Earth’s history offers a model of life under an orange sky
The Earth is a decidedly blue planet. The concentration of oxygen in our atmosphere is not only critical to respiration, but also scatters light from the sun to give us a blue sky, both from the ground and from space. This fact not only makes for some amazing views of our home, but also gives us a shortcut to find potential homes of other lifeforms as well. In our search of exoplanets resembling Earth, it makes sense that we look for more blue planets harboring oxygen in the same way. However, this may be limiting our search too much, as data suggest that the Earth hasn’t always been blue. Two and a half billion years ago, our sapphire globe was actually orange, because the atmosphere had not yet had time to develop into the air we breath today.
Before we were blue
The orange from Earth’s youth was thanks to cyanobacteria and their methane waste. The methane would built up in the atmosphere and be broken up by energy from the sun, resulting in a thick layer of orange hydrocarbons. Cyanobacteria aren’t the only reason such a hyrdrocarbon layer could exist, as Saturn’s moon Titan displays a similar hue without expectations of life existing there. However, this model for life existing under an orange sky means that we may want to widen our search for Earth-like planets in the universe. Orange planets might not be move-in-ready to the same degree as an oxygen rich planet, but it may still be home for living things.
Fortunately, searching for orange planets doesn’t mean we’ll suddenly have a bunch of red herrings from unsuitable orange planets. Looking at the ratio light reflected from carbon dioxide to methane should help us sort between orange atmospheres from biological versus geological sources. This is the same analysis that gives us little reason to expect cyanobacteria on a place like the aforementioned Titan.
Young universes were blue
A young, orange planet is of course counter to the color-coding of the universe at large, particularly stars. It’s estimated that while the universe has an average color of beige these days, earlier on hotter-burning stars were casting more blue light. As a star cools, the amount of energy it exudes decreases, and the visible light emitted shifts down the color spectrum, with red hot suns actually being the coolest, oldest stars. The comparison to orange planets isn’t great though, because the color on the planets is coming from molecules scattering light, not the original light from the star itself.
My first grader asked: Are white stars the hottest?
While it might seem like white, as combination of the full spectrum of visible light, should encompass and therefore exceed the properties of blue light, that’s not the case here. White stars burn hotter than warmer hues, but blue stars are the hottest, partly thanks to the changes in what fuel is being burned in the star.
Source: Could 'Pale Orange Worlds' Lead Us to Alien Life? by Irene Klotz, Discovery News