The solar system: the planets and what holds them together

One star and its leftovers

The most honest way to describe the solar system is this: there is the Sun, and then there are the leftovers.

NASA gives the number: the Sun holds 99.8% of the mass of the entire solar system. Eight planets, hundreds of moons, millions of asteroids and comets divide up what remains — less than two parts in a thousand. If the solar system were a tonne of material, everything other than the Sun would come to barely two kilograms.

That imbalance isn’t a quiz fact. It explains everything else: a mass concentrated like that keeps bodies billions of kilometres away in orbit, and 4.6 billion years ago it decided where every piece would end up.

How it all began

About 4.6 billion years ago, somewhere in the Milky Way, a dense cloud of interstellar gas and dust began to collapse under its own weight. As it contracted it spun faster and flattened out, the way pizza dough does when it’s spun: sphere to disc.

At the centre, material piled up until pressure and temperature were high enough to start nuclear fusion. The Sun was born, having taken over 99% of the available matter. In the leftover disc, dust grains started bumping into each other and sticking, building bodies that grew step by step: pebbles, then mountains, then planets.

Practical example: the age of that process comes from radiometric dating of the oldest meteorites — fragments that never melted and stayed as they were. They’re the cosmic equivalent of tree rings, and reading them is why we can say “4.6 billion” rather than “a very long time ago”.

The line that separates rock from ice

Why are the planets near the Sun small and stony, and the distant ones enormous? The answer is the temperature of the disc.

Close to the star it was too hot for substances like water to stay solid, so only the heat-resistant material was left: rock and metal. Not much of it, so small planets. Beyond a certain distance — what astronomers call the frost line — water and other volatile compounds could freeze, and the supply of solid building material jumped.

Out there the planetary cores grew fast, and once massive enough their gravity hauled in vast amounts of hydrogen and helium from the disc. The giants are the result. Jupiter is eleven times Earth’s width, and it would take a thousand planets like ours to fill it.

The eight planets, in two families

	Terrestrial planets	Giant planets
Which	Mercury, Venus, Earth, Mars	Jupiter, Saturn (gas), Uranus, Neptune (ice)
Surface	Solid, something to land on	None: the atmosphere fades into liquid
Made of	Rock and metals	Hydrogen and helium, plus frozen water and ammonia in the outer two
Moons	None or a few	Dozens to hundreds: as of March 2026 the IAU counts 101 for Jupiter and 285 for Saturn
Rings	No	Yes, all four

The four inner planets are built alike but ended up very differently. Venus is wrapped in a crushing carbon dioxide atmosphere, 93 times Earth’s surface pressure: it is the hottest planet in the system at 467 °C, a greenhouse effect that ran away. Earth, at the right distance, has liquid water on its surface, a water cycle that keeps stirring it, and plants running photosynthesis on that star’s light. Mars, smaller and colder, lost nearly all of its atmosphere. Same kind of planet, three outcomes.

Volcanism isn’t an Earth speciality either, and elsewhere it runs on a different engine from the one behind volcanoes and earthquakes here. Io, a moon of Jupiter, has more active volcanoes than anywhere else in the solar system: hundreds of them, with lava fountains tens of kilometres high. What feeds them isn’t leftover heat from formation. It is gravity: Jupiter pulls one way, the moons Europa and Ganymede the other, and Io’s interior flexes constantly and heats up by friction.

On the giants the numbers turn strange. A day on Jupiter lasts 9.9 hours, the shortest in the solar system, while its year runs to nearly 12 Earth years. Neptune, 30 astronomical units from the Sun, takes 165 Earth years to go round once: since its discovery in 1846 it has completed barely one orbit.

Practical example: Neptune was found with mathematics before anyone saw it. The irregularities in Uranus’s orbit didn’t add up; Urbain Le Verrier calculated where the body causing them had to be, and in 1846 Johann Galle looked from the Berlin Observatory and found it, essentially where the calculations said it would be.

What holds it all together

Here is the question nearly everyone asks as a child: if the Sun pulls on the planets, why don’t they end up inside it?

They are falling in, constantly — they just keep missing. Every planet carries sideways speed inherited from the spinning disc. The Sun’s gravity bends that motion inward; the sideways speed carries the planet past. The compromise between the two is a closed curve: an orbit.

Practical example: tie a weight to a string and swing it above your head. Your hand pulls the weight towards the centre, as gravity would; the weight never reaches your hand because it is moving sideways. If the string snapped, the weight wouldn’t fall on you — it would fly off in a straight line. Earth works the same way.

Orbits aren’t perfect circles but ellipses, slightly squashed ones, and that has a practical consequence: the distance from the Sun changes over the year, and planets move faster when they are closer. The distances, too, are far larger than the diagrams suggest. Astronomers use the astronomical unit (AU), the average Earth-Sun distance: about 150 million kilometres. Jupiter sits at 5.2 AU, Neptune at 30. On a model where Earth is one metre from the Sun, Neptune would be thirty metres away, and every planet would be a speck.

What was left over: rock, ice and comets

Between Mars and Jupiter lies the main asteroid belt: somewhere between 1.1 and 1.9 million bodies larger than a kilometre across, plus a great many smaller fragments. It isn’t the wreckage of an exploded planet, as the story often goes, but a planet that never happened: Jupiter’s gravity kept stirring those bodies up, so they smashed into each other instead of sticking together. All the asteroids combined still weigh less than the Moon.

Beyond Neptune the Kuiper Belt begins, a ring of icy bodies where Pluto lives. Much further out is the Oort Cloud, the shell that long-period comets come from: it extends from 5,000 to 100,000 astronomical units, out to roughly 1.6 light-years from the Sun. That is where our star’s gravitational hold fades into interstellar space.

Meanwhile the bubble of particles streaming from the Sun, the heliosphere, starts giving way to the interstellar medium between 80 and 100 astronomical units out. That is where the solar wind slows abruptly, at the termination shock: Voyager 1 crossed it in 2004, Voyager 2 in 2007. True interstellar space, past the heliopause, came much later for both — in 2012 and 2018.

Why Pluto is no longer a planet

On 24 August 2006, in Prague, the International Astronomical Union voted on a formal definition. A planet is a body that orbits the Sun; has enough mass for its own gravity to pull it into a nearly round shape; and has cleared the neighbourhood around its orbit.

Pluto meets the first two and fails the third. Not because of its size — size appears nowhere in the criteria — but because it shares its region with a crowd of trans-Neptunian bodies, discovered in growing numbers from the 1990s onward. Hence the category of dwarf planets: round bodies, not satellites, that haven’t swept their surroundings clear. Pluto is the prototype, and five dwarf planets are officially recognised.

The reclassification took nothing away from Pluto, which remains one of the most studied objects out there: the New Horizons probe flew past it on 14 July 2015. It has five moons — Charon, Nix, Hydra, Kerberos and Styx — a diameter of about 2,377 kilometres, and orbits at an average 39 astronomical units, taking 248 Earth years to go round once: in that time Neptune, whose year runs to 165, gets round one and a half times.

The solar system is moving

One last thing the posters leave out: we are not sitting still. The whole solar system orbits the centre of the Milky Way at about 829,000 km/h, taking 230 million years to complete one circuit. Put the two numbers together and you get a sense of how young the Sun is on a galactic scale: since it formed, it has been round about twenty times.