Strange New Worlds
Each pinpoint of light in the night sky is a sun, and each sun is potentially at the center of a planetary system like—or perhaps not so like—our own. All told, there are some 400 billion stars within our galaxy. How many have planets? How many planets are potentially habitable? And on how many has life actually taken hold?
As soon as astrobiology casts its net beyond the solar system, it runs into the daunting problem of interstellar distances. Imagine the Earth to be hollow and a dozen oranges scattered fairly evenly around the cavernous void inside. That gives some idea how far stars are apart in relationship to their size. Even the nearest star is almost unimaginably remote—10,000 times farther away than Pluto. If it were accessible by interstate highway, you would need about 50 million years to drive there. And that's the nearest star. Most of those specks of light in the night sky are tens or hundreds of times farther away. How can we possibly learn about any strange new worlds that might accompany them? And, assuming there are plenty of worlds out there, what determines whether they can support life, and of what kind?
One way to assess the biological potential of other stars and their worlds is to think about hypothetical planetary systems. Following their favorite rule of thumb, "Where there's water, there might be life," astrobiologists like to ask: Where around a star could water exist in its liquid state? The answer leads to the idea of a habitable zone (HZ). Put an Earth-sized planet in a star's habitable zone and its temperature would be moderate enough for it to support surface water and therefore features like rivers, lakes and oceans. A straightforward equation in physics, the Stefan-Boltzmann equation, gives