But what is the reason for picking that as a criteria? What do we know that makes that more likely than any other criteria?
There is no reason other than that we would have 5 planets which all have a unique and common trait. There is no other unique trait which only 4 or 5 planets have in common.
I know, why would the rotation matter. But it's better than looking for tidally locked planets as there is a crapload of tidally locked planets. Or planets with similar temperature, rotational periods etc. There are plenty of planets sharing those traits.
For the inverted rotation trait it's exactly 4 planets, 1 in each system (if that is in fact true, I still haven't checked)
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