Venus

Venus is Terra's evil twin, the deadly and hellish to the Earth's (relative) peace and tranquility. Unless you're in Sci-Fi, in which case we conquered the planet and terraformed it into Earth 2.0 or better.

Overview

Venus, being the brightest object in the night sky that isn't Luna, has likely been seen in the night sky since the days man was not too far off from animal (i.e. still primitive hunter gatherers). It had a tendency to be confused for being two bodies in ancient times (the "morning star" and "evening star"), though civilizations as far back as Sumeria realized they were just seeing the same celestial body twice in a day. It took until Pythagoras proved Venus, without a doubt, was both the Morning and Evening Stars in 600 BCE to finally lay the two-body belief fully to rest, however.

In the Middle Ages, Venus as one of the five classical planets would be associated in alchemy with copper and Friday. It would also be associated with femininity, leading to its alchemical symbol being repurposed as representing Hot Chicks forevermore. On the more scientific side of things during the Renaissance, observations of Venus showed it orbited between the Earth and Mercury and had phases, contributing to the shift to heliocentric models at the time. However, knowledge of Venus hit a roadblock in the 18th century after discovery of its atmosphere - with it being so thick, it was impossible to gleam what Venus was like below it with the methods of the time.

Modern times picked up the slack as rocketry took off (literally), leading to the advent of probes using radar to map Venus' surface. It was these radar scans that had us discover how shit Venus really is, showing signs of giant mountains of fire and rivers of lava, all ultimately driven by runaway absorption of the Sun's rays. The Soviets famously made the first landing on the surface of the hothouse, though the probe didn't last long before becoming unusable partially-molten scrap.

Venus is the 2nd planet from Sol and is notable for being similar to Earth in size and overall composition. However, that's where the similarities end - with a ridiculously thick carbon dioxide atmosphere both crushing you and causing the planet to be fuck-off hot, standing on Venus in the present day is, for all intents and purposes, instantly deadly. There's a reason why it needs to be terraformed in sci-fi stories, after all.

Warhammer 40,000

Since we are in Grimdark, there's no 'Second Earth' hippie bullshit here. At least, not in the Age of the Imperium - the Dark Age of Technology may have seen an actual verdant Venus instead of one we just know is habitable. Kinda familiar sounding to Destiny (having its own Venus being a terraformed paradise that gets turned unsafe by some cataclysm), though both are ripping off much older sci-fi stories.

In the grim darkness of the 41st milennium, Venus lives a double life - an Industrial World on the surface, the planet itself is not much of note besides its identity. However, it is also host to the finest luxury orbitals in the entire Galaxy, and is essentially the summer getaway for the top dogs of the Imperial hierarchy. Emperor knows how much Slaaneshi corruption has weasled its way into these hubs of hedonism. In that sense, it can also be considered an arechetypal Pleasure World, though unlike most the pleasure is most certainly not to be found on the surface of the world.