The planets in our solar system never line up in one perfectly straight line like they show in the movies. If you look at a two-dimensional plot of the planets and their orbits on a piece of paper you may be lead to believe that all the planets will circle around to the same line eventually. In reality, the planets do not all orbit perfectly in the same plane. Instead, they swing about on different orbits in three dimensional space. For this reason, they will never be perfectly aligned. It’s like waiting for a swarm of flies circling your head to all line up. It is not going to happen. When astronomers use words like “planetary alignment”, they don’t mean a literal lining up. They just mean that some of the planets are in the same general region of the sky. And this type of “alignment” almost never happens to all the planets, but instead happens to two or three planets at one time.
When was the last time all of the planets were aligned?
When astrologers speak of the planets being aligned (something which doesn’t really concern astronomers) they don’t mean that the planets will actually all lie on a straight line at some instant of time. One calculation of alignments within around thirty degrees (about as close as they can get) shows that the last such alignment was in 561 BC, and the next will be in 2854. The eight planets plus Pluto are somewhat aligned every 500 years, and are grouped within 30 degrees every one to three alignments.