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.

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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.

The researchers add that some of the water vapor will work its way into the stratosphere, where ultraviolet radiation from the sun will break up water molecules into their constituents, oxygen and hydrogen. Hydrogen is such a light element that the liberated hydrogen atoms will be lost to space. That process will remove much of the water vapor from the atmosphere, leaving an intensifying sun to bake Earth's surface under an increasingly cloudless sky.

How much warmer will Earth get? Others have estimated that when the sun begins its shift to red-gianthood, it will be twice as bright as it is now, meaning Earth will be at least 20 percent hotter than today, says Scott Kenyon, a senior scientist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

"The ultimate fate of Earth, if it survived at all as a separate body, would be to become a molten remnant," Schroeder and Smith write.

Initially, their calculations suggested that as the sun loses mass in its red-giant phase, the orbits of remaining planets – including any remnant Earth –actually would grow more distant, since the strength of gravity depends not just on the distance between two objects, but also on their masses as well.

But as the work proceeded, and the duo accounted for the sun finally ceasing to rotate, they found that tidal interactions between Earth and the sun would increase the drag Earth experiences, gradually guiding it on an inward spiral. The "molten, remnant" planet becomes an asterisk in the cosmic history book some 7.6 billion years from now, they estimate.

Assuming that the human race – or whatever it evolves into – lasts close to a billion years, there may be a couple of outs, Schroeder and Smith say. One is Mars, which under the new regime is squarely in the sun's relocated habitable zone, at least for a while.

The other is conceptually, if not technologically, more daring – divert an object the size of a large asteroid, perhaps an object from the Kuiper Belt, into a relatively close encounter with Earth every 6,000 years. The goal is to alter Earth's orbit, to move it farther from the sun.

As far back as 2001, a team of researchers estimated that by using gravity assists from Jupiter and Saturn, the amount of energy needed to divert the object would be enormous, but significantly less than the energy needed to travel between stars.

The aim would have to be perfect to place the object into the right orbit, or the asteroid itself could plow into the planet or destabilize Earth's orbit.

But humans have a couple of billion years to figure it out.

Of course, the end of the world as astronomers foresee it is so far down on the list of astronomical events humans could worry about that it all vanishes, Dr. Kenyon notes.

As for possible safe havens, he adds, "If all we learn in a few billion years from now is how to go from here to Mars, we'll be in a sorry state."

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