As of 2014, there is no single piece of concrete evidence to prove the existence of alien life, although most scientists today agree that the potential of discovering life outside Earth is much greater than previously thought. According to Discovery News, alien life is considered highly probable and will likely be discovered by the year 2024.
Fictional accounts of aliens found in mainstream media aren't considered an accurate representation of alien life. Some scientists believe that aliens will be simple life forms such as bacteria or single celled organisms while others believe that more complex forms of life exist. Within the Milky Way galaxy, there are several planets that may be able to support some type of life.
NBC News has stated that over half of the galaxies visible from Earth's telescopes may have planets and solar systems similar to our own. Scientists often determine the probability of alien life by examining the availability of water on other planets. Currently, there are several planets that are thought to have some form of water available, including Mars and Titan, one of Saturn's moons.
The debate of whether alien life exists is hinged on the lack of definite proof of other species outside Earth. According to New Scientist, there are some examples that point to irrefutable evidence. For example, methane has been found on Mars. The presence of methane is linked to the metabolic process that occurs when life forms process organic compounds for energy. This evidence is controversial and has only been accepted as proof of alien life by a portion of scientists.