The solar system is our cosmic neighborhood: one ordinary star, eight planets, five officially recognized dwarf planets with many more candidates, hundreds of moons, and countless smaller bodies. For most of history the planets were points of light. In the past sixty years robotic spacecraft have visited every planet, and these wandering lights have become real places, each with its own weather, landscapes and puzzles.

How it formed

About 4.6 billion years ago, a cloud of gas and dust began to collapse under its own gravity, perhaps nudged by a nearby exploding star. Most of the material fell to the center and became the Sun. The rest flattened into a spinning disk, and within it grains of dust stuck together into pebbles, boulders and eventually planets. This nebular hypothesis, first suggested in the eighteenth century, explains why all the planets orbit in nearly the same flat plane and in the same direction. We know the solar system's age from the decay of radioactive elements in the oldest meteorites, rocks that have changed little since the beginning.

Two kinds of planets

Close to the young Sun it was too hot for ice to survive, so only rock and metal could gather. That produced the four small, rocky terrestrial planets: Mercury, Venus, Earth and Mars. Farther out, ice was plentiful and the growing planets became massive enough to pull in huge amounts of hydrogen and helium. That produced the giant planets: the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune. Jupiter alone has more than twice the mass of all the other planets combined.

Each world teaches something. Venus, nearly Earth's twin in size, has a crushing carbon dioxide atmosphere and a surface hot enough to melt lead, about 465 degrees Celsius, the result of a runaway greenhouse effect. Mars is cold and dry today, but its dried riverbeds and minerals that form in water show that liquid water once flowed there, and rovers are searching its rocks for signs of ancient life.

Moons that might hold oceans

Some of the most exciting places in the solar system are moons. Jupiter's moon Europa appears to hide a salty ocean beneath its icy crust, holding more water than all of Earth's oceans. In 2005 the Cassini spacecraft saw jets of water vapor and ice spraying from cracks near the south pole of Saturn's small moon Enceladus, fed by an ocean below. Saturn's largest moon, Titan, has a thick atmosphere and lakes, not of water but of liquid methane and ethane. Wherever there is liquid water and a source of energy, scientists ask whether life could exist.

Small bodies and the Pluto question

Between Mars and Jupiter lies the asteroid belt, rocky leftovers that never formed a planet. Beyond Neptune lies the Kuiper belt, a vast zone of icy bodies. Farther still, astronomers believe, is the Oort cloud, a huge sphere of icy objects that may be the source of long-period comets, though no one has yet observed it directly. When a comet falls toward the Sun, its ice turns to gas and streams away in a glowing tail.

Pluto, discovered by Clyde Tombaugh in 1930, was counted as the ninth planet for 76 years. Then astronomers began finding similar icy worlds in the Kuiper belt, including Eris in 2005, which seemed about as large. Either there would soon be dozens of planets, or the definition needed tightening. In 2006 the International Astronomical Union defined a planet as a body that orbits the Sun, is massive enough to pull itself into a nearly round shape, and has cleared its orbital neighborhood of other objects. Pluto fails the third test and became a dwarf planet. Some scientists still dispute the definition. The episode is a reminder that categories are human tools, revised when we learn more, while Pluto itself did not change at all.

Our robotic explorers

The twin Voyager spacecraft, launched in 1977, flew past Jupiter, Saturn, Uranus and Neptune and are still sending data. In 2012 Voyager 1 became the first human-made object to enter interstellar space, beyond the bubble of particles blown out by the Sun. Around the end of 2026 it is expected to reach a distance of one light-day from Earth, so that a radio message takes a full day to reach it. In 2015 the New Horizons probe flew past Pluto and revealed mountains of water ice and a vast plain of frozen nitrogen.

Worked example: a solar system you can walk

Diagrams in books squeeze the planets close together. To feel the real scale, shrink everything so the Sun is a ball 10 centimeters across, about the size of a grapefruit.

• Earth becomes a grain of sand about 1 millimeter wide, roughly 11 meters from the Sun.
• Jupiter becomes a marble about 1 centimeter wide, roughly 56 meters away.
• Neptune becomes a bead about 3.5 millimeters wide, more than 300 meters away.
• Proxima Centauri, the nearest star, would be another grapefruit nearly 3,000 kilometers away.

The solar system is almost entirely empty space, and the gaps between stars are vastly emptier still. That scale shapes everything from how long spacecraft take to travel to why we may never visit another star in person.