What is the solar system? The Sun, planets and beyond

The solar system is the Sun and everything that orbits it: eight planets and their moons, dwarf planets such as Pluto, and billions of smaller rocky and icy bodies, from asteroids to comets. It formed about 4.6 billion years ago from a spinning cloud of gas and dust. The Sun holds more than 99% of its mass, and its gravity reaches out so far that the most distant comets take millions of years to orbit once.

In short

• The eight planets come in two families: four small rocky worlds close to the Sun, and four giants of gas and ice farther out.
• Beyond Neptune lie the Kuiper belt of icy bodies, where Pluto orbits, and far beyond that, the Oort cloud of comets.
• Nearly all of the solar system is empty space: shrink the Sun to a 30 cm ball and Earth is a 2.8 mm bead 32 m away.

What's in the solar system?

At the center is the Sun, an ordinary star. Around it, in roughly the same flat plane and all in the same direction, orbit the planets. A planet, as the International Astronomical Union (IAU) defined it in 2006, orbits the Sun, is massive enough to be pulled into a round shape, and has cleared its orbit of other large objects. A dwarf planet meets the first two tests but not the third. The IAU recognizes five: Ceres, Pluto, Eris, Haumea and Makemake. Why Pluto was moved to that group is the subject of why Pluto is not a planet.

Planet	Family	Average distance from the Sun	Diameter	What stands out
Mercury	Rocky	0.39 AU	4,879 km	Smallest planet; no real atmosphere
Venus	Rocky	0.72 AU	12,104 km	Hottest planet, under thick carbon dioxide
Earth	Rocky	1 AU	12,756 km	The only known world with life and oceans of liquid water on its surface
Mars	Rocky	1.52 AU	6,792 km	Cold desert; once had flowing water
Jupiter	Gas giant	5.2 AU	142,984 km	More mass than all other planets combined
Saturn	Gas giant	9.6 AU	120,536 km	Brightest rings; the most known moons
Uranus	Ice giant	19.2 AU	51,118 km	Rolls on its side, tilted about 98°
Neptune	Ice giant	30.1 AU	49,528 km	Fastest winds in the solar system

An astronomical unit (AU) is the average Earth–Sun distance, about 150 million km. Figures are from NASA's planetary fact sheets. The rocky planets, Venus and Mars among them, are made of rock and metal. The giants are mostly hydrogen and helium (Jupiter, Saturn) or water, ammonia and methane ices around rocky cores (Uranus, Neptune), with no solid surface to stand on.

Moons are numerous, and their count keeps changing as telescopes find small ones. As of November 2026, NASA's Jet Propulsion Laboratory lists 460 moons of the planets and Pluto, not counting our own Moon, in its discovery table. Saturn has the most: 293 confirmed moons as of August 2026, according to NASA.

Between Mars and Jupiter is the asteroid belt, millions of rocky bodies, the largest of which is the dwarf planet Ceres. Beyond Neptune, from about 30 to 50 AU, is the Kuiper belt, millions of icy bodies including Pluto, according to NASA.

How big is the solar system, really?

Pictures in books squeeze the planets close together so they fit on the page. The real spacing is hard to believe until you do the arithmetic. Shrink the Sun, 1,392,700 km wide, to a ball 30 cm across. Every size and distance then shrinks by the same factor, about 4.6 billion.

Object	Real size or distance	In the model
The Sun	1.39 million km wide	a 30 cm ball
Earth	12,756 km wide, 1 AU out	a 2.8 mm bead, 32 m away
Jupiter	142,984 km wide, 5.2 AU out	a 3 cm marble, 168 m away
Neptune	49,528 km wide, 30 AU out	a 1 cm pea, almost 1 km away
Pluto	2,377 km wide, about 40 AU out	a half-millimeter grain, 1.3 km away
The Oort cloud	from about 2,000 to 100,000 AU	from 65 km to about 3,200 km away
The nearest star, Proxima Centauri	about 268,000 AU	about 8,600 km away, a 4.5 cm ball, since it is a small red dwarf

In this model the planets fit in a town, but the solar system's outer edge is as far away as another continent. Light, the fastest thing there is, takes 8 minutes to get from the Sun to Earth and about 4 hours to reach Neptune.

The Oort cloud is a theorized shell of icy bodies, proposed by Jan Oort in 1950 to explain where long-period comets come from, according to NASA; its objects are too small and too far away for telescopes to see them out there. Voyager 1, launched in 1977 and the farthest spacecraft from Earth, left the bubble of the Sun's wind for interstellar space in 2012, but NASA estimates it won't reach the Oort cloud for about 300 years.

How did the solar system form?

About 4.6 billion years ago, a cloud of gas and dust collapsed under its own gravity, perhaps nudged by a nearby exploding star. As it shrank, it spun faster and flattened into a disk, which is why the planets orbit in nearly one plane and in the same direction. Most of the material fell to the center and became the Sun.

In the disk, dust grains stuck together into pebbles, then boulders, then bodies kilometers wide that collided and merged into planets. Distance from the young Sun decided what kind. Close in, it was too hot for ice, so only rock and metal could clump together, making small rocky planets. Farther out, beyond what astronomers call the frost line, ice was plentiful too, so cores grew big enough to pull in vast amounts of hydrogen and helium gas and become giants. The leftovers survive as asteroids, Kuiper belt objects and comets, which is why scientists study them for clues to the beginning.

Where is the solar system?

The solar system sits in the Orion Arm of the Milky Way, about halfway out from the galaxy's center. It carries the whole system around that center at about 830,000 km per hour, and one orbit takes about 230 million years, according to NASA's solar system facts. It is one planetary system among what are likely billions in our galaxy alone: astronomers have confirmed thousands of planets around other stars. Our place in the wider universe is ordinary. What may be rare is a planet like Earth.

Still argued

Some astronomers think a ninth planet, several times Earth's mass, may orbit far beyond Neptune, because a group of distant Kuiper belt objects seems to share oddly aligned orbits. Others argue the alignment is a side effect of which objects telescopes happen to see. No such planet has been found.

Check yourself

• What three tests does a body have to pass to count as a planet, and which one does Pluto fail?
• Why are the inner planets small and rocky while the outer ones are giants?
• In a model where the Sun is a 30 cm ball, roughly how far away would Earth be, and how far the nearest star?

Sources

• Solar system facts — NASA
• Planetary fact sheets — NASA Space Science Data Coordinated Archive
• Planetary satellite discovery circumstances — NASA Jet Propulsion Laboratory
• Kuiper belt: facts — NASA
• Oort cloud: facts — NASA
• Saturn moons — NASA