Mercury orbits the Sun in just 88 days, faster than any other planet. Despite being closest to the Sun, it is not the hottest planet — with virtually no atmosphere to trap heat, it swings between extreme highs and lows depending on whether a spot faces the Sun.

Mercury has no single discoverer — it is bright and fast-moving enough to have been tracked with the naked eye and observed by ancient Mesopotamian civilizations for thousands of years, and later recorded by Greek, Roman, and Chinese observers. Its rapid, erratic motion across the sky earned it its name, after the swift messenger god. The first close-up data came from NASA's Mariner 10 flybys (1974–75), and the first spacecraft to orbit Mercury was NASA's MESSENGER mission, which arrived in 2011.

Venus is the hottest planet in the solar system because its dense carbon dioxide atmosphere traps heat in a runaway greenhouse effect. It also rotates backward (retrograde) compared to most other planets, and a single day on Venus is longer than its year.

As the brightest planet in Earth's sky, Venus has been observed since prehistory and was recorded independently by nearly every ancient civilization, including the Babylonians, Maya, and Egyptians. Galileo Galilei's telescopic observations of Venus's phases in 1610 provided important evidence that planets orbit the Sun rather than the Earth. The Soviet Venera 7 became the first spacecraft to make a successful soft landing on another planet and transmit data from its surface, reaching Venus in 1970, and NASA's Magellan mission mapped the planet's surface using radar in the early 1990s.

Earth sits in the Sun's "habitable zone," where temperatures allow liquid water to exist on the surface. It is the only planet known to support life, and its active plate tectonics constantly reshape its surface over geological time.

Earth was not "discovered" as a planet in the way the others were — for most of human history it was understood as the fixed center of the universe. The idea that Earth itself is a planet orbiting the Sun took hold gradually, driven by the work of Nicolaus Copernicus (1543), Galileo Galilei, and Johannes Kepler in the 16th and 17th centuries, and became firmly established science after Isaac Newton explained the physics of orbits in 1687.

Mars gets its color from iron oxide (rust) covering its surface. It hosts Olympus Mons, the largest known volcano in the solar system at roughly three times the height of Mount Everest, and Valles Marineris, a canyon system that stretches nearly the width of the continental United States.

Mars's reddish glow made it visible and notable to ancient sky-watchers for millennia, including the Egyptians, Babylonians, and Chinese. Galileo first observed Mars through a telescope in 1610. In 1877, Italian astronomer Giovanni Schiaparelli mapped surface markings he called "canali" (channels), which were mistranslated as "canals" and fueled decades of speculation about Martian civilizations — later disproven. NASA's Viking 1 and Viking 2 became the first spacecraft to successfully land on Mars and operate there, in 1976.

Jupiter's Great Red Spot is a storm larger than Earth that has raged for at least 190 years. Jupiter's enormous gravity strongly influences the orbits of comets and asteroids: it ejects many objects from the solar system entirely, but it can also redirect others toward the inner planets, making its overall effect on Earth's impact rate more complex than the old "protective shield" idea suggests.

Jupiter has been tracked by naked-eye astronomers since antiquity. Its modern scientific significance began in January 1610, when Galileo Galilei used one of the first telescopes to observe four points of light orbiting it — Io, Europa, Ganymede, and Callisto, now known as the Galilean moons. This was the first direct observation of objects orbiting a body other than Earth, and it became a key piece of evidence supporting the Copernican, Sun-centered model of the solar system.

Saturn's rings are made of billions of individual chunks of ice and rock, ranging from dust-sized grains to boulders. Saturn is the least dense planet in the solar system — it is, in principle, light enough to float in a large enough body of water.

Saturn is the farthest planet easily visible to the naked eye and has been known since antiquity. Galileo observed it in 1610 but his telescope could not resolve the rings clearly — he described strange "handles" on either side of the planet. In 1655, Dutch astronomer Christiaan Huygens correctly identified these as a ring system and discovered Saturn's largest moon, Titan. In 1675, Italian-French astronomer Giovanni Cassini discovered the dark gap within the rings now known as the Cassini Division.

Uranus is tipped over on its side, with an axial tilt of about 98 degrees — likely the result of a massive collision early in its history. Methane in its atmosphere absorbs red light, giving the planet its pale blue-green color and helping make it the coldest planet in the solar system.

Uranus was the first planet discovered using a telescope rather than known since antiquity. British astronomer William Herschel spotted it on 13 March 1781 while surveying the sky from his garden in Bath, England, initially mistaking it for a comet. Follow-up observations confirmed it was a planet. It was eventually named Uranus, after the Greek god of the sky, at the suggestion of astronomer Johann Bode — the only planet named after a Greek rather than Roman deity.

Neptune has the fastest planetary winds in the solar system, reaching speeds of about 1,300 mph (2,100 km/h). Like Uranus, its blue color comes from methane in its atmosphere, and it takes about 165 Earth years to complete a single orbit of the Sun.

Neptune is the only planet found through mathematical prediction rather than direct sky-scanning. Astronomers noticed that Uranus's orbit was being pulled slightly out of place by the gravity of an unseen object. French mathematician Urbain Le Verrier and, independently, British astronomer John Couch Adams calculated where such a planet should be. On 23 September 1846, German astronomer Johann Galle at the Berlin Observatory pointed his telescope to Le Verrier's predicted position and found Neptune within about one degree of it on the very first night of searching.