Venus

Venus is the second planet from the Sun and the brightest natural object in the night sky after the Moon — easily visible as the "evening star" or "morning star" depending on its orbital position. The planet appears nearly white-yellow and shines so brilliantly it is frequently mistaken for an aircraft, a UFO, or another planet. Venus is the third-brightest object in the entire sky after the Sun and Moon.

Despite being nicknamed "Earth's sister planet" for its similar size and mass, Venus is one of the most hostile environments in the solar system. The atmosphere is 96.5% carbon dioxide with sulfuric acid cloud layers, atmospheric pressure 92 times Earth's at the surface (equivalent to being 3,000 feet underwater), and surface temperatures averaging 867°F — hot enough to melt lead. The temperature is constant day and night because the dense atmosphere distributes heat evenly across the planet.

Venus is the textbook example of runaway greenhouse warming. The planet may have once had oceans, but its closer-to-the-Sun position caused enough water to evaporate that the resulting water vapor (a powerful greenhouse gas) trapped heat, evaporating more water, in a feedback loop that eventually drove off all the planet's surface water and left the dense CO2 atmosphere behind.

The surface has been mapped by Soviet Venera landers (which survived only minutes in the lethal conditions before dying) and by orbital radar mapping that penetrates the cloud layer. The planet shows volcanic plains, mountains, impact craters, and unusual landforms not seen on any other planet. Whether Venus has active volcanism today is a question of intense ongoing research.

Venus rotates backwards relative to the other planets — east to west rather than west to east — and extremely slowly. A single Venusian day (sunrise to sunrise) takes about 117 Earth days. The planet's year (one orbit around the Sun) is just 225 Earth days, meaning a Venus year is shorter than a Venus day.

From Earth, Venus shows phases like the Moon — full when on the opposite side of the Sun, crescent when between Earth and the Sun. Galileo's 1610 telescope observation of the phases of Venus was a critical observational confirmation that Venus orbits the Sun rather than Earth.