Venus

Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker and denser than Earth and any other rocky body in the Solar System. Its atmosphere is composed of mostly carbon dioxide (CO₂), with a global sulfuric acid cloud cover and no liquid water. At the mean surface level the atmosphere reaches a temperature of 737 K (464 °C; 867 °F) and a pressure 92 times greater than Earth's at sea level, turning the lowest layer of the atmosphere into a supercritical fluid. Venus is the third brightest object in Earth's sky, after the Moon and the Sun, and, like Mercury, appears always relatively close to the Sun, either as a "morning star" or an "evening star", resulting from orbiting closer (inferior) to the Sun than Earth.

The orbits of Venus and Earth make the two planets approach each other in synodic periods of 1.6 years. In the course of this Venus comes closer to Earth than any other planet, while on average Mercury stays closer to Earth (and any other planet) due to its more inferior orbit. Venus is frequently used for gravity assists, as a waypoint for interplanetary flights. Venus has no moons and has a very slow retrograde rotation about its axis, a result of competing forces of solar tidal locking and differential heating of Venus's massive atmosphere. As a result a Venusian day is 116.75 Earth days long, about half a Venusian solar year, which is 224.7 Earth days long.

Venus has a weak magnetosphere, lacking an internal dynamo it is induced by the solar wind and the atmosphere interacting. Internally, Venus has a core, mantle, and crust. Internal heat escapes through active volcanism, resulting in resurfacing instead of plate tectonics. Venus may have had liquid surface water early in its history with a habitable environment, before a runaway greenhouse effect evaporated any water and turned Venus into its present state. Conditions at the cloud layer of Venus have been identified as possibly favourable for life on Venus, with possible biomarkers having been found in 2020, which has spurred new research and missions to Venus.

Humans have been observing Venus throughout history across the globe, featuring it and its prominence in fiction, mythology, astrology and astronomy, acquiring particular importance in many cultures. With telescopes the phases of Venus became discernable and were in 1613 soon presented as decisive evidence disproving the then dominant geocentric model and supporting the heliocentric model. In 1961 Venus became the target of the first interplanetary space flight (Venera 1), while only the second interplanetary probe returned data, passing Venus the next year (Mariner 2). Venus also became the first planet other than Earth to be impacted (1967, Venera 4) and landed on (1970, Venera 7). The data returned revealed a planet with a thick CO₂ atmosphere and its resulting infernal greenhouse effect, raising concerns about global warming and climate change on Earth. As of 2025 the only probe active and reaching Venus is the Solar Orbiter, performing flybys until 2030. The next planned Venus mission is not set to launch before summer 2026 (Venus Life Finder).