Earth’s Place in the Universe

Our Solar System

Imagine travelling at 67,000 miles per hour. Guess what? You are! That’s how fast Earth spins around the sun. Earth isn’t alone of course. We’re just one speedy little planet in a whole solar system.

Put simply, a solar system is a collection of objects that orbit a star or sun. Along with Earth, our solar system includes seven other planets. There are also dwarf planets like Pluto, which are smaller than normal planets but still directly orbit the sun, and are nearly round. There are also at least 150 assorted moons, which orbit planets, along with millions of smaller, irregularly shaped chunks of space rock called asteroids, comets and meteorites.

What keeps it all together? Gravity! The gravitational pull of the sun at the center of the solar system locks the planets and other objects into orbits around it.

Our Solar System

At Home in the Milky Way Galaxy

Our solar system seems unimaginably big. But incredibly, it’s just one of hundreds of billions of solar systems in a galaxy called the Milky Way. And the Milky Way is just one of billions of galaxies in the universe!

The Milky Way is shaped like an enormous disc. The whole Milky Way spins, too. It’s shaped like a spiral with pinwheel “arms.” Our cozy little solar system is somewhere about halfway out a spiral called the Orion Arm.

If you look at the sky on a very dark and clear night, you might see the Milky Way. Look for a glittering swath of dense stars, almost like a bright cloud. Imagine seeing a disc from the edge. That’s exactly what you are seeing: a side view of the Milky Way Galaxy.

Our solar system formed about 4.5 billion years ago from a cloud of cosmic dust. The tiny particles to came together to form the Sun and the planets. As the Sun grew bigger, its gravity also got stronger. That gravity pulled in more and more matter.

Eventually, 99% of the matter in the dust cloud merged into the Sun. The other 1% of matter became all the other objects in our solar system.

The planets

Going Round the Sun

The planets in our solar system are (in order from the closest to the Sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Mercury, Venus, Earth and Mars are relatively small, rocky planets. We call these the terrestrial planets. Beyond Mars is a zone of rock chunks called the asteroid belt. Beyond the asteroid belt are the giant planets Jupiter, with over 50 moons, and Saturn, with its distinctive rings. These planets are much larger than the terrestrial planets and made mostly of gases. Uranus and Neptune are large, icy planets.

There are also at least five dwarf planets in our solar system. A dwarf planet is an object that has enough mass to be nearly round like a planet but doesn’t behave like a regular planet. Unlike moons, which orbit planets, dwarf planets circle the Sun directly but unlike full sized planets, dwarf planets don’t have the mass to clear other objects out of their orbit like the eight real planets. The dwarf planets we know of are Ceres, Pluto (which was previously considered the ninth planet), Eris, Makemake, and Haumea.

We call one orbit around the Sun a year. On Earth, a single orbit takes 365.25 days. Mercury, which is closest to the Sun, takes only 88 days for a full orbit. Mars, which is the next planet past Earth, has a year that is more than twice as long as ours: 687 days.

Saturn takes 29 Earth years to complete one circle around the Sun. A year on Neptune, the farthest planet in our solar system, equals 165 Earth years. You would have to wait a long time for your birthday party if you lived on Neptune!

What else is in our solar system

What else is in our solar system

Planets, dwarf plants, and moons aren’t the only objects that orbit our sun. Asteroids, comets, and meteors are much smaller chunks of rock and ice that also orbit the Sun. Most are irregularly shaped. With all these objects in different orbits around the Sun, sometimes one will enter a collision course with Earth.

Meteors are small and usually burn up as they enter Earth’s atmosphere, causing bright streaks across the sky. When Earth travels through an area of the solar system with lots of meteors, we get meteor showers.

Asteroids are bigger than meteors, sometimes miles across. When an asteroid does hit the Earth, the impact can cause big environmental changes. About 66 million years ago, an asteroid at least six miles across slammed into Earth. It caused dust, smoke, and big temperature changes. The dinosaurs went extinct after that. Fortunately, comets and asteroids almost never run into planets.

Hello Out There?

Every year, scientists learn more about our solar system. They’ve found so many amazing things, like ancient stream channels on Mars, moons with salty oceans under miles of ice, and strange objects spinning around our sun that we never knew existed.

So far, scientists haven’t found any direct evidence for life anywhere else in the solar system. Among other things, life requires water and heat. So those are what scientists look for first. If we ever do find life somewhere else in the solar system, it might not be on a planet. It could be on one of the many moons in the solar system. And that life might be very simple, like bacteria. Even so, it would be a very exciting find.

Meanwhile, we will keep orbiting the sun, glad that Earth is in just the right spot to keep us warm and safe in our very own solar system.

Written by Laura McCamy

Edited by Beth Geiger, MS Geology

Illustrated by Renee Barthelemy