“and all of the planets are lined in the sky”
No Doubt – Stricken

The simple response to this lyric is that it can’t happen – you will never get all of the planets exactly lined up in the sky. That makes for a very short blog post though so let’s go into a bit more detail!

There are two ways that you could think of the planets lining up. The first is them being in a straight line going out from the Sun. People tend to think of the solar system in this way because it’s how the planets are often shown in graphics like the one below. The problem with this picture though (besides the fact that the distances between the planets are far larger, and Pluto is included at the end!) is that the planets in fact all orbit in slightly different planes around the Sun; in other words each orbit is slightly tilted compared to the others. This means that at any one time, some of the planets will be higher up or lower down compared to the imaginary straight line drawn out from the Sun. If you flatten the solar system down to a 2D representation then I would assume that  the planets will pretty much line up at some point in the future but I couldn’t easily find an online orrery that could shown me when!

The solar system. Credit: NASA

The second way to think about this is the planets being lined up in the sky as we see them from Earth. This is probably what No Doubt were going for in this song. We can often see planets in the night sky (as discussed in the Jimmy Eat World post) but which planets we see depends on where they are in their orbit compared to the Earth. For example, if a planet is on the other side to the Sun from us then we won’t see it. This means that it is very rare for all of the planets to be visible from Earth at the same time. According to these posts, the next time the planets will roughly be lined up will be 6th May 2492 as you can see in the below screenshot from stellarium; unfortunately I doubt any of us will be around to see it!

Stellarium screenshot showing the visibility of all the planets in 2492. Credit: Stellarium/Jen Gupta

It is more common to have two or three planets in the sky at the same time, although because of the tilt of their orbits they often don’t appear in a straight line (e.g. see Mercury, Venus and Jupiter form a triangle in this post). However, occasionally the bright (naked-eye) planets do approximately line up; the last such alignment was in 2002 and a similar alignment will be visible in September 2040. If you want to see some planets tonight, currently (early January 2015) you can spot Mercury, Venus and Mars in the south-west just after sunset.

Finally, every so often a hoax goes round that a planetary alignment (of the first kind) will cause people to float into the air due to the combined gravitational pull of the planets. This is of course nonsense, but thanks to a photoshopped tweet it has been doing the rounds again, with weightlessness scheduled for tomorrow (4th January 2015). Thankfully, Universe Today has done a great job of writing about this so I don’t have to!