On the NASA website, Dr. Eric Christian answered this same question, saying, “If you are talking about an ‘exact’ line, it has probably never happened. Even having all 9 planets in a rough line is very infrequent (millions of years, depending upon how ‘rough’ a line you’re willing to take.”
Depending on a planet’s rotation around the sun, a few may line up when their years coincide, but to have them all line up at once is very rare. This is because while the earth takes one year to rotate around the sun, Mars takes two, and so on for every planet. The timing just is too different for a full alignment to happen with any kind of regularity.
Depends what you mean by align. They have some kind of alignment at all times.
If you mean opposition, see http://en.wikipedia.org/wiki/Opposition_%28planets%29. If you mean conjunction, see http://en.wikipedia.org/wiki/Astronomical_conjunction. There are many other kinds of alignments that the planets may have.
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