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Don't panic: Earth has at least 1.75 billion years to go, scientists say

A team of British researchers has developed a model for determining how long a planet can expect to be within its sun’s habitable zone before the brightening star cooks the planet's liquid water into steam.

By Contributor / September 19, 2013

A team of scientists predicts that Earth has about 1.75 billion years before the sun is too bright for life here.

Heinz-Peter Bader/Reuters

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Earth has at least 1.75 billion years left, scientists have found. That means that Homo sapiens, in the unlikely event that the species will persist all that time, have used up about 0.01142857142 percent of their time on Earth so far.

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A team of British researchers has developed a model for determining how long a planet can expect to be within its sun’s habitable zone – the sweet spot just far enough to the sun so that the planet’s water doesn’t sizzle into vapor but just close enough to the sun so that it doesn’t freeze.

The model, reported in the journal Astrobiology aimed at assessing which planets outside our solar system, called exoplanets, might be in that comfortable zone long enough for intelligent life to make its gradual appearance there. In doing so, it also offers a prediction for Earth’s remaining time.

Most exoplanet research is underpinned by one all-important question: “Is this planet habitable?” To answer that question, scientists often begin by asking if that planet falls within the star’s habitable zone, where liquid water, an ingredient thought to be critical for life, could be available.

But just how the bounds of the habitable zone are calculated and plotted has been the subject of much debate in recent years, as exoplanet research now includes not just hunting those planets, but also classifying them.

Researchers, for example, have debated what effect cloud cover might have on the range in which a planet might be habitable. Perhaps, clouds might keep a planet close to the sun cooler than it otherwise would have been, protecting its surface water reserves from evaporation, researchers have proposed.

Still, as the latest paper’s authors note, what is not controversial is that the habitable zone, however it is defined, fluctuates over time. Over billions of years, a star’s brightness increases, and planets once in that sweet spot begin to broil.

“Toward the end of a planet’s [habitable zone] lifetime, steadily increasing stellar luminosity is likely to result in a runaway greenhouse event, which would represent a catastrophic and terminal extinction event for any surface biosphere present on the planet,” write the authors, in the paper.

So, in the hunt for extraterrestrial life, the question, “is it habitable?” is not meaningful without also answering, “for how long is it habitable?”

That’s because life takes a long time to develop – or at least so it seems based on our experience here on Earth. Here, on this planet formed about 4.5 billion years ago, we didn’t get single-celled organisms, called prokaryotes, until about 3.6 billion years ago, and bacteria that could photosynthesize didn’t pop up until 200 million years after that. Fish then turned up about 500 millions of years ago, then insects about 300 million years ago, and then dinosaurs about 200 million years ago. [Editor's note: An earlier version misstated the times of the emergence of photosynthesizing organisms and of insects.]

Humans, following up on the evolution of mammals, birds, and flowers, have spent just 200,000 years on this planet. We are, essentially, the scrubby, ultra-thin tip of an eraser, topping a long pencil of time that precedes us.

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