[Shorel]

No, by being in a plane, the seven planets form a polygon.

If you are in a planet, there is a line to any other planet. Two planets form a line. Three planets form a triangle. Four planets form some irregular polygon of four sides. And so on.

In this case, the area of the polygon formed by the seven planets is minimal, for a period of several years, and they are effectively, almost in a line.

This is not a common occurrence.

[throwawayk7h]

I believe the person you're responding to was observing that the planets will appear to be in a line to an observer on the surface of one of those planets.

[gweinberg]

Polygon? So on? Come on, you know you want to say "heptagon".