Habitable zone exoplanets

This artist's representation shows a planet, right, orbiting the star Alpha Centauri B, center, a member of the triple star system that is closest to Earth. This exoplanet sat too close to its star for liquid water to exist. Now, scientists say a star's habitable zone might stretch a little bit closer in, which could alter the way researchers hunt for planets. (L. Calcada / European Southern Observatory)

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The habitable zone around sun-like stars might be a little wider – or thinner – depending on how big you thought the habitable zone was in the first place, suggests new research in the journal Nature.

The findings, based on 3-D models of the runaway greenhouse gas effect, may alter the estimated number of habitable planets around sun-like stars in our galaxy — and they may also may affect how future planet-hunting space telescopes are designed and built.

The habitable zone is the doughnut-shaped "Goldilocks" region around a star where a planet would be warm enough to have liquid water and cool enough to keep it from evaporating away. In the sun’s case, Earth sits near the inside boundary of the habitable zone, and cold Mars sits closer to the far end of it. Gassy, super-hot Venus sits so close to the sun that it experienced a runaway greenhouse effect early in its history and whatever water may have existed has long since evaporated.

Researchers wonder when the same thing might happen to Earth, as the sun grows hotter and our planet warms over the eons.

“This destabilizing greenhouse feedback can 'run away' until the oceans have completely evaporated,” the study authors wrote.

Other scientists using data from NASA’s now-crippled Kepler space telescope have estimated that 22% of sun-like stars in the Milky Way have an Earth-size planet in the habitable zone, which — depending on how many sun-like stars are in the galaxy, 50 billion or 100 billion — comes out to as many as 22 billion potential Earth-like planets.

But those scientists estimated the habitable zone of a sun-like star to be from 0.5 of an astronomical unit — half the distance from the sun to Earth — to 2 astronomical units, or AU, James Kasting and Chester Harman of Penn State University wrote in a commentary on the Nature paper.

“This is a curious choice, because the completely desiccated planet Venus orbits at 0.72 AU, suggesting that the habitable-zone inner edge is somewhere beyond that distance,” Kasting and Harman wrote, noting that bone-dry Venus clearly isn’t habitable.

Other estimates tried to model at what point water would start to fill upper layers of the atmosphere and get burned off by the sun – a phenomenon known as "moist greenhouse" – and at what point all the water would simply evaporate from the oceans and fill the air with moisture. Neither of these scenarios are good for life. That previous research put the habitable zone at about 0.99 AU, which would mean that Earth, at 1 AU, is perhaps a little too close to the warmer inner edge of the habitable zone.

But these previous models have relied on one-dimensional models of the atmosphere. The current study uses a three-dimensional model that takes into account factors like clouds and variations in relative humidity.

Ultimately, the authors of the Nature paper put the inner edge of the habitable zone – the edge closer to the sun – at 0.95 AU. The difference, 0.04 AU, doesn’t sound like much, but comes out to 3.7 million miles – and it gives Earth a decent amount of breathing room.

“This has strong implications for the possibility of liquid water existing on Venus early in its history, and extends the size of the habitable zone around other stars,” the Nature study authors wrote.

But it also means that the estimated 22 billion Earth-like planets around sun-like stars, based on a minimum distance from the sun of 0.5 AU, “could be too high by a factor of almost 2,” Kasting and Harman said.

This means future telescopes would have to focus on the planets farther from their stars, which take longer to orbit their star and thus take more time to pin down. The findings could have implications for how future telescopes are designed, the commentators said.

“This difference is important because a smaller value … implies that a future space telescope designed to observe and characterize such planets must be correspondingly larger,” Kasting and Harman wrote.

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