All planets in our solar system cannot align in a straight line. The unique orbits of each planet around the Sun include slightly different tilt angles and speeds within three-dimensional space, which makes such a perfect alignment practically impossible.
Each planet orbits on its own distinct plane, tilted at small angles relative to Earth’s orbital plane (the ecliptic). Due to these orbital tilts, the planets never reach the exact same three-dimensional coordinates necessary for a straight line.
When ignoring these tilts and considering only a top-down view, an exact alignment of all eight planets occurs roughly once every 400 billion to several trillion years. However, when factoring in the actual three-dimensional orbital tilts, the likelihood becomes astronomically rare—around once every 86 billion trillion trillion trillion years.
Long before such an alignment could ever occur, the Sun will exhaust its fuel and become a red giant, expanding enough to engulf the inner planets in about 5 billion years. When individuals mention planets lining up, they typically refer to a visual phenomenon known as a planetary parade.
Since all the planets orbit the Sun within a relatively flat, disk-like system, multiple planets can appear to group together in the same section of the night sky from our perspective on Earth. This creates the illusion that the planets form a line in the sky, even though they are actually millions of miles apart in space.
Even during such groupings, the gravitational influence of the other planets on Earth is negligible; only the Moon and the Sun exert enough gravity to noticeably affect Earth, shaping our daily tides and seasonal changes.
Content for this question contributed by Caryn Franklin, resident of Cincinnati, Ohio, USA
