| Earth received its sweetest valentine on Feb. 14, 1990, when this planet's most distant traveler — the Voyager 1 spacecraft — looked back from a point past the orbit of Neptune and took one last picture of home. In that famous photograph, Earth is just a "pale blue dot," Carl Sagan would later write. Everything that makes our world so wonderful — its vast oceans, its breathable atmosphere — is represented by a single azure pixel on a field of impenetrable black. But, had Voyager taken this photograph 3 billion years ago, the dot would have looked entirely different. That's according to research from NASA Goddard astrobiologist Giada Arney. During an era known as the Archean, from 4 billion to 2.5 billion years ago, photosynthetic organisms had not yet taken over the Earth, filling the atmosphere with oxygen. Instead, the planet was probably shrouded in a haze of organic molecules that scattered orange light, making our planet appear pale apricot. In fact, if evolution had taken a different turn, our planet might have become lilac instead of blue. Research suggests that many organisms in Earth's early oceans relied on a purple pigment for photosynthesis, rather than green chlorophyll. It appears that chlorophyll-based photosynthesis was more efficient, since green plants now dominate the planet. But the descendants of those plum-colored organisms can still be found in places where oxygen is scarce, like certain salty lakes or the murky bottom of a flower vase that has been left out too long. (Weird side note: Most of the light emitted by the sun is actually in the blue-green part of the electromagnetic spectrum. But chlorophyll reflects, rather than absorbs, most of this light — that's why plants look green. Why are plants not using the most abundant type of light in our sky? It may be a form of protection, preventing fragile compounds in the plant from being broken down by too much energy.) The pale orange and purple dots don't just represent what was, or what could have been; they are a testament to what still might be, elsewhere in the universe. In the next 10 years, scientists studying exoplanets will gain access to telescopes capable of picking out the pinprick of a planet amid the glare of its star. The images they capture will look much like the photograph from Voyager: just a single pixel of light in the middle of so much darkness. But the color of that light can say a lot. The exoplanets that scientists eventually see might glow orange, like early Earth, or purple, like the Earth-that-might-have-been. Or maybe they use another photosynthetic pigment entirely. Researchers can imagine a world orbiting a faint sun, on which every photon of light is precious. This planet might host plants that absorb every wavelength of visible light, making their leaves black (such plants do exist on Earth — like the black-leafed Ophiopogon planiscapus). Scientists can learn even more about an exoplanet by separating its light out into its component parts — a technique called spectroscopy. The molecules in an atmosphere absorb light at signature wavelengths, and researchers can determine the composition of a body by seeking out those gaps in its spectrum. For example, had Voyager conducted spectroscopy on its image of Earth, it would have found the hallmarks of water vapor and ozone. By looking at spectra taken by satellites in orbit around Earth, scientists are also able to tell that the amount of carbon dioxide in our atmosphere is increasing. For a cautionary tale about what happens to a planet with too much carbon dioxide, you need only look to the pale yellow dot of Venus. That world's CO2 and sulfuric acid atmosphere traps heat, giving the planet a hellish average temperature of 880 degrees Fahrenheit. It's fun to consider all the colorful possibilities. But each one makes me even more grateful I live on this pale blue dot. — Sarah | 
Twitter
Facebook
