Venus is hotter than Mercury despite being farther from the Sun

Common sense says the closest planet to the Sun should be the hottest. Mercury orbits at roughly half Venus’s distance, baking in direct sunlight with no atmosphere to soften the blow. And yet Venus is the hottest planet in the solar system, with an average surface temperature of about 467°C (872°F) — hot enough to melt lead. Mercury, despite its front-row seat, tops out around 430°C in daylight and plunges far below freezing at night.

The culprit is air, or rather an enormous amount of it. Venus is wrapped in a crushing atmosphere of carbon dioxide, roughly 90 times denser than Earth’s, draped in clouds of sulfuric acid. That blanket lets sunlight filter down but traps the heat radiating back up — the same greenhouse effect that warms Earth, dialed to a catastrophic extreme.

It is the textbook case of a runaway greenhouse: a planet that cooked itself.

Scientists think Venus may once have had oceans. As the young Sun brightened, that water evaporated, flooding the atmosphere with vapor and CO₂ that trapped ever more heat in a vicious feedback loop. The water was eventually lost to space, leaving carbon dioxide to dominate.

The result is a world where the temperature barely changes between day and night or pole to equator — Mercury swings wildly, but Venus’s thick air spreads its furnace heat with brutal, suffocating uniformity.