The Darkest Worlds

by Seth Shostak

Imagine a world where the Sun doesn't shine -- ever. A place where there are nights but no days, and where the term "year" has no meaning. On such an unlit world, you'll never see anything in the sky brighter than the puny sparkle of the stars.

Welcome to orphan planets -- worlds that have been ripped from the bosom of their birth and flung into the dark expanses of interstellar space. While such perpetually sombrous locales may sound like freakish oddities, the truth is that planetary orphans could be as common as carbuncles.

This surprising possibility has been suggested by theoreticians working to understand how stars and planets are born. Their models reveal that infant planets -- freshly hatched from the swirling disks of gas and dust that accompany star birth -- don't always play nicely. They often have highly eccentric orbits that provoke strong gravitational interactions with their nest-mates. The resulting tug-of-war accelerates some of them to high speed -- sufficient to kick them out of their natal homes altogether.

Consequently, a newly minted planetary system is soon purged of some of its members. And recent results from astronomers who study the occasional gravitational lensing of unknown worlds by intervening stars suggest that orphan planets could be at least as numerous as the stars.

In other words, there could be hundreds of billions of orphan worlds shuffling through our galaxy. Finding and studying these rogue objects is clearly worthwhile.

But it's stunningly hard. The planets and moons of our solar system are blatantly visible because they reflect sunlight. Without the nearby Sun, these planets would be cryptic and dark on the sky. Nonetheless, and with the right instrument, they might still give away their presence because they're slightly warm. Consequently, they emit a dull, infrared glow. Earth, for example, is slowly leaking the energy of its molten innards into space. The heat flux is only about 0.1 watts per square meter -- miniscule in comparison with the roughly 700 watts per square meter of sunlight bathing the landscapes of our planet. But to a sensitive infrared telescope, even this tepid flush might be detectable.

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