Mercury sits closer to the Sun, yet Venus is the hottest world we know. The answer is an atmosphere that traps heat better than any oven door.
The wrong winner
Line the planets up by distance and Mercury should win the heat contest — it orbits at barely a third of our distance from the Sun. And at noon, Mercury does roast at around 430 °C. But with almost no atmosphere, its heat escapes to space the moment the Sun sets, plunging the night side to about −180 °C. Mercury doesn't hold heat; it borrows it.
A blanket 92 atmospheres thick
Venus took the opposite path. Its atmosphere is almost pure carbon dioxide, pressing down with 92 times Earth's surface pressure — standing on Venus would feel like being under a kilometer of ocean. Sunlight filters in, warms the ground, and the outgoing heat simply cannot leave: CO₂ absorbs it and radiates it back down. The result is a planet-wide 465 °C, day and night, pole to pole — hot enough to melt lead, and hotter than Mercury ever gets.
A warning written in rock
Venus is Earth's twin in size and gravity, which is exactly what makes it unsettling. Scientists believe it may once have had oceans, lost to a runaway greenhouse effect: heat evaporates water, water vapor traps more heat, and the spiral feeds itself until the oceans boil away. Venus is the textbook case of how a climate can tip — one of the reasons planetary scientists keep sending probes into that crushing sky.