We're getting close—oh, so close—to being able to shed more light on Earth-size exoplanets. So far, we know of about 30 sitting in their stars' habitable zones, where liquid water can exist. With the next generation of telescopes coming online in 2019 and beyond, we might finally be able to figure out what's happening in their atmospheres.

Imagine we look at one of these worlds and see Earth-like ratios of nitrogen, oxygen, carbon dioxide, methane and ammonia—a perfect blue dot that, as far as we can tell, looks just us. Seriously, imagine! Earth 2.0, right in our celestial backyard, possibly hosting a range of unimaginable flora and fauna.

The trouble is, this a very human-centric vision of life on other worlds. Though life arose on Earth about 3.5 billion years ago, the proliferation of multicelluar life began relatively recently, about 600 million years ago.

"When we say 'life' in just our everyday vernacular, people only really attribute that to animal life or plant life," said Sarah Rugheimer, an astronomer and astrobiologist at the University of St Andrews in Scotland. "By and large, they don't imagine a planet that's all microbial mats and microbes, which is honestly what most of Earth's history is."

Why does this matter? Because microbes drive the basic composition of Earth's atmosphere, and that atmosphere has changed drastically over time. On early microbial Earth, the dominant atmospheric gases were carbon dioxide and methane. It was hardly a place for humans, yet very much full of life. Therefore, when we train our next-generation telescopes on Earth-size exoplanets, we must be prepared for a wide range of results—many of which could still indicate the presence of life, despite looking nothing like modern-day Earth.

Scientists like Rugheimer, and her colleague (and former Ph.D. advisor), Lisa Kaltenegger, are already preparing for those results. With a new paper accepted for publication in The Astophysical Journal, Rugheimer and Kaltenegger are helping create a hypothetical guidebook to Earth-size exoplanet atmospheres.

Imagining Earth at other stars

We have a pretty good idea of how Earth's atmosphere evolved over time, and how that corresponds to the appearance of different forms of life. And though life on other worlds may evolve differently, we can still use Earth's history as a baseline to help interpret what we see on Earth-size exoplanets.

Rugheimer and Kaltenegger modeled Earth's atmosphere at four distinct points in history: before microbes (3.9 billion years ago), after microbes and the first rise of oxygen (2 billion years ago), during the second rise of oxygen (800 million years ago), and Earth as it is today. At each of these points, oxygen, methane and carbon dioxide were in drastically different abundances.