The Earth is the only known example of a habitable planet in our universe. As such, we must base our definition of life off of what we can see on our planet. The two main prerequisites for life are the presence of liquid water and a rocky geography (so that carbon is readily available). However, the ability of these requirements to exist on a planet also depends on many other factors including the characteristics of the host star and the planet’s atmosphere, and the location of the planet in relation to the habitable zone.
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Rocky planets provide a stable platform upon which liquid water can be maintained thus allowing life to develop (Mason 2008). The geochemical processes of rocky planets provide a solid surface and thus conditions favorable to life; a surface made of minerals which permits liquid water to be stable and allows the correct temperatures necessary for life (Mason 2008) (Fishbaugh, et al. 2007). Biogenic elements are readily available on rocky planets because they are natural consequences of the geological processes that occur on such planets (Jakosky 1998). These biogenic elements, such as carbon, are necessary elements of organic molecules and thus the building blocks of life (Jakosky 1998).
Carbon is the chemical element that plays a “key…central role” (Jakosky 1998, 95) in all organic molecules on the Earth (Ollivier, et al. 2009). Organic carbon macromolecules are the building blocks of cells because the element will readily form a variety of molecular structures; all of which play important roles in molecular structures (Lunine 1999) (Fishbaugh, et al. 2007). The element is the most likely to form covalent bonds and has a remarkable tendency to bond with itself because it has four valence electrons (Lunine 1999) (Jakosky 1998) (Ollivier, et al. 2009). Carbon forms a larger variety of chemical bonds than any other element (Jakosky 1998). The element is also so prevalent in biological structures because it is the fourth most abundant element in the universe (Lunine 1999). In the vast family of organic compounds, carbon is the essential component (Jones 2008). However, carbon is not enough for life to emerge; water is also essential.
When we look at Earth, the abundance of bacteria (the simplest form of life), is limited only by the availability of water (Fishbaugh, et al. 2007). Water is also the one common component used by life forms on Earth (Mason 2008). Liquid water is a polar solvent, thus providing a medium in which molecules can dissolve and react with each other (Ollivier, et al. 2009). While water is not the only polar solvent it is the most common because hydrogen is the most abundant element in the universe and oxygen is the third most prevalent (Lunine 1999) (Ollivier, et al. 2009). Because of its abundance in the cosmos, water is the most common polar solvent (Lunine 1999). However, water’s “importance for life is not due solely to [its] abundance: The properties of water are crucial”(Lunine 1999, 143). Water exists as a liquid over a wide range of temperature, a range much wider than other polar solvents (Lunine 1999). This ability allows it to be in liquid form at temperatures that are suited for organic reactions; if it’s too cold reaction times are too slow to allow biological processes and it it’s too warm organic bonds are too easily broken (Lunine 1999). Its ability to maintain a liquid state across a wide range of temperatures is due to hydrogen atoms forming bonds of modest strength with those of other water molecules, or hydrogen bonding (Lunine 1999). Hydrogen bonding produces a stronger bond than those of similar sized molecules and is what allows water to be such a good polar solvent (Lunine 1999). The main mass of a living cell is water, and its ability to function as medium for biochemical processes allows diffusion in and out of cells (Lunine 1999) (Ollivier, et al. 2009). The salinity and acidity inside a cell body have to be carefully regulated, which could not happen without water’s ability to act as a medium for biochemical processes to occur (Lunine 1999). Water allows cells to transport nutrients in and out and to remove the undesired products of their biochemical reactions (Lunine 1999). Because of its unique properties as a universal solvent, water is a hugely important component of life. However, for water to exist on a planet in liquid form, the planet must be the correct distance from the host star, or in the habitable zone.
The habitable zone is the area around a star in which energy emitted is at a level that allows a planet’s surface to remain the right temperature for liquid water to exist, (between 0 and 100 degrees Celsius) (Clark 1998). For this to happen, the planet cannot be too far from the star, where water would freeze, or too close to the star, where water wouldn’t be able to condense (Clark 1998). This “perfect area” is known as the continuously habitable zone. Being in the habitable zone also keeps planets from having huge temperature differences and therefore being unsuitable for life. Mercury is a good example of this, as it has temperatures ranging from -150 to 400 degrees Celsius from night to day (Fishbaugh, et al. 2007). However, location of the habitable zone depends on the mass of the star (Hanslmeier 2009). As a star ages, it becomes brighter and emits stronger energy, which pushes the habitable zone outwards (Hanslmeier 2009). Therefore for a planet to have liquid water and for life to evolve to higher forms, it must remain within the habitable zone (Hanslmeier 2009). The host star defines the habitable zone and thus the ability of planets in its orbit to produce life. The energy needed for life comes from the host star. While rocky planets have some internal heat from geothermal processes, this heat source is only a tiny fraction of that emitted by the host star (Hanslmeier 2009). Earth’s internal heat is only 1/20,000 as much as that provided by the Sun (Hanslmeier 2009). The mass of the host star is the most important parameter in how much energy it emits; the more mass it has, the higher the temperature will be on its surrounding planets (Hanslmeier 2009).
However, our universe has many exceptions to the habitable zone. For example, earlier in its history, Mars had liquid water on its surface, yet the sun would have been smaller, putting Mars outside of the habitable zone (Clark 1998). The “complications presented by our own solar system show that our rigid definition of habitable zones is in fact, somewhat loose, and that we should always keep this in mind when looking for planets in ‘habitable zones’ around other stars” (Clark, 198). As shown by Mars, being in the habitable zone does not guarantee that a planet will have the necessary requisites for life to emerge. The Earth, Venus, and Mars have all had liquid water at some point, showing the latter two to be exceptions to the habitable zone requirement. This phenomenon can be explained by looking at their different atmospheric densities. The Earth maintains moderate temperatures unlike its two neighbors which are desert worlds (Mason 2008). That the Earth is within in the habitable zone, thus the correct distance from the Sun, is a dominating factor in what allows the planet to maintain life (Mason 2008).
Venus, which is inside the inner boundary of the habitable zone, has a thick atmosphere that causes a “runaway greenhouse effect” (Mason 2008). The enormous greenhouse effect traps solar radiation causing a large temperature increase (Lunine 1999). While Venus has a thick layer of sulfuric clouds that reflect more sunlight than Earth’s clouds it still has a much higher temperature than the Earth (Lunine 1999). So while Venus’s surface receives less sunlight than Earth’s its temperature is still much hotter. Because of the high temperature it is too hot for water to be present in its liquid form (Jakosky 1998). It is thought that earlier in its history, Venus had oceans but the increase in the Sun’s mass caused them to evaporate (Jakosky 1998). The moist greenhouse effect on the planet caused water to evaporate into the atmosphere where ultra violet radiation split hydrogen and oxygen (Hanslmeier 2009). The hydrogen escaped from the atmosphere while the oxygen was then free to bond with other elements (Hanslmeier 2009). Today, gaseous water vapor and oxygen are both scarce in Venus’s atmosphere with water vapor only in 10 parts per million (Lunine 1999). Venus also has most of its carbon in its atmosphere. Carbon must be in sufficient supply for life to emerge, and the difference between Venus and the Earth is that while carbon on Venus is almost all located in its atmosphere, the Earth has a similar amount of carbon trapped in its crust in carbonates and other carbon compounds (Lunine 1999) (Jones 2008). This amplifies Venus’s greenhouse effect and also further hinders the ability of the planet to create life. The thick carbon dioxide atmosphere of Venus is almost 100 times the mass of Earth’s atmosphere (Jones 2008). This causes the planet to sustain an average surface temperature of 467 degrees Celsius, far above the level where life can exist (Jones 2008).
Mars was in fact a good candidate for life. At one point in time, it is thought that its environment was comparable to the conditions on Earth, (a rocky planet with liquid water present), when life arose (Fishbaugh, et al. 2007). Gullies on the surface of Mars have been seen to be changing proving that water was flowing on the planet’s surface at some point (Jones 2008). However, now Mars is in a state of “runaway glaciation”, where the atmosphere is too thin to hold in the smaller amount of heat the planet receives from solar radiation (Mason 2008). Mars is too cold for liquid water to be stable; it freezes instantly (Jones 2008). There is still a possibility that there could be subsurface, microbial life, as there has been proof of liquid water beneath Mars’s permanent ice caps, but it is very hard to find evidence of such life (Jones 2008).
While all three of these planets have atmospheres that have large amounts of carbon dioxide and can thus have some form of greenhouse warming only the Earth has the right balance of position in relation to the Sun and thickness of atmosphere (Jakosky 1998). The Earth is in the perfect spot between the two states of its neighbors, and is therefore able to maintain liquid water at its surface, unlike Venus and Mars. Earth’s atmosphere is the correct thickness to keep in heat, warming the surface enough for life to grow, without causing overheating and taking away the ability of water to stay in its liquid state (Jakosky 1998). These two factors combine to control Earth’s climate because they decide the amount of sunlight that is transferred through the atmosphere.
To more deeply understand what is required for life to develop we can also look at other planets and moons within our solar system. Titan, one of Jupiter’s moons is one such example. Titan is rich in the pre-biotic elements necessary for life, like carbon, and scientists have detected water vapor in its atmosphere (Clark 1998). As Titan is far out of the habitable zone, any water in the atmosphere should have condensed out (Clark 1998). However, one explanation for the water vapor is that it is being constantly replenished. The replenishing could just be coming from small cometary impacts but there is some thought that Titan resembles an early Earth and that heat is all that’s needed for life to appear (Clark 1998).
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Another of Jupiter’s moons, Europa, has proved to be an interesting study. Based on its density, Europa is probably around 15 percent or more made of water (Jakosky 1998). The moon is thought to be covered in water ice but with a widespread ocean of liquid water underneath (Jones 2008). The cracks on its surface seem to be filled with clean ice water, pointing to the presence of water ice and carbon-bearing materials welling up from beneath the surface (Lunine 1999) (Jakosky 1998). However the water could also be spread out through different layers instead of focused at the surface; it is difficult to tell without closer means of examination (Jakosky 1998). The abundance of ice at the surface is verified, due to the characteristic wavelengths of sunlight reflected from its surface (Jakosky 1998). Europa’s icy surface is so bright that it reflects most of the sunlight hitting it (Lunine 1999). Some amount of liquid water on the planet is likely, because for the surface to be changing there must either be water erupting from underneath the ice layer, or the ice must be warm enough to flow in some areas (Jakosky 1998). Europa has a rocky mantle, and an iron rich core (Jones 2008). With its rocky geography and the presence of liquid water, the moon could produce life if it had sufficient heat. However, the moon is far out of the habitable zone thus the only heat it receives is through tidal heating, orbital and rotational energy that heats the moon as it circles Jupiter (Jones 2008). Europa has mutually gravitational pulls against two of Saturn’s other satellites, Io and Ganymede (Lunine 1999). Because the three moons’ orbital periods are multiples of each other, Europa’s is twice that of Io’s and Ganymede’s is twice that of Europa’s, they continue to tug gravitationally on each other (Lunine 1999). This tugging causes a slight twist as each of the moons vary their orbital speeds which results in a friction that heats the moons (Lunine 1999). Io, with the smallest orbit, gets enough heat that it has a large amount of volcanic activity (Lunine 1999). However, as Europa is twice the distance away from Jupiter as Io, the moon receives half the amount of heat which is not enough to melt its icy surface (Lunine 1999). It is highly possible that if Europa received the correct amount of heat as a catalyst, life would emerge on the moon; if it is not already present in the moon’s mantle ocean, (which is likely in areas with higher heating) (Lunine 1999).
Based on what we have seen to be necessary for life on Earth, we can predict what signs to look for on other planets as indications of life. The planet must be rocky and have liquid water present. However, to fulfill this second requirement of water, the planet must have the right temperature. This can be achieved by being in the habitable zone and having the correct atmospheric density. Yet if the planet is outside the zone, with the right density of atmosphere the temperature could still be a temperature that allows liquid water. If the planet is outside the outer edge of the habitable zone, like Mars, to keep it from becoming too cold the atmosphere must be thick enough to cause a greenhouse effect and raise temperature. Mars cannot achieve this effect because it is too small and thus cannot retain its atmosphere. Therefore the planet must also have a mass that is large enough to keep its atmosphere. So exceptions to these general guidelines could still produce life and must not be overlooked in the search for life on other planets.
One way to look for life on other planets is to study atmospheric biospheres. A huge sign of life is a chemical equilibrium. Abiotic processes cannot result in a chemical equilibrium (Clark 1998). For example, finding oxygen and hydrocarbons together is a strong sign of life (Clark 1998). The two chemicals are incompatible over time but are produced in abundance by living organisms, therefore when they are present together in an atmosphere it suggests that they are in a state of constant production (Clark 1998). A combination of atmospheric components whose simultaneous existence is unlikely through non-biological processes is a huge indicator of the possibility of life on a planet (Mason 2008). Water vapor can be treated with a similar attitude; if it is prevalent in a planet’s atmosphere there is a strong possibility of life or that the planet could support life (Clark 1998). This is known as an atmospheric signature of life; the detection of an atmospheric component in a large enough quantity that it cannot be explained by being a product of non-biological processes (Mason 2008). Oxygen is also a very reactive substance and combines easily with other substances. As such, if it is not being constantly replaced it would combine with surface chemicals and be lost (Clark 1998). This can be seen on Mars, where oxygen bonded with the ferrous material of the planet’s surface, turning it red (Clark 1998). So it can be inferred that any planet with water vapor in the atmosphere has a constant production of oxygen and could be suitable for life.
When looking at other planets for the presence of life, we can also like at surface signatures. If a planet is significantly covered by plant life, its spectral signature will be distinct (Mason 2008). Because of photosynthesis, plants are in the green color family, causing a “strong discontinuity in plant reflectance at the red edge” (Mason 2008). Temporal signatures can also be studied in the search for life. Over time, certain patterns can be seen in atmospheric makeups (Mason 2008). For example, if an atmosphere was looked at once and there was an abundance of carbon dioxide, it would likely be seen as non-biologically produced (Mason 2008). Yet if the same atmosphere was looked at over a longer period of time, it could be seen that there were periodic variations in carbon dioxide and methane. This variation has been seen on the earth and is caused by organisms so the gases could now be classified as biologically produced thus pointing to life on the planet in question (Mason 2008).
Recently there has been a string of discoveries of planets that could support life. While all the prospects are for now too far away for close enough inspection, they are all likely candidates based on the criteria that has been discussed. One such planet is even being heralded as a “super-Earth” (Wall 2012). This newest discovery, called HD 40307g is inside its host star’s habitable zone and is only 42 light years away (Wall 2012). This is much closer than previous finds, like Kepler 22b discovered in 2011 which is 600 light years away, and telescopes could even be produced in the near future that would be strong enough to directly image the planet (Wall 2012).While the host star is less luminous than our sun, the planet is still in the correct position as to be inside the habitable zone (Wall 2012). HD 40307g is the outermost of six planets orbiting the host star and is the only one thought to be a candidate for extraterrestrial life, (the other five are inside the inner edge of the habitable zone making their temperatures likely too hot for life) (Wall 2012). The planet is thought to be seven times the size of the Earth for now it is impossible to tell from this distance if the planet is rocky (Wall 2012). One scientist even put the chance of HD 40307g being solid at 50/50 (Wall 2012). However, experts have also noted that the planet lacks tidal locking, increasing the chances that it could have Earth-like conditions and the right climate and atmosphere to support life (Wall 2012).
This recent spate of planet discoveries started in the 1990s when astrobiology emerged as an exciting new field. Researchers began to find many exoplanets yet there were often puzzling anomalies that accompanied these discoveries (Kaufman 2011). Among the new finds, it was more common for a planet to have a highly eccentric orbit than a near-circular orbit like those found in our solar system (Kaufman 2011). Solar systems were also found that seemed like ours yet had seemingly impossible variations; for example Jupiter-like planets orbiting extremely close to their host star at the same distance of the Earth to the Sun (Kaufman 2011). On these puzzling discoveries, Paul Butler, a preeminent scientist in this field, said “having our solar system as a model can be worse than having a sample of zero because it leads you down one road and you don’t imagine the others” (Kaufman 2011). This idea must be kept in mind as the search for life on other planets continues; while we can form guidelines on what a planet needs to produce life based on the Earth, the chances that life will develop in exactly the same way elsewhere in the universe are minuscule. We must continue to search using the factors that have been discussed, and yet keep our minds open to possibilities of developed life that are beyond our wildest imagination.
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