There's a fascinating reason Earth's pale blue color might help us find aliens

In 1972, the Apollo 17 crew captured the first photograph of Earth in full view. The shot, appropriately named "Blue Marble," shows our planet roiling in shades of blue and white against the dark backdrop of space.

Eighteen years later, the Voyager 1 space probe snapped a photo from billions of miles away as it traveled past Pluto in 1990. In the image, dubbed "Pale Blue Dot," Earth appears as a lonely pixel, a faint shade of blue in the vastness of space.

Now, scientists believe that Earth's iconic blue color might signal something deeper than the beauty of the spinning dot that we call home. It might help us find life on other distant planets.

That's according to University of Washington researchers Joshua Krissansen-Totton, a graduate student, and David Catling, an astrobiologist and professor in Earth and space sciences, set out to explore in a study recently published in The Astrophysical Journal. They found that, while we probably shouldn't base our search for life on the color of other worlds alone, the color blue might come in handy when selecting which planets to study deeper.

Why is the sky blue?

It's a question you might remember asking as a child.

The visible light from the sun is made up of different wavelengths, which we see as colors when they are reflected off of surfaces. When the light travels through our atmosphere, it bumps into molecules of gas, dust, and water and scatters.

Dust and water particles are pretty big, and all of the different wavelengths, or colors, bounce off them in the same way. So the light they reflect back to us appears white.

But when the light collides with tiny particles, like gas molecules, that are smaller than the wavelengths of the light, a special type of scattering occurs. When this scattering happens, the molecules seem to be better at scattering colors with shorter wavelengths, like blue, rather than those with longer wavelengths, like red.

All that strong scattering makes the whole sky, which is filled with gas molecules, appear blue. This blueness even scatters into space, which is what makes Earth so photogenic. Even our oceans, which are actually colorless, reflect blue light.

The oxygen in our air, a product of Earth's bustling inner life, chemically reacts with molecules in our atmosphere that would otherwise form an murky haze, like that of Venus, Jupiter and Saturn, which is one of the reasons the researchers think a planet's color might just say something about its ability to harbor life.

But before you set the GPS in your spaceship for the nearest pale blue dot, you might want to do a little extra investigating. The pale blue color alone doesn't always mean a planet is habitable.

Krissansen-Totton and Catling looked at the way different wavelengths of light are absorbed and reflected in various real and theoretical exoplanets to investigate whether or not this pale blue might be indicative of life elsewhere in the universe. They found that Earth's signature hue can be closely imitated by uninhabitable planets.

It turns out planets and moons don't require life to scatter blue in their atmosphere. Gas- and ice giant exoplanets, as well as sub-Neptune-sized worlds with hydrogen-dominated or thick, water-steam envelopes all have the potential to mimic Earth's color, Astrobiology Magazine reports. Even our rusty next door neighbor, now dubbed the Red Planet, may once have been gentle blue.

"Mars may have even been a pale blue dot earlier in its history when it had a thicker carbon dioxide atmosphere," Krissansen-Totton told Astrobiology Magazine.

That being said, Krissansen-Totton added, Earth's spectrum has unique signatures that could come in handy in the search for extraterrestrial life.

How blue can help in the search for life

Every planet has a spectral signature, which shows how its atmosphere treats different wavelengths of light. In Earth's spectral signature, the scattering stretches through different wavelengths in an up-and-down U-shape, something the researchers haven't seen other planets imitate.

The unique shape of Earth's spectral signature is a product of the life teeming inside it. Light scatters in the bluer portion of the spectrum because of the nitrogen and oxygen in our atmosphere. Some light is absorbed in the greener portion because of the ozone in our atmosphere, which like the oxygen we breathe would not exist without life. And finally there's the reflectance of red light, which is partially caused by Earth's widespread vegetation, Astrobiology Magazine reports.

"Finding a terrestrial planet in the habitable zone with a U-shape spectrum would be incredibly exciting," Krissansen-Totton said.

By combining the search for pale blue worlds in habitable zones with the search for similar U-shape signatures, we might get clues about their ability to sustain life, narrowing down the pool of planets to investigate and bringing us closer to finding life in the universe.