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Age matters. Searching for alien life on planets orbiting older stars may be fruitless because they always become prohibitively hot or cold.

The search for life on other worlds has focused on planets in what’s known as the habitable zone – the ring around stars where it’s the right temperature for liquid water.

That has led some to target planets orbiting red dwarf stars, as their smaller size and cooler temperatures mean planets in the habitable zone are closer in, and so easier to spot.

But we should also look for planets whose stars are the right age, regardless of their size, say and at the University of Tokyo, Japan.

Because stars grow brighter with age, planets at the inner edge of the habitable zone eventually enter a “runaway greenhouse mode”, in which their oceans boil away. Meanwhile, planets at the outer edge lose heat-trapping gases from their atmospheres over time as volcanic activity decreases, so they enter an ice-covered “snowball state”. Kadoya and Tajika built a model of how planets heat or cool over time, and found that both fates set in after about 3 billion years.

Smaller stars don’t actually provide life-friendly environments for any longer than sun-sized stars, says Kadoya. This has implications for planned exoplanet missions like the Transiting Exoplanet Survey Satellite due to launch in 2017, and the Giant Magellan Telescope under construction in Chile, both of which will target smaller red dwarf stars.

“For the purpose of detecting planets with life, it is important to concentrate on the young planetary systems,” says Kadoya. Checking the dwarf stars for life is still a good idea just because of their sheer numbers, though, he adds.

Earth’s future

At 4 billion years old, Earth is well and truly over the hill, according to the model. Its ongoing life-friendly nature is probably due to its optimum positioning from the sun, says Tajika. “The orbital radius of the Earth is in the range of the most long-lived conditions for the warm climate mode,” he says.

Unfortunately, the perfect climate of our planet is unlikely to endure forever, Tajika says. “Earth will be in the runaway greenhouse mode in the future because it is in the inner habitable zone.”

But there might be another thing keeping planets habitable, says at Macquarie University in Sydney, Australia: life itself.

“The researchers have ignored the effect of life,” he says. “The ability of life to modulate the surface conditions means that any planet on which life evolves is probably more robust to climatic change than these models predict.”

Then again, mathematical models for habitable planets are riddled with uncertainties, says at the University of New South Wales, Sydney.

“It’s hard to test these models,” he says. “In this case, they’ve based the model on Earth chemistry, but we don’t know how well this would match other planets.”

Journal reference: The Astrophysical Journal Letters, DOI: