Question: Is our search for life elsewhere too narrowly defined? ‘‘When scientists say they are looking for ‘life’, or the conditions for ‘life’ on Mars or elsewhere in the universe, why do they assume that ‘life’ anywhere else is going to be similar to what it is on Earth, when alien life forms would have developed in environments completely different from ours and therefore not require what we assume to be substances ‘essential for life’?’’

You are right that we do not have a good idea about the variety of life that could exist elsewhere.

Scientists have speculated that some life in the universe could be based on silicon rather than carbon, but in the universe, carbon is about 15 times more abundant than silicon. Maybe some life forms are not based on water, but are based on other liquids such as the methane/ethane lakes of Titan.

NASA’s Mars rover 'Opportunity'. Credit:EPA/NASA

You are right that most scientists begin with what they are most familiar with: water and carbon-based life. We make this assumption not because we think this is the only possibility, but because this is what we know how to look for.

You look under the lamp post for your keys not because that is the only place you could have lost them, but because that is the only place where you can see anything. As scientists, our job is to invent new instruments and move the lamp post so we can look further afield.