Mercury It is the smallest planet in our solar system

 Mercury 

It is the smallest planet in our solar system and the one closest to the Sun.

Quick facts:

Distance from Sun: about 58 million km, 36 million miles

Diameter: 4,879 km. That is a bit bigger than our Moon

Orbit: 88 Earth days to go around the Sun

Day: 59 Earth days to rotate once

Year: so a day on Mercury is longer than its year

Temperature: -173C to 427C, -279F to 801F. No atmosphere to hold heat in

What it is like:

Rocky surface covered in craters, kind of like the Moon

Iron core that makes up about 75 percent of the planet

Almost no atmosphere, just a very thin exosphere of atoms blown off the surface

No moons and no rings

Why it is tricky to study:

Because it is so close to the Sun it is hard to see from Earth, only at dawn or dusk

NASA sent MESSENGER which orbited from 2011 to 2015 and mapped most of it

ESA and JAXA have BepiColombo on the way now, it should arrive in 2027

Fun bit:

Even though Venus is farther from the Sun, Mercury can actually be hotter on the surface because Venus has thick clouds that reflect sunlight.