Extraterrestrial life
[15] Planetary systems emerged, and the first organic compounds may have formed in the protoplanetary disk of dust grains that would eventually create rocky planets like Earth.
Although probes can withstand conditions that may be lethal to humans, the distances cause time delays: the New Horizons took nine years after launch to reach Pluto.
Jovian planets or gas giants are not considered habitable even if they orbit close enough to their stars as hot Jupiters, due to crushing atmospheric pressures.
Life on other planets may have started, evolved, given birth to extraterrestrial intelligences, and perhaps even faced a planetary extinction event millions or even billions of years ago.
The proponents of this hypothesis consider that very little evidence suggests the existence of extraterrestrial life and that at this point it is just a desired result and not a reasonable scientific explanation for any gathered data.
[35][better source needed] The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy.
Zhi-Wei Wang and Samuel L. Braunstein suggest that a random universe capable of supporting life is likely to be just barely able to do so, giving a potential explanation to the Fermi paradox.
[45] If extraterrestrial life exists, it could range from simple microorganisms and multicellular organisms similar to animals or plants, to complex alien intelligences akin to humans.
[49] Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on terrestrial planets with a chemical make-up and temperature range similar to that of Earth.
[58] Scientists from the University of Oxford analysed it from the perspective of evolutionary theory and wrote in a study in the International Journal of Astrobiology that aliens may be similar to humans.
Fossil evidence as well as many historical theories backed up by years of research and studies have marked environments like hydrothermal vents or acidic hot springs as some of the first places that life could have originated on Earth.
For example, the hydrothermal vents found on the ocean floor are known to support many chemosynthetic processes[7] which allow organisms to utilize energy through reduced chemical compounds that fix carbon.
[74] An experiment on the two Viking Mars landers reported gas emissions from heated Martian soil samples that some scientists argue are consistent with the presence of living microorganisms.
[93] In December 2023, astronomers reported the first time discovery, in the plumes of Enceladus, moon of the planet Saturn, of hydrogen cyanide, a possible chemical essential for life[94] as we know it, as well as other organic molecules, some of which are yet to be better identified and understood.
According to the researchers, "these [newly discovered] compounds could potentially support extant microbial communities or drive complex organic synthesis leading to the origin of life.
The natural abundance of carbon, which is also relatively reactive, makes it likely to be a basic component of the development of a potential extraterrestrial technological civilization, as it is on Earth.
[10] One sign that a planet probably already contains life is the presence of an atmosphere with significant amounts of oxygen, since that gas is highly reactive and generally would not last long without constant replenishment.
Scholars from Ancient Greece were the first to consider that the universe is inherently understandable and rejected explanations based on supernatural incomprehensible forces, such as the myth of the Sun being pulled across the sky in the chariot of Apollo.
However, as the Greek atomist doctrine held that the world was created by random movements of atoms, with no need for a creator deity, it became associated with atheism, and the dispute intertwined with religious ones.
[130] The best-known early-modern proponent of ideas of extraterrestrial life was the Italian philosopher Giordano Bruno, who argued in the 16th century for an infinite universe in which every star is surrounded by its own planetary system.
[135][136] Speculation about life on Mars increased in the late 19th century, following telescopic observation of apparent Martian canals – which soon, however, turned out to be optical illusions.
The expansion of the genre of extraterrestrials in fiction influenced the popular perception over the real-life topic, making people eager to jump to conclusions about the discovery of aliens.
[147] Astrobiology, however, remains constrained by the current lack of extraterrestrial life-forms to study, as all life on Earth comes from the same ancestor, and it is hard to infer general characteristics from a group with a single example to analyse.
The public interest in extraterrestrial life and the lack of discoveries by mainstream science led to the emergence of pseudosciences that provided affirmative, if questionable, answers to the existence of aliens.
[151] Looking beyond the pseudosciences, Lewis White Beck strove to elevate the level of public discourse on the topic of extraterrestrial life by tracing the evolution of philosophical thought over the centuries from ancient times into the modern era.
His review of the contributions made by Lucretius, Plutarch, Aristotle, Copernicus, Immanuel Kant, John Wilkins, Charles Darwin and Karl Marx demonstrated that even in modern times, humanity could be profoundly influenced in its search for extraterrestrial life by subtle and comforting archetypal ideas which are largely derived from firmly held religious, philosophical and existential belief systems.
On a positive note, however, Beck further argued that even if the search for extraterrestrial life proves to be unsuccessful, the endeavor itself could have beneficial consequences by assisting humanity in its attempt to actualize superior ways of living here on Earth.
Life may still exist elsewhere in the Solar System in unicellular form, but the advances in spacecraft allow to send robots to study samples in situ, with tools of growing complexity and reliability.
At the same time, the data returned by space probes, and giant strides in detection methods, have allowed science to begin delineating habitability criteria on other worlds, and to confirm that at least other planets are plentiful, though aliens remain a question mark.
[156] In 2000, geologist and paleontologist Peter Ward and astrobiologist Donald Brownlee published a book entitled Rare Earth: Why Complex Life is Uncommon in the Universe.
