Science fictions gave us a wrong idea about aliens. Similarly, NASA gave us a wrong idea about habitable planet. None of these words are as we commonly know it.
Alien : This is something like foreigner. Foreigner is someone who comes from another country. It doesn’t matter how he/she looks like. Similarly, an alien is a living creature that comes from a different planet. These creatures do not necessarily look like what we see on Hollywood movies. An alien may look like an octopus, may look like an insect, may look like a plant or may even be invisible.
Habitable : For around fifty years NASA is searching for habitable planets to find aliens on that. Their definition of habitable planet is, a planet that looks like earth. The word habitable actually means – suitable for living. Bring a fish out of water and see how long it can live. Try to keep yourself underwater for ten minute (without oxygen). What is habitable for you, can kill a fish in a minute. There are some beetles that live their entire life around dung. Billions of bacteria live their entire life inside your mouth. Habitable is different for different creatures. There is NO ideal habitable.
Some aliens may breath nitrogen, some aliens may drink acid, some aliens may feel comfortable in fire. Aliens may be of any shapes or sizes. Alien may live in any type of environment. It is a stupidity, to search earth-like planets to find aliens on it. Aliens may live on any planet. But, if you find any alien on an earth-like planets, they would be more likely to look like creatures on earth. They may not look like human, but at least may look like some animals on earth.
SM Élite 