Astronomy Cast Ep. 172: William Herschel - REMASTER By Fraser Cain & Dr. Pamela Gay From January 11, 2010. Ancient astronomers knew of 5 planets, Mercury, Venus, Mars, Jupiter and Saturn – 6 if you count Earth. And then in 1781, William Herschel discovered an entirely new planet, Uranus, boosting the number of planets to 7. Let’s learn about Herschel, his equipment, his discoveries, and his sister Caroline – an accomplished astronomer all on her own!