E.T. may actually live in a world that is nothing like Earth. Astronomers are now thinking of expanding the habitability zone of a planet to include a wider set of criteria.

For a long time the idea that only Earth-like planets can host life has been the standard for astronomers in the search for alien life. Now that there are already more than 700 exoplanets discovered and of that number only a few are in the so-called habitable zone, scientists are saying that limiting the search for extraterrestrial life in Earth-like planets could exclude other possibilities of alien life.

For scientists like Dirk Schulze-Makuch, an astrobiologist with the Washington State University School of Earth and Environmental Sciences and Abel Mendez, a modeling expert from the University of Puerto Rico, the narrow model for life-sustaining planets should be replaced by a wider criterion of what constitutes the habitable zone.

The concept of the habitable zone was popularized by astronomer Michael Hart in the 1970s. According to Hart the Earth was the only possible home for life in the galaxy. The habitable zone or the Goldilocks zone is the zone where a planet is just hot enough for liquid water to exist on the surface.

"The first question is whether Earth-like conditions can be found on other worlds, since we know empirically that those conditions could harbor life," Schulze-Makuch said. "The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not."

Life as we know it can't exist without water. However, extremeophiles that can survive in the most severe environments have made scientists rethink their definition of life. Life could actually survive in other parts of the galaxy that doesn't have water.

The new index that Schulze-Makuch and his co-authors propose is divided into two different indices- an Earth Similarity Index (ESI) for categorizing a planet's more earth-like features and a Planetary Habitability Index (PHI) for describing a variety of chemical and physical parameters that are theoretically conducive to life in more extreme, less-earthlike conditions.

Alien life could be based on other elements than carbon and might not even need a water-based environment. Life could adapt to use whatever liquid at hand. Under the proposed index, planets could get a low ESI score but still rate a better PHI score. This will give scientists are a more complex view of planetary habitability.