Alignments of the fast-moving inner planets can occur as regularly as every few months or so, while groupings of the slower outer planets — Jupiter, Saturn, Uranus, and Neptune — occur far less often, but last longer when they do.
About every 100 years or so, six or more planets “line up” and appear together within a small area of the sky. A well-publicized conjunction of this type occurred May 5, 2000, when the Moon and all of the planets except Uranus, Neptune, and Pluto (which was still a planet then) lined up within 15 degrees or so of the Sun. Such gatherings have occurred tens of thousands of times in the past, with no observed physical consequences.