Mercury is the smallest and closest to the sun of all the eight planets. Mercury is a hot planet since its the closet one to the sun. It is 35,980,000 miles away from the sun. It might seem like a lot and that the heat would never reach Mercury but now think about it if the heat can reach earth and its way farther than Mercury. It probably will be hot. Mercury has a radious of 1,516 miles. Its surface area is 28.88 million square miles.