One in Five Stars Has Earth-sized Planet in Habitable Zone

[Xithras]

57. Yep. "Habitable" has a different meaning for scientists than for "normal people".

The planet can have a sulfuric acid atmosphere, be tide locked to its star and have 500 degree summer days, and scientists will call it "habitable" if its the right size, type, and in the right location.

"Earth-like" simply means that it's habitable AND has liquid water. Again, that doesn't mean much. The Earths atmosphere was almost exclusively Nitrogen and Carbon Dioxide until a couple billion years ago, and would have stayed that way if not for the evolution of cyanobacteria. Those cyanobacteria, an evolved life form unique to our own planet, created all of the oxygen that we breathe today and sequestered much of the carbon dioxide in our atmosphere. If they hadn't existed, our planet would look like Mars with oceans, and would probably lack any life more advanced than simple algae.

The evolution of cyanobacteria, and the fact that their photosynthesis resulted in the creation of free oxygen, were the result of evolutionary processes on our own planet. The odds of those exact evolutionary processes being duplicated on another planet, and of a planet having a human-breathable oxygen atmosphere as a result, are incredibly small. There probably aren't more than a handful of them in our entire galaxy, and there's a substantial possibility that we are standing on the ONLY one.