Earth has a birthday today.
If it's yours, here's wishing you a happy one. But just for fun, let's suppose there is life on all of them. And suppose a person was born on each of the other eight at the same precise moment you were here on Earth. When you had your 1st birthday, your twin on Mercury would have been just a bit over 4 years old. That's because Mercury orbits the sun slightly more than 4 times for each orbit of Earth.