text: aliens means extra terrestrial life\n\nExtraterrestrial life is life that may exist and originate outside the planet Earth, the only place in the universe currently known by humans to support life. Its existence is currently hypothetical; there is yet no evidence of extraterrestrial life that has been widely accepted by the scientific community.\n\nMost scientists hold that if extraterrestrial life exists, its evolution would have occurred independently in different places in the universe. An alternative hypothesis, held by a minority, is panspermia, which suggests that life in the universe could have stemmed from a smaller number of points of origin, and then spread across the universe, from habitable planet to habitable planet. These two hypotheses are not mutually exclusive.\n\nSpeculative forms of extraterrestrial life range from sapient beings seen in works of science fiction to life at the much smaller scale of bacteria.\n\nExtraterrestrial life forms, especially intelligent ones, are often referred to in popular culture as aliens or ETs. The putative study and theorisation of ET life is known as astrobiology or xenobiology.\n\n\nPossible basis of extraterrestrial life\n\n[edit] Biochemistry\n\n    Main articles: Biochemistry and Alternative biochemistry\n\nAll life on Earth is based on the building block element carbon with water as the solvent in which bio-chemical reactions take place.\n\n    * Water is useful because it has a neutral pH, and due to its continued dissociation between hydroxide and hydronium ions.\n\n        2H2O → H3O+ + OH-\n\n    As a result, it can dissolve both positive metallic ions and negative non metallic ions with equal ability. Furthermore, the fact that organic molecules can be either hydrophobic (repelled by water) or hydrophilic (soluble in water) creates the ability of organic compounds to orient themselves to form water enclosing membranes. The fact that solid water (ice) is less dense than liquid water also means that ice floats, thereby preventing the Earth's oceans from slowly freezing solid. Additionally, the Van der Waals forces between water molecules gives it an ability to store energy with evaporation, which upon condensation is released. This helps moderate climate, cooling the tropics and warming the poles, helping to maintain a thermodynamic stability needed for life.\n\n    * Carbon is fundamental to conventional terrestrial life for its immense flexibility in creating covalent chemical bonds with a variety of non-metallic elements, principally nitrogen, oxygen and hydrogen. Carbon dioxide and water together enable the storage of solar energy in sugars, such as glucose. The oxidisation of glucose releases biochemical energy needed to fuel all other biochemical reactions\n\n        6CO2 + 6H2O + sun energy → C6H12O6 + 6O2\n        C6H12O6 + 6O2 → 6CO2 + 6H2O + biochemical energy\n\n    The ability to form organic acids (-COOH) and amine bases (NH2-) gives it the possibility of neutralisation dehydrating reactions to build long polymer peptides and catalytic proteins from monomer amino acids, and with phosphates to build not only DNA, the information storing molecule of inheritance, but also adenosine triphosphate (ATP) the principle energy "currency" of cellular life.\n\nGiven their relative abundance and usefulness in sustaining life it has long been assumed that life forms elsewhere in the universe will also utilize these basic components. However, other elements and solvents might be capable of providing a basis for life. Silicon is usually considered the most likely alternative to carbon, though this remains improbable. Silicon life forms are proposed to have a crystalline morphology, and are theorized to be able to exist in high temperatures, such as planets closer to the sun. Life forms based in ammonia rather than water are also considered, though this solution appears less optimal than water.[1]\n\nIndeed, technically life is little more than any s