Today Earth is a “pale blue dot,” as famously coined by the late Carl Sagan. But 2.5 billion years ago, it would have been a “pale orange dot” owing to the methane produced by organisms – and finding exoplanets that look similar could suggest they, too, have life on their surface.

This is according to a new study by Giada Arney from the University of Washington and her colleagues, presented at the American Astronomical Society’s Division for Planetary Sciences conference in Maryland this week. The research used geological data to examine what Earth would have looked like in the Archaean era, 2.5 billion years ago, and see if this appearance could help in the hunt for habitable exoplanets. Turns out, it might.

During this era, methane molecules in the atmosphere broken down by light formed complex hydrocarbons, organic compounds of hydrogen and carbon, giving Earth an orange haze, or smog, that would have been visible from space. There are two possible sources for such methane – biological, namely life, or geological processes, such as on Saturn's moon Titan. Thus, finding exoplanets with an orange hue could suggest they are going through a similar process to that which occured on the younger Earth.

As Arney explains, to find out if an exoplanet’s orange hue comes from biological or geological processes, you’ll need to find out how much carbon dioxide is there. If it’s a lot, it could be Earth-like; if not, it’s probably more like Titan.

“If we saw a hydrocarbon haze in an exoplanet’s atmosphere, it could suggest a methane source consistent with biological methane production, but it isn’t enough to just detect the haze,” she told IFLScience. “To argue that the haze is a sign of life, you’d have to also characterize the background atmosphere, particularly the amount of carbon dioxide.”

^{Arney said Archaean Earth's hue would have been similar to Titan, shown. NASA}

On Earth, our haze – which would have extended 20 to 70 kilometers (12 to 43 miles) into the atmosphere – eventually gave way to the relatively bluer appearance we have today as the amount of oxygen grew. Oxygen destroys methane, so once levels started rising 2.5 billion years ago, the haze disappeared for good. But it’s a good indicator of biosignatures.

At the moment, we know of a few exoplanets that have hazes or clouds in their atmospheres, but we can’t be sure they are Earth-like hydrocarbon hazes just yet. Future telescopes like the James Webb Space Telescope (JWST), though, could potentially characterize them.

Perhaps, in the continuing search for worlds like Earth, we may have to start looking for more of these “pale orange dots.”

The study will be published in the journal Astrobiology.