Given that it's the closest planet to the sun, Mercury seems like it should be the hottest planet in our solar system.

However, at a blistering 900 degrees Fahrenheit (480 degrees Celsius), Venus tops Mercury's 800 F (430 C) highest surface temperature, despite being an average of 31 million miles (50 million kilometers) farther from the sun. So how can the second planet from our star be hotter than the closest planet to it?

It all comes down to reflectivity, atmospheric composition and geological history, experts told Live Science.

Totally different atmospheres

A planet's distance from its star is not the only factor that influences the planet's temperature.

"Distance tells us how much sunlight arrives at a planet, but it does not tell us how much is reflected … absorbed, how efficiently heat escapes, or how effectively the atmosphere transports heat around the planet," Stephen Kane, an astrophysicist who studies planetary habitability at the University of California, Riverside, told Live Science in an email. "Those properties can be just as important as distance, and sometimes much more important."

Mercury makes the case in miniature. According to Kane, the planet has essentially no atmosphere, so incoming sunlight strikes bare rock directly, heating it to extreme temperatures during the day. But with barely anything overhead to trap that warmth, Mercury radiates it straight back into space the moment the sun sets. As a result, nighttime temperatures plunge from roughly 800 F (430 C) during the day to about minus 290 F (minus 180 C) at night ‪—‬ a swing of well over 1,000 degrees, he added.

Venus tells the opposite story. Wrapped in an atmosphere that's roughly 90 times as dense as Earth's and consists almost entirely of carbon dioxide, Venus traps heat so effectively that its surface temperature barely changes at all, no matter where the sun happens to be, Kane explained.

A permanent blanket

Sunlight arrives at a planet mostly as near-infrared radiation and visible light, which pass through atmospheres consisting primarily of gases such as nitrogen, oxygen and carbon dioxide with relatively little trouble. Once the ground and lower atmosphere absorb that energy, though, they re-release it as infrared radiation, the kind of energy we feel as heat. Carbon dioxide happens to be excellent at grabbing and holding on to infrared radiation, Kane noted.

On Venus, the atmosphere is so deep and loaded with carbon dioxide that infrared energy leaving the surface gets absorbed and re-emitted over and over before any of it finally escapes into space. Some of that energy gets redirected back downward, Kane said, further warming the planet's lower atmosphere and surface. However, the heat isn't trapped forever, since energy conservation means that Venus must eventually release the same amount of energy it absorbs.

Notably, Venus doesn't actually soak up more sunlight than Mercury overall. Thick cloud cover reflects roughly three-quarters of incoming sunlight back into space before it ever reaches the ground, and only about 3% of the sunlight that arrives at Venus makes it down to the surface, Kane said. In fact, if that atmosphere and cloud deck were stripped away, a bare-rock Venus would actually run cooler than Mercury, since it would be receiving only about 29% as much sunlight from the start, he explained. It's the blanket, not the sunbathing, that makes Venus the hotter world.

Where the blanket came from

Venus' atmospheric blanket didn't spring into existence all at once, and how it formed is its own open question. Venus shows strong evidence of past and present volcanic and tectonic activity acting to smother the planet in greenhouse gases, Paul Byrne, a planetary scientist and associate professor of Earth, environmental and planetary sciences at Washington University in St. Louis, told Live Science in an email.

This volcanic activity has been so intense that Bryne describes modern Venus as sitting in a "post-runaway greenhouse" state, where its extreme heat has become a self-sustaining process regardless of what the planet's interior is doing at any given moment.

The heat isn't being generated from below; it's a consequence of the atmosphere Venus already has, locked in place by the same infrared-trapping effect described above.

Taken together, the experts' answers point to the same underlying lesson: How close a planet sits to its star is only the opening chapter of its climate story. What kind of atmosphere it has and how effectively that atmosphere holds on to heat are usually responsible for the rest.

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