A pale blue dot may not be the only hint of life beyond the solar system. New research suggests astronomers pay attention to pale orange worlds as well, since they may resemble what Earth looked like earlier in its history.
During Earth's so-called Archean era, some 2.5- to 3.8 billion years ago, the atmosphere had little oxygen and much more methane, thanks in large part to organisms called cyanobacteria that filled the seas.
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Computer models show that methane released by the bacteria left Earth periodically wrapped in an orange blanket of hydrocarbons, the result of sunlight breaking down the methane and molecules recombining in the atmosphere.
A similar phenomenon can be seen today at Saturn's moon Titan, though its hydrocarbon haze is not tied to biological activity.
"In the later Archean, when there was a lot of methane there were times when our entire planet was enshrouded by hydrocarbons," Giada Arney, an astronomy graduate student at the University of Washington, said at the American Astronomical Society's planetary sciences meeting in Maryland this week.
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"When we look at Earth through time, we would see that Earth has looked very different at different epochs in its geological history. When we ask the question ‘What does an Earth-like planet look like?' the answer depends on the time period that we're thinking about.
"Pale orange dots can be Earth-like planets too," Arney said.
Baby Earth's blanket of hydrocarbons helped shield the planet from damaging ultraviolet radiation, much like ozone does today. The haze also helped Earth cool off, by reflecting solar heat back into space. Gradually, Earth's climate and environment changed, leading to the rich diversity of life that exists today.
Astronomers may be able to find chemical footprints of similar early processes unfolding beyond the solar system.
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Detecting the pale orange glow of an extrasolar planet should be a good place to start.
Simulations at the school's Virtual Planet Laboratory show that measuring the ratio of carbon dioxide to methane in a planet's atmosphere would help distinguish whether the hydrocarbon haze is caused by biological or geological processes.
"If you can characterize the haze, it may be a sign of life itself," Arney said.