[paul.fr]

Look at the nice picture.



now, will the planets ever all be in one straight line? Is that even possible?

[DoctorBeaver]

Yes & yes  []

[DrDick]

Astrologers like to put great significance on such alignments, which happen very rarely. 

When astrologers speak of the planets being aligned (something which doesn't really concern astronomers) they don't mean that the planets will actually all lie on a straight line at some instant of time. One calculation of alignments within around thirty degrees (about as close as they can get) shows that the last such alignment was in 561 BC, and the next will be in 2854. All nine planets are somewhat aligned every 500 years, and are grouped within 30 degrees every 1 to 3 alignments.

From "Curious About Astronomy?" http://curious.astro.cornell.edu/question.php?number=203

[syhprum]

There was a very good straight line alignment of all the naked eye planets (inculding Mercury) about seven years ago