The solar system consists of the Sun, eight planets, at least five dwarf planets, hundreds of moons, millions of asteroids and comets, and vast stretches of gas, dust, and charged particles filling the space in between. The Sun alone accounts for more than 99% of the system’s total mass, with everything else, every planet, moon, and chunk of rock, making up the remainder.
The Sun
The Sun is a medium-sized star made up of about 92% hydrogen and 8% helium, with only trace amounts of heavier elements like oxygen, carbon, and iron. Deep in its core, hydrogen atoms fuse together to form helium, releasing the energy that lights and heats the entire solar system. This process has been running for roughly 4.6 billion years and will continue for another five billion or so.
Beyond producing light and heat, the Sun constantly releases a stream of charged particles called the solar wind. This plasma races outward at over a million miles per hour, carrying the Sun’s magnetic field with it. As the Sun rotates once every 27 days, it winds those field lines into a vast spiral that reaches every planet. The solar wind inflates an enormous bubble around the entire solar system called the heliosphere, which acts as a kind of shield against radiation from other stars.
The Four Rocky Planets
The inner solar system holds four small, dense worlds with solid surfaces: Mercury, Venus, Earth, and Mars. All four are built primarily from rock and metal, which is why they’re called terrestrial (from the Latin word for Earth).
Mercury, the closest to the Sun, has virtually no atmosphere. Its surface swings between extreme heat and cold, yet ice persists in permanently shadowed craters near its poles. Venus is wrapped in a thick blanket of carbon dioxide so dense that its surface pressure is 92 times what you feel at sea level on Earth. That atmosphere traps heat so effectively that Venus is hotter than Mercury despite being farther from the Sun. Earth is the only planet with large liquid-water oceans, an oxygen-rich atmosphere, and active plate tectonics. Mars has a thin carbon dioxide atmosphere, roughly 100 times thinner than Earth’s, but it still supports wispy water-vapor clouds and polar caps made of both water ice and carbon dioxide ice.
The Gas and Ice Giants
Beyond Mars, the four outer planets are vastly larger and made mostly of gases and ices rather than rock. They split into two pairs with meaningfully different compositions.
Jupiter and Saturn are gas giants. Their bulk is overwhelmingly hydrogen and helium, compressed into liquid and even metallic states deep inside. Saturn is famous for its spectacular ring system, made mostly of water ice crystals ranging from dust-sized specks to boulder-sized chunks, with varying amounts of dust and other impurities giving different rings slightly different colors. Jupiter also has rings, though they’re far thinner and darker.
Uranus and Neptune are often called ice giants because their interiors contain far less hydrogen and helium and much more of what planetary scientists call “ices,” primarily water, ammonia, and methane. Conventional models suggest these planets could be up to 90% ice by mass, with a small rocky core at the center and a relatively thin outer envelope of hydrogen and helium. More recent modeling suggests Uranus and Neptune might actually contain more rock than ice, perhaps up to 70% silicates mixed with hydrogen and helium. The debate is ongoing, but either way these two worlds are chemically distinct from Jupiter and Saturn. Both Uranus and Neptune also have ring systems, though much fainter than Saturn’s.
Dwarf Planets
A dwarf planet orbits the Sun and has enough mass to pull itself into a roughly round shape, but unlike a full planet, it hasn’t cleared its orbital neighborhood of other similarly sized debris. The International Astronomical Union currently recognizes five: Ceres, Pluto, Haumea, Makemake, and Eris, listed in order of distance from the Sun.
Ceres sits within the asteroid belt between Mars and Jupiter and is by far the smallest of the five. The other four orbit in the cold outer reaches beyond Neptune. Pluto, the most famous, was reclassified from planet to dwarf planet in 2006 under the IAU’s updated criteria. Many astronomers suspect dozens more objects in the outer solar system will eventually qualify, but only these five have been formally recognized so far.
Moons
Natural satellites orbit six of the eight planets, plus several dwarf planets and even some asteroids. The count varies as new discoveries roll in. Saturn currently holds the record with 274 confirmed moons after astronomers verified 128 additional small moons in March 2025. Jupiter follows with well over 90. Most of these are small, irregularly shaped bodies only a few kilometers across, likely captured asteroids or collision fragments. A handful are worlds in their own right: Jupiter’s moon Europa hides a liquid-water ocean beneath an ice shell, and Saturn’s moon Titan has a thick atmosphere and lakes of liquid methane on its surface.
Earth has one moon, and Mars has two tiny ones. Mercury and Venus have none.
Asteroids and the Asteroid Belt
Millions of rocky and metallic objects orbit the Sun, and the majority cluster in the asteroid belt between Mars and Jupiter. That belt contains an estimated 1.1 to 1.9 million asteroids larger than one kilometer across, plus millions of smaller ones. Despite those numbers, the belt is mostly empty space. If you gathered every asteroid together, their combined mass would still be less than that of Earth’s Moon.
Not all asteroids stay in the belt. Near-Earth asteroids cross or come close to our planet’s orbit, and Trojan asteroids share Jupiter’s orbit, clustered at gravitationally stable points ahead of and behind the planet.
Comets, the Kuiper Belt, and the Oort Cloud
Comets are icy bodies that develop glowing tails of gas and dust when they swing close enough to the Sun for their surface ice to vaporize. They originate from two distant reservoirs.
The Kuiper Belt is a broad ring of icy objects just beyond Neptune’s orbit, stretching from about 30 to 50 astronomical units (AU) from the Sun. One AU equals the distance from Earth to the Sun, roughly 93 million miles. Pluto and the other outer dwarf planets live here. Short-period comets, those that complete an orbit in under 200 years, typically originate from the Kuiper Belt.
Far beyond that lies the Oort Cloud, a spherical shell of icy bodies whose inner edge may begin around 1,000 AU from the Sun and whose outer edge extends to roughly 100,000 AU, nearly halfway to the nearest star. Long-period comets, which can take thousands or even millions of years to orbit the Sun, are thought to originate here. The Oort Cloud is considered the outermost boundary of the solar system by scientific consensus, even though no spacecraft has come close to reaching it.
The Space Between
The gaps between planets are not truly empty. The interplanetary medium is filled with the solar wind’s charged particles, the Sun’s magnetic field lines spiraling outward, and scattered grains of dust from asteroid collisions and comet tails. This tenuous material is far thinner than any vacuum we can create on Earth, but it has real effects: it shapes comet tails, erodes asteroid surfaces over millions of years, and interacts with planetary magnetic fields to create phenomena like Earth’s auroras.
At the outer edge of the heliosphere, the solar wind slows below the speed of sound at a boundary called the termination shock, then passes through a turbulent region called the heliosheath before reaching the heliopause, where the Sun’s influence gives way to interstellar space. NASA’s Voyager 1 and Voyager 2 spacecraft have both crossed this boundary, confirming it lies roughly 120 AU from the Sun. Beyond the heliopause, the Oort Cloud’s icy bodies still orbit the Sun, but the space around them is no longer dominated by solar wind. It belongs to the galaxy.