Far-Out Earths 
We Earthlings have a pleasant climate in part because we’re located just the right distance from the Sun. Unlike Venus, we’re not so close as to be too hot, and unlike Mars, we’re not so far as to be too cold.
But two scientists recently suggested that rocky planets like Earth can have a mild climate even if they’re many times farther from their parent stars than Earth is. That means that millions of life-supporting planets could exist at much greater distances from their stars than currently thought possible.
Earth owes much of its climate not just to its location but also to its atmosphere, which contains water vapor and carbon dioxide. These greenhouse gases trap solar heat and raise the average temperature above freezing.
At large distances from the Sun, though, both gases freeze and lose their power to warm the air. But one greenhouse gas stays a gas, even at the low temperatures that prevail far from the Sun: hydrogen, the lightest and most abundant element in the cosmos.
The scientists calculated that a rocky planet like Earth with a thick atmosphere of hydrogen could stay warm even if it were a billion miles from the Sun, which is about as far out as Saturn is. And despite the thick atmosphere, enough sunlight would filter through for plants to conduct photosynthesis.
In fact, if there are any astronomers on such a far-out world, they may wonder whether life could exist on a planet located a mere 93 million miles from its sun.
Script by Ken Croswell, Copyright 2011