What happens when all nine planets line up?

The answer to this question is pretty straightforward.
Nothing!

From a scientific perspective there is no reason to believe
there would be any significant effect at all if all the planets were to
line up. First of all, it is literally impossible for all nine to line up
absolutely perfectly, so any alignment would only be a relatively close
alignment. Second of all, the amount of gravitational pull one planet
exerts on another planet is extremely small, given the distances. Planets
do have an extremely small effect on one another and the tiny shifts in
planetary motion due to mutual gravitational pull are actually measurable
by very precise instruments, but the effects, like I said, are very small
indeed. Claims that planetary alignments could somehow induce
earthquakes, floods of other such phenomenon are just plain wrong.

By comparison, the gravitational effect the moon exerts on the
earth is far larger. The existence of tides is an example of this. Any
conceivable effect one planet might have on another would be massively
outweighed by the effects a planet’s moons have on it.

I am assuming that your question arises because you have heard
some sort of claim about the significance of the alignment of certain
planets. Certainly astrologers would see great significance in such
events, as well as some self-proclaimed prophets and mystics. Some have
even tried to connect astronomical events such as planetary alignments
with specific biblical events such as the birth of Jesus. One thing which
can be said with certainty is that the Bible assigns absolutely no
significance to planetary alignments. Another safe statement is that
science will lend no credence to claims that accidental planetary
alignments which do occur at random times have any significant effect on
the earth. As to the claims of certain religions or philosophies
regarding the significance of planetary alignments, I would prefer to let
the validity of such claims and those who make them speak for themselves.

John Oakes