Throughout history, interest in the search for life on other worlds have waxed and waned. Yet many exciting developments have emerged over the past decades in the search for planetary-sized bodies in nearby stars, and this interest has never been greater.

What is considered a habitable planet? Since liquid water is essential for supporting all forms of life, it is crucial that conditions on a planet allow water to exist in liquid form for it to be considered suitable for supporting life. If the planet is too close to the star, water is unable to condense; conversely if the planet is too far from the star, water will forever be frozen. And the area in between? This sweet spot is known as the habitable zone of a star.

The habitable zone, or sometimes referred to as the Goldilocks zone, is broadly defined as the annulus around a star in which the flux of stellar energy allows a terrestrial planet’s surface to remain at the right temperature for liquid water to exist. In other words, the average temperature of the planetary surface must remain from zero to 100 degrees Celsius. This also means that the habitable zones must be different around stars of different spectral types. Depending on the type of star, the habitable zone is likely to be different.

Earth – fortunately for us – exists within the habitable zone of our own sun and fulfils all necessary conditions which allow water to exist in liquid form. However, being in the habitable zone doesn’t necessarily mean that it will be able to support life. Other strong mitigating factors, usually related to the atmosphere of the planet, can create temperature differences which make a planet within the habitable zone of a star uninhabitable. The planets Mars and Venus in our own solar system are cases in point. For example, while Venus lies within the habitable zone of our sun, its thick atmosphere traps solar radiation and creates a greenhouse effect that sends temperatures on the planet soaring, rendering the planet uninhabitable. Mars, on the other hand, poses a different set of problems. While Mars was supposed to have been outside the habitable zone, there is now good evidence that running water once existed on the Red Planet. What all these means is that it is very challenging for scientists and researchers to potentially find habitable planets, much less Earth-like ones.

Of course, the overriding assumption here is that life as we know it – carbon-based life forms which require oxygen and methane for survival – requires liquid water for survival. What happens if that assumption is wrong? Could there be extraterrestrial life out there which does not depend on water for survival?