Which planet is closest to the Sun?

Mercury orbits nearest the Sun, circling it in about 88 Earth days, which is why the Romans named it after their swift messenger god. Being closest does not make it the hottest planet overall, though, and that catches people out. Mercury is small and its gravity is too weak to hold a proper atmosphere, so it has almost nothing to trap heat. Daytime surface temperatures climb to several hundred degrees while the night side plunges far below freezing, a swing wider than on any other planet. Venus, further out, is wrapped in a thick carbon dioxide atmosphere that traps heat so effectively that its surface stays hotter than Mercury's day side all the time. Mercury is also hard to observe from Earth because it never strays far from the Sun in our sky, appearing only briefly near the horizon at dawn or dusk. Its surface is heavily cratered, much like the Moon, since it has no weather or plate tectonics to erase impact scars.

Mercury has ice hidden in deep polar craters whose floors never see sunlight.