A Beginner's Tour of the Solar System

Our solar system is the only planetary system we can explore up close,

and it is far more varied than the neat diagrams from school suggest.

It holds one star, eight planets, hundreds of moons, millions of asteroids and a vast frozen outer region we have barely begun to map.

This tour moves outward from the Sun, one region at a time.

By the end you will know what each planet is made of,

why the solar system is arranged the way it is,

and which worlds you can find in your own night sky without a telescope.

The Solar System at a Glance

At its simplest, the solar system is the Sun and everything held in orbit by its gravity.

That includes the planets and their moons, but also dwarf planets such as Pluto, rocky asteroids,

icy comets and a thin stream of dust and gas.

All of it formed together about 4.6 billion years ago from a spinning cloud of gas and dust.

The eight planets fall into three families. The four closest to the Sun are small and rocky.

The next two are gas giants made mostly of hydrogen and helium.

The outer two are ice giants, rich in water, ammonia and methane beneath their hydrogen-rich atmospheres.

Distances in the table use the astronomical unit, or AU,

which is the average distance between Earth and the Sun: about 150 million km (93 million miles).

The Sun at the Center

The Sun is an ordinary middle-aged star, but from our point of view it is everything.

It contains about 99.8 percent of all the mass in the solar system, which is why every planet,

asteroid and comet circles it rather than the other way around.

Its light takes a little over eight minutes to reach Earth.

The Sun also explains why the planets differ so much.

Close to the young Sun it was too warm for water and other ices to freeze,

so only rock and metal could clump together into planets.

Farther out, beyond what astronomers call the frost line, ice was plentiful and planets could grow much larger.

That single temperature pattern explains a great deal of what follows.

The Inner Rocky Planets

The four inner planets are built from rock and metal, with solid surfaces you could, in principle, stand on.

They are small compared with the giants, and between them they have only three moons: Earth's Moon and the two tiny moons of Mars.

• Mercury is the smallest planet and the closest to the Sun. It has almost no atmosphere, so its days are scorching and its nights are bitterly cold.
• Venus is nearly Earth's size, but a thick carbon dioxide atmosphere traps heat so well that its surface is hot enough to melt lead.
• Earth is the largest of the rocky planets and the only world known to have liquid water on its surface and life.
• Mars is about half as wide as Earth, cold and dry today, yet covered in valleys and old riverbeds carved by water long ago.

The Asteroid Belt: A Dividing Line

Between Mars and Jupiter lies the main asteroid belt, a broad ring of rocky and metallic bodies left over from the solar system's birth.

The largest object there, Ceres, is round enough to count as a dwarf planet, while most asteroids are irregular chunks far smaller than a city.

Despite what science fiction shows, the belt is mostly empty space.

Spacecraft have crossed it many times without needing to dodge anything.

If you could gather every asteroid in it into one ball, the combined mass would be only a few percent of our Moon's.

The Giant Planets

Past the asteroid belt, the scale of the solar system changes dramatically.

The four giant planets hold nearly all of the mass that is not in the Sun,

and each has a ring system and a large family of known moons,

with new small ones still being discovered. None of them has a solid surface in the familiar sense;

their atmospheres simply grow denser and hotter with depth.

Jupiter and Saturn, the Gas Giants

Jupiter is about 11 times as wide as Earth and more massive than all the other planets combined. Its striped clouds and the Great Red Spot,

a storm wider than Earth, make it a favorite telescope target.

Saturn is a little smaller and so low in density that its average is less than that of water.

Its famous rings are made of countless pieces of ice and rock.

Uranus and Neptune, the Ice Giants

Uranus and Neptune are each about four times as wide as Earth.

Methane in their upper atmospheres absorbs red light, which gives both a blue or blue-green color.

Uranus is tipped so far over that it essentially rolls around the Sun on its side, while Neptune,

the most distant planet, has the fastest winds measured anywhere in the solar system.

Beyond Neptune: The Frozen Frontier

The planets end at Neptune, but the solar system does not.

Past it lies the Kuiper Belt, a doughnut-shaped zone of icy bodies that includes Pluto,

which the International Astronomical Union reclassified as a dwarf planet in 2006.

NASA's New Horizons spacecraft flew past Pluto in 2015 and revealed mountains of water ice and smooth plains of frozen nitrogen.

Much farther out, astronomers think a vast,

roughly spherical shell of icy objects called the Oort Cloud surrounds the whole system,

possibly reaching a good part of the way toward the nearest stars. It has never been observed directly.

Its existence is inferred from long-period comets that fall inward from every direction in the sky.

How to See the Planets Yourself

You do not need special equipment to start exploring.

Five planets are easy to see with the naked eye and have been known since ancient times: Mercury, Venus, Mars, Jupiter and Saturn.

Here is how to find them.

1. Look along the Sun's path. The planets orbit in nearly the same plane, so they always appear close to the ecliptic, the line the Sun and Moon follow across the sky.
2. Watch for steady light. Stars twinkle noticeably, especially near the horizon, while planets usually shine with a calmer, steadier glow.
3. Use a sky app or chart. Planets drift against the background stars from week to week, so check a current chart to learn which ones are up tonight.
4. Try binoculars. A pair held steady can show Jupiter's four largest moons as tiny points of light lined up beside the planet.
5. Add a small telescope later. At modest magnification, Saturn's rings and the changing phases of Venus come into view, a memorable first sight for anyone.

Mercury and Venus never stray far from the Sun in the sky, so look for them shortly after sunset or before sunrise.

Never point binoculars or a telescope anywhere near the Sun,

and never look at the Sun itself without certified solar filters,

because even a brief glance through optics can cause permanent eye damage.

Frequently Asked Questions

How many planets are in the solar system?

There are eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Astronomers also recognize several dwarf planets, including Pluto, Ceres and Eris, and more may join that list as distant icy bodies are studied.

Why is Pluto no longer called a planet?

In 2006 the International Astronomical Union defined a planet as a round body orbiting the Sun that has cleared its orbital neighborhood of other material.

Pluto is round and orbits the Sun, but it shares its region with many other Kuiper Belt objects,

so it was placed in the new dwarf planet category.

Can you see Uranus and Neptune without a telescope?

Uranus sits right at the edge of naked-eye visibility and can be glimpsed from a very dark site if you know exactly where to look.

Neptune is always too faint for the unaided eye, but binoculars and a good star chart will show it as a dim, star-like point.

Where to Go From Here

The solar system rewards curiosity at every level.

Start by learning which bright "stars" in your evening sky are really planets, then pick one world at a time to read about and observe.

Each has its own story, from Mercury's extreme days to the icy mysteries beyond Neptune, and every one of them is worth a closer look.