Pluto
One cannot say for sure if it is the smallest planet in space, but it is the smallest planet in our Solar System.
The Earth is the only planet in our Solar System with one Moon
Earth is the major planet in our solar system that has only one moon, which is called the Moon.
Only Earth has exactly one moon.
The planet Mercury is not only the smallest of the terrestrial planets in our Solar system, but it is also the one closest to the Sun.
Within our Solar System, the Earth is the only planet with one moon.
Only Earth has exactly one moon.
Many planets have only one moon. For example, Earth only has one moon. There are likely to be billions of planets in our universe with just one moon. As you can imagine, the list of these would take a long time to write out. We will only have discovered a small amount of actual planets that fit this description though.
Earth
jupiter
Well no outer planet has only one moon they all either have more than 1 or none at all.