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The Darkest Worlds

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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.

The bottom line is that finding orphan planets -- small, faint, and located who-knows-where - is not for the faint of heart. The task is comparable to observing a match flame at the distance of Pluto. The WISE satellite, a hi-tech, space-based infrared telescope especially suited for such work, has found only a few.

Mary Barsony, an astrophysicist at the SETI Institute, is trying to substantially increase the sample size. Her strategy is to take advantage of the fact that planets are born hot. Using the Subaru telescope on Hawaii's big island, Barsony's team looks for orphan worlds in nearby star-forming regions. Here the stars are in their infancy, and their planetary spawn will still be warm -- hence more visible.

Of course, all planets inevitably cool with time. But by examining stellar nurseries, Barsony greatly increases the chances of detecting orphan worlds -- planets that are no more than a few million years old, and not yet drifted far from the neighborhoods where they were born.

The strategy's clever, but the task is still challenging. Dim stars can mimic the appearance of a footloose planet, so sorting the wolves from the sheep requires a detailed spectroscopic search for the tell-tale signature of a low-mass object. So far, Barsony's team has found about a thousand candidates.

If most of these turn out to be orphans, Barsony will have validated our suspicion that a mammoth number of planets are roaming free.

Lacking sunlight to warm their surface or to fuel plant growth, these Stygian worlds may strike you as guaranteed to be deader than two-week old salad. But this simple assumption could be wrong. Life on dark worlds might be as common as dirt.

Metabolism could be sustained by the left-over heat from these rogue planets' fiery birth, not to mention the additional warmth generated by radioactive decay of elements in their cores. As noted, this inherited heat slowly subsides. But for larger planets, the cooling time can be billions of years.

Think of the Earth, whose warm interior is still adequate to slide continents around, uplift new mountains, and -- not incidentally -- power exotic life forms at ocean vents. Jupiter, a world far larger than Earth, is so warm that it currently radiates more internal heat than it receives from the Sun.

In addition, Barsony points out that orphan planets might have moons. For any planet with multiple satellites, "tidal heating" -- a type of frictional warming caused by mutual gravitational tugs -- might keep temperatures temperate. In our own solar system, tidal heating is the reason why Europa, and other moons such as Ganymede, Callisto and Enceladus, could have enormous, sub-surface oceans -- and maybe even life.

This is a truly remarkable idea: The possibility of biology on planets and moons that, mere decades ago, we hadn't even imagined were there. Untethered worlds that ply the loneliest spaces of the Galaxy like dark ships, freighted with living things and passing silently in the eternal night of space. It's a long way from the "M-class" planets of Star Trek lore.