Planetary habitability is the measure of a planet’s or a natural satellite’s potential to develop and sustain life. Habitable environments do not need to contain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia. As the existence of life beyond Earth is unknown, planetary habitability is largely an extrapolation of conditions on Earth and the characteristics of the Sun and Solar System which appear favorable to life’s flourishing—in particular those factors that have sustained complex, multi-cellular organisms and not just simpler, unicellular creatures. Research and theory in this regard is a component of planetary science and the emerging discipline of astrobiology.
An absolute requirement for life is an energy source, and the notion of planetary habitability implies that many other geophysical, geochemical, and astrophysical criteria must be met before an astronomical body can support life. In its astrobiology roadmap, NASA has defined the principal habitability criteria as “extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism.”
In determining the habitability potential of a body, studies focus on its bulk composition, orbital properties, atmosphere, and potential chemical interactions. Stellar characteristics of importance include mass and luminosity, stable variability, and high metallicity. Rocky, terrestrial-type planets and moons with the potential for Earth-like chemistry are a primary focus of astrobiological research, although more speculative habitability theories occasionally examine alternative biochemistries and other types of astronomical bodies.
There are currently 33 planets currently identified that can potentially sustain life. Of these 33 planets, 22 of them are much larger than Earth – the other 11 are relatively the same size as Earth. Scientists are still finding new planets capable of sustaining life, even as recent as May 2nd, 2016, where three planets were found (Click here for info).
All planets identified are categorized with quite a large amount of information on them, this will be as close as we could possibly get to the new frontier for a long time, but science will, hopefully, someday allow us to visit these new worlds.