Our Solar System and Others

ORIGINS Discoveries of planetary systems around other stars are providing insights into the process by which planets form. The exoplanets are also possible abodes for life outside our own Solar System. A stronomers have long speculated about the origin of our Solar System. They have noted regularities in the way the planets orbit the Sun and in the spacing of the planetary orbits. But until recently, astronomers have been limited to studying one planetary system: our own. In 1600, Giordano Bruno was burned, naked, at the stake, in part for wondering about the “plurality of worlds,” to use the term of that day. Finally, after hundreds of years of wondering whether they existed, astronomers discovered another planetary system in 1992, though it was around a very strange type of star. And in the last two decades, planetary systems galore have been discovered around stars like the Sun. While twenty years ago we knew of only the handful of planets orbiting our Sun, as of this writing (2012) we know of about 1000 planets around other stars, as well as more than 2000 additional good candidates (see the Chapter Opener), the latter mostly from NASA’s Kepler spacecraft (■ Fig. 9–1 ).