If a planet needs to remain in the habitable zone of a sun and fulfil all necessary planetary conditions so as to support life, what conditions on such a planet is required to bring forth intelligent life-forms?

We cannot be sure how likely life-forms can emerge on other worlds. We only know that the evolution of even the simplest creatures on Earth appears to follow so random and unlikely a sequence of events, and that it makes evolution of complex life elsewhere seem very unlikely.

The six most abundant elements in the cosmos are hydrogen, helium, oxygen, carbon, neon and nitrogen – unsurprisingly the chemical make-up of life on Earth is based on the cosmic abundance of these elements. Since life is such a complex phenomenon – and we know that matter tends to form more and more complex structures – it is inevitable that complex molecules such as carbon and silicon form the core of even larger complex molecules.

For Earth, the availability of water is what makes life possible – water, as a solvent, is able to facilitate different forms of chemical reactions. However, could other forms of chemistry be able to sustain alternative forms of life? For example, could life flourish at extreme ends of the temperature range if another complex molecule other than water acts as a solvent to react for an alternative world chemistry?

Any such debate is mere speculation at this time. It is even more futile to speculate what forms of alternative technology could be produced by intelligent life-forms if they were not carbon-based, or either living in ultra-cold or ultra-hot environments.

Science fiction authors are possibly the most speculative of all when it comes to this topic, and the exploration of other worlds is one of the most enduring themes of science fiction. During the budding years of science fiction, Mars was the most romanticised and written-about planet in our solar system. Similarly, Venus was a popular topic during the early-to-mid 20th century.

Not stopping at fantasising about the intrasolar planets, science fiction authors have also created thousands of fictional planets, following some basic scientific principles that help them explain the forms extraterrestrial life might take on these planets. Such planets either explore unusual social environments – primitive cultures, political or religious extremes, pseudo-medieval societies – or contain unusual physical environments such as single-climate planets in artic, jungle, desert or barren worlds.