Solar System and Scale

Our solar system is the Sun and everything that orbits around it: eight planets, dozens of moons, millions of asteroids, comets, and tiny bits of dust. Everything in the solar system is held in place by the Sun’s gravity. The Sun is so heavy that even Neptune, billions of kilometers away, still travels in a giant slow circle around it.

The Eight Planets in Order

Starting closest to the Sun and moving outward, the planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. A useful sentence to remember the order is "My Very Educated Mother Just Served Us Noodles" - the first letter of each word matches the first letter of each planet.

1. Mercury - the closest planet to the Sun. Tiny, rocky, no real atmosphere. Days are scorching hot, nights are freezing cold.
2. Venus - the hottest planet. A thick blanket of carbon dioxide traps heat at around 460 deg C (about 860 deg F), even hotter than Mercury.
3. Earth - our home. The only planet we know of with liquid water on the surface and life.
4. Mars - the red planet. Reddish iron-oxide dust covers the surface. Has frozen water at the poles.
5. Jupiter - the largest planet. A gas giant with a famous storm called the Great Red Spot, bigger than Earth itself.
6. Saturn - the planet with the most beautiful rings, made of icy chunks.
7. Uranus - a pale blue ice giant that rotates tipped on its side, like a rolling ball.
8. Neptune - the farthest planet. Deep blue, with the fastest winds in the solar system.

Two Families of Planets

The first four planets (Mercury, Venus, Earth, Mars) are the ROCKY planets - small, dense, with solid surfaces you could walk on. The next four (Jupiter, Saturn, Uranus, Neptune) are the GIANT planets - huge balls of gas and ice with no real surface. Between Mars and Jupiter sits the asteroid belt, a wide ring of leftover rock and metal that never formed into a planet.

A Sense of Scale

Pictures of the solar system in books are NEVER to scale. The Sun is so huge that about 109 Earths could fit across its face, and over a million Earths could fit inside it. Jupiter is more than ten times wider than Earth. If a textbook drew them to true size, the Sun would fill the whole page and Earth would be a single dot of ink.

• Sun - about 109 times wider than Earth.
• Jupiter - about 11 times wider than Earth, the largest planet.
• Earth and Venus - almost the same size, the two biggest rocky planets.
• Mercury - the smallest planet, only a little bigger than our Moon.

Distances in Space

Distances are even harder to picture than sizes. Astronomers use a unit called the astronomical unit (AU). One AU is the distance from the Earth to the Sun, about 150 million kilometers (about 93 million miles). Mars is about 1.5 AU from the Sun. Jupiter is about 5 AU. Neptune is about 30 AU. Light from the Sun takes only 8 minutes to reach Earth - but more than 4 hours to reach Neptune. Space is mostly empty.