We Should Look for Orange Exoplanets Because That's What Earth Used to Look Like

The Earth is the only planet we know of (so far) to host life, so astronomers use it as a guide when searching the cosmos for potentially habitable worlds. The dream has been to use advanced telescopes to find similar pale blue dots beyond our solar system. However, new research suggests pale blue dots are not the only type of worlds we should be searching for.

A new study led by Giada Arney of the University of Washington shows we should not rule out pale orange dots—hazy worlds similar to Saturn's moon Titan—in our search for life beyond Earth.

Arney is studying a period early in the Earth's history known as the Archean era. By analyzing geological data and combining it with computer simulations, she determined what the Earth would have looked like some 2.5 billions years ago. Her research was published in the journal Astrobiology, and presented last week at the 47th annual Division of Planetary Sciences meeting in National Harbor, Maryland.

The Archaen atmosphere had very little oxygen and much higher concentrations of methane. If we were to go back in time, we would not be able to survive on the Earth's surface without a supply of breathable oxygen and a spacesuit specifically designed to withstand intense solar radiation. Computer models show the methane-rich atmosphere covered the planet in a hazy orange hydrocarbon shroud.

The hazy shroud of hydrocarbons is formed when methane molecules in the atmosphere are zapped by ultraviolet light from the Sun. The radiation breaks apart the methane molecules into its constituents: hydrogen and carbon. The molecules then recombine to form more complex organic compounds, enveloping the planet in an orange haze. The haze is visible from space and closely resembles the smog we see on Earth surrounding industrial cities.

Atmospheric methane is produced one of two ways: either via geological processes like we see on Titan, or via biological activity. Finding other worlds shrouded in a similar haze could mean these planets are very similar to the early Earth and might even be able to support life.

Since haze produced by geological processes looks identical to haze produced by biological processes, the next step to figuring out whether a world is habitable is to measure how much carbon dioxide is in the atmosphere.

"The haze formation is controlled by the ratio of methane to carbon dioxide," Arney explained. "If there's an abundance, then we can attribute the haze to biology, and a planet could be Earthlike. On the other hand, if there's very little carbon dioxide, then the planet is probably more like Titan."

Earth's methane and Titan's methane come from two very different sources, so the resulting haze particles would have different spectral signatures. This means the haze could be thought of as a biosignature or the chemical fingerprint of life. "If we can characterize the haze, it could be a sign of habitability," Arney explained.

Right now, the Earth relies on the ozone layer to help shield itself from harmful UV rays. However, the early Earth didn't have an ozone layer, so a thick blanket of hydrocarbon haze protected it damaging solar rays. The haze helped cool the Earth, Arney said, by reflecting solar heat back into space. Gradually, over millions of years, the climate and environment changed, evolving into the lush, habitable planet we live on today.

Observing the chemical signatures of processes similar to the ones that took place on the early Earth will help astronomers in their search for life beyond Earth. As such, we need to expand our search to include all potential Earthlike planets, including the pale orange dots of the Universe.