One star and everything that orbits it
The solar system is the Sun and everything held in orbit around it by gravity: eight planets, hundreds of moons, millions of asteroids, and a vast number of comets and icy bodies. The Sun holds almost all of the mass, commonly given as about 99.8 percent. Everything else is a thin scattering of leftovers by comparison.
The whole family formed roughly 4.6 billion years ago from a collapsing cloud of gas and dust. Most of the material fell into the center and became the Sun. The rest flattened into a spinning disk, and over time grains stuck together into larger bodies, which grew into planets. That origin explains a pattern you can see in the map today: the planets all orbit in roughly the same plane and in the same direction.
Distances are easier to grasp with a unit called the astronomical unit, the average distance from Earth to the Sun, about 150 million kilometers. Sunlight takes a little over eight minutes to cover it. Neptune sits at roughly 30 of those units, so light needs several hours to reach it.
The Sun
The Sun is a star, a huge ball of hot gas that shines because of nuclear reactions in its core. It is an ordinary, middle-aged star by galactic standards, and it is expected to keep shining steadily for billions of years more. Its gravity organizes the entire system, and its light and wind shape the conditions on every world. The Sun is also an active body, and the outbursts known as solar storms can disturb satellites and power systems on Earth.
The inner, rocky planets
The four planets closest to the Sun are small, dense and rocky.
- Mercury is the smallest planet and the closest to the Sun. With almost no atmosphere, it swings between extreme heat and cold.
- Venus is similar to Earth in size, but its thick carbon dioxide atmosphere traps heat and makes it the hottest planet.
- Earth is the only world known to host life, with liquid water at the surface and a protective magnetic field.
- Mars is a cold, dry desert today, though it shows many signs of a wetter past.
Inside the Earth, as inside the other rocky planets, heavier material sank toward a metallic core while lighter rock formed the outer layers. Rocky planets also tend to carry the scars of their history. Craters are preserved on airless worlds, while on Earth erosion and moving crust erase most of them.
The asteroid belt
Between Mars and Jupiter lies a broad region full of rocky and metallic fragments. These bodies are left over from the era of planet building, and Jupiter’s gravity likely kept them from merging into a single planet. Despite what movies suggest, the belt is mostly empty space, and spacecraft can cross it without close calls. The largest object there, Ceres, is classed as a dwarf planet.
The giant planets
Beyond the belt, the solar system changes character. Jupiter and Saturn are the gas giants, made mostly of hydrogen and helium with no solid surface to stand on. Jupiter alone holds more than twice the mass of all the other planets combined. Both have bands of fast winds, enormous storms, and large families of moons, and Saturn is famous for its bright rings.
Uranus and Neptune are usually called ice giants. They are smaller than Jupiter and Saturn and contain a larger share of heavier substances such as water, ammonia and methane, which are “ices” in the astronomer’s sense even though they exist as hot, dense fluids deep inside. Uranus is unusual because it is tipped on its side, so it rolls around its orbit rather than spinning upright like a top.
Moons
Most planets beyond Mars have moons, and the count changes as new small ones are found. Some are small captured rocks. Others are worlds in their own right. Ganymede, in Jupiter’s system, is larger than Mercury. Titan, around Saturn, has a thick atmosphere and lakes of liquid hydrocarbons. Io is intensely volcanic, and several others hide salty water beneath their ice, which makes them targets in the search for extraterrestrial life. Our own Moon is unusually large compared with Earth, and it influences tides.
Dwarf planets and the outer reaches
Past Neptune lies a wide, cold region of icy bodies called the Kuiper Belt. Pluto is its best-known member and is now classified as a dwarf planet, a category created in 2006. Further out, thought to extend a very long way from the Sun, is the Oort cloud, a hypothesized shell of icy objects that is the likely home of long-period comets. It has never been observed directly.
Comets are mixtures of ice, dust and rock. When one nears the Sun, heat vaporizes some of the ice and produces a glowing coma and tail. Asteroids and comets are best seen as two kinds of preserved building material rather than as rivals.
Why the planets differ
Temperature during formation explains much of the layout. Close to the young Sun it was too warm for ices to condense, so only rock and metal could build planets. Farther out, ice added extra solid material, allowing some cores to grow large enough to gather huge envelopes of gas. Details are still debated, and some evidence suggests the giant planets may have shifted their orbits early in history.
How we know what we know
Much of our knowledge comes from telescopes, including optical telescopes on the ground and in space, and from robotic spacecraft that have flown past or orbited every planet. Landers and rovers have studied Mars directly, and sample return missions have brought material back from asteroids. Spacecraft can also measure magnetic fields, atmospheres and the composition of surfaces.
The solar system is also our reference case. Because astronomers now know of thousands of planets around other stars through exoplanet detection, they use our home system as a yardstick to see what is typical and what may be unusual. Anyone who wants to build intuition for these scales might enjoy planetarium shows, which compress the distances and sizes into something visible.
Our system is a single example of how a star can end up with planets, and it is still being explored. Every new mission tends to raise as many questions as it answers.