The End of Planetary Habitability

For nearly all of Earth history, life was limited to creatures so small that they are invisible to the naked eye. Casual inspection during all that time would have suggested that it was a failed planet. In other planetary systems, primitive life might flourish but never advance to the point where forests and flying animals even get a serious chance to evolve. Stars with short lifetimes, unstable planetary atmospheres, changes in orbital or spin axis, massive extinctions, impacts, crustal catastrophes, the cessation of plate tectonics, or any of a whole raft of other problems could prevent the evolution of advanced life or its prolonged survival. And on Earth itself, complex life has thrived only for the last 10% of the planet's existence.

Perhaps the most predictable aspect of advanced life (if it exists) on other planets around other stars is that its tenure is limited and that eventually any such life—and even some of the planets—will perish. Like individual organisms, planets and their grand environments have life spans. All planets with life eventually become extinct. This final outcome may be brought about by external sources such as impacts or a nearby supernova, by internal effects such as atmospheric or biological catastrophe, or (if all else fails) by increase in the brightness of the central star. This will be the ultimate fate of Earth: Life on our planet will eventually be roasted out of existence. The sun is slowly getting brighter. It is now 30% brighter than it was in the early history of the planet. Over the next 4 billion years it will double in brightness. Even if life survives this travail, it will soon be stilled. About 4 billion years from now, the sun will begin to expand rapidly in size, and its brightness will dramatically increase. The sun will become a red giant, as did the stars Antares in the constellation Scorpio and Betelgeuse in the constellation Orion. In a billion-year time span, its brightness will increase over 5000 times.

At the very beginning of this process, Earth's oceans will vaporize, driving our precious water supply into space. In the final stages of its transformation into a red giant, the sun will expand to the point where it will nearly reach the orbit of Earth. The Universe will be one living planet poorer.

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