habitable?

Essence of life; what is the most important factor in determining if a planet is habitable?

Abstract

Planetary habitability is often described as the existence of liquid water, and to look for life in our solar system and beyond, scientists need to first understand how and why some planets are habitable while others are not. Astrophysical and geochemical factors affecting the development of a terrestrial planet have been investigated using photoelectrographs, spectrographs, and observational studies. Research indicates habitability is largely influenced by luminosity and distances from host star, planetary core composition and the presence of water. Optimal conditions for creation and sustainability of life are reviewed.

Keywords; habitability, astrophysical, geochemical, sustainability, luminosity, cosmic.

Introduction

Planetary habitability is a planets potential to have habitable environments that are both hospitable and suitable for creation of life. Habitable environments may not necessarily contain natural life; life may develop directly on a planet or be transported from another cosmic body(Tanton et al 2013). For this review, I restrict the definition by describing a planet as habitable if carbon- and water-based life can reside on the surface or near-surface of a terrestrial area. The paramount requirement for sustainable life is an energy source, and it has been established that many other geophysical, geochemical, and astrophysical conditions must be met before a cosmological body can support life- the existence of an individual human being or animal. However, what factor is most vital for organisms to create and sustain life on a planet? Does availability of water trump the importance of a planet’s core temperature? How do they compare to the importance of geochemical factors determining atmospheric composition? The review explores the different factors in determining if a planet is habitable, and determining if there is a key influence.

The liquid water habitable zone

The presence of liquid water on earth played a key role in the development of organic microorganisms and certainly boosted their evolution into more complex forms, existence of Darwinian evolution is generally considered to be an essential feature of life ( Lazcano et al 2008). Assuming all life would indeed be C4 Carbon based, liquid water would consequential for sustaining life. The liquid water habitable zone (HZ) describes the orbital distance at which a terrestrial planet can preserve above-freezing conditions through regulation by the carbon cycle (Misra et al 2016). However, a fair few factors come into determining the size of the habitable zone.

Temperature and Distance

The temperature range is of paramount importance when determining the size of any habitable zone, the long term thermal limits for ectothermic metazoans and invertebrates lies between -2 and 47.4’C(Silva et al 2013) . Too high and via a strong greenhouse effect cascading the photo-dissociation of water vapor and hydrogen, an arid inhospitable oven would form, eventually denaturing all organic material. Too cold and condensation of Carbon Dioxide would create a thick endothermic atmosphere, resulting in formation frozen water which would make life impossible. Thus, a tailored temperature is dependent not only on the distance from the host star, but also its mass and luminosity. The inverse square law is used to extrapolate circumstellar habitable zone, given that the distance from the sun to earth is 1 Astronomical Unit(AU) and the sun is 1 Solar Mass.

[pic 1]

Figure 1: The calculated distance of habitable zones, based on distance from host stars(AU)and mass of host stars(Solar mass).( Gliese 581, ESO gallery)

In accordance to the inverse square law and figure 1 shown above, a proportional relationship between mass (Solar mass) and distance(AU) can be deduced. The greater the mass of the host star, the habitable zone is at a further distance from host star. This zone is at approximately 0.95 to 1.37 AU for G-stars (0.8-1.2 solar masses), but pushes towards the right as the host star ages and leaves the Main Sequence (Lopez et al 2005).

Multiple studies have shown there are uncertainties that may change the width of habitable zones, which infringes on sustainability of life towards edges of any habitable zone.

 Focusing on planets on the outer edges, it can be noted that these planets cannot maintain stable climates, they instead oscillate between periods of extreme frost and temperate climate. Planets with such ‘limit cycles’- a closed trajectory orbit, would have conditions inimical for sustaining life (Misra et al 2016).

Unfortunately, habitability is not just a simple algorithm relating luminosity and distance, planets located well in their respective habitable zones have shown to be inhospitable to life, Venus and Mars as prime examples.

 Furthermore, there is constant debate over the fact that luminosity is affected by spectral typing of the host star (Silva et al 2013), influencing the habitable zone and arguing to nullify the entire inverse square law theory.

Spectral types and Stellar Evolution

The universe is home to 7 main types of stars, classified in the Morgan-Keenan system range from the hottest(O-type) to coolest (M-type) (Morgan et al 1973). This spectrum is further divided as per luminosity.  Focusing on the Main-sequence of stars, which make up 90% of the stars in the universe, it is understood that with time their spectral properties change. These changes include, emission temperature, radiance and wavelength of light, all of which affect planetary habitability. The conditions of the surface of planets is entirely controlled by radiative output of the star. A stars’ effective surface temperature and luminosity mirror reactions ongoing in the stars’ core. The cascade of nuclear reactions in the core cause the stars’ stability to deviate (Kopparapu et al 2013), cycling through episodes of ultra violet(UV) and X-ray radiations, energetic flares and shock ejections. All these hazardous emissions can be coupled with ionizing particles such as XUVs and gamma rays (Gudel et al 2014) which hinder any nearby planets ambitions to not only create life, but even just to sustain it. This proves to show the importance of structural stability in time.