Do planets always revolve around their star in the same sense of rotation?

Answer

No, and when it was first shown, it was a surprise. Because according to our insights into how planets form (explained here several times, see for example www.ikhebeenvragen.be/scientist/question3.jsp?id=29338) the direction of movement of a planet should neatly follow the direction of rotation of the central star. The physical cause for this is that both star and planetary system around it get their rotational motion from the cloud in which they were born together. That cloud used to be much larger than both star and planetary system, and as it contracted everything started to rotate faster and faster, in the same direction, namely that of the original rotation.

But in practice we see that some planets (a significant minority) revolve in the ‘wrong’ direction around their star. So something special must have happened there. The only good explanation one finds, then, is that two planets once came close to each other, and so completely changed their motion as a result of their mutual attraction at the meeting; one went ‘wrong’, and the other catapulted away. The picture is somewhat confirmed by the observation that such ‘wrong-moving’ planets orbit at a fairly large angle to the equatorial plane of the star, which is again exceptional; indeed, in collisions the pieces fly off in quite random directions.

What we don’t see are systems where one planet moves in one direction, and the other moves in the other. A bit like the traffic in England and here: either we all drive on the left or we all drive on the right. Wrong-way drivers cause accidents, and planetary systems where there would once have been such wrong-way drivers have already passed that accident, and are now back in line, left or right.