Editing Extraterrestrial life (section)

==Context==
{{Life in the Universe}}
Initially, after the [[Big Bang]], the universe was too hot to allow life. It is estimated that the temperature of the universe was around 10 billion K at the one second mark.<ref>{{Cite web |date=2020-09-01 |title=Overview - NASA Science |url=https://science.nasa.gov/universe/overview/#:~:text=NASA-,Big%20Bang%20and%20Nucleosynthesis,of%20fog%20that%20scattered%20light. |access-date=2025-02-12 |language=en-US}}</ref> [[Chronology of the universe|15 million years later]], it cooled to temperate levels, but the elements that make up living things did not exist yet. The only freely available elements at that point were [[hydrogen]] and [[helium]]. [[Carbon]] and [[oxygen]] (and later, [[water]]) would not appear until 50 million years later, created through stellar fusion. At that point, the difficulty for life to appear was not the temperature, but the scarcity of free heavy elements.<ref>{{cite web |url= https://www.scientificamerican.com/article/when-did-life-first-emerge-in-the-universe/|title= When Did Life First Emerge in the Universe?|author= Avi Loeb|date= April 4, 2021|publisher= Scientific American|accessdate=April 17, 2023}}</ref> [[Planetary system]]s emerged, and the first [[organic compound]]s may have formed in the [[protoplanetary disk]] of [[cosmic dust|dust grains]] that would eventually create rocky planets like Earth. Although Earth was in a molten state after its birth and may have burned any organics that fell in it, it would have been more receptive once it cooled down.<ref>{{cite web |last=Moskowitz |first=Clara|author-link= Clara Moskowitz |title=Life's Building Blocks May Have Formed in Dust Around Young Sun |url=http://www.space.com/15089-life-building-blocks-young-sun-dust.html |date=29 March 2012 |publisher=[[Space.com]] |access-date=30 March 2012}}</ref> Once the right conditions on Earth were met, life started by a chemical process known as [[abiogenesis]]. Alternatively, life may have formed less frequently, then spread – by [[meteoroid]]s, for example – between [[habitable planet]]s in a process called [[panspermia]].<ref name="USRA-2010">{{cite conference |url=http://www.lpi.usra.edu/meetings/abscicon2010/pdf/5224.pdf |title=Panspermia: A Promising Field of Research |conference=Astrobiology Science Conference 2010: Evolution and Life: Surviving Catastrophes and Extremes on Earth and Beyond. 20–26 April 2010. League City, Texas. |first=P. H. |last=Rampelotto |date=April 2010 |bibcode=2010LPICo1538.5224R}}</ref><ref>{{cite book |first1=Guillermo |last1=Gonzalez |first2=Jay Wesley |last2=Richards |title=The privileged planet: how our place in the cosmos is designed for discovery |pages=343–345 |publisher=Regnery Publishing |year=2004 |isbn=978-0-89526-065-9 |url=https://books.google.com/books?id=KFdu4CyQ1k0C&pg=PA343}}</ref>

During most of its [[stellar evolution]] stars combine hydrogen nuclei to make helium nuclei by stellar fusion, and the comparatively lighter weight of helium allows the star to release the extra energy. The process continues until the star uses all of its available fuel, with the speed of consumption being related to the size of the star. During its last stages, stars start combining helium nuclei to form carbon nuclei. The higher-sized stars can further combine carbon nuclei to create oxygen and silicon, oxygen into neon and sulfur, and so on until iron. In the end, the star blows much of its content back into the stellar medium, where it would join clouds that would eventually become new generations of stars and planets. Many of those materials are the raw components of life on Earth. As this process takes place in all the universe, said materials are ubiquitous in the cosmos and not a rarity from the Solar System.<ref>Bennet, pp. 60-63</ref>  

[[Earth]] is a planet in the [[Solar System]], a planetary system formed by a star at the center, the [[Sun]], and the objects that orbit it: other planets, moons, asteroids, and comets. The sun is part of the [[Milky Way]], a [[galaxy]]. The Milky Way is part of the [[Local Group]], a [[galaxy group]] that is in turn part of the [[Laniakea Supercluster]]. The [[universe]] is composed of all similar structures in existence.<ref>Bennett, p. 53</ref> The immense distances between celestial objects is a difficulty for the study of extraterrestrial life. So far, humans have only set foot on the [[Moon]] and sent robotic probes to other planets and moons in the Solar System. 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]].<ref name="bennett55">Bennet, p. 55</ref> No probe has ever reached extrasolar planetary systems. The ''[[Voyager 2]]'' has left the Solar System at a speed of 50,000 kilometers per hour, if it headed towards the [[Alpha Centauri]] system, the closest one to Earth at 4.4 light years, it would reach it in 100,000 years. Under current technology, such systems can only be studied by telescopes, which have limitations.<ref name="bennett55"/> It is estimated that [[dark matter]] has a larger amount of combined matter than stars and gas clouds, but as it plays no role on the stellar evolution of stars and planets, it is usually not taken into account by astrobiology.<ref>Bennet, pp. 57-58</ref> 

There is an area around a star, the [[circumstellar habitable zone]] or "Goldilocks zone", where water may be at the right temperature to exist in liquid form at a planetary surface. This area is neither too close to the star, where water would become steam, nor too far away, where water would be frozen as ice. However, although useful as an approximation, [[planetary habitability]] is complex and defined by several factors. Being in the habitable zone is not enough for a planet to be habitable, not even to actually have such liquid water. Venus is located in the habitable zone of the Solar System but does not have liquid water because of the conditions of its atmosphere. Jovian planets or [[gas giant]]s are not considered habitable even if they orbit close enough to their stars as [[hot Jupiter]]s, due to crushing atmospheric pressures.<ref name="Neighbors" /> The actual distances for the habitable zones vary according to the type of star, and even the [[solar activity]] of each specific star influences the local habitability. The type of star also defines the time the habitable zone will exist, as its presence and limits will change along with the star's stellar evolution.<ref>{{cite web |url= https://www.space.com/goldilocks-zone-habitable-area-life|title= Goldilocks zone: Everything you need to know about the habitable sweet spot|author= Vicky Stein|date= February 16, 2023|publisher= Space.com|accessdate=April 22, 2023}}</ref>

The Big Bang took place 13.8 billion years ago, the Solar System was formed 4.6 billion years ago, and the first hominids appeared 6 million years ago. 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 brief times of existence of Earth's species, when considered from a cosmic perspective, may suggest that extraterrestrial life may be equally fleeting under such a scale.<ref>Bennet, p. 65</ref>  

During a period of about 7 million years, from about 10 to 17 million after the Big Bang, the background temperature was between {{cvt|373 and 273|K|C F}}, allowing the possibility of [[liquid water]] if any planets existed. 
[[Avi Loeb]] (2014) speculated that [[Abiogenesis|primitive life]] might in principle have appeared during this window, which he called "the Habitable Epoch of the Early Universe".<ref name="IJA-2014October">{{cite journal |last=Loeb |first=Abraham |author-link=Abraham Loeb |title=The Habitable Epoch of the Early Universe |url=https://www.cfa.harvard.edu/~loeb/habitable.pdf |date=October 2014 |journal=[[International Journal of Astrobiology]] |volume=13 |issue=4 |pages=337–339 |doi=10.1017/S1473550414000196 |access-date=15 December 2014 |bibcode=2014IJAsB..13..337L |arxiv = 1312.0613 |s2cid=2777386 }}</ref><ref name="NYT-20141202">{{cite news |last=Dreifus |first=Claudia |author-link=Claudia Dreifus |title=Much-Discussed Views That Go Way Back – Avi Loeb Ponders the Early Universe, Nature and Life |url=https://www.nytimes.com/2014/12/02/science/avi-loeb-ponders-the-early-universe-nature-and-life.html |date=2 December 2014 |work=[[The New York Times]] |access-date=3 December 2014 }}</ref>

Life on Earth is quite ubiquitous across the planet and has adapted over time to almost all the available environments in it, [[extremophile]]s and the [[deep biosphere]] thrive at even the most hostile ones. As a result, it is inferred that life in other celestial bodies may be equally adaptive. However, the origin of life is unrelated to its ease of adaptation and may have stricter requirements. A celestial body may not have any life on it, even if it were habitable.<ref>Aguilera Mochon, pp. 9–10</ref>