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[edit]
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, largely due to "Venus" being the Roman name of their interpretation of the Greek Goddess of Love, Aphrodite, 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.
This led to Pulp Fantasy and thus Sword & Sorcery interpreting Venus as a steaming, primordial world; a planet of turbulent shallow seas, swelting swamps and steamy tropical jungles, often full of dinosaurs - a young and vibrant world compared to the dessicating husk of Mars.
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. During this time, it was also found that Venus is the only Solar System planet to rotate opposite of the direction it's orbiting in, suggesting it got reversed by a celestial backhand from a protoplanet instead of gaining a moon like Earth.
Venus is the 2nd planet from Sol and is notable for being similar to Earth in size, gravity, and overall composition. However, that's where the similarities end - with a ridiculously thick carbon dioxide atmosphere with copious clouds of sulphuric acid crushing you, dissolving you, and causing the planet to be fuck-off hot at the same time, 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. Visions on how to terraform Venus are diverse. Some think we should try to convert all the CO2 into oxygen and graphite via bacterial photosynthesis. Alternative proposals suggest creating super mountains a dozen times higher than Terran Mount Everest or Martian Olympus Mons with island peaks for humans to live on to avoid the crushing pressures and hellhole temperatures at ground level. Others propose creating floating cloud cities (like Star War’s Bespin) sailing atop Venus’ upper clouds. Both of the latter theories have the settlements’ temperatures and air pressures closer to the Sahara or Death Valley than literal subterranean hell. Such habitats would need to be robustly built to withstand strong winds and acids but is easier than building a pressure vessel thicker than a submarine. It also helps that breathable air is a floating gas lighter than CO2. Either way, the planet has a bit of a PR problem regarding colonization theory compared to Luna, Mars, or the Asteroid Belts due to popular culture.
Warhammer 40,000[edit]
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.
Before the age of the Imperium, the only history we know of Venus is that it was once ruled by War Witches who fielded armies of Litho-Gholem. The early Iron Warriors Space Marine Legion led the conquest of Venus during the Mehr Yasht campaign.
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 living in the deep cloud-hives of the planet. 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.