A widely reported study in the Proceedings of the National Academy of Sciences that there could be as many as 40 billion habitable Earth-like planets in the Milky Way galaxy caused quite a stir recently.
The astronomers who authored the study were not claiming to have found life elsewhere in the Cosmos. Rather more modestly, they had discovered an indispensable precursor for life: turf, and lots of it. Turf, that is, in the form of innumerable Earth-sized planets orbiting sun-like stars within the Goldilocks zone – where surface temperatures are “just right,” per the fairy tale. Just right, that is, for water to exist as a liquid, not as ice, nor vapor.
The announcement that Earth-like exoplanets are widespread in our galaxy – and presumably others as well – inevitably reinvigorated contemplation of that venerable theme not just of science fiction but of fringe science as well: interstellar travel.
Might humanity someday colonize the galaxy as we once peopled all the continents of the Home Planet? Could futuristic starships ply the vast gulfs of interstellar space as wooden sailing ships once plied the vast gulfs of the sea? Might this help avoid destruction of the Earth by offloading human pressures onto other worlds, thereby providing an outlet, or escape valve, alleviating escalating ecological strains on the home planet from our inborn penchant for constant expansion?
As a devotee of science fiction and an avid amateur astronomer, much as it pains me, a reality check is in order.
The late ecologist Garrett Hardin once considered the prospects for interstellar travel in an essay called “Overpopulation: Escape to the Stars?” For fans of Star Trek, Star Wars and Carl Sagan’s popular PBS series Cosmos, it makes for depressing reading – puncturing our inflated hopes for a dazzling future of truly “dancing with the stars.”
The first downer is simply the daunting distance. It’s a cliché that the stars twinkling above are far, far away, but just how far away they actually are is truly astounding. Alpha Centauri is the closest at 4.3 light-years. (A light-year measures distance, not time; it’s how far light travels in a year, almost six trillion miles.) That’s about 25 trillion (25,000,000,000,000) miles away.
However, the nearest star that might have a habitable planet is 12 light-years away, or about 70 trillion miles. Assuming that our Sun were the size of a tennis ball on a South Carolina beach, then this nearest star would be roughly 2,600 miles away on a California beach. Even with vastly advanced technology, it would take centuries to reach even this closest system, at staggering cost. Starfarers who boarded a star-bound vessel would depart Earth forever, in contrast to most science fiction, where galaxy trotters hop back and forth as if it were a simple flight between L.A. and San Francisco.
German astrophysicist Sebastian von Hoerner thought it might be possible someday to build a starship that could reach 3% of the speed of light, or 22 million mph. This would be almost a thousand times faster than the Apollo voyages to the moon, which attained 0.0037% of the speed of light, or 25,000 mph. Even at 22 million mph, it would take almost 360 years – multiple human generations – to reach the nearest star with a habitable planet.
While this might be a worthy endeavor someday in and of itself, it would be far too little and far too late to save the Earth from overpopulation. As Hardin pointed out, during the 28 years in which world population increased by 2.2 billion people, NASA managed to put 12 men on the moon for a few hours. Whoa.