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A Planet Someone Might Call Home

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Well, they've finally done it. Although astronomers have bagged nearly 500 exoplanets in the last 15 years, they've all been Christmas presents that, once unwrapped, weren't all that exciting. They consisted of worlds that were too big -- or too hot or cold -- for life-as-we-know-it. Now, finally, researchers have uncovered a planet that might be a cousin of Earth.

This is good news, as it begins to redress a discouraging fact: We've found precious few other solar systems that resemble our own. Years ago, astronomers blithely assumed that other systems would follow the same blueprint as the Sun's -- small, rocky (and potentially habitable) planets orbiting close to their host star, and giant, smelly gas giants farther out.

But many of the planets found in the last 15 years have marched to the beat of a different drummer, sporting big, bulbous worlds that hugged their star so tightly, a year could be as short as 100 hours.

Sure, these so-called "hot Jupiters" were interesting from an astrophysical point of view. They challenged researchers to understand how they formed, and how they managed to fall into such constricted orbits. But astrophysics aside, these toasty worlds were clearly not the sorts of locales where life could set up shop. That would be biology in a blast furnace.

In other words, the planet hunters were returning from the field with the wrong kind of trophies. Planets similar to Earth weren't part of the prize.

Astronomers explained that this was simply due to what they call "selection effects". Their search techniques were attuned to finding big planets, close-in to their suns. So when asked "where are our planetary doppelgangers?", many astronomers whistled in the dark: Sure, they declared, there could be many Earth-like worlds out there, but we need other techniques to find them.

Now there's growing reason to think that this optimism is justified. Researchers using the Keck telescope in Hawaii have found a planet that could, at least in principle, be awash in oceans and blanketed with an atmosphere. The planet is located in the star system Gliese 581 -- a mere 20 light-years away.

Mind you, this isn't Earth's identical twin. Gliese 581 is a small dwarf star, only a fraction the size and brightness of our Sun. Therefore, any planets that are in its so-called "habitable zone", which is to say at the right distance to possibly have liquid oceans on their surfaces, will be in smaller orbits than for the much hotter Sun. The new planet is in such a tight track around its star that it has succumbed to what's called "tidal locking". This means that one side perpetually faces Gliese 581, much as one side of the Moon faces Earth.

So one hemisphere of this new world, which has a diameter only about 50 percent larger than Earth, will be substantially and permanently warmer than the other. But tidal locking need not rule out life, which might establish itself somewhere in the sweet spot straddling the sunny and dark sides of this world.

The obvious question is: could this new planet have inhabitants? We don't yet know, although there's been at least one attempt to find out. The SETI Institute, as part of a decade-long survey of nearly a thousand star systems, twice swung its antennas in the direction of Gliese 581, hoping to pick up radio signals that would prove that someone was home. No transmissions were detected.

That should provoke disappointment, but not discouragement. After all, you could have looked at Earth with a radio telescope for most of its 4-1/2 billion-year history, and failed to find any signals. Even if this newly discovered world is carpeted with creatures, they might not yet be of the sort able to build a radio transmitter.

The important thing about Gliese 581's newly discovered habitable planet is not that we should expect to find some Glieseans, but that planetary cousins of the Earth may, indeed, be as common as corn flakes. And if that's true, then our expectations for finding our opposite numbers among the stars will get a significant boost.

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