Retroactive continuity , or retcon for short, is a literary device in which facts in the world of a fictional work that have been established through the narrative itself are adjusted, ignored, supplemented, or contradicted by a subsequently published work that recontextualizes or breaks continuity with the former.
157-533: The Time Lords are a fictional ancient race of extraterrestrial people in the British science fiction television series Doctor Who . Time Lords are so named for their command of time travel technology and their non-linear perception of time. Originally, they were described as a powerful and wise race from the planet Gallifrey , from which the Doctor was a renegade; details beyond this were very limited for
314-610: A Time Lord can regenerate twelve times (thirteen incarnations in all). There were exceptions to this rule, however: when the Master reached the end of his regenerative cycle, he took possession of the body of another person to continue living. In " The Five Doctors ", the Master was offered a new cycle of regenerations by the High Council to save the Doctor from the Death Zone, which may indicate that there are methods to circumvent
471-680: A Time Lord. This story element was notably featured in Series 3 ; the Doctor uses it to hide himself from the Family of Blood and becomes a schoolteacher in Edwardian England. His nemesis the Master used it to disguise himself as a human to escape the Time War. The story element is later revisited in the Series 12 episode " Fugitive of the Judoon " when a mysterious, unknown incarnation of
628-416: A basic component of the development of a potential extraterrestrial technological civilization, as it is on Earth. Fossil fuels may likely be generated and used on such worlds as well. The abundance of chlorofluorocarbons in the atmosphere can also be a clear technosignature, considering their role in ozone depletion . Light pollution may be another technosignature, as multiple lights on the night side of
785-611: A catalog of microorganisms, with the way each one reacts to sunlight. The goal is to help with the search for similar organisms in exoplanets, as the starlight reflected by planets rich in such organisms would have a specific spectrum, unlike that of starlight reflected from lifeless planets. If Earth was studied from afar with this system, it would reveal a shade of green, as a result of the abundance of plants with photosynthesis. In August 2011, NASA studied meteorites found on Antarctica, finding adenine , guanine , hypoxanthine and xanthine . Adenine and guanine are components of DNA, and
942-487: A collection of science fiction short stories originally published in Super Science Stories and Astounding Science Fiction from 1940 to 1950. Compiled into a single publication by Gnome Press in 1950, the collection features a framing sequence in which the stories are told to a reporter by Dr. Susan Calvin , chief robopsychologist at U.S. Robots and Mechanical Men , Inc. Changes necessary to fit
1099-446: A crack in time to grant him a full new regeneration cycle. The revelation in 2020's The Timeless Children that The Doctor is, in fact, not Gallifreyan and instead had their DNA used to give Gallifreyans the ability to regenerate, calls into doubt whether or not the Doctor does in fact have a limit to their regenerations, or if they merely believed they did due to all other Time Lords being limited. The Time Lord homeworld, Gallifrey ,
1256-671: A different concept of ageing from humans. In " The Stolen Earth ", the Tenth Doctor refers to when who he thought was his original incarnation was a "kid" at 90 years old. However, within a specific incarnation, a Time Lord is able to age, albeit much more slowly than a human. The War Doctor and Eleventh Doctor , over the course of the Last Great Time War and the Battle of Trenzalore , respectively, are seen to age within their respective incarnations to what would appear to
1413-594: A female. The Doctor also regenerated as a woman in " Twice Upon A Time ", as the Thirteenth Doctor . Whether or not Time Lords can recognise each other across regenerations is not made entirely clear: In " The Impossible Astronaut ", a future version of the Eleventh Doctor is shot, causing him to begin his regeneration cycle. He is shot again before the regeneration completes, causing him to die instantly. However, in " The Wedding of River Song ", it
1570-532: A fictional work is in All-Star Squadron #18 (February 1983) from DC Comics . The series was set on DC's Earth-Two , an alternate universe in which Golden Age comic characters age in real time. All-Star Squadron was set during World War II on Earth-Two; as it was in the past of an alternate universe, all its events had repercussions on the contemporary continuity of the DC multiverse. Each issue changed
1727-425: A hand. Also seen in " Journey's End ", is the apparent ability to siphon off regeneration energy in order to cancel the effect of changing appearance; which requires them to have a "bio-matching receptacle" (in this case the Doctor's severed hand), which is usually impractical. However, this "non-regeneration" was revealed as "counting" towards the Doctor's twelve possible regenerations during the events of " The Time of
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#17327911193811884-496: A human to be old age; both conflicts are suggested within the series to last hundreds if not thousands of Earth years. In The Two Doctors , the Second Doctor states that the "Rassilon Imprimatur" allows Time Lords to safely travel through time, becoming symbionts with their TARDISes, and that the reason other species are incapable of developing time travel is that they lack the imprimatur. However, he implies later that he
2041-481: A living entity must have the capacity to replicate itself, the capacity to avoid damage/decay, and the capacity to acquire and process resources in support of the first two capacities. Life on Earth started with an RNA world and later evolved to its current form, where some of the RNA tasks were transferred to DNA and proteins . Extraterrestrial life may still be stuck using RNA, or evolve into other configurations. It
2198-446: A major cultural impact, especially extraterrestrials in fiction . Science fiction has communicated scientific ideas, imagined a range of possibilities, and influenced public interest in and perspectives on extraterrestrial life. One shared space is the debate over the wisdom of attempting communication with extraterrestrial intelligence. Some encourage aggressive methods to try to contact intelligent extraterrestrial life. Others – citing
2355-534: A math equation, none of its values were known at the time. Although some values may eventually be measured, others are based on social sciences and are not knowable by their very nature. This does not allow one to make noteworthy conclusions from the equation. Based on observations from the Hubble Space Telescope , there are nearly 2 trillion galaxies in the observable universe. It is estimated that at least ten per cent of all Sun-like stars have
2512-472: A mixed aromatic - aliphatic structure") that could be created naturally, and rapidly, by stars . It is still unclear if those compounds played a role in the creation of life on Earth, but Sun Kwok, of the University of Hong Kong, thinks so. "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life." In August 2012, and in
2669-623: A new personality. Regenerations can be traumatic. In Castrovalva , the Fifth Doctor requires the use of a Zero Room, a chamber shielded from the outside universe that provides an area of calm for him to recuperate. The Time Lord's personality also sometimes goes through a period of instability following a regeneration, such as in The Christmas Invasion . It was first stated in The Deadly Assassin that
2826-650: 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. The immense distances between celestial objects
2983-466: A potential explanation to the Fermi paradox. 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 . When scientists talk about extraterrestrial life, they consider all those types. Although it is possible that extraterrestrial life may have other configurations, scientists use
3140-538: A race called Time Lords, and in Spearhead from Space (1970), the Doctor's earlier description of himself as a human is retconned when the Third Doctor explicitly states that he is not human. In The Time Warrior (1973–1974), the name of the Doctor's home planet, Gallifrey , was revealed on screen for the first time. The Time Lords are considered one of the oldest and most technologically powerful races in
3297-570: A reasonable scientific explanation for any gathered data. In 1961, astronomer and astrophysicist Frank Drake devised the Drake equation as a way to stimulate scientific dialogue at a meeting on the search for extraterrestrial intelligence (SETI). The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilisations in the Milky Way galaxy . The Drake equation is: where: and Drake's proposed estimates are as follows, but numbers on
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#17327911193813454-443: A result has been conceived, even if hypothetical. Evolution requires life to be divided into individual organisms, and no alternative organisation has been satisfactorily proposed either. At the basic level, membranes define the limit of a cell, between it and its environment, while remaining partially open to exchange energy and resources with it. The evolution from simple cells to eukaryotes, and from them to multicellular lifeforms,
3611-419: A rocky planet can be a sign of advanced technological development. However, modern telescopes are not strong enough to study exoplanets with the required level of detail to perceive it. Retroactive continuity There are various motivations for applying retroactive continuity, including: Retcons are used by authors to increase their creative freedom, on the assumption that the changes are unimportant to
3768-468: A scale. Life on Earth is quite ubiquitous across the planet and has adapted over time to almost all the available environments in it, extremophiles 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
3925-476: A series of factors that range from the location in the galaxy and the configuration of the Solar System to local characteristics of the planet, and that it is unlikely that all such requirements are simultaneously met by another planet. 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
4082-576: A series' continuity. Later stories may contradict the previous ones or explicitly establish that they never happened. A notable example of subtractive retconning is the X-Men film series . After X-Men: The Last Stand faced criticism for abruptly killing off characters such as Cyclops and Jean Grey , its sequel, X-Men: Days of Future Past , features the character Wolverine traveling back in time to 1973 to prevent an assassination that, if carried out, would lead to mutant extinction. The result of this
4239-483: A short period) a massive burst of radiation that would have killed anything else instantly. However, the radiation burst caused enough damage to start a regeneration. Time Lords are extremely long-lived, routinely counting their ages in terms of centuries; the Second Doctor claimed in The War Games that Time Lords could live "practically forever, barring accidents". The series has suggested that Time Lords have
4396-579: A solvent other than water. The low temperatures required would add an extra problem, the difficulty to kickstart a process of abiogenesis to create life in the first place. Norman Horowitz , head of the Jet Propulsion Laboratory bioscience section for the Mariner and Viking missions from 1965 to 1976 considered that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to
4553-440: A star's light and heat, and so its lifeforms would not grow beyond a certain complexity. There is also research in assessing the capacity of life for developing intelligence. It has been suggested that this capacity arises with the number of potential niches a planet contains, and that the complexity of life itself is reflected in the information density of planetary environments, which in turn can be computed from its niches. It
4710-660: A system of planets, i.e. there are 6.25 × 10 stars with planets orbiting them in the observable universe. Even if it is assumed that only one out of a billion of these stars has planets supporting life, there would be some 6.25 billion life-supporting planetary systems in the observable universe. A 2013 study based on results from the Kepler spacecraft estimated that the Milky Way contains at least as many planets as it does stars, resulting in 100–400 billion exoplanets. The apparent contradiction between high estimates of
4867-519: A trial is a "very rare" event for the Time Lords. Exceptions to this policy are made only in extreme circumstances when they feel they have to, such as where the Doctor calls them for help in the serial. At the start of the 2005 television series, Gallifrey was thought to have been destroyed and the Time Lords functionally extinct as a result of a mutually destructive Time War with the Dalek race;
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5024-686: A warrior greater than either", which is "half-Dalek, half-Time Lord", while in " Hell Bent " (2015), the General, while describing the prophecy of the Hybrid, mentions the Time Lords as one of two warrior races along with the Daleks . In " Before the Flood " (2015), the Fisher King describes the Time Lords as "cowardly, vain curators, who suddenly remembered they had teeth, and became the most warlike race in
5181-567: A wide range of events and individuals, so long as they do not cross back into their own timeline. Over subsequent episodes their history, their development of time manipulation, and their internal politics were touched upon, with Time Lord society portrayed as a stagnated ceremony-bound oligarchy and their past having descended into myth and legend . The Doctor became at times an ally, being appointed their president during his fourth , fifth , and twelfth incarnations and assisting them on many other occasions. In an audio commentary recorded for
5338-461: A world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde , in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422 , which is located 400 light years from Earth. Glycolaldehyde is needed to form ribonucleic acid , or RNA, which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to
5495-402: Is life that originates from another world rather than on Earth . No extraterrestrial life has yet been scientifically conclusively detected. Such life might range from simple forms such as prokaryotes to intelligent beings , possibly bringing forth civilizations that might be far more advanced than humans. The Drake equation speculates about the existence of sapient life elsewhere in
5652-601: 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 . No probe has ever reached extrasolar planetary systems. The Voyager 2 has left
5809-631: Is a new timeline where Jean and Cyclops never died. Retroactive continuity is similar to, but not the same as, plot inconsistencies introduced accidentally or through lack of concern for continuity; retconning, by comparison, is done deliberately. For example, the ongoing continuity contradictions on episodic TV series such as The Simpsons (in which the timeline of the family's history must be continually shifted forward to explain why they are not getting any older) reflects intentionally lost continuity, not genuine retcons. However, in series with generally tight continuity, retcons are sometimes created after
5966-494: Is able to interface with his spaceship in ways that only a 'time-sensitive' is supposed to be able to. In " Utopia ", the Tenth Doctor states that he finds it difficult to look at Captain Jack Harkness because Jack's existence has become fixed in time and space. In the Series 4 episode " Journey's End ", the Tenth Doctor was shown to use his telepathic abilities to wipe Donna Noble's mind of certain memories, specifically
6123-501: Is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon , such as methane , ethane or propane . Another unknown aspect of potential extraterrestrial life would be the chemical elements that would compose it. Life on Earth is largely composed of carbon, but there could be other hypothetical types of biochemistry . A replacement for carbon would need to be able to create complex molecules, store information required for evolution, and be freely available in
6280-493: Is an Earth-like planet in the fictional constellation of Kasterborous. It is located in a binary star system 250 million light years from Earth. The points in time when Gallifrey appears are never definitively stated. As the planet is often reached by means of time travel, its relative present could conceivably exist almost anywhere in the Earth's past or future, as well as anywhere in the conceivable universe. From space, Gallifrey
6437-416: Is being carried out both directly and indirectly. As of September 2017 , 3,667 exoplanets in 2,747 systems have been identified , and other planets and moons in the Solar System hold the potential for hosting primitive life such as microorganisms . As of 8 February 2021, an updated status of studies considering the possible detection of lifeforms on Venus (via phosphine ) and Mars (via methane )
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6594-403: Is common knowledge that the conditions on other planets in the solar system, in addition to the many galaxies outside of the Milky Way galaxy , are very harsh and seem to be too extreme to harbor any life. The environmental conditions on these planets can have intense UV radiation paired with extreme temperatures, lack of water, and much more that can lead to conditions that don't seem to favor
6751-485: 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 giants are not considered habitable even if they orbit close enough to their stars as hot Jupiters , due to crushing atmospheric pressures. The actual distances for
6908-520: Is embodied in the Copernican principle , which states that Earth does not occupy a unique position in the Universe, and the mediocrity principle , which states that there is nothing special about life on Earth. Other authors consider instead that life in the cosmos, or at least multicellular life, may be actually rare. The Rare Earth hypothesis maintains that life on Earth is possible because of
7065-493: Is fatal to a number of humans, and appears unaffected by the energy whip wielded by the Sycorax in " The Christmas Invasion ". In " Smith and Jones " the Tenth Doctor says that the radiation given off by X-rays pose no real threat to Time Lords, and proceeds to absorb an amount that would be lethal to a human, which he subsequently expels through his foot. "The End of Time" shows the Tenth Doctor as being capable of surviving (for
7222-439: Is likely devoid of life. However, Venus is still of interest to astrobiologists, as it is a terrestrial planet that was likely similar to Earth in its early stages and developed in a different way. There is a greenhouse effect , the surface is the hottest in the Solar System, sulfuric acid clouds, all surface liquid water is lost, and it has a thick carbon-dioxide atmosphere with huge pressure. Comparing both helps to understand
7379-444: Is no guarantee that it is transmitting radio communications in the direction of Earth. The length of time required for a signal to travel across space means that a potential answer may arrive decades or centuries after the initial message. The atmosphere of Earth is rich in nitrogen dioxide as a result of air pollution , which can be detectable. The natural abundance of carbon, which is also relatively reactive, makes it likely to be
7536-657: Is not guaranteed. The Cambrian explosion took place thousands of millions of years after the origin of life, and its causes are not fully known yet. On the other hand, the jump to multicellularity took place several times, which suggests that it could be a case of convergent evolution , and so likely to take place on other planets as well. Palaeontologist Simon Conway Morris considers that convergent evolution would lead to kingdoms similar to our plants and animals, and that many features are likely to develop in alien animals as well, such as bilateral symmetry , limbs , digestive systems and heads with sensory organs . Scientists from
7693-400: Is revealed this was a shape shifting android the Doctor used to fake his death, making this questionable. It's a bit dodgy, this process, you never know what you're gonna end up with. In cases of non-fatal injury, Time Lords who have recently regenerated can use left over cellular energy to heal and even regrow severed limbs, as seen in " The Christmas Invasion " where the Tenth Doctor regrows
7850-534: Is seen as a yellow-orange planet and was close enough to central space lanes for spacecraft to require clearance from Gallifreyan Space Traffic Control as they pass through its system. The planet was protected from physical attack by an impenetrable barrier called the quantum force field, and from teleportation incursions by the transduction barrier—which could be reinforced to repel most levels of this type of technological attack. The Doctor's granddaughter Susan first describes her home world (not named as "Gallifrey" at
8007-548: Is seen using this method to query a cat about the goings-on of the flat in " The Lodger ". In " A Good Man Goes to War " and " Closing Time " he is apparently able to even understand babies, as well as horses in " A Town Called Mercy ". In " The Girl in the Fireplace ", the Tenth Doctor reads the mind of Madame de Pompadour —and in the process, to his surprise, she is able to read his mind as well. In " The End of Time ",
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#17327911193818164-432: Is so common in superhero comics that the term " comic book death " has been coined for it. An early example of this type of retcon is the return of Sherlock Holmes , whom writer Arthur Conan Doyle apparently killed off in " The Final Problem " in 1893, only to bring him back, in large part because of readers' responses, with " The Empty House " in 1903. The character Zorro was retconned early in his existence. In
8321-401: Is somehow innately able to sense which events in time are 'fixed' and which are in 'flux'. The Eleventh Doctor slightly amends what was said earlier in " The Doctor's Wife ", saying that he could only sense if there were other Time Lords in this universe. In the original series episode Warriors' Gate , Romana is called a 'time-sensitive' by a marauding slaver and, though she seems to deny this,
8478-452: Is speculated by Donna Noble in " The Doctor's Daughter " (2008) to be a surviving Time Lord, though the Doctor initially rejects the suggestion. Two other Time Lord-like beings appeared in " Journey's End " (2008): Donna, briefly empowered with the mind and knowledge of a Time Lord, and a half-human clone of the Tenth Doctor. Donna's memories related to the Doctor, as well as her Time Lord knowledge, are buried in order to save her life, while
8635-470: Is still conscious and able to return to return to the TARDIS and Pandorica respectively. In Death of the Doctor , the Eleventh Doctor responds to a question from Clyde Langer by saying he can regenerate "507" times. Early news reports, before the episode was broadcast, suggested he would say there is no limit to the number of regenerations. Writer Russell T Davies stated in an interview with SFX that
8792-502: Is suddenly announced that "Oceania was not after all in war with Eurasia. Oceania was at war with Eastasia and Eurasia was an ally" (Part Two, Ch. 9), there is an immediate intensive effort to change "all reports and records, newspapers, books, pamphlets, films, sound-tracks and photographs" and make them all record a war with Eastasia rather than one with Eurasia. "Often it was enough to merely substitute one name for another, but any detailed report of events demanded care and imagination. Even
8949-466: Is that the chemical elements that make up life, such as carbon and water, are ubiquitous in the universe. The third is that the physical laws are universal, which means that the forces that would facilitate or prevent the existence of life would be the same ones as on Earth. According to this argument, made by scientists such as Carl Sagan and Stephen Hawking , it would be improbable for life not to exist somewhere else other than Earth. This argument
9106-421: Is that there are natural sources of such signals as well, such as gamma-ray bursts and supernovae, and the difference between a natural signal and an artificial one would be in its specific patterns. Astronomers intend to use artificial intelligence for this, as it can manage large amounts of data and is devoid of biases and preconceptions. Besides, even if there is an advanced extraterrestrial civilization, there
9263-447: Is unclear if our biochemistry is the most efficient one that could be generated, or which elements would follow a similar pattern. However, it is likely that, even if cells had a different composition to those from Earth, they would still have a cell membrane . Life on Earth jumped from prokaryotes to eukaryotes and from unicellular organisms to multicellular organisms through evolution . So far no alternative process to achieve such
9420-510: The 2005 series , the Ninth Doctor claims that he can sense the movement of the Earth through space as well as being able to perceive the past and all possible futures. He is also able to concentrate and time his motions well enough to step safely through the blades of a rapidly spinning fan, and later claims that if any Time Lords still existed, he would be able to sense them. As the Tenth Doctor he repeats this assertion, adding also that he
9577-479: The Doctor Who universe. In The Time Warrior , the Time Lords are characterised by Sontaran military intelligence, quoted by Commander Linx , as "a race of great technical achievement, but lacking the morale to withstand a determined assault". In " The Witch's Familiar " (2015), Davros mentions a prophecy on the Doctor's world that spoke of a hybrid made up of "two great warrior races forced together to create
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#17327911193819734-742: The Krillitane Mr Finch in " School Reunion " (2006). In " Father's Day " (2005), the Ninth Doctor remarks that before Time Lords were "all gone", they would have prevented or repaired paradoxes such as that which attracted the Reapers to 1987 Earth. In " Rise of the Cybermen " (2006), the Tenth Doctor mentions, "When the Time Lords kept their eye on everything, you could pop between realities , home in time for tea. Then they died, and took it all with them. Walls of reality closed,
9891-610: The Ninth Doctor describes his planet as "just rocks and dust" in " The End of the World " (2005), and mentions in " Dalek " (2005) that the Time Lords "burnt" with the Daleks at the end of the "Last Great Time War", and the Tenth Doctor tells the Master in " The Sound of Drums " (2007) that the Time Lords are "dead" and "all [they've] got is each other". The Doctor describes himself as the last of his kind and says his planet burned on numerous other occasions, as do other individuals, such as
10048-778: The Rio Tinto river in Spain. NASA officials soon distanced NASA from the scientists' claims, and Stoker herself backed off from her initial assertions. In November 2011, NASA launched the Mars Science Laboratory that landed the Curiosity rover on Mars. It is designed to assess the past and present habitability on Mars using a variety of scientific instruments. The rover landed on Mars at Gale Crater in August 2012. A group of scientists at Cornell University started
10205-516: The Series 5 episode " The Big Bang " the Doctor telepathically left a message in Amy Pond 's head before sealing her into the Pandorica so that she would know what was happening when she woke up. Time Lords also have the ability to regenerate their bodies when their current body is mortally wounded. This process results in their body undergoing a transformation, gaining a new physical form and
10362-459: The hydrothermal vents found on the ocean floor are known to support many chemosynthetic processes which allow organisms to utilize energy through reduced chemical compounds that fix carbon. In return, these reactions will allow for organisms to live in relatively low oxygenated environments while maintaining enough energy to support themselves. The early Earth environment was reducing and therefore, these carbon fixing compounds were necessary for
10519-485: The 15th century, acknowledged the possibility Jesus could have visited extraterrestrial worlds to redeem their inhabitants. Nicholas of Cusa wrote in 1440 that Earth is "a brilliant star" like other celestial objects visible in space; which would appear similar to the Sun , from an exterior perspective, due to a layer of "fiery brightness" in the outer layer of the atmosphere. He theorised all extraterrestrial bodies could be inhabited by men, plants, and animals, including
10676-444: The 2009 DVD release of The War Games (1969), producer Derrick Sherwin mentioned how in a discussion with the serial's co-writer Terrance Dicks the previous day, Dicks was "absolutely certain" that Sherwin created the Time Lords for the serial, although Sherwin could not remember himself. Later in the commentary, Dicks recalled Sherwin telling him in the discussions with Dicks and Dicks' fellow co-writer Malcolm Hulke that because
10833-578: The Daleks into believing that she was dead. The Doctor also shows a greater tolerance to cold compared to humans in The Seeds of Doom and " Planet of the Ood " and even Romana in The Ribos Operation , and in " 42 ", the Tenth Doctor states he is able to survive at absolute zero for a short period of time. In " World War Three ", the Doctor is able to shake off an electrocution attempt which
10990-517: The Doctor had always been established as being on the run from his own people, that if he has to appeal to them, the Doctor would be in trouble. In a 2016 interview with The Essential Doctor Who magazine, Dicks mentioned how when Sherwin and he were discussing The War Games one day, Sherwin said, "He belongs to this mysterious race called the Time Lords, doesn't he?" with "everything" ultimately coming from that discussion. In The War Games DVD commentary, Sherwin mentioned that he recalled hearing about
11147-778: The Doctor " (2013), it is shown that the Eighth Doctor regenerates into the War Doctor to fight in the Time War. Many years later, as shown during " The Day of the Doctor " (2013) and also described by the Partisan in "The End of Time", the War Doctor originally planned to use a Time Lord weapon known as the Moment to destroy the Time Lords and Daleks. However, after being shown the Tenth and Eleventh Doctors during "The Day of
11304-537: The Doctor " concerned this supposed destruction and their eventual survival. They developed a culture of custodianship and time-related technologies based on this perception which includes strictly controlled space /time travel machines (known as " TARDISes ") and monitoring devices to travel through time and to prevent time from being subverted or abused—although actual action was described as rare in practice due to their traditional policy of strict non-interference and neutrality. They can act to manipulate timelines of
11461-500: The Doctor ". In "The End of Time", the Tenth Doctor was able to postpone his regeneration long enough so that he could travel in time and space to see his past companions for one last time before he regenerated. The Fifth Doctor had also shown a similar ability in The Caves of Androzani , fighting off the effects of an impending regeneration so he can return to Androzani Minor to save his companion Peri. Time Lords appear to have
11618-542: The Doctor (played by Jo Martin ), is revealed to have been hiding on Earth as a tour guide named “Ruth Clayton” using a Chameleon Arch. Time Lords can communicate by telepathy , and can link their minds to share information and enhance their powers. In Castrovalva , the Doctor activates the TARDIS' Zero Room mentally. Additionally, both the Doctor and the Master demonstrate significant hypnotic abilities which may be supplemented by their telepathic abilities. These powers were elaborated upon from 2005. The Eleventh Doctor
11775-425: The Doctor engages in astral projection , but warns that if he is disturbed while doing so, his mind could become severed from his body and he could die. In " Last of the Time Lords ", the Doctor telepathically interfaces with a network tapped into the human population who collectively chant his name. The focus of psychic energy granted the Doctor the ability to de-age himself, float through the air, deflect shots from
11932-512: The Doctor forever. This again implies that regeneration is not inevitable and can indeed be refused. Upon encountering the remains of fellow Time Lord the Corsair in " The Doctor's Wife ", the Doctor refers to the Corsair as both male and female, hinting that Time Lords can switch genders upon their regenerations; this is confirmed in " Dark Water ", in which the Master , previously seen in various male incarnations for over forty years, returned as
12089-419: The Doctor remarks upon her ability, she comments that he should have stayed in university. However, despite showing several appearances, Romana regenerated only once on that occasion. In " Utopia ", the Master, just before regeneration, claimed that he would become "young and strong", implying that he could choose the form of his new body. The human-Time Lord hybrid River Song in " Let's Kill Hitler " claimed she
12246-485: The Doctor spends centuries believing he burnt Gallifrey until the Eleventh Doctor's time; this is because the time streams are out of sync after the War Doctor meets his future selves and the altered events are only known to the Doctor once they occur in the Eleventh Doctor's experience and become part of his memory. Indeed, earlier on in the episode, both the Tenth and the Eleventh Doctors mistakenly believe that
12403-422: The Doctor there to begin with for questioning—and exile him in "Hell Bent" before running off again. Time Lords and human beings look alike, however they differ in many respects. Physiological differences from humans include two hearts which normally beat at 170 beats per minute, and a "respiratory bypass system" that allows them to survive strangulation. The Twelfth Doctor was able to survive direct exposure to
12560-455: The Doctor", he works together with them to change the assumed outcome of the Time War : thirteen incarnations of the Doctor team up together to freeze Gallifrey in time and place it outside of their universe (protecting it and the remaining Time Lords), while the Daleks destroy themselves in their own crossfire once Gallifrey is gone. The War Doctor does not retain the memory of these events and
12717-480: The Doctor's, Madame Vastra , reminds the Doctor that the Time Lord race developed due to their billions of years' exposure to the time vortex. The Doctor then recalls that Rory and Amy had spent their wedding night in the TARDIS; therefore it is theorised by Vastra that River's conception mirrored that of the Time Lords' genesis and therefore she herself developed Time Lord genetic characteristics. In " The Night of
12874-519: The Master uses the same technique, allowing the Tenth Doctor to hear the drumming sound the Master constantly hears. The Doctor later displays his telepathic communion powers in " Fear Her " and in " The Shakespeare Code ", where by using his mind melding technique he is partially able to relieve a man of his mental illness as he traces back through his memories. In " Planet of the Ood ", the Tenth Doctor seems able to temporarily confer some degree of telepathy on his companion Donna Noble, so that she can hear
13031-586: The Master's laser screwdriver, and telekinetically disarm the Master. In addition, Time Lords may be clairvoyant , or have additional time-related senses. In The Time Monster , and Invasion of the Dinosaurs , the Third Doctor is able to resist fields of slow time, being able to move through them even though others are paralysed . In City of Death , both the Fourth Doctor and Romana notice distortions and jumps in time that no one else does. In
13188-478: 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. 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
13345-478: The Sun. Descartes wrote that there was no means to prove the stars were not inhabited by "intelligent creatures", but their existence was a matter of speculation. When considering the atmospheric composition and ecosystems hosted by extraterrestrial bodies, extraterrestrial life can seem more speculation than reality, due to the harsh conditions and disparate chemical composition of the atmospheres, when compared to
13502-455: The Tenth Doctor explains to Wilfred Mott that a Time Lord can die before they have a chance to regenerate, in which case they die outright. In The Deadly Assassin at least one of the murders was carried out with a 'staser', possibly a weapon designed to both kill and prevent regeneration (stasers are also stated to have little effect on non-living tissue). In the Series 4 episode " Turn Left ",
13659-422: The Tenth Doctor's body is shown on a stretcher following the parallel events of " The Runaway Bride ". A UNIT officer states that the Doctor's death must have been too quick to allow for regeneration. In Destiny of the Daleks , Romana showed the ability to rapidly change form several times in a row during her first regeneration, and apparently had the ability to change into whatever appearance she desired. When
13816-411: The Time Lords are attempting to return to the universe and will come back if he speaks his true name. Due to "half the universe" surrounding Trenzalore, a siege begins that lasts centuries as the Doctor knows that if the Time Lords return, the Time War will begin anew. With the Doctor on the verge of death, Clara Oswald pleads with the Time Lords through the crack to intervene and save the Doctor. Through
13973-528: The Time Lords at the beginning of the series, but as no one else remembered this, it "might have come out of [his] dreams". Elaborating on this genesis in a 2014 interview in Doctor Who Magazine , Sherwin said of The War Games , "It was a case of what shall we do, how can we end this? Let's go back to the beginning and say [the Doctor] was a Time Lord, a renegade Time Lord, a pain in the arse for
14130-409: The Time Lords have the ability to grant more regenerations: at the behest of Clara Oswald they granted the Doctor himself a new cycle when he was at the point of death from old age, having used up his entire cycle. Regeneration, regardless of how many regenerations the individual Time Lord has already undergone, is a conditional and non-inevitable phenomenon. This is stated in " The End of Time " when
14287-463: The Time War. During the episode "The Doctor's Wife" it is revealed that several Time Lords and their TARDISes had been trapped and destroyed by an entity called House who lived in a separate bubble universe. In " A Good Man Goes to War " (2011), it is revealed that the daughter of Amy Pond and Rory Williams , Melody Pond (who later goes by her transliterated name " River Song "), has been born with Time Lord-like genetic traits. An old acquaintance of
14444-625: 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. The planetary context would also have an influence: a planet with higher gravity would have smaller animals, and other types of stars can lead to non-green photosynthesizers . The amount of energy available would also affect biodiversity , as an ecosystem sustained by black smokers or hydrothermal vents would have less energy available than those sustained by
14601-441: The War Doctor killed all of the Time Lords on the last day of the Time War. After being informed that the plan to save Gallifrey was successful, the Eleventh Doctor sets out to find Gallifrey and restore the Time Lords. In " The Time of the Doctor ", the Doctor visits the planet Trenzalore where he discovers a question being broadcast through all of time and space through a crack in the universe: "Doctor Who?" The Doctor realizes that
14758-483: The ability to stay conscious for moments after events that would outright kill other lifeforms instantly, giving them the opportunity to regenerate. This is seen in Logopolis (fall from a great height), and The Caves of Androzani (fatal disease). In both " The Stolen Earth " and " The Big Bang ", the Doctor is shot by a Dalek's energy weapon, which has almost always been shown to instantly kill any other lifeform, yet
14915-410: The area surrounding these hydrothermal vents which can suggest that some form of life can be supported even in the harshest of environments like the other planets in the solar system. The aspects of these harsh environments that make them ideal for the origin of life on Earth, as well as the possibility of creation of life on other planets, is the chemical reactions forming spontaneously. For example,
15072-568: The atom speeds, either too fast or too slow, make it difficult for specific ones to meet and start chemical reactions. A liquid medium also allows the transport of nutrients and substances required for metabolism. 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. Life based on ammonia rather than water has been suggested as an alternative, though this solvent appears less suitable than water. It
15229-581: The audience compared to the new story which can be told. Retcons can be diegetic or nondiegetic. For instance, by using time travel or parallel universes , an author may diegetically reintroduce a popular character they had previously killed off. More subtle and nondiegetic methods would be ignoring or expunging minor plot points to remove narrative elements the author doesn't have interest in writing. Retcons are common in pulp fiction , and especially in comic books by long-established publishers such as DC and Marvel . The long history of popular titles and
15386-465: The best name for it a few months back: 'Retroactive Continuity'. Has kind of a ring to it, don't you think?" Retcons sometimes add information that seemingly contradicts previous information. This frequently takes the form of a character who was shown to have died but is later revealed to have somehow survived. This is a common practice in horror films , which may end with the death of a monster that goes on to appear in one or more sequels. The technique
15543-404: The clone lives out his existence in a parallel universe with Rose Tyler . " The End of Time " (2009–10) shows the High Council of Time Lords, led by Lord President Rassilon , attempting to escape the Time War by materialising Gallifrey in the place of Earth at Christmas. However, the Tenth Doctor destroys the device which allows their passage into the present, sending them back into the events of
15700-402: The cold temperatures would make such chemistry take place at a very slow pace. Water is rock-solid on the surface, but Titan does have a subsurface water ocean like several other moons. However, it is of such a great depth that it would be very difficult to access it for study. The science that searches and studies life in the universe, both on Earth and elsewhere, is called astrobiology . With
15857-465: The crack, the Time Lords grant the Doctor a new regeneration cycle, saving his life before sealing the crack again. In " Hell Bent " (2015), it is revealed that Gallifrey returned to the universe around the time of its end. After escaping his confession dial in " Heaven Sent " (2015), still furious over the death of Clara Oswald in " Face the Raven " (2015), the Doctor manages to depose Rassilon—who had put
16014-469: The creation or maintenance of extraterrestrial life. However, there has been much historical evidence that some of the earliest and most basic forms of life on Earth originated in some extreme environments that seem unlikely to have harbored life at least at one point in Earth's history. 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
16171-444: The difficulty for life to appear was not the temperature, but the scarcity of free heavy elements. 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 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. Once
16328-455: The end of Season 8, to be still alive when actor Patrick Duffy wanted to return to the series. This season is sometimes referred to as the "Dream Season" and was referred to humorously in later TV series such as Family Guy as a "gas-leak year". Other series such as St. Elsewhere , Newhart , and Roseanne would notably employ the same technique. Unpopular stories are sometimes later ignored by publishers, and effectively erased from
16485-413: 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. Earth is a planet in the Solar System ,
16642-506: The fact to explain continuity errors. Such was the case in The Flintstones , where Wilma Flintstone was mistakenly given two separate maiden names over the course of the series: "Pebble" and "Slaghoople". Though the term "retcon" did not yet exist when George Orwell wrote Nineteen Eighty-Four , the totalitarian regime depicted in that book is involved in a constant, large-scale retconning of past records. For example, when it
16799-423: The first decade of the series. They later became integral to many episodes and stories as their role in the universe developed. For the first eight years after the series resumed in 2005, the Time Lords were said to have been destroyed during the Last Great Time War at some point in the show's continuity between the television movie in 1996 and the show's revival. In 2013, the 50th anniversary special " The Day of
16956-403: The first hominids appeared 60 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
17113-466: The first places that life could have originated on Earth. These environments can be considered extreme when compared to the typical ecosystems that the majority of life on Earth now inhabit, as hydrothermal vents are scorching hot due to the magma escaping from the Earth's mantle and meeting the much colder oceanic water. Even in today's world, there can be a diverse population of bacteria found inhabiting
17270-410: The formation of planets, eventually arriving on young planets early in their formation. 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 as we know it, as well as other organic molecules , some of which are yet to be better identified and understood. According to
17427-512: The galaxy". In the very distant past, the Time Lords fought a genocidal war against the Great Vampires, which led to such a catastrophic loss of life that the Time Lords renounced violence. In The War Games , the Second Doctor mentions that the Time Lords' "great powers" are hardly ever used due to their policy of non-intervention into the affairs of other planets, and that they instead observe and gather knowledge. Because of this, holding
17584-455: The giant planets themselves are highly unlikely to have life, there is much hope to find it on moons orbiting these planets. Europa , from the Jovian system, has a subsurface ocean below a thick layer of ice. Ganymede and Callisto also have subsurface oceans, but life is less likely in them because water is sandwiched between layers of solid ice. Europa would have contact between the ocean and
17741-404: 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 stars stellar evolution. The Big Bang took place 14 billion years ago, the Solar System was formed 4 and a half billion years ago, and
17898-571: The hierarchy of lifeforms from Earth for simplicity, as it is the only one known to exist. The first basic requirement for life is an environment with non-equilibrium thermodynamics , which means that the thermodynamic equilibrium must be broken by a source of energy. The traditional sources of energy in the cosmos are the stars, such as for life on Earth, which depends on the energy of the sun. However, there are other alternative energy sources, such as volcanoes , plate tectonics , and hydrothermal vents . There are ecosystems on Earth in deep areas of
18055-545: The history of the fictional world in which it was set. In the letters column , a reader remarked that the comic "must make you [the creators] feel at times as if you're painting yourself into a corner", and, "Your matching of Golden Age comics history with new plotlines has been an artistic (and I hope financial!) success." Writer Roy Thomas responded, "we like to think that an enthusiastic ALL-STAR booster at one of Adam Malin 's Creation Conventions in San Diego came up with
18212-440: The life-abundant Earth. However, there are many extreme and chemically harsh ecosystems on Earth that do support forms of life and are often hypothesized to be the origin of life on Earth. Hydrothermal vents , acidic hot springs, and volcanic lakes are examples of life forming under difficult circumstances, provide parallels to the extreme environments on other planets and support the possibility of extraterrestrial life. Since
18369-487: The line was not intended to be taken seriously and is instead a commentary. He said that the "thirteen lives" rule was too deeply entrenched in the viewer consciousness for his throwaway line to affect it. It is revealed in " The Time of the Doctor " that this was in fact false and that due to his various regenerations, the Eleventh Doctor was in fact his last incarnation. However, the Time Lords intervened through
18526-543: The medium. To create DNA , RNA , or a close analog, such an element should be able to bind its atoms with many others, creating complex and stable molecules. It should be able to create at least three covalent bonds: two for making long strings and at least a third to add new links and allow for diverse information. Only nine elements meet this requirement: boron , nitrogen , phosphorus , arsenic , antimony (three bonds), carbon , silicon , germanium and tin (four bonds). As for abundance, carbon, nitrogen, and silicon are
18683-409: The memories of her travels in the TARDIS and to 'implant' a defence mechanism which is activated in " The End of Time ". The War Games showed that other Time Lords are also able to erase people's memories, as in that story, Jamie and Zoe 's travels with the Doctor were erased from their memory, and the council of Time Lords also put a memory block on the Doctor so he could not pilot the TARDIS. In
18840-478: The mid-20th century, active research has taken place to look for signs of extraterrestrial life, encompassing searches for current and historic extraterrestrial life, and a narrower search for extraterrestrial intelligent life . Depending on the category of search, methods range from analysis of telescope and specimen data to radios used to detect and transmit communications. The concept of extraterrestrial life, and particularly extraterrestrial intelligence, has had
18997-572: The most abundant ones in the universe, far more than the others. On Earth's crust the most abundant of those elements is silicon, in the Hydrosphere it is carbon and in the atmosphere, it is carbon and nitrogen. Silicon, however, has disadvantages over carbon. The molecules formed with silicon atoms are less stable, and more vulnerable to acids, oxygen, and light. An ecosystem of silicon-based lifeforms would require very low temperatures, high atmospheric pressure , an atmosphere devoid of oxygen, and
19154-504: The native planet that may not be caused by natural causes. There are three main types of techno-signatures considered: interstellar communications , effects on the atmosphere, and planetary-sized structures such as Dyson spheres . Organizations such as the SETI Institute search the cosmos for potential forms of communication. They started with radio waves , and now search for laser pulses as well. The challenge for this search
19311-576: The new version included the name of the company (originally the Finmark Robot Corporation), new, earlier references to the Three Laws of Robotics , and new interpolated scenes featuring Dr. Calvin herself. The TV series Dallas annulled its entire Season 9 as just the dream of another character, Pam Ewing . Writers did this to offer a supposedly plausible reason for the major character of Bobby Ewing , who had died onscreen at
19468-404: The number of writers who contribute stories can often create situations that demand clarification or revision. Retcons also appear in manga , soap operas , serial dramas , movie sequels, cartoons , professional wrestling angles , video games , radio series, and other forms of serial fiction . They are used in role-playing games such as Dungeons & Dragons . An early published use of
19625-418: The ocean that do not receive sunlight, and take energy from black smokers instead. Magnetic fields and radioactivity have also been proposed as sources of energy, although they would be less efficient ones. Life on Earth requires water in a liquid state as a solvent in which biochemical reactions take place. It is highly unlikely that an abiogenesis process can start within a gaseous or solid medium:
19782-568: The origin of extraterrestrial life. The Solar System has a wide variety of planets, dwarf planets, and moons, and each one is studied for its potential to host life. Each one has its own specific conditions that may benefit or harm life. So far, the only lifeforms found are those from Earth. No extraterrestrial intelligence other than humans exists or has ever existed within the Solar System. Astrobiologist Mary Voytek points out that it would be unlikely to find large ecosystems, as they would have already been detected by now. The inner Solar System
19939-426: The original 1919 novel, The Curse of Capistrano , Zorro ends his adventures by revealing his identity, a plot point that was carried over to the 1920 film adaptation The Mark of Zorro . In order to have further stories starring Zorro, author Johnston McCulley kept all the elements of his original story, but retroactively ignored its ending. One notable example is Isaac Asimov ’s 1950 fixup novel I, Robot ,
20096-436: The other Time Lords who stole his TARDIS and buggered off around the universe. So if he's going to be called to book let's bring in the Time Lords." Early on in the series, the Doctor was identified as a human being; however, his home planet, which from the start of the series is explicitly established as not being Earth , was not named. In The War Games (1969), the Doctor's people appeared, who from then on are known as
20253-468: The others are used in other biological processes. The studies ruled out pollution of the meteorites on Earth, as those components would not be freely available the way they were found in the samples. This discovery suggests that several organic molecules that serve as building blocks of life may be generated within asteroids and comets. In October 2011, scientists reported that cosmic dust contains complex organic compounds ("amorphous organic solids with
20410-401: The phrase "retroactive continuity" is found in theologian E. Frank Tupper's 1973 book The Theology of Wolfhart Pannenberg : "Pannenberg's conception of retroactive continuity ultimately means that history flows fundamentally from the future into the past, that the future is not basically a product of the past." A printed use of "retroactive continuity" referring to the altering of history in
20567-553: The planetary core ceased to generate a magnetic field, solar winds removed the atmosphere and the planet became vulnerable to solar radiation. Ancient life-forms may still have left fossilised remains, and microbes may still survive deep underground. As mentioned, the gas giants and ice giants are unlikely to contain life. The most distant solar system bodies, found in the Kuiper Belt and outwards, are locked in permanent deep-freeze, but cannot be ruled out completely. Although
20724-511: The precise differences that lead to beneficial or harmful conditions for life. And despite the conditions against life on Venus , there are suspicions that microbial life-forms may still survive in high-altitude clouds. Mars is a cold and almost airless desert, inhospitable to life. However, recent studies revealed that water on Mars used to be quite abundant, forming rivers, lakes, and perhaps even oceans. Mars may have been habitable back then, and life on Mars may have been possible. But when
20881-456: The probability of the existence of extraterrestrial civilisations and the lack of evidence for such civilisations is known as the Fermi paradox . Dennis W. Sciama claimed that life's existence in the universe depends on various fundamental constants. 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
21038-476: The probe to study ocean water. Still, Cassini detected complex organic molecules, salts, evidence of hydrothermal activity, hydrogen, and methane. Titan is the only celestial body in the Solar System besides Earth that has liquid bodies on the surface. It has rivers, lakes, and rain of hydrocarbons, methane, and ethane, and even a cycle similar to Earth's water cycle . This special context encourages speculations about lifeforms with different biochemistry, but
21195-438: The problems of survival of life on other planets. However, he also considered that the conditions found on Mars were incompatible with carbon based life. Even if extraterrestrial life is based on carbon and uses water as a solvent, like Earth life, it may still have a radically different biochemistry . Life is generally considered to be a product of natural selection . It has been proposed that to undergo natural selection
21352-433: The researchers, "these [newly discovered] compounds could potentially support extant microbial communities or drive complex organic synthesis leading to the origin of life ." Although most searches are focused on the biology of extraterrestrial life, an extraterrestrial intelligence capable enough to develop a civilization may be detectable by other means as well. Technology may generate technosignatures , effects on
21509-426: 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 meteoroids , for example – between habitable planets in a process called panspermia . 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
21666-405: The right side of the equation are agreed as speculative and open to substitution: 10,000 = 5 ⋅ 0.5 ⋅ 2 ⋅ 1 ⋅ 0.2 ⋅ 1 ⋅ 10,000 {\displaystyle 10{,}000=5\cdot 0.5\cdot 2\cdot 1\cdot 0.2\cdot 1\cdot 10{,}000} The Drake equation has proved controversial since, although it is written as
21823-422: The rocky surface, which helps the chemical reactions. It may be difficult to dig so deep in order to study those oceans, though. Enceladus , a tiny moon of Saturn with another subsurface ocean, may not need to be dug, as it releases water to space in eruption columns . The space probe Cassini flew inside one of these, but could not make a full study because NASA did not expect this phenomenon and did not equip
21980-532: The same samples suggests that a non-biological reaction is a more likely hypothesis. In February 2005 NASA scientists reported they may have found some evidence of extraterrestrial life on Mars. The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center , based their claim on methane signatures found in Mars's atmosphere resembling the methane production of some forms of primitive life on Earth, as well as on their own study of primitive life near
22137-657: The scanners being programmed to "detox" humans and therefore being unaware of what elements the Doctor requires. A Time Lord is able to conceal their Time Lord nature, and become a human, by using the Chameleon Arch – a device that stores their "essence" and memories in an innocuous device such as a fob watch , and replaces them with false counterparts until the object is later re-opened. The process allows them to disguise themselves as humans physiologically and psychologically, meaning they only have one heart and are stripped of non-human powers, and of any memory of having been
22294-399: 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
22451-482: The stellar evolution of stars and planets, it is usually not taken into account by astrobiology. 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
22608-508: The study of Earth's life, the only known form of life, astrobiology seeks to study how life starts and evolves and the requirements for its continuous existence. This helps to determine what to look for when searching for life in other celestial bodies. This is a complex area of study, and uses the combined perspectives of several scientific disciplines, such as astronomy , biology , chemistry , geology , oceanography , and atmospheric sciences . The scientific search for extraterrestrial life
22765-539: The survival and possible origin of life on Earth . With the little amount of information that scientists have found regarding the atmosphere on other planets in the Milky Way galaxy and beyond, the atmospheres are most likely reducing or with very low oxygen levels, especially when compared with Earth's atmosphere. If there were the necessary elements and ions on these planets, the same carbon fixing, reduced chemical compounds occurring around hydrothermal vents could also occur on these planets' surfaces and possibly result in
22922-471: The telepathic song of the Ood. When she is unable to bear the song, the Doctor removes the ability. In " The Lodger ", the Eleventh Doctor (pressed for time and needing to convey a great deal of information to someone) smashed his forehead into another person's forehead, causing a massive instantaneous transfer of information. The Doctor also contacts the Time Lords by going into a trance and creating an assembling box in The War Games . In The Two Doctors ,
23079-636: The tendency of technologically advanced human societies to enslave or destroy less advanced societies – argue it may be dangerous to actively draw attention to Earth. Initially, after the Big Bang the universe was too hot to allow life. 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,
23236-486: The time) as having bright, silver-leafed trees and a burnt orange sky at night in the serial The Sensorites (1964). This casts an amber tint on anything outside the city, as seen in The Invasion of Time . However, Gallifrey's sky appears blue and Earth-like in " The Five Doctors " (1983) within the isolated Death Zone. Extraterrestrial life Extraterrestrial life , or alien life (colloquially, alien ),
23393-509: The twelve regeneration limit. The Master says in " The Sound of Drums " that the Time Lords "resurrected" him to fight in the Time War . It was revealed in The Brain of Morbius that the Time Lords also use the Elixir of Life in extreme cases, where regeneration is not possible. It is confirmed in " The Time of the Doctor " that a Time Lord can normally regenerate only twelve times but that
23550-608: The universe. The science of extraterrestrial life is known as astrobiology . Speculation about the possibility of inhabited worlds beyond Earth dates back to antiquity. Early Christian writers discussed the idea of a "plurality of worlds" as proposed by earlier thinkers such as Democritus ; Augustine references Epicurus 's idea of innumerable worlds "throughout the boundless immensity of space" in The City of God . Pre-modern writers typically assumed extraterrestrial "worlds" are inhabited by living beings. William Vorilong , in
23707-445: The unusual properties of the meteorite were eventually explained as the result of inorganic processes, the controversy over its discovery laid the groundwork for the development of astrobiology. 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. Lack of corroborating evidence from other experiments on
23864-565: The vacuum of space in " Oxygen ". Time Lords also seem to have an increased resilience to higher frequencies of sound, as seen in " The Christmas Invasion " and " Partners in Crime ". If severely injured, Time Lords can go into a healing coma which lowers their body temperature to below freezing which the Doctor did in Planet of the Daleks . In the serial Destiny of the Daleks , Romana was able to voluntarily stop both of her hearts beating, to fool
24021-553: The worlds were sealed. Everything became that bit less kind." In " The Satan Pit " (2006), the Tenth Doctor states that his people "practically invented black holes. Well, in fact, they did." Both the Beast (in "The Satan Pit") and the Doctor (in "The Sound of Drums" and " The Doctor's Wife ", 2011) believe the Doctor ended the War by killing all of the Time Lords and many of the Daleks. The Tenth Doctor's artificially created "daughter" Jenny
24178-425: Was "focusing on a dress size", but subsequently weighed herself, seeming unsure of how her new body had truly developed. The Doctor said on several occasions he wished he was "ginger", which he has seemed unable to control in previous regenerations. In " Last of the Time Lords ", when the Master is fatally wounded, he chooses not to regenerate, essentially committing suicide rather than regenerate and be kept prisoner by
24335-417: Was habitable. It is unclear if life and intelligent life are ubiquitous in the cosmos or rare. The hypothesis of ubiquitous extraterrestrial life relies on three main ideas. The first one, the size of the universe allows for plenty of planets to have a similar habitability to Earth, and the age of the universe gives enough time for a long process analog to the history of Earth to happen there. The second
24492-656: Was lying about at least some of this information to mislead the Sontarans . At the beginning of The Trial of a Time Lord , the Sixth Doctor suggests that a number of elder Time Lords were able to use their combined mental energy to summon his TARDIS against his will. In the episode " Cold Blood ", the Eleventh Doctor experiences excruciating pain when the Silurian attempts to decontaminate him of surface bacteria. The Doctor states this would kill him, most likely due to
24649-399: Was reported. Scientists search for biosignatures within the Solar System by studying planetary surfaces and examining meteorites . Some claim to have identified evidence that microbial life has existed on Mars. In 1996, a controversial report stated that structures resembling nanobacteria were discovered in a meteorite, ALH84001 , formed of rock ejected from Mars . Although all
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