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Moving the Earth: a planetary survival guide

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But long before that, in 1.1 billion years, the Sun will grow 11% brighter, raising average terrestrial temperatures to around 50 °C (120 °F). That will warm the oceans so much that they evaporate without boiling, like a pan of water left on a sunny kitchen counter.

Plants and animals will have a very tough time adapting to that hothouse, although some single-celled organisms called Archaea might survive. But only for a while. Once the water vapour is in the atmosphere, ultraviolet light from the Sun will split the water molecules, and the hydrogen needed to build living cells will slowly leak into space. If our descendants - or other intelligent life-forms that follow us - want to survive, they'll have to migrate elsewhere. But where and how?

One approach would be to fire up rockets and move to another planet. Back in 1930, British science-fiction author Olaf Stapledon wrote about a future where our descendants fled to Venus, and later Neptune, when the Earth became uninhabitable. Eminent scientists such as Stephen Hawking have endorsed the idea of establishing colonies on the Moon or other planets so humanity would survive any disaster that wiped out life on Earth.

Yet evacuating all 6.7 billion Earthlings would take the equivalent of a billion space shuttle launches. Even if we could launch 1000 shuttles a day, it would still take 2700 years to move the whole planet's population.

Then there's the matter of taking care of people once they reached their new home. Moving to any other planet would require "terraforming" it to provide food, water and oxygen to support colonists. Why not bring our own planet along with the resources we would need?

Tiny change

Elementary physics tells us that we actually can move the planets. Launching a rocket into space pushes the Earth a bit in the opposite direction, like the recoil from a gun.

Science-fiction author and trained physicist Stanley Schmidt exploited this fact in his novel The Sins of the Fathers, in which aliens built giant rocket engines at the South Pole to move the Earth. (Read about other sci-fi novels and films that have tackled the problem of moving worlds.)

In real life, however, the Earth is so massive that a rocket would have little effect on its motion. Launching a billion 10-tonne rockets in exactly the same direction would change the Earth's velocity by just 20 nanometres per second - peanuts compared to the planet's current speed of 30 kilometres per second.

A few astronomers have tackled the problem of moving planets, but not for dealing with emergencies on human time scales. They're actually devising thought experiments to understand the dynamics of planetary systems, says Greg Laughlin of the University of California, Santa Cruz. So processes that occur on geologic time scales work perfectly well.

Moving out

Planetary dynamics seemed simple and orderly when we knew only our own solar system, but that changed with the discovery of "hot Jupiters" on tight orbits around other stars. The planets couldn't have formed in the scorching regions where they orbit - there was not enough gas and dust there to amass such giant worlds. Instead, they must have migrated there from more distant birthplaces.

To understand how planetary systems might rearrange themselves, Laughlin, his Santa Cruz colleague Don Korycansky, and University of Michigan astronomer Fred Adams posed themselves the problem of how to move the Earth so the warming Sun didn't cook the planet.

For the purposes of their calculation, the three chose the Earth's final destination as an orbit 1.5 times its present distance from the Sun, at what is now the orbit of Mars. In 6.3 billion years, when the Sun is in its red-giant stage and is 2.2 times brighter than today, a planet at that distance will receive about as much sunlight as the Earth receives today.

Moving the Earth to a circular orbit at that distance requires increasing its orbital energy by about 30%. That would be possible, they say, by changing the orbits of icy bodies in the distant solar system so they would pass close to the Earth, transferring some of their orbital energy to the planet.

The objects lie in a ring of icy bodies beyond Neptune called the Kuiper belt and in an even more distant shell of comets called the Oort cloud. Because they are far from the Sun, the objects have relatively low orbital energy, so they could be nudged using methods being developed to deflect asteroids away from the Earth.

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When the Sun expands into a red giant several billion years from now, the Earth will be dragged into its atmosphere (Illustration: Mark Garlick/HELAS)


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