Extraterrestrial life

Some important international efforts in the search for extraterrestrial life. In the sense of the clock needles from the top left:

• The search for extra-solar planets (image: Kepler telescope)
• Listen to extraterrestrial signals that indicate intelligence (image: Allen Telescope Array)
• Robotic Solar System Exploration (Image: Curiosity rover in Mars)

The term extraterrestrial life refers to the hypothetical type of life that may exist outside Planet Earth and that did not originate on it.

Such life can range from simple prokaryotes (or life forms) to beings with civilizations much more advanced than humanity. The Drake equation speculates on the existence of intelligent life elsewhere in the universe. The science of extraterrestrial life in all its forms is known as astrobiology.

A growing portion of the scientific community is inclined to believe that some form of extraterrestrial life may exist where conditions are right, although it is generally considered that such life probably exists only in basic forms. An alternative hypothesis is panspermia, which suggests that life could arise in one place and then spread to other habitable planets. These two hypotheses are not mutually exclusive. There is speculation about extraterrestrial life forms ranging from bacteria, which is the majority position, to other more evolved forms of life, which may have developed intelligence of some kind. The discipline that studies the viability and possible characteristics of extraterrestrial life is called exobiology.

Due to a lack of evidence for or against, any scientific approach to the subject always takes the form of guesswork and estimation. Although it is worth mentioning that the subject also has a large number of informal and para-scientific theories, which easily exceed the criteria of any scientific epistemology, for example, making unfalsifiable statements according to Popper's criteria, and are therefore considered pseudosciences.

Universal biochemistry

All life on Earth requires chemical elements, hydrogen, oxygen, carbon, nitrogen, sulfur, phosphorus, as well as many others in smaller quantities, such as certain minerals; It also requires liquid water as the solvent in which the reactions take place. Sufficient amounts of carbon and other constituent elements of life, together with water, would make possible the formation of living organisms on other planets with a chemistry, pressure and temperature similar to those of the Earth. Since the Earth and other planets are made of "star dust", it is very likely that other planets have formed with a similar composition of chemical elements as the terrestrial ones. The combination of carbon and water in the form of carbohydrates, like sugar, can be a source of chemical energy on which life depends, while also providing elements of genetic structure and coding^{[quote required]}. Pure water is useful, as it has a neutral pH due to the continued dissociation between its hydronium and hydroxide ions. As a result, it can dissolve both positive (metallic) and negative (non-metallic) types of ions with equal ability.

Due to their relative abundance and usefulness in sustaining life, many have hypothesized that all forms of life, wherever they occur, would also use these scale materials. Even so, other elements and solvents can provide a certain base of life. Silicon has been pointed to as a possible alternative to carbon; Based on this element, life forms with a crystalline morphology have been proposed, theoretically capable of existing in high-temperature conditions, such as on planets that orbit very close to their star.

Life forms based on other solvents have also been suggested, since there are chemical compounds capable of maintaining their liquid state in different temperature ranges, thus expanding the habitable zones considered viable. Thus, for example, ammonia is studied as an alternative solvent to water. Life in an ocean of ammonia could appear on a planet much further away from its star.

Technically, life is basically a self-replicating reaction, so on this simple premise life could arise under a wide range of different conditions and ingredients, although the carbon-oxygen pathway seems the most optimal and conductive. There are even theories that self-replicating reactions could occur in the plasma of a star, although this would be a highly extreme and unconventional type of life.

World Literature

There was a dramatic change in thinking with the invention of the telescope and heliocentrism. Once it became clear that Earth was merely one planet among innumerable bodies in the universe, the theory of extraterrestrial life began to become a topic in the scientific community. One of the first was the Italian philosopher Giordano Bruno, who argued in the 16th century that for an infinite universe in which all the stars were surrounded by their own planetary system, there would be other worlds with "no less virtue nor a different nature from our earth"; and, like Earth, "contain animals and inhabitants".

The possibility of extraterrestrial life was a commonplace of polite speech during the 17th century, although in the poem Paradise Lost (1667) Milton used this theme cautiously when the angel suggests to Adam the possibility of extraterrestrial life. life on the moon:

Fontanelle expanded the creative sphere of the Creator, instead of denying it, in his work Conversations on the Plurality of Worlds. And in The excursion" (1728), David Mallet exclaimed: 'Ten thousand worlds shine; each one with the load of him / Of populated worlds & # 34;.

In literature, another example would be The Other World: Societies and Governments of the Moon, by the poet Cyrano de Bergerac, where extraterrestrial societies are presented as humorous or ironic parodies of terrestrial society.

In 1752, Voltaire published the short story Micromegas, which advances many of the notions that are later recurrently expressed in emerging and contemporary science fiction. In particular, the idea that aliens can travel between the stars and come to Earth (going so far as to suggest some light propulsion, analogous to a sun sail), and that they are fundamentally different from humans (in this case, in size, life span and number of senses).

The science fiction genre takes place during the 19th century with Jules Verne in Around the Moon (1870), which offers a discussion of the possibility of life on the Moon, but with the conclusion that it is barren, and The War of the Worlds, by H. G. Wells (1898), with the popular idea of the "martian invasion”.

Scientific approaches to extraterrestrial life

Because it is a phenomenon that for the moment remains essentially outside the scope of science (since there are no data available, and therefore the possibility of experimenting and refuting the hypotheses), there is no discipline " formal" that studies extraterrestrial life, nor any academic curriculum that trains experts in it. Those who have approached the subject scientifically are generally experts in various fields, who for purely personal interest have developed hypotheses about the possibilities of life on other worlds, and have shared their points of view through some medium. Despite this, an enormous amount of serious papers and publications have emerged on the subject, so that one can speak of a quasi-science dedicated to studying and theorizing about this phenomenon, despite the lack of evidence. The proto-science that studies extraterrestrial life is called exobiology or astrobiology, and is essentially dedicated to speculating about the limits in which, according to our scientific knowledge, life could occur.

Questions and arguments

There are many questions about what alien life might be like, for which science still has no answer, such as:

• Could it not be carbon-based? (for example, be silicon-based?)
• What is the probability that life evolves, and to what level of complexity (unicellular, multicellular, intelligent, technologically advanced)?
• What conditions requires the evolution of life?

Detractors of the idea that extraterrestrial life may exist indicate that it is not scientific to hypothesize about unknown or proven facts, such as life forms that are not based on carbon, advanced ecosystems that are not rich in hormone gases, or planets with biospheres significantly different from that of the Earth itself (average temperature, type of star they orbit, satellites, geology, etc.).

Principle of mediocrity

Because the only example of life we know of in the universe is life on planet Earth, those interested in the subject following a rational approach often follow the scientific principle of mediocrity, stating that life in the universe Planet Earth is not a special case, and therefore life as we know it can be considered a typical example of what life everywhere would be like. This presumption is relevant, since it strongly determines the actions undertaken by those who seek to scientifically prove the existence of life outside of Earth. Said principle of mediocrity, despite its conjectural status, allows us to venture some predictions about the possible attributes of extraterrestrial life. In particular, it is admitted that there are universal attributes of life. For example, it is accepted that Darwinian evolution is universally valid, and that all potential living creatures owe their characteristics to a process of natural selection, both on Earth and elsewhere in the universe.

There are other quasi-universal attributes or characteristics in species that, when successively repeated in different ways in different species in the terrestrial biosphere—a process characterized as convergent evolution—are considered highly probable in a hypothetical alien biosphere. Among these characteristics, it is worth highlighting the appearance of the senses, the limbs adapted for different environments and, most likely, photosynthesis when we talk about the plant kingdom.

In this sense, there is a great diversity of forms that extraterrestrial life could take. There are other more particular attributes that are often taken for granted, but according to experts, since they do not respond better than others to an evolutionary need and do not occur in all species present in the same habitat, they may vary or not exist, such as for example organs like the human hand, or a human-like position of eyes, nose and mouth. There are also other attributes, among them for example the skeleton, which although they are considered a necessity for creatures of a certain size, could be very different from what we know. Thus, for example, the vertebral column would be a terrestrial invention, since it does not occur in all organisms on planet Earth.

The detractors of this hypothesis of convergent evolution indicate that for it to exist, among other factors, very similar environmental conditions must occur, which by statistics is very difficult for them to occur, since the existence of planets with significantly biospheres is not known. similar to that of Earth.

Special Earth Hypothesis

In opposition to the principle of mediocrity, there are those who affirm that life on Earth is not a mediocre case, and that the conditions necessary for its appearance are so unique and particular that it may well be possible that very few exist, or even only one planet with life in the universe: Earth.

Proponents of this hypothesis claim that life on Earth, and in particular human life, seems to depend on a long and extremely lucky chain of events and circumstances, which could well be unrepeatable even on the cosmic scale. For example, it is regularly mentioned that without a Moon as large as Earth's, the planet would tend to precess much more significantly, drastically changing its inclination in its rotation and thus chaotically affecting the climate and, quite possibly,, making life as we know it impossible.

Other apparent lucky coincidences are also mentioned, such as the fact that the Sun is in a place in the Milky Way relatively free of supernovae, as opposed to the galactic center, or that the Sun is just the right size to give enough energy, and last long enough for life to have appeared.

Another positive coincidence for life on Earth is the existence of a planet the size of Jupiter, as the authors of the book Rare Earth point out, in a stable, almost circular orbit and at a sufficient distance from Earth to trap numerous comets and asteroids that would otherwise end up impacting the planet, ruining all fledgling life. These, among many other coincidences, separately may seem trivial, but together they make Earth a cosmically special place.

However, since the end of the 20th century, and as a result of new discoveries, such as the existence of organic molecules in space, the presumed existence of an ocean of liquid water on Europa, or the proven fact that planets extrasolar are relatively common, and therefore some of them could present feasible conditions for life, have meant that this hypothesis is no longer shared by a large part of the scientific community.

Other theories

Panspermia is the theory that life on Earth came from space, speculating that life arrived from other celestial bodies (perhaps from extrasolar planets) in the form of spores, traveling on meteors and cosmic dust that would be spewed into the space by meteoric collisions. There is a variant of this theory, which states that life is strictly native to the solar system, but that it did spread to Earth (or even, from Earth to other bodies) through spores in meteors; This theory is called transpermia. However, and accepting, of course, the validity of the aforementioned theories, it is necessary not to lose sight of another scientific approach to extraterrestrial life: its search through radio signals from deep space. During the last few months, a lot has been said and written about picking up signals that supposedly come from extremely distant galaxies. However, it is necessary to wait until, through the Scientific Research Methodology, it is denied or, rather, it is corroborated that said signals are real.

Speculation about possible extraterrestrial life forms, especially intelligent ones, as well as their possible civilizations and relationships with human beings have been and are also covered by science fiction and ufology.

Search for Alien Life

Scientists search for extraterrestrial life primarily in three ways:

• Direct search, that is, the observation of microbial life or of any kind in the celestial bodies that humanity comes to visit.
• Indirect detection, or the detection of distinctive features or brands of life in celestial bodies through advanced telescopes.
• Listen to artificial signalswhich would allow to detect true alien civilizations that emit electromagnetic radiation as a byproduct of their technological advancement.

Direct search

Sources of Encelado; image obtained by Cassini/Huygens

Because, in practice, the only celestial bodies that humans can visit are those in our solar system, the direct search for extraterrestrial life has been limited to that system; mainly in search of microscopic life, either fossil or active. However, not all bodies in the solar system are considered suitable for the presence of life. Currently, possible search targets are considered to be:

• Mars, an idea supported by the theorized past and present similarities between Earth and that planet: mainly the already confirmed presence of water in abundance and a dense atmosphere in the remote past. These facts, and the relative proximity of Mars, have caused it to be the celestial body most explored by astrobiology: it has already been looking for evidence of biological activity in Mars during NASA Viking 1 and Viking 2 missions, with ambiguous results but generally considered negative. There are strong controversies about these evidences of microbial existence in Mars. An experiment of the Viking Mars Lander reported gas emissions from the hot Martian soil, which some argue are consistent with the presence of microbes. However, lack of evidence or corroboration through other experiments in the Viking Mars Lander suggests that a non-biological reaction is the most successful hypothesis.

• Meteorites from Mars that have fallen on Earth: due to the relative proximity between Mars and Earth and the low Martian gravity, it is accepted as a fact that certain meteors are martian rocks ejected by major meteoric shocks, that after wandering for millions of years in space, they clash randomly against our planet. On 6 August 1996, NASA experts publicly revealed that such a meteorite named ALH84001, found in Antarctica, showed evidence of possible microscopic biological activity; in particular, traces similar to fossilized bacteria. This study has been widely discussed and criticized in its conclusions, keeping up to this day in the polemic.

• The moon of Jupiter, Europe: from the passage of Voyager 1 and Voyager 2 missions it is believed that Europe has a liquid water ocean of several kilometers deep, under the ice that covers its surface. Because of the proven fact that tide forces and orbital resonance provoke the warming of Io (the largest moon closest to Jupiter), originating vulcanism, it is believed that Europe would present similar volcanic activity, being the second in distance just after Io. Therefore, it is believed that there could be sources of heat and dumping of chemicals in that ocean. Examples of abysmal ecosystems that depend on geothermal activity for their livelihood are known on Earth, so that such a possibility in Europe cannot be excluded, leading even experts to propose an unmanned mission to Europe, consisting of an alunizaje probe and a robotic submarine capable of penetrating the thick layer of ice. This mission is not currently in the plans of any space agency, and if it were to be realized, it would take several decades in the future.

• Saturn's moon. During the Cassini-Huygens mission of NASA and ESA it was discovered that Encélado ejects large quantities of water into space through enormous geysers, revealing the presence of an active cryovulcanism and a very high possibility of reservoirs of liquid water under the icy surface. Concealed is a surprise for astrobiology and planetology, as no one expected to find such activity on such a small moon; but now the experts consider that it could be even more feasible than Europe as a place of search for life, since it is believed that its surface ice layers are much thinner, making it easier to access groundwater. The known fact that water escapes outer space is evidence of it.

Indirect search

Terrestrial Planet Finder - A space telescope project dedicated to photographing extra-solar planets.

Due to the recently acquired ability to detect extrasolar planets or exoplanets orbiting stars other than our Sun, there has been strong interest among the astronomical community in discovering worlds comparable in size and properties to Earth; planets that are just beginning to be detected. There is also strong interest in the possibility of actually observing such worlds using much more sophisticated telescopes than are currently available.

To date there is only one example of direct observation of an extrasolar planet (see GQ Lupi); and although it is beginning to be possible to detect Earth-sized planets (see Gliese 876) in other systems, taking pictures of them is still not possible, because the available instruments are not sensitive enough. to separate the enormous brightness of the star from that of its planets. That may change in the near future, when telescopes like NASA's Terrestrial Planet Finder or ESA's Project Darwin come online. Among the functions of such devices is to obtain photographs of the planets, and to detect their fundamental properties, such as their temperature, or the presence or absence of an atmosphere, as well as details about their composition (by means of spectroscopy).

Some people believe that such methods would make it possible to detect parallel worlds where biological processes comparable to those present on Earth exist. The idea is supported by the fact that the light that reflects our planet carries with it "marks" that reveal the presence of life; for example, the presence of a high level of oxygen, and certain variations of the infrared spectrum, which reveal the presence of vegetation.

Of course, such detection methods assume that life on Earth is a mediocre case, and that the characteristics of light reflected from Earth are shared by all cases. This detection method has the advantage of allowing the detection of worlds with primitive life (and which do not transmit radio waves as SETI expects), provided that such life has modified the atmosphere, in a manner analogous to how life Earth's atmosphere has changed since its appearance.

Listen for artificial signals

On the other hand, it has been theorized that any technological society will be transmitting information: electromagnetic radiation generated by humans is detectable within a radius of more than 50 light years from Earth, and is constantly expanding. The SETI project (Search for Extraterrestrial Intelligence) or "Search for Extraterrestrial Intelligence", analyzes the data collected by large radio telescopes and analyzes them looking for artificial patterns using supercomputers, as well as a large distributed computing project in the world; SETI@home. To date, however, only the Wow! has been remarkable in this search.

Messages with no destination

Over time there have also been a series of initiatives in the opposite direction: not to look for the signal of a possible extraterrestrial intelligence, but to inform potential civilizations that are listening of our presence. The first was the so-called Arecibo Message, launched in 1974 in the direction of the M13 star cluster. On board the probes Pioneer 10 (in the direction of the star Aldebaran) and Pioneer 11 (in the direction of the constellation of Aquila) there are two messages (see Pioneer Plaque ) destined for a possible civilization alien who could intercept the probes. The same occurs in the case of the Voyager Golden Record, in the Voyager 1 probes (in the direction of the Ophiuchus constellation) and Voyager 2 (in the direction of the star Ross 248). More recently, in 2008, a team of Ukrainian scientists has sent messages in the direction of the Gliese 876 system. of the British band The Beatles in the direction of the star Polaris which is 431 light years from earth, using a 70m antenna at DSN's outside Madrid in order to celebrate the 50th anniversary of NASA, the 45th anniversary of the Deep Spacial Network (DSN) and the 40th anniversary of the song.

Several SETI scientists have warned that trying to contact hypothetical extraterrestrial civilizations by sending radio transmissions into space is reckless, unscientific, unethical, and potentially catastrophic.