Astronomers redefine habitable zone around stars

A comparison of planetary orbits between our solar system and the Gliese 581 system
Even before astronomers had discovered planets in other solar systems, they had already defined what's known as a star's 'habitable zone' — the band of space surrounding a star where the temperature is just right for there to be liquid water present on the surface of a planet.

The definition for this zone has been the same for about 20 years, ever since it was first set by James Kasting, a professor at Penn State University that studies planetary atmospheres. However, with recent updates to the HITRAN (high-resolution transmission molecular absorption) and HITEMP (high-temperature spectroscopic absorption parameters) databases, both of which contain the spectral data for water and several molecules that contain carbon and oxygen, astronomers have now redefined the habitable zone, shifting it slightly further out from its previous location.

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Under the old definition, Venus was just along the inner edge of our Sun's habitable zone, Earth was firmly in the middle of the zone and Mars was marginally on the outer edge of it Now, Venus is much too close, Earth orbits along the inner edge of the zone and Mars is much closer the middle.

The new range for the zone has implications for planets outside of our system as well. Some worlds that had been identified as potentially habitable are now outside the zone and other planets that were left off the list now quality as potentially habitable.

The new habitable zone and planets that fall within its limits

"Right now as I see it as a significant change," said Abel Méndez, an astronomer at the University of Puerto Rico at Arecibo, who manages the Habitable Exoplanet Catalog. "Many of those planets that we believe were inside are now outside. But on the other side, it extends the habitable zone's outer edge, so a few planets that are farther away might fall inside the habitable zone now."

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One specific example of this is the exoplanet Gliese 581d — a possibly rocky planet about 3 times as massive as Earth orbiting a star about 20 light years away in the constellation Libra — which was outside its star's habitable zone under the old system, but is in the middle of the new zone, raising its chances of being habitable by a considerable amount.

"That will be a big change for that particular planet," said Méndez. "That means the prospects for life on the planet will be much better."

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