Which planet is the hottest in the solar system?

Venus is the hottest planet in the Solar System. Its average surface temperature is about 737 K, or roughly 464°C (867°F), hot enough to melt lead.

The main reason is Venus's extremely dense atmosphere, which is composed of about 96.5% carbon dioxide. This produces an intense greenhouse effect: sunlight warms the surface, while the thick atmosphere absorbs and re-radiates infrared heat, trapping much of it near the ground. Surface pressure is about 93 times Earth's sea-level pressure.

Mercury is closer to the Sun, so it receives more sunlight, but it has almost no atmosphere to retain heat. Its temperatures therefore swing dramatically between day and night. Venus stays uniformly scorching, including on its nightside. Its slow rotation and thick atmosphere also help distribute heat around the planet, making its surface hotter overall than Mercury's.