Why did Earth thrive and our sister planet, Venus, died? From the fires of a sun's birth... twin planets emerged. Then their paths diverged. Nature draped one world in the greens and blues of life. While enveloping the other in acid clouds... high heat... and volcanic flows. Why did Venus take such a disastrous turn?

For as long as we have gazed upon the stars, they have offered few signs... that somewhere out there... are worlds as rich and diverse as our own. Recently, though, astronomers have found ways to see into the bright lights of nearby stars.

They've been discovering planets at a rapid clip... using observatories like NASA's Kepler space telescope... A French observatory known as Corot ... .And an array of ground-based instruments. The count is approaching 500... and rising. These alien worlds run the gamut... from great gas giants many times the size of our Jupiter... to rocky, charred remnants that burned when their parent star exploded.

Some have wild elliptical orbits... swinging far out into space... then diving into scorching stellar winds. Still others orbit so close to their parent stars that their surfaces are likely bathed in molten rock. Amid these hostile realms, a few bear tantalizing hints of water or ice... ingredients needed to nurture life as we know it. The race to find other Earths has raised anew the ancient question... whether, out in the folds of our galaxy, planets like our own are abundant... and life commonplace? Or whether Earth is a rare Garden of Eden in a barren universe?

With so little direct evidence of these other worlds to go on, we have only the stories of planets within our own solar system to gauge the chances of finding another Earth. Consider, for example, a world that has long had the look and feel of a life-bearing planet. Except for the moon, there's no brighter light in our night skies than the planet Venus... known as both the morning and the evening star.

The ancient Romans named it for their goddess of beauty and love. In time, the master painters transformed this classical symbol into an erotic figure. It was a scientist, Galileo Galilei, who demystified planet Venus... charting its phases as it moved around the sun, drawing it into the ranks of the other planets.

With a similar size and weight, Venus became known as Earth's sister planet. But how Earth-like is it? The Russian scientist Mikkhail Lomonosov caught a tantalizing hint in 1761. As Venus passed in front of the Sun, he witnessed a hair thin luminescence on its edge.

Venus, he found, has an atmosphere. Later observations revealed a thick layer of clouds. Astronomers imagined they were made of water vapor, like those on Earth. Did they obscure stormy, wet conditions below? And did anyone, or anything, live there?

NASA sent Mariner 2 to Venus in 1962... in the first-ever close planetary encounter. Its instruments showed that Venus is nothing at all like Earth. Rather, it's extremely hot, with an atmosphere made up mostly of carbon dioxide.

The data showed that Venus rotates very slowly... only once every 243 Earth days... and it goes in the opposite direction. American and Soviet scientists found out just how strange Venus is when they sent a series of landers down to take direct readings.

Surface temperatures are almost 900 degrees Fahrenheit, hot enough to melt lead, with the air pressure 90 times higher than at sea level on Earth. The air is so thick that it's not a gas, but a "supercritical fluid." Liquid CO2. On our planet, the only naturally occurring source is in the high-temperature, high-pressure environments of undersea volcanoes. It comes in handy for extracting caffeine from coffee beans... or drycleaning our clothes.

You just wouldn't want to have to breathe it. The Soviet Venera landers sent back pictures showing that Venus is a vast garden of rock, with no water in sight. In fact, if you were to smooth out the surface of Venus, all the water in the atmosphere would be just 3 centimeters deep. Compare that to Earth... where the oceans would form a layer 3 kilometers deep.

If you could land on Venus, you'd be treated to tranquil vistas and sunset skies, painted in orange hues. The winds are light, only a few miles per hour... but the air is so thick that a breeze would knock you over. Look up and you'd see fast-moving clouds... streaking around the planet at 300 kilometers per hour. These clouds form a dense high-altitude layer, from 45 to 66 kilometers above the surface.

The clouds are so dense and reflective that Venus absorbs much less solar energy than Earth, even though it's 30% closer to the Sun.