Feel the Scale
The Sun holds about 99.8 percent of all the mass in the solar system. Everything else, every planet, moon, asteroid and comet, shares the remaining sliver. Astronomers measure distances here in astronomical units: one AU is the average Earth–Sun distance, about 149.6 million kilometres. Sunlight crosses it in roughly 8 minutes and 20 seconds.
Shrink the Sun to a football and Earth becomes a peppercorn about 25 metres away. Jupiter is a large marble at around 130 metres, and Neptune a smaller marble nearly 750 metres off. Space is mostly empty, which is why spacecraft take years to cross it.
The Rocky Four
Mercury is the smallest planet and closest to the Sun, a cratered world with almost no atmosphere. Venus is nearly Earth's size but wrapped in thick carbon dioxide; its runaway greenhouse effect pushes surface temperatures to around 465 °C, hotter than Mercury. Earth is the only world known to host life and liquid surface oceans.
Mars is half Earth's diameter, red from iron oxide dust, and home to Olympus Mons, a shield volcano rising roughly 22 km, and Valles Marineris, a canyon system some 4,000 km long. Dry riverbeds and lake deltas, such as Jezero Crater, show that water once flowed there.
Gas Giants and Ice Giants
Jupiter is more massive than all the other planets combined. Its Great Red Spot is a storm wider than Earth that has been observed for centuries. Saturn is so low in density that it would float in a large enough bathtub; its rings are made mostly of water ice, ranging from dust to house-sized chunks.
Beyond them lie the ice giants. William Herschel discovered Uranus in 1781, the first planet found with a telescope. It is tipped over by about 98 degrees, rolling around the Sun on its side. Neptune was found in 1846 at Berlin by Johann Galle, guided by Urbain Le Verrier's mathematical prediction, a triumph of physics over guesswork.
Jupiter
Fifth from the Sun. Cloud belts and the four Galilean moons are visible in a small scope.
How to observe it →Saturn
Sixth from the Sun. The rings tilt toward and away from us over a roughly 29-year orbit.
Cassini's story →Moons Worth Knowing
Our Moon orbits at an average of 384,400 km and always shows us the same face because its rotation is tidally locked to its orbit. Galileo spotted Jupiter's four largest moons in January 1610: Io, the most volcanically active body known; Europa, with a global ocean beneath its ice shell; Ganymede, larger than Mercury; and cratered Callisto.
Saturn's Titan has a thick nitrogen atmosphere and lakes of liquid methane and ethane. ESA's Huygens probe landed there on 14 January 2005. Tiny Enceladus sprays plumes of water vapour and ice from its south pole, discovered by NASA's Cassini spacecraft, making it a prime target in the search for life.
Dwarf Planets, Asteroids and Comets
Between Mars and Jupiter lies the main asteroid belt. Its largest member, Ceres, is classed as a dwarf planet. On 24 August 2006, the International Astronomical Union's General Assembly in Prague defined a planet as a body that orbits the Sun, is rounded by its own gravity and has cleared its orbital neighbourhood. Pluto fails the third test and became a dwarf planet.
Pluto lives in the Kuiper Belt, a ring of icy bodies beyond Neptune. NASA's New Horizons flew past Pluto in 2015 and past the snowman-shaped Kuiper Belt object Arrokoth on 1 January 2019. Further out still, the hypothesised Oort Cloud is thought to supply long-period comets. Halley's Comet, the most famous periodic comet, is next due near the Sun in 2061.
What You Can Actually See
Binoculars: lunar maria and craters, Jupiter's moons as tiny points, Venus's crescent phase near its closest approach. Small telescope at 25x and up: Saturn's rings. At 100x or more on a steady night: Jupiter's belts and the Great Red Spot, Mars's polar caps when it is close. Mars comes nearest at opposition, roughly every 26 months.