Are we not the only Earth out there?
You stand in a perpetual sunset, beneath an eerie, reddish-orange sky laced with thin clouds. At the edge of a vast sea, solid ground rises slowly from the water, giving way to lowlands covered in vegetation. The plants bask in temperatures reaching 40 degrees Fahrenheit (4 degrees Celsius), but their leaves aren't green -- they're black and spread open wide to absorb the scant energy washing across the landscape.
You've come to this paradise from your permanent home, an outpost located on the dark, frozen side of the planet. You hike down the lowland hills to the water's edge. As you gaze at the horizon, you vow that, next year, you'll bring the whole family so they can enjoy the color and heat and light. Then you realize that next year is just 37 days away, and you feel suddenly small and insignificant in a vast, overwhelming universe.
This could be your future Earth. No, really.
The scene we just described is an artistic interpretation of what Gliese 581g -- a potential Earth-like planet discovered in 2010 -- might be like if we could travel the 20.5 light-years to get to it. Granted, astronomers haven't confirmed its existence, but that hasn't stopped a few from running computer simulations to predict 581g's climate and overall habitability.
The models suggest that this strangely familiar world, which races around red-dwarf Gliese 581 in just 37 days, keeping one face pointed at the star at all times, may be covered in water and may possess an atmosphere containing large amounts of carbon dioxide. If so, a greenhouse effect just might heat the region directly facing the host star, producing an ice-covered planet with a large area of liquid water in the middle that looks like the iris of an eye. This "eyeball Earth" could support life, including photosynthetic organisms with black pigments especially suited to absorb the weak light filtering through the thick atmosphere.
Even if Gliese 581g turns out to be a figment of astronomy's imagination, it stands as a symbol of what could be humanity's greatest triumph: finding a habitable planet outside of our solar system. A few years ago, this seemed a dream of fools and sci-fi fanatics. Now, thanks to advanced planet-hunting techniques and some serious equipment, such as the Kepler space telescope, astronomers are locating thousands of candidate planets outside of our solar system -- what they call exoplanets -- and are coming to a sobering, almost frightening realization: The universe may be filled with billions of planets, some of which most certainly resemble Earth.