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Astrobiology, the science of extraterrestrial life, is almost entirely theoretical,[note 1] but still plays an important role in such arenas as the search for extraterrestrial intelligence (SETI) or ET life in general.
 Assumptions about life
Astrobiologists have to tread a fine line between assuming that some commonalities we observe in life on Earth will be required elsewhere, and still working with the limitations that the chemistry and physics of the Universe impose on possible biologies. For instance, there is no reason to expect that life elsewhere would be based on DNA or protein structures like ours. However, a way of transmitting information from one generation to another is a vital component for what we define as "life". Whatever "alien DNA" is, it would probably have similar features, such as a polymeric structure and an auto-catalytic copying mechanism based on hydrogen bonding (or other bonding perhaps), but not the exact same molecular structure. It is also considered that temperatures and pressures leading to the presence of liquid water are key factors, due to water's role as a very flexible polar solvent, though other solvents have been suggested. 
When considering the chemistry of potential extraterrestrial life, it is also highly likely that molecules with carbon or hydrocarbon structures will form a vital part of the picture, due to carbon's ability to form many kinds of molecular bonds, thus forming the basis of huge numbers of potential chemical processes. When people involved in projects like SETI want to know "what to look for" in order to have a good idea of "where to look", they turn to astrobiologists to try to improve their odds.
 Rocky planets
Some rocky planets, such as Earth, are capable of supporting carbon-based "life as we know it". Conditions necessary for life are generally thought to be:
- Orbit around a yellow star (like the Sun), a red giant, or a stable combination of those stars. Stars redder than the sun may also be life sustaining, as may stars slightly hotter.
- Presence of water, carbon and other organic elements (believed to be fairly common, at least on planets that formed around stars with a decent level of heavy element enrichment — astronomers call this "metallicity" (In Astronomy all elements heavier than helium[note 2] are called "metals").
- Presence of water, and carbon-based organic molecules. The former is believed to be fairly common, while higher-resolution spectroscopy equipment has started to detect fairly large organic molecules.
- Orbit within the habitable zone (distance from star where the temperatures allow water to exist as ice, liquid water and vapor).
- Presence of an active core that causes volcanic and tectonic activity as well as a magnetic field to protect the surface from cosmic and solar radiation (life evolving despite this is perhaps possible but probably far more difficult).
- The size and density of the planet have to be such that the planet's gravity can hold an atmosphere, but is not so strong as to crush lifeforms.
- The planet's solar system resides in the habitable zone of its galaxy, thus avoiding extreme galactic radiation and gravitational tides.
Out of all the known planets, Gliese 581c, Gliese 581d and Mars are the only planets (besides Earth) to meet these criteria even remotely. Europa, one of Jupiter's satellites, is too cold to support life on its surface, but may have life in subsurface liquid oceans. However, it should be noted that current methods for detecting exoplanets are biased towards finding ones that certainly aren't Earth-like. Moons of gas giants, notably Europa may also harbour life. Titan, a satellite of Saturn, is likewise too cold for liquid water, but has the right surface conditions for liquid methane, and could hypothetically support life that used methane as a working fluid. Allegedly, the majority of scientists at a conference hosted by the European Space Agency believe Mars has, or used to have, life.
 Habitable zone
The Goldilocks Zone, Habitable Zone, or Life Zone is the relatively narrow range of distance from a star within which an orbiting planet (or in some cases a planet's moons) may be able to sustain life, at least the carbon-based forms we know and love, because the temperature is right for water to remain liquid. Water is a common enough material in the universe and acts as a very flexible biochemical solvent. The phrase "Goldilocks Zone" is a reference to the story of Goldilocks and the Three Bears, since, like the porridge which Goldilocks chose to eat in the bears' house, conditions in the Goldilocks Zone are neither "too hot" nor "too cold", but are "just right". The definition of a Goldilocks Zone has expanded to include moons such as Enceladus that exist within a zone where warming occurs through tidal friction by their planet (Saturn, in the case of Enceladus), as opposed to external heating by a star, thus expanding a lot the places where water-based life could appear.
In our solar system the inner edge of the zone lies between the orbits of Venus and Earth, while the outer edge extends roughly as far as the orbit of Mars. The habitable zone changes as the stellar activity of the star system changes. Earth is the only Goldilocks planet in our own solar system. We would be unable to survive on the nearest neighbouring planets, Venus (too hot) or Mars (too cold). Non-Goldilocks temperatures also correspond with differences in the physical geology and chemical composition of the environment. Earth's almost circular orbit keeps us firmly within the Goldilocks Zone at all times.
Identifying other habitable or inhabited planets is a major preoccupation in space exploration and colonisation, as well as a perennial theme in science fiction. The most compelling reason for this fascination with Goldilocks planets is the possibility that they could already support extraterrestrial life, which is arguably the Holy Grail of space exploration. If observing atmospheric conditions and life signs on such distant planets becomes possible, we could discover species unknown on Earth, and studying the development of life elsewhere could provide new insights into such aspects of biology as the origin of life and the degree of chance involved in evolution. The discovery of Goldilocks Zone planets attracts interest from the media and the public as well as the scientific community. Recent examples include planets in the Gliese 581 solar system, approximately 20 light years from Earth, including Gliese 581 c, d, and g. Gliese 581 g,[note 3] discovered in 2010, is thought to be the planet most closely resembling Earth of any which have been discovered yet. 22% of stars like our sun may have Goldilocks planets similar to Earth according to data from the Kepler telescope.
We must remember not to limit ourselves to searching for life on Earth-like planets inside habitable zones, as alternative scenarios have been proposed, such as volcanic activity providing heat on planets or moons outside of the habitable zone (similar to "smokers" on Earth's ocean floors), silicon-based life that can survive where carbon-based life cannot, and even floating, balloon-like lifeforms living in the atmospheres of gas giants. Other forms of life unknown to scientists today may have evolved elsewhere in the Universe to thrive in conditions that are not Goldilocks at all.
Many other factors are also required to make a planet suitable for life. Atmospheric composition is one, as the greenhouse effect can either raise the temperature enough to keep surface water liquid, or make it too hot for liquid water, and the mass of the planet is also important for maintaining this atmosphere. Planets that are too small do not have sufficient gravity to hold on to their atmospheres in the face of the onslaught of the solar wind.
The range of the Goldilocks Zone also changes throughout a star's lifespan, slowly edging out further away from the star as it ages. Therefore a planet needs to remain inside the zone as it drifts outward. This favours planets which form just on the edge of the zone early in a solar system's life, as the zone will then spread outwards slowly as the star ages and heats up.
 Carbon-based life
All known life on Earth is "carbon-based", which means that the most important atom in the molecules that compose life is carbon. One of the simplest reasons for why carbon is the most important atom in biology is its chemical properties and how they react.
Carbon is able to form long chains, as well as more complex structures, cyclic rings, aromatic rings and polycyclic systems such as steroids. This gives rise to an entire discipline, organic chemistry, devoted to the study of carbon-based molecules. The reason for this is the unique structure of the carbon atom: carbon has 4 electrons on its outer shell and because carbon, like every other atom, tries[note 4] to attain the most stable configuration, which is an outer shell which is effectively "full". So, by a fairly unremarkable quirk of quantum mechanics and thermodynamics, carbon will bond with up to four other atoms, such as hydrogen, oxygen, or nitrogen.
Carbon is not the only atom with this bonding structure: silicon is very similar to carbon (it is the next element down from carbon in the periodic table, and so has the same valence structure, but with another shell of electrons under it) and although it binds less readily than carbon, some bacteria are known to incorporate silicate structures in their physiology. Silicon-based life, however, is highly unlikely, as large silicate structures become unstable, don't display the diversity of organic compounds, and silicon dioxide, the product of respiration, is solid in standard conditions. In the absence of chemical evidence to suggest it can form appropriate structures, "silicon-based" life remains a pipe dream of science fiction writers. So, barring anything truly spectacularly different, any life "as we know it" will almost certainly be carbon-based.[note 5]
The above assumptions have some interesting exceptions, which may suggest a greater possibility of extraterrestrial life.
No, we're not talking about kinky folks, we're talking about any of a number of microscopic organisms that have adapted to live in what we would consider extreme environments here on Earth. There have been documented findings of life in places formerly believed inhospitable, such as thermal vents on the ocean floor, extremely hot or cold water, high or low pressure environments, highly acidic or alkaline environments, up to 3.6 km underground, inside rocks,[note 6] and in locations without oxygen. Sometimes entire ecosystems have been discovered based on these critters.
One of the key factors about extremophiles on thermal vents is the rapidity with which these vents are colonised. While such colonies are hardly evidence of a second genesis on Earth, they do show that life, as it is, can thrive in very unusual circumstances. Importantly, many deep sea extremophiles can survive without the help of the Sun's energy, so the first trophic level in such ecosystems is not photosynthesising plants, but bacteria that use the heat and reactive chemicals from the vents to provide chemical energy. This is particularly key to the potential existence for life on planets in the outer Solar System, where the solar flux from the sun is greatly reduced.
Extremophiles indicate that life can thrive in places that common sense would tell us should be barren and devoid of life. The discovery of these organisms in the 1970s led biologists to believe life could possibly exist in similar environments on planets (or moons) that were previously thought to be inhospitable. Because of the conditions that we have actually observed life thriving in, it is certainly not impossible for bacterial life to still be present on Mars, particularly in the polar regions where there is ice and potentially liquid water, or extremely cold places such as asteroids. One of the other prime candidates for life within the solar system are the oceans that are suspected to be under the ice on Europa, one of Jupiter's moons. While it is widely accepted that Europa is theoretically capable of hosting bacterial life (and until a probe is sent to successfully dig under it, we will be unable to confirm this idea), some recent theories have led to the idea that cosmic rays may be capable of oxygenating underwater oceans to similar levels found on Earth, thus macroscopic life may also be a very realistic possibility.
 Elliptical orbits
A planet in our location but orbiting the Sun in a much wider ellipse would dip in and out of the Goldilocks Zone during its orbit, and so possibly could not sustain life. Some scientists speculate that life may be able to survive in planets with elliptical orbits that take them out of the habitable zone from time to time. All that may be required is liquid water at some point in a planet's orbit. Life could hibernate during cold conditions or could shelter in watery places during hot conditions. 
 Plasma life forms
Experiments have showed that at specified condition blobs of gaseous plasma can grow, replicate and communicate. Also according to computer simulation it is possible for interstellar clouds of spacial dust to store information since dust particles could join together to form double-helix structures similar to DNA and even divide to create two identical copies of the same structure.
 Boltzmann brains
In statistical mechanics and quantum mechanics, anything can pop out of vacuum energy by random thermal or quantum fluctuation. Theoretically speaking, it is possible that a conscious entity (i.e., a Boltzmann brain) could be formed from such random fluctuations given enough time. It is to be noted that the probability of this occurring is ridiculously small.
 Artificial life
With our own teetering steps into artificial life just beginning, one should not make the mistake of assuming that all alien life is organic in origin.
The universe may well contain numerous genetically engineered and/or mechanical civilisations which have either superseded or outlasted their creators. They may also have capabilities that organic life does not, or have massive lifespans and intelligence. Such beings might be capable of traveling between stars for centuries or millenia without dying. Additionally, it might be possible for ETs to cheat death by uploading themselves onto machines.
 Scope of astrobiology
Given all this, the question becomes "how much life is out there?" Given the relative ease of communicating versus travelling to other planets, the more relevant question may be "how much life out there could we talk to?" The Drake Equation attempts to provide the framework of an answer.
 Religious reactions
Some creationists outright deny the existence of extraterrestrial life, because the Bible doesn't explicitly mention it. However, in the Book of Job there is mention of a council that is held with the sons of God. It has been thought that these "sons" are the leaders of other created civilizations, but since there was no concept for extraterrestrials at the time the Bible was written, it far more likely refers to angels. There are those that believe that if life were found on another planet, it would most likely be very simple, not fully-formed advanced organisms created instantaneously, like in the Genesis story. As such, they argue that life on Earth is very, very, very finely tuned to exist, and probably doesn't anywhere else. But these people don't take in account that our God is a creator and He will keep on creating.
Although the Catholic Church does not require its members to be creationists, Church officials have weighed in on the issue of extraterrestrial life. When the planet 51 Pegasi b was discovered in 1995, one bishop proclaimed that if any life existed on said world, it would have souls in need of salvation. (51 Pegasi b is a "hot Jupiter", and if any lifelike critters existed in its atmosphere they wouldn't resemble life as we know it.) A cardinal[who?] was quick to denounce this claim, however, saying that they didn't know if original sin occurred on other worlds. Jesuit members of the Vatican Observatory have made slightly more informed
decisions assumptions regarding the souls of extraterrestrial life.
 See also
- Astrobiology Magazine
- Encyclopedia of Astrobiology, Astronomy, and Spaceflight, David Darling
- ↑ However, some definitions of astrobiology include the study of such things as how plants grow in zero-G, which has some level of actual testing.
- ↑ And thus not dating back to primordial nucleosynthesis but to nuclear reactions within stars
- ↑ Sadly its existence has proven to be highly doubtful. Go to Kepler-438b instead, at least if you can live with the intense stellar activity of its daystar
- ↑ This does not mean it "tries" in the sense that people make conscious efforts to attain goals. It means that in the state alluded to, (the absolute value of) the binding energy (the sum of the kinetic energy and potential energy) is lowest — the carbon atom is effectively "rolling downhill" when it forms bonds that fill these shells. Note that by convention potential energy and binding energy have negative signs.
- ↑ The more so as any hypothetical silicon-based life is extremely unlikely to arise and survive in any environment that fosters carbon-based life. For one thing, it would require a significantly different regime of liquid medium, pressure, temperature, and chemical availability; for another, its prognosis in any hypothesised competition against carbon-based life is quite poor owing to carbon's greater elemental abundance, the greater stability of its compounds, and its lower energy requirements for building complex structures (causing greater rapidity of replication, mutation, and evolution).
- ↑ Think of the Horta, albeit on the bacterial level
- ↑ http://www.clearleadinc.com/site/extraterrestrial-life.html
- ↑ See the Wikipedia article on Habitability of red dwarf systems.
- ↑ See the Wikipedia article on Extraterrestrial_life#Direct_search.
- ↑ Are we not the only Earth out there?
- ↑ To the outermost regions of the Solar System
- ↑ 22% of Sun-like Stars have Earth-sized Planets in the Habitable Zone
- ↑ New "Devil Worm" Is Deepest-Living Animal: Species evolved to withstand heat and crushing pressure by Dave Mosher, National Geographic News, June 2, 2011.
- ↑ http://news.sciencemag.org/plants-animals/2010/04/scienceshot-animals-live-without-oxygen
- ↑ Universe Today - Europa Capable of Supporting Life
- ↑ NASA: Extreme forms of life can survive on planets with weird orbits
- ↑ http://www.newscientist.com/article/dn4174-plasma-blobs-hint-at-new-form-of-life.html#.U8sfg7HIwfw
- ↑ http://www.newscientist.com/article/mg19526174.500-life-in-interstellar-dust.html
- ↑ http://physicsworld.com/cws/article/news/2007/aug/15/helices-swirl-in-space-dust-simulations
- ↑ http://www.newscientist.com/article/mg19526171.100-spooks-in-space.html
- ↑ http://arxiv.org/abs/hep-th/0611043
- ↑ http://physics.about.com/od/thermodynamics/f/BoltzmannBrains.htm