Adventure movies of the 1980s and 1990s were obsessed with the possibility of the planets of the solar system aligning , bringing with it the end of the world or the return of an ancient and powerful curse. The reality however is less apocalyptic, although even more interesting. For one thing, the planets can’t align perfectly , and for another, even if they did, we could only feel it on Earth with the most sensitive instruments .
It is impossible to align the planets because, although the solar system is flat, it is not perfectly flat . The solar system was formed from one of the countless clouds of gas and dust that populated the Milky Way at its inception. This cloud was contracting by its own gravity , concentrating most of its mass in the center and creating small packages orbiting around this central region. Although the cloud was approximately spherical at its origin, the collisions of the particles that made it up were canceling any movement in a direction other than the main one , the average of all the individual directions.
This resulted in the planets and their moons, asteroids, and other objects all orbiting in more or less the same plane . This process of course was not perfect. Partly because the time that has passed since it began is finite, but also because there came a time when these collisions became so infrequent that they ceased to have any effect . Today we have the eight main planets orbiting in a configuration very close to a perfect plane. Mars, Jupiter, Uranus, and Neptune orbit within two degrees of Earth’s orbit , with Uranus being the closest to Earth’s. Saturn’s orbit is tilted almost two and a half degrees with respect to Earth’s, while Venus’s is over three degrees and Mercury’s is seven degrees .
Therefore, even if the planets were to align seen from “above” the solar system, the reality is that they would not be completely aligned , but forming a certain zigzag in the vertical coordinate. And this is considering only the planets themselves, which are by far the most massive objects in the solar system and therefore the ones that were most affected by that process of collision and arrangement. Dwarf planets clearly show the limitations of this process, with orbits having inclinations of tens of degrees . Pluto for example orbits with an inclination of more than 17 degrees. Ceres is nearby with a 10-degree tilt, while Eris , the only dwarf planet more massive than Pluto (that we currently know of, at least) has an orbital tilt of 44 degrees.
But what if all these orbits had the same inclination as Earth’s? Could there be a planetary alignment? The short answer is no , the long answer is no, before the Sun dies and takes Mercury, Venus and possibly Earth with it. One way to roughly find out how often their planets realign is to find the least common multiple of their orbital periods , the time it takes to complete one orbit around the Sun. If they started out perfectly aligned, we’ll have to wait that many years to let them all return to the same position. Taking the period of each planet until Neptune and rounding it to the nearest day, we obtain that these alignments occur every two trillion years . Bearing in mind that the age of the universe is less than 14 billion (that is, eight zeros would still be missing to reach the other figure) and that the Sun will die in about 5 billion years, that time would have to be greatly reduced to reach to see a planetary alignment.
Just for the four rocky planets, Mercury, Venus, Earth and Mars , to align, it would take almost a million years . The closest we can hope to an alignment is for them all to be in the same region of space . It seems that the next time this happens will be in May of the year 2492 , when the other seven planets will be visible simultaneously in the night sky . But still, Mercury will appear to the east and Jupiter to the west, with the rest of the planets in between.
But is that even if it really happened, the effects on Earth would be completely negligible . The only bodies that affect us with their gravity are the Sun , around which the Earth orbits, and the Moon , which creates the tides and keeps the Earth’s axis of rotation stable. The added effect of the rest of the planets barely accounts for a small percentage of the effect of the Moon . Therefore, it would perhaps mean an increase of a couple of centimeters in the highest tides on the planet, but little more.
Reference:
NASA Solar System Exploration Factsheet, NASA, Archived from the original on February 2004
