Which planet is closest to the Sun?

Mercury orbits at about 58 million kilometres, a little over a third of our distance, and goes round in 88 days. It is also the smallest planet, barely larger than our own Moon, and it has essentially no atmosphere - the solar wind strips away anything it manages to acquire.

That is why it is not the hottest. Mercury's daytime surface reaches around 430 degrees Celsius and its night falls to about minus 180, because there is no air to hold the heat once the Sun goes down. Venus, half again as far out, sits at about 465 degrees all over, all the time, because its thick carbon dioxide atmosphere will not let the heat leave. Proximity sets the input; atmosphere decides what is kept.

There is a pleasing oddity in the orbital mechanics. Because planets spend most of their time on the far side of their orbits from each other, and Mercury's orbit is the smallest, Mercury is on average the closest planet to Earth - and to Mars, and to Jupiter, and in fact to every other planet in the Solar System, including Neptune.