It is not possible for all nine planets to align perfectly because the axes around which they rotate tilt at different angles. The planets do end up within 90 degrees of each other about once every 200 years.Continue Reading
In 561 B.C., all nine planets appeared in the same 30-degree sector, which is about the closest approximation of perfect alignment that the planets can reach. Astronomers expect all nine planets to line up this closely again in 2854.
Sometimes a subgroup of the planets approaches alignment. For example, in 2000, Mars, Jupiter, Venus, Saturn and Mercury were found within the same 50-degree sector.Learn more about Planets