Extraterrestrial life is defined as life that does not originate from Earth. Referred to as alien life, or simply aliens, these hypothetical forms of life range from simple bacteria-like organisms to beings far more advanced than humans.
Alien life, has been theorized to exist in the Solar System and quite possibly throughout the Universe. This theory relies on the vast size and consistent physical laws of the observable Universe.
No samples of extraterrestrial life have been found. However, various controversial claims have been made for evidence of extraterrestrial life. Beliefs that some unidentified flying objects are of extraterrestrial origin along with claims of alien abduction are dismissed by most scientists. Most UFO sightings are explained either as sightings of Earth-based aircraft or known astronomical objects,
Several theories have been proposed about the possible basis of alien life from a biochemical, evolutionary or morphological.
All life on Earth is based upon 26 chemical elements. However, about 95% of this life is built upon only six of these elements : carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur; abbreviated CHNOPS by astrobiologists Carbon is fundamental to 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 oxidation of glucose releases biochemical energy needed to fuel all other biochemical reactions. Due to their relative abundance and usefulness in sustaining life, many have hypothesized that life forms elsewhere in the universe would utilize these basic materials. However, other elements and solvents could provide a basis for life.
Many have considered evolution and morphology is one basis for the extraterrestrial life. Alien describe as having light green or grey skin, with a large head, as well as four limbs fundamentally humanoid. Intelligent aliens could communicate through gestures, as deaf humans do, or by sounds created from structures unrelated to breathing, which happens on Earth when, for instance, cicadas vibrate their wings, or crickets rub their legs. 
Some scientist believes that the aliens came to earth to visit. They proposed that if earth can provide life, why not the other planets outside the solar system that has been discovered just recently. Many scientists tried to win their proposition but the majority of the science community believes that extraterrestrial life does not exist not unless they would be able to witness it in action.
