When doomsday comes to planet Earth, it will leave the third rock from the sun a scorched, sterile cinder devoid of living organisms. Virtually everyone can agree on that.

But May 21, 2011, as the start of a five-month period that ends with an October 2011 doomsday? That's off by at least a billion years, give or take 50 million, according to a recent study by a pair of astronomers in Mexico and Britain. ("Recent" on time scales that astronomers work with, anyway.)

And the cause they cite is considerably different than the one invoked by a California minister for an October end of the world. [ csm ]

The scenario the two astronomers' calculations spin out – in a paper in the Monthly Notices of the Royal Astronomical Society in Britain in 2008 – is based on what they say is an improved understanding of how stars like the sun evolve over their history. In addition, they examine the effect those changes would have on the sun's so-called habitable zone – a Goldilocks region where temperatures are "just right" to allow liquid water to pool and remain on the surface of a planet.

Over the years, the typical one-sentence version of Earth's demise has tended to read something like this: In about 5 billion years, the sun will expand into a red-giant star, engulfing Mercury, Venus, and Earth as it grows.

Poof, Earth is toast.

But if the calculations of Klaus-Peter Schroeder and Robert Smith are in the ballpark, the end of life on Earth would come much sooner – but with the potential for a haven on the fourth rock from the sun, Mars.

The story begins some 4.57 billion years ago, when the young sun's nuclear furnace ignited and stabilized. Back then, solar physicists estimate, the sun was 30 percent dimmer than it is today. As it has matured, it has brightened at a pace of about 1 percent every 110 million years.

Over that period, the two explain, Earth's climate system has adjusted to the increase in the sun's output, keeping the planet's average temperature within a livable range and with plenty of water on hand. Orbiting 93 million miles from the sun, Earth finds itself nicely placed in the sun's habitable zone.

But over the next billion years, the duo says, the sun's output will rise by another 10 percent.

As the sun continues to warm, it will in effect push the inner edge of the habitable zone out beyond Earth's orbit. So when, some 5 billion years from now, the sun enters its red-giant phase, the habitable zone will have long since left Earth behind to embrace Mars.

Thus, roughly 4 billion years before Earth is overrun by a swelling sun, the planet will already have warmed enough to drive water from rivers, lakes, and oceans into the atmosphere as water vapor, according to Dr. Schroeder, from Universidad de Guanajuato, in Mexico, and Dr. Smith of the University of Sussex, in England.

Water vapor is the atmosphere's most abundant greenhouse gas, so the buildup of water vapor, as well as increased releases of carbon dioxide from soils and from dying, decaying vegetation, would amplify the heating that the sun delivers.

finish story :CSM