Astrobiology
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Astrobiology is the science of extraterrestrial life. As such, it is almost entirely theoretical[1], but still plays an important role in such arenas as the search for extraterrestrial intelligence (SETI) or ET life in general.
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[edit] Life out there
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 solvent, though other solvents have been suggested. [2]
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.
[edit] Life on 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:
- 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).
- 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.
- 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.
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. 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.[3]
The "habitable zone" is a range around a star within which temperatures are such that water could exist as a solid, liquid, and vapor on a planet that describes an orbit in this region. This also depends partly on the heat generated internally by a planet, such as due to nuclear decay processes.
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.
We must remember not to limit ourselves to searching for life on Earthlike 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.
[edit] 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[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.[5]
[edit] Extremophiles
No, we're not talking about kinky folks or Xtreme Sports enthusiasts, 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, and even inside rocks.[6] 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.[7]
[edit] Plasma life form
Experiments have showed that at specified condition blobs of gaseous plasma can grow, replicate and communicate.[8] 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.[9][10]
[edit] Boltzmann brains
In statistical mechanics and quantum mechanics anything can pops out of vacuum energy by random thermal or quantum fluctuation. Altough the probability is ridiculously small, a conscious entity can emerge of vacuum giving enought time. Those spooky space brains are called Boltzmann brains.[11][12]
[edit] 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.
Of course, creationists think all life is artificial, but that is another matter.
[edit] The Drake Equation
- See the main article: The Drake Equation
The Drake equation is a famous formulation that would allow one to calculate the number of advanced civilizations (defined as having invented radio so they can, in theory, be contacted) in our own Milky Way galaxy at a specific moment in time. It was first presented in 1961 by astronomer Frank Drake.
It is usually written in the following form:

where
- N represents the number of civilizations with which we might be able to communicate;
- R* is the average rate of star formation in our galaxy
- fp is the fraction of those stars that have planets
- ne is the average number of planets that can potentially support life per star that has planets
- fℓ is the fraction of the above that actually go on to develop life at some point
- fi is the fraction of the above that actually go on to develop intelligent life
- fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
- L is the length of time such civilizations release detectable signals into space.
While the Drake Equation may look long and scary, it can be seen that it merely starts with the number of stars and then defines the fraction that will take a step in the right direction for life, starting with the number that even have planets for life to form on (we ignore the possibility that contactable life can form or exist on stars) and ending with the probability that they'll develop technology and stick around. Although the Drake equation does not include all possible variables, it is generally accepted as a helpful, though not authoritative, tool for the field of exobiology. Some variables, such as the rate of star formation are known or can be calculated, others such as the number of planets and the number that can support life are currently unknown, but obtainable. The remaining values are the most controversial and really, anyone's guess is as good as anyone else's with regards to these. Life could be very easy to start, but difficult to get to the "advanced" stage, or it may be inevitable that intelligent species will emerge once life has started - the answer to this is completely unknown. Guessing at these values allows the number of civilisations to be guesstimated, so the numbers do vary wildly depending on how likely an individual thinks each component is. Given the number of stars in the galaxy (and in the entire universe) even very conservative figures for the unknown values give a large number of civilisations that could be contacted.
A counter argument to the Drake Equation was proposed by Italian physicist Enrico Fermi. The Fermi Paradox asks the question "If life is common in the universe why have we not found proof for it?" A similar concept is called "the great silence" which asks "If travel is difficult but communication is easy, why have we not detected radio waves of civilizations?"
[edit] Creationists and Astrobiology
Many creationists outright deny the existence of extraterrestrial life, because the Bible doesn't mention it, and 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.
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.
[edit] See also
- Exotheology
- Gliese 581c
- Rocky Planet
- Mars
- Life
- Drake Equation
- SETI - The Search For Extraterrestrial Intelligence
- Some of the ideas of Richard C. Hoagland
[edit] External links
- EvoWiki:Exobiology
- Seti.org
- Astrobiology Magazine
- Encyclopedia of Astrobiology, Astronomy, and Spaceflight, David Darling
[edit] Footnotes
- ↑ Some definitions of astrobiology include the study of such things as how plants grow in zero-G.
- ↑ http://www.clearleadinc.com/site/extraterrestrial-life.html
- ↑ http://en.wikipedia.org/wiki/Extraterrestrial_life#Direct_search
- ↑ 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
- ↑ Universe Today - Europa Capable of Supporting Life
- ↑ 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