The ESA’s Venus Express has shown that the rotation rate of Venus is Slowing Down and at an amazingly fast rate. Venus is already an odd planet in that it rotates in the opposite direction that the sun and other planets do. However the length of its day had increased 6.5 minutes in just 16 years. While that may not look like much, it is blazingly fast by planetary terms where the Earths rotational period increase increased by only a few seconds in the same time
This poses a problem for 4.5 billion years usualy given as the age of the Solar System becuses projecting back the rotation rate gets imposibly fast way befor the billion year mark.