In looking for other planets that might have life, astronomers focus especially on finding planets that

Liquid water

Explanation:

It is possible that life exists on other planets. Earth is not unique to sustain life. Life may not exist in multi-cellular form but unicellular only just like when life began on Earth.

As water is very important of every life form on Earth. The planet must have such conditions to maintain water in liquid form. Unicellular organisms are composed of water majorly. Thus, when astronomers are searching for life, they focus on finding liquid water.

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