Venus and Mercury are the two planets closest to the Sun. So it seems reasonable to assume that Mercury, the closest of all, should also be the hottest.
It isn't.
Venus holds that record, with an average surface temperature of about 465°C — hot enough to melt lead.
Mercury has almost no atmosphere
Mercury receives far more solar energy than Venus because it is much closer to the Sun. But Mercury has almost no substantial atmosphere to retain that heat.
Its temperature therefore changes dramatically. The side facing the Sun can become extremely hot, while the night side becomes extraordinarily cold.
Venus is very different.
Venus is wrapped in a massive atmosphere
Venus has an extremely dense atmosphere composed mostly of carbon dioxide. Thick clouds containing sulfuric acid cover the planet.
Sunlight can reach and heat the planet, but its dense atmosphere makes it extremely difficult for that energy to escape back into space.
The result is an exceptionally powerful greenhouse effect.
A runaway greenhouse
Earth also has a natural greenhouse effect. Without it, our planet would be much colder.
Venus demonstrates what can happen under radically different planetary conditions. Its carbon-dioxide-rich atmosphere traps enormous amounts of heat, keeping temperatures extremely high across the planet.
This is why Venus remains hotter than Mercury despite being farther from the Sun.
Could you stand on Venus?
Venus does have a solid, rocky surface. But surviving there would be extraordinarily difficult.
Besides temperatures around 465°C, the atmospheric pressure at the surface is roughly 90 times the pressure at Earth's sea level.
Several spacecraft have reached the surface, including probes from the Soviet Venera programme, but they survived only for relatively short periods in the extreme environment.
One planet, one remarkable lesson
Distance from the Sun isn't the only thing that determines how hot a planet becomes.
Mercury shows what happens when a planet receives intense sunlight but has little atmosphere to retain the heat. Venus shows the opposite extreme: a dense atmosphere capable of trapping enormous amounts of energy.
And that is why the second planet from the Sun — not the first — is the hottest planet in our Solar System.