Various researchers have claimed that there could be 40 billion to 100 billion habitable planets in the Milky Way but recent research throws cold water on that:
These results are certainly encouraging, since they suggest that the Milky Way could be teeming with life. Unfortunately, more recent research into extra-solar planets has cast doubt on these previous estimates. This is especially the case where tidally-locked planets that orbit M-type (red dwarf) stars are concerned.
In addition, research into how life evolved on Earth has shown that water alone does not guarantee life – nor, for that matter, does the presence of oxygen gas. Further to this, Schwieterman and his colleagues considered two other major biosignatures that are essential to life as we know it – carbon dioxide and carbon monoxide. Matt Williams, “Complex Life Might Require a Very Narrow Habitable Zone” at Universe Today
He goes on to raise many additional issues as well.
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See also: Astrophysicist: Alien Hunters, Please Stop Using The Drake Equation