"In the mid-1990s, astronomers made history when they detected three planets orbiting stars in the Milky Way. The planets were nothing like Earth, however: They were giant gas balls like Jupiter or Saturn. More than five hundred planets have been found since then, yet none of them could support life. Now, armed with more powerful technology, planet hunters are racing to find a true twin of Earth. Science writer Michael D. Lemonick has unique access...