What is the solar system? The Sun, 8 planets and more, explained simply
By the WhyCosmos team · Last updated:
The solar system is the Sun and everything held in orbit around it by the Sun's gravity: eight planets and their moons, dwarf planets such as Pluto, and countless asteroids, comets and smaller bodies. It formed about 4.6 billion years ago from a collapsing cloud of gas and dust, and the Sun holds about 99.8 percent of its mass.
Most children meet the solar system as a list of eight names to memorise, usually drawn as eight balls lined up neatly beside the Sun. Almost everything about that picture is out of scale. This page covers the planets in order, how the solar system formed, what really happened to Pluto, and where the solar system actually ends.
How to explain it to your child
Explain it to an 8-year-old
The solar system is the Sun's family. The Sun is in the middle, and eight planets travel around it in big loops, along with many moons, rocks and icy objects. Earth is the third planet from the Sun. The Sun's pull, called gravity, keeps the whole family together, the way a string keeps a ball going round when you swing it.
Explain it to a 13-year-old
The solar system is a planetary system bound by the Sun's gravity. It formed about 4.6 billion years ago when a cloud of gas and dust collapsed: most of the material became the Sun, and a spinning disc of leftovers built up the planets. The four inner planets are small and rocky; the four outer planets are giants made mostly of gas and ice. Beyond Neptune lie the Kuiper Belt and, much farther out, the Oort Cloud, a vast shell of icy bodies thought to surround the whole system.
Key facts
| Age | About 4.6 billion years |
|---|---|
| At the centre | The Sun, holding about 99.8 percent of the solar system's mass |
| Planets | Eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune |
| Dwarf planets recognised by the IAU | Five: Ceres, Pluto, Haumea, Makemake and Eris |
| Largest planet | Jupiter, about 11 times as wide as Earth |
| Hottest planet | Venus, around 465°C at the surface |
| Earth's distance from the Sun | About 150 million km, called one astronomical unit (AU) |
| Farthest spacecraft | Voyager 1, in interstellar space since 2012 |
What are the 8 planets in order from the Sun?
| # | Planet | Type | One fact worth remembering |
|---|---|---|---|
| 1 | Mercury | Rocky | The smallest planet and the closest to the Sun; its year lasts just 88 Earth days |
| 2 | Venus | Rocky | The hottest planet, because its thick carbon dioxide atmosphere traps heat |
| 3 | Earth | Rocky | The only world known to have life and liquid water on its surface |
| 4 | Mars | Rocky | Red because of iron oxide, or rust, in its dust; home to Olympus Mons, the tallest volcano in the solar system |
| 5 | Jupiter | Gas giant | The largest planet; more than 1,300 Earths could fit inside it |
| 6 | Saturn | Gas giant | Famous for its bright rings, and the planet with the most known moons: 285 as of March 2026 |
| 7 | Uranus | Ice giant | Spins tipped over on its side, tilted about 98 degrees |
| 8 | Neptune | Ice giant | The farthest planet, about 30 times Earth's distance from the Sun, with the fastest winds measured in the solar system |
The four inner planets are called rocky or terrestrial planets because they have solid surfaces. Jupiter and Saturn are gas giants; Uranus and Neptune are ice giants, built largely from water, methane and ammonia around a small rocky core. A memory line many children use: My Very Educated Mother Just Served Us Noodles.
What else is in the solar system?
- Moons: hundreds of them. Earth has one; Saturn has 285 known moons and Jupiter 101, as of March 2026.
- Dwarf planets: round worlds such as Pluto and Ceres that orbit the Sun but share their region of space with many other objects.
- Asteroids: rocky and metallic leftovers, most of them in the asteroid belt between Mars and Jupiter.
- Comets: icy bodies that grow glowing tails when they come close to the Sun.
- The Kuiper Belt: a broad ring of icy bodies beyond Neptune, where Pluto lives.
- The Oort Cloud: a vast, distant shell of icy bodies thought to surround the whole solar system, and the source of long-period comets.
How did the solar system form?
About 4.6 billion years ago, a cloud of gas and dust began to collapse under its own gravity. Most of the material fell to the centre, grew hot and dense enough for nuclear fusion to start, and became the Sun. The rest flattened into a spinning disc around the young star.
Inside that disc, dust grains stuck together into pebbles, then boulders, then bodies kilometres across, which collided and built up the planets over millions of years. Close to the hot Sun only rock and metal could survive, so the inner planets are rocky. Farther out it was cold enough for ice to form, giving the outer planets enough material to grow huge and pull in vast amounts of gas.
The leftovers that never became planets, the asteroids and comets, are time capsules from that era. That is why space agencies have brought samples home from them: Japan's Hayabusa2 from the asteroid Ryugu in 2020, and NASA's OSIRIS-REx from the asteroid Bennu in 2023.
Why is Pluto not a planet anymore?
Pluto is still part of the solar system; only its label changed. In August 2006 the International Astronomical Union (IAU) agreed a definition: a planet must orbit the Sun, be massive enough to pull itself into a round shape, and have cleared the neighbourhood around its orbit of other objects. Pluto passes the first two tests but not the third, because it shares its region, the Kuiper Belt, with many other icy bodies. So it was reclassified as a dwarf planet.
The decision followed the discovery of Eris, a distant world about a quarter more massive than Pluto, though slightly smaller across. Had Pluto stayed a planet, Eris and possibly many others would have had to join it. When NASA's New Horizons spacecraft flew past Pluto in July 2015, it found a surprisingly active world with mountains of water ice and a vast heart-shaped plain of frozen nitrogen.
How big is the solar system, and where does it end?
Distances in the solar system are so large that astronomers use the astronomical unit (AU), Earth's average distance from the Sun of about 150 million km. Neptune orbits at about 30 AU. Sunlight takes about 8 minutes to reach Earth and about 4 hours to reach Neptune.
To feel the scale, shrink the Sun to a football about 22 cm wide. Earth becomes a peppercorn about 24 metres away, just beyond the far end of a cricket pitch. Jupiter is a grape about 120 metres away, and Neptune a small bead about 700 metres away. The nearest star, a small red dwarf, would be a marble about 3 cm across, roughly 6,300 km away, a little less than the distance from Delhi to London. That is why classroom drawings are never to scale.
There is no wall at the edge. One boundary is the heliopause, where the Sun's stream of charged particles, the solar wind, gives way to interstellar space. NASA's Voyager 1 crossed it in August 2012 and is now more than 25 billion km from Earth. The Sun's gravity reaches much farther still, out to the Oort Cloud.
The solar system fills the first three lessons of Astronomy 101, the WhyCosmos live course for ages 8 to 16: the Sun, Moon and Earth; the other planets; and the scale of the solar system.
Questions parents and kids ask
The solar system is the Sun and all the objects that travel around it because of its gravity: eight planets, their moons, dwarf planets, asteroids and comets.
There are eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The IAU also recognises five dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris.
Since 2006 Pluto has been classed as a dwarf planet because, unlike the eight planets, it has not cleared its orbit of other objects: it shares the Kuiper Belt with many icy bodies. It is still part of the solar system.
Jupiter. It is about 11 times as wide as Earth, and more than 1,300 Earths could fit inside it.
No. The solar system is one star, the Sun, with everything that orbits it. The Milky Way is a whole galaxy of hundreds of billions of stars, and our solar system is one tiny part of it.
Not yet. Two spacecraft, Voyager 1 and Voyager 2, have crossed into interstellar space, but Voyager 1 took about 35 years to get there and Voyager 2 about 41. Even at Voyager 1's speed, reaching the nearest star would take tens of thousands of years.
Your child asks questions like this every week.WhyCosmos runs live online astronomy classes for ages 8–16, taught by real astrophysicists — where questions like these are the curriculum.
Sources and further reading
NASA: Our solar system · NASA: Dwarf planets · IAU: New moons of Saturn and Jupiter (March 2026) · NASA: Voyager