Welcome to the last day on Earth. Humanity's descendants have long since died or fled, oceans have long since vaporized, and the life-giving sun has ballooned into a swollen giant filling the sky.

Scientists have debated for years what will happen to our planet when the sun's fusion furnace begins to run out of fuel and swell into a red giant a few billion years from now. The most recent simulations suggest that Earth will end up being swallowed by the dying sun.

The impending doom is more dire than any fictional villain could ever wish upon a world. Yet the planet need not perish if future civilizations can somehow move Earth out beyond the danger zone. Barring that, a clever escape plan might prove useful.

Red dawn

The fate of life on our planet has always depended upon the sun's destiny. Astrophysicists can trace both the sun's past and future based on their understanding of stellar evolution, which comes from observing stars of many types and at different stages of life.

Larger stars typically meet their end in spectacular supernova explosions, and leave behind either neutron stars or black holes. But mid-size stars like the sun experience a more gradual transformation, after they consume the last of their hydrogen fuel and start burning helium.

Helium burning leads to higher core temperatures that would cause the sun to start swelling into a red giant, around 5 billion years from now. Simulations show the sun eventually expanding to around 250 times its current size.

Previous studies showed that expanding sun would engulf Mercury and Venus, while Mars would remain safely out of reach. But Earth remained in a zone of uncertainty because of its location between those planets. A faint chance existed that the sun would lose too much mass before getting too big, and would allow the Earth to escape into a wider orbit as the sun loses its gravitational grip.

Final sunset

Now any hope for Earth's final salvation may have finally died. British astronomers ran a simulation in early 2008 which included the sun's weakened gravitational pull, and the Earth moving outward in response.

"If that were the only effect, the Earth would indeed escape final destruction," said Robert Smith, emeritus reader at the U.K.'s University of Sussex. "However, the tenuous outer atmosphere of the sun extends a long way beyond its visible surface, and it turns out the Earth would actually be orbiting within these very low density outer layers."

That low-density gas would cause enough drag for the Earth to drift inwards, even as tidal forces caused by the Earth's gravity force the nearest side of the sun to bulge outwards. The Earth eventually drifts into the bulge and ends up vaporized, around 7.6 billion years from now.

However, life has even less time than the planet. Most scientists agree that every living thing faces certain extinction 1 billion years from now, when the sun's growing brightness transforms Earth into a global desert. Dropping carbon dioxide levels would starve plants of the ability to conduct photosynthesis, and that would lead to the inevitable death of all living things.

Life after Earth

A long shot exists for life to survive Earth's fate, but it would involve some novel solutions or a serious space colonization effort.

One team at Santa Cruz University in California has proposed capturing a passing asteroid and using its gravitational effects to "nudge" Earth's orbit outward. A continuous asteroid passage every 6,000 years or so could keep Earth at a comfortable distance and give life another 5 billion years on the planet.

People leery of miscalculations with the asteroid solution could also turn to good old fashioned engineering. Humans may find new homes among the asteroid belt, the outer planets, in artificial colonies, or perhaps beyond this star system. "Space is disease and danger wrapped in darkness and silence," complains Dr. Leonard "Bones" McCoy of "Star Trek," but even life aboard a starship would beat species extinction.

"A safer solution may be to build a fleet of interplanetary 'life rafts' that could maneuver themselves always out of reach of the sun, but close enough to use its energy," Smith suggested.

The sun will eventually lose most of its mass as it becomes a white dwarf, and could come to resemble other burnt-out star systems spotted by NASA's Spitzer Space Telescope in a 2009 study. About 1-3 percent of white dwarf stars seem to contain dust and rocky debris, which may represent remnants of rocky planets such as Earth.

By that time, humanity should have either found its new foothold in the universe or long since ceased to exist.

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