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

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The potential for a star to host a habitable planet increases as it moves along the main sequence, various factors cause the distance between the star and the habitable zone, as well as its width, to increase with time, a consequence of stellar evolution (Lopez et al 2005).

The greater the mass of the star, the faster it burns through its fuel. This is due to larger gravitational forces on its core, increasing core density, temperature, and rate of reaction (Kopparapu et al 2013). However, given that the habitable zone moves further away with time, and as Giant sequence stars eventually cool down, life could be sustained temporarily. Following the Helium flash that indicates the cooling period of such stars, any planet 7 to 22 AU away would be in a habitable zone with temperate climate. A problem arises in that at this point, these stars only have a life of 10^9 years remaining, enough to sustain life temporarily but not create it, highlighting the importance of longevity.

Habitable zones are more commonly found around stars of the K G F and M classification. Our sun a yellow dwarf of the classification GV2, has a stable core. Due to its mass, the expected time spent on the main sequence is only 10 billion years, on the contrary a red dwarf which would have half the Suns mass, could remain in the main sequence for up to 80 billion years. Potentially adequate time for live to evolve.

At the start of their lifespan Red Dwarf stars are known to produce stellar flares that could double the stars’ brightness. As a Red star grows its luminosity may increase exponentially, causing devastating effects as an individual solar flare has the potential to wipe out a nearby planet’s atmosphere, annihilating all life on it. However, with time (1.2 billion years) and reduction in energy flux, they readily become stable.

Remembering the aforementioned astrophysical factors, the inverse square law theory and the fact that Red Dwarf stars have an average mass of 0.5 Solar Mass Units, it is understood that the formed habitable zones are very close to the host star. Any planet in this habitable zone would be under strong gravitational influence causing a phenomenon known as ‘Tidal locking’ to occur. This is when a planet only undergoes one rotation on its own axis for every individual orbit around the host star (Yang et al 2014). Having only one side of the planet constantly face its star, causes great extremes in temperature, due to differential heating such planet would face extreme weather and an unstable atmosphere. Sustaining life in such planets would be near impossible, contradicting the theory that life could evolve given such dwarf planets were stable for a long duration.

Effects of Obliquity and Rotation Rate

The climate of a planet is largely influenced by its obliquity- the angle of tilt for axis rotation. Planets with high obliquities seem to have warmer climates and minimal counts of low clouds. The climate warms by 21 Kelvin when the obliquity increases from 0° to 90(Wang et al 2016). For planets in the inner edge of the habitable zone (closer to star) this increased warming with greater tilt results in a significantly strong greenhouse effect. Holes in the planet’s atmosphere result in formations of hotspots and a rise in surface temperature. Unbearable temperatures and formation of scorched land decreases the planets habitability. On the outer edge, increased obliquity corresponds to a decrease in surface temperature, resulting in frozen landscape, without liquid water, organic life is unsustainable. Thus, an increase in planetary obliquity results in the narrowing of the habitable zone width.

Planetary rotation rate is another key factor in determining atmospheric circulation (Yang et al 2014). Planets with low obliquity allow the formation of a variety of clouds, which have dominant control over a planets radiation balance. Clouds are solely responsible for maintaining planetary albedo- the reflection of unnecessary celestial radiation. This reflection aids in keeping greenhouse gas levels at secure levels and maintaining surface temperatures. Planets that rotate slow have a weak Coriolis force-the seeming force felt due to rotation, the opposite for those that rotate fast. Planets with low Coriolis force allows for cloud cohesion, increasing planetary albedo. Planets rotating rapidly cause clouds to thin, increasing the greenhouse effect and decreasing planetary habitability.

Fast rotation also results in a significant pole to equator temperature difference. As heat cannot diffuse efficiently, it dissipates with latitude in an unstable manner (Zsom et al 2013).

MAGNETOSPHERIC STRUCTURE

Although a star’s core stabilizes with time and the probability of it housing a habitable star increases as it moves up the Main Sequence, theoretically it always has a habitable zone. A potentially habitable planet may orbit an unstable or highly reactive star; which would be vastly susceptible to damage. High reactive stars emit hazardous wavelengths of light due to unstable nuclear reactions in its core. Stellar winds composed of UV,EUV, X ray and Gamma rays are perilous spectra that can have detrimental effects on a planet’s atmosphere. In order to sustain itself, a such planet needs strong internal pressure that opposes stripping of the atmospheric layers, this is achieved via a strong intrinsic magnetic field ( Cohen et al 2014).

A strong magnetic field negates atmospheric erosion and mass loss, which affects the planets environment and habitability.

Conclusion

In accordance to the definition of life and habitability used in this review article water must be present in its liquid state for life to form. The corresponding habitable zone is the range of distances from the star where a planet can have liquid water on its surface. This range is influenced by a chain of factors, primarily the luminosity of the host star. However planetary habitability should be viewed as a web, a series of integrated factors that work together. Although a web may have weak points, all factors must be acknowledged due to their strong influence on one another. Thus, I would conclude that habitability is far from unidimensional, that no one factor can guarantee sustainability of life, but instead a series of factors must align.

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