It is not if, but when scientists really can claim that they have found extra-terrestrial life. The Hubble telescope is the key.
When scientists studied the planet named HD189733b through the Hubble telescope they also found the key on how they shall find life out in space
 
For the very first time researchers have managed to identify an organic molecule on another planet. The planet, HD189733b are located some 63 light years from earth, in the Vulpecula constellation. There they found that light which had passed through this planets atmosphere contained traces of metan, carbon monoxide and water wapour. These molecules are all close related to life on earth.

This does not indicate that this planet have life, since it is extremely hot. Surface temperature is 700 degrees Celsius. But the spectroscopic methods allows researchers to look for certain chemical components in other planets atmospheres.  When they do find a planet with atmosphere containing water vapour, free oxygen and also metan, then it is a significant clue that life exists on that planet. Hubble will likely produce a lot of exoplanet weather maps in the future.

The observable universe are thought to contain about 80 billion galaxies. More or less. It would be a miracle if earth is the only planet which contains life.

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Image Credit: NASA, ESA, and G. Bacon (STScI)