Why the Planets Align: Life Inside Two Differentially Rotating Disks

The Milky Way as seen from Colorado. (Credit: Joel Tonyan/Flickr) January was a good month for the planets. Scientists reported theoretical evidence for “Planet 9” and the alignment of Mercury, Venus, Saturn, Mars and Jupiter in the morning sky encouraged many people to wake up extra early to see the show. As you likely know, these planetary alignments occur because the solar system is largely shaped like a flattened disk with the ecliptic being the plane that holds Earth’s orbit around the sun. The orbit of Mercury is inclined 7 degrees relative to the ecliptic, more than twice the inclination of any other planet. That small range of orbital inclinations relative to the ecliptic means that any time it's viewed edge-on, the range of the planets would be very small in the direction perpendicular to Earth's orbit.