Alien life may flourish on purple planets
"Find a purple planet, and you may have spotted alien life. Some of the first Earthlings were purple bacteria that ruled the planet about 3 billion years ago. If any Earth-like exoplanets host similar microbes, their distinctive hue will be visible from space."
Characterizing the purple Earth: Modelling the globally-integrated spectral variability of the Archean Earth
"The ongoing searches for exoplanetary systems have revealed a wealth of planets with diverse physical properties. Planets even smaller than the Earth have already been detected, and the efforts of future missions are placed on the discovery, and perhaps characterization, of small rocky exoplanets within the habitable zone of their stars."
Purple Bacteria on Earth Could Survive Alien Light
"Purple bacteria contain pigments that allow them to use sunlight as their source of energy, hence their color. Small as they are, these microbes can teach us a lot about life on Earth, because they have been around longer than most other organisms on the planet."