Scientists using an observational technique that exploits Albert Einstein's theory of gravity report the discovery of a planet just 5.5 times Earth's mass. The new world, located in Sagittarius toward the Milky Way's center, orbits a cool M-dwarf star 21,500 light-years away. "This finding means that Earth-mass planets are not that uncommon," says Kailash Sahu of Baltimore's Space Telescope Science Institute and a member of the discovery team. "If we found one, there must be more." The new world is the first discovered around another star that agrees with astronomers' theories of how planetary systems form. Princeton astronomer Bohdan Paczynski...