It took many years of searching before Pluto was first discovered in 1930. Long before that, in the late 1800s, an astronomer named Percival Lowell noticed that two planets, Uranus and Neptune, did not go around the sun in a perfect circle as they should. This discovery meant that something else—another large body—must be pulling the two planets out of orbit. The search for a ninth planet began. On February 18, 1930, a young astronomer named Clyde Tombaugh found an object among the stars. After several colleagues confirmed his observations, Tombaugh's discovery was finally named the ninth planet. It was called Pluto after the Roman god of the underworld.
Which best describes the pattern of this paragraph?
sequence
compare and contrast
cause and effect