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.

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.

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.

All this suggests that good candidates for life outside our solar system must have enough time in their star’s sweet spot – more than 4.5 billion years, it seems – for that life to burgeon.

In search of those planets on which Earth’s life-hunting resources are best spent, the team modeled the expected habitable zone lifetime for seven confirmed exoplanets and 27 of the Kepler telescope’s exoplanet candidates, as well as the lifetime for Earth.

Earth, according to the model, has a habitable zone lifetime as long as 7.79 billion years, meaning that the planet has about 3.29 billion years left, though the scientists said that the figure could be as low as 1.75 billion years. So, we are about 70 percent of the way through our planet’s lifespan, and about 0.01 percent of the way through humans’ lifetime on Earth, should we survive another 1.75 billion years – which scientists say is unlikely, given that the planet is expected to become warmer and warmer as the sun brightens.

Rising temperatures would be catastrophic to humans far before the Earth reached its broiling end, the researchers said.

Mars, though, has a much longer habitable zone lifetime than does Earth. The Red Planet might someday as a prime viewing platform for our descendants a billions of years from now to observe Earth as it roasts.

“If we ever needed to move to another planet, Mars is probably our best bet. It’s very close and will remain in the habitable zone until the end of the Sun’s lifetime – six billion years from now,” said Andrew Rushby, a researcher at the University of East Anglia, in a statement.

In the far-flung cosmos, planets orbiting low mass stars have longer habitable zone lifetimes, the authors found. For example, Gliese 581d, a possible planet in the constellation Libra, has a lifetime of between 42.4 to 54.7 billion years – plenty of time for life to swell there, the authors said. Since its star, 581, is considered to be small and slow-burning, its planets have often been floated as possible life-harbors. In October 2008, the Ukrainian National Space Agency beamed a radio signal toward the star’s system; it should arrive in the 581 galactic ZIP code in 2029.

Of course, the habitable zone lifetime is not the sole factor in determining whether or not a planet is habitable, the researchers said. Even if a planet is within the sun’s habitable zone for billions and billions of years – enough time, theoretically, for life and all its dramas and stories, big and small, to slowly unfold – that does not mean that the planet has life of any kind, let alone intelligent life.

For life to be there, the planet must also have experienced untold numbers of happy accidents – as well as eluded unimaginable numbers of unhappy accidents – of the sort that produced wet, green and oxygen-rich Earth. Mars, for example, is in our sun’s habitable zone. But, after a catastrophic event tore up its atmosphere some 4 billion years ago, its wet and warm days ebbed into cold and dry ones, its green and featured landscape into a red-brown and blank one.

“The planets in our sample are unlikely to conform to all these conditions,” wrote the authors, citing Earth’s life-conducive planetary mass and composition, plate tectonics, and atmospheric pressure and composition.

“To date, a true Earth analog planet has not been detected,” the authors wrote.