It's only a small planet very much like Earth, and it flies in orbit around a small star much like the sun. It's 120 trillion miles away as most of us count - but only 20 light-years as astronomers see it.

After 11 years studying that planet's path in space through the eyes of the world's largest telescope, astronomers could announce recently that they had finally discovered the very first "exoplanet" whose orbit around its own sun places it clearly inside that distant solar system's "habitable zone," where gravity and temperatures make water plus an atmosphere - and life - clearly possible.

It defies imagination to recognize that if life is at least possible beyond our Earth and our life-giving sun, it could be ubiquitous on some planets everywhere in the galaxy - in some form or other, whether recognizable to us or not.

The news of the planet called Gliese581g came from Steven Vogt of UC Santa Cruz and Paul Butler of the Carnegie Institution in Washington, two veteran searchers for planets far beyond our solar system, and although Butler wouldn't speculate, Vogt said forthrightly that he puts the odds at "100 percent" that some form of life must exist on that planet right now.

Why was Vogt so certain? The best answer is that life on Earth exists literally everywhere, in wildly varied forms and in every conceivable environment, so why not in other environments elsewhere?

Microbes, after all, are even now chewing on hydrocarbons from the disastrous BP oil spill that persists at the bottom of the Gulf of Mexico 2 miles down, where it's perpetually dark and airless.

Even more advanced forms of life, like giant clams, crabs, shrimp and strange tube worms, are now thriving in near-boiling water at the mouths of volcanoes deep undersea. Such life was discovered more than 20 years ago 8,000 feet down in the Galapagos Rift Zone, where giant plates of the Earth's crust are slowly splitting apart and sulfur-eating bacteria have evolved in the airless total darkness to provide nourishing energy for the higher life-forms around them.

The highly radioactive wastewater and toxic sludge left over from the Hanford plutonium reactors in the state of Washington would kill any animal or human instantly. But bacteria called Deinococcus radiodurans have been discovered multiplying there and withstanding radiation a thousand times the levels deadly to humans.

In the sulfurous gases of a deep, dark South African gold mine, where oxygen is nonexistent and temperatures run higher than 140 degrees, there's a thriving bacterium with a whip-like tail called Desulforudis audaxviator. That creature gets its energy from the decay products of uranium, creates its oxygen from saltwater circulating in the rocks around it, and nourishes itself with organic molecules from water, carbon and the ammonia in the rocks of its happy habitat.

Hardy microbes thrive in the boiling fumaroles and geysers of Mount Lassen and Yellowstone, while others have evolved to colonize the polar ice caps of the Arctic and the frigid glaciers of Antarctica.

All these organisms are known as "extremophiles," and there's no extreme environment on Earth today that isn't home to one or more of them - some with genetic lineages millions of years old.

In fact, every ecological niche on Earth is occupied today by the living creatures that have evolved to fit in there. And, as Vogt said, in Earth's early days our planet must have been bombarded again and again by giant asteroids that destroyed all life here, and each time the sterilized Earth recovered, and life emerged and evolved again and again.

So why not on a planet like Gliese 581g? - or not on that planet, perhaps, but on any one of the hundreds of other "habitable zones" in solar systems throughout the galaxy that are yet to be discovered as the new science of "exoplanet" research continues.