Extrasolar Planetary Systems and Life in other Solar Systems

The family of planets grew for the first time in written history when, in 1781, William Herschel discovered a new planet, Uranus, which he first took for a comet (Chap. 11). Once an additional planet was found, even though accidentally, the possibility of others was more readily considered. In the late 1700s, this was also inspired by the empirical Titius-Bode law which at that time seemed to predict the distances of the known planets through Uranus nicely except for a prediction of a nonexistent planet at 2.8 AU (see Box 11.1).