How can we know if we're looking at habitable exo-Earths or hellish exo-Venuses?
The differences between Earth and Venus are obvious to us. One is radiant with life and adorned with glittering seas, and the other is a scorching, glowering hellhole, its volcanic surface shrouded by thick clouds and visible only with radar. But the difference wasn’t always clear. In fact, we used to call Venus Earth’s sister planet.
source: Phys.org