There are 8 planets that orbit the Sun at distances ranging from 40% of Earth’s distance to 30 times Earth’s distance. Then there are dwarf planets, such as Pluto, that are in orbit around the Sun and comets, whose orbits take them close to the Sun. The solar system also contains moons—more than 200 in total—that orbit planets and even some dwarf planets and

asteroids. Spacecraft have explored every planet in the solar system and have visited smaller bodies, such as moons, asteroids, and comets. Earth has been studied from space and compared with all the other worlds in our solar system; all of these missions together have made it possible to assemble a field guide to the planets and other bodies of our solar system.

 

 

Organizing the Solar System

 

Our solar system is enormous, and the Sun is pretty much at the center of it, at a distance of 150 million km, or 93 million mi, from Earth. The Sun has a diameter of almost 1.4 million km, or 870,000 mi, which is 1% of the distance between Earth and the Sun—in other words, Earth is roughly 100 Suns away from the Sun! The Sun’s diameter is more than 100 times bigger than Earth. In terms of Earth’s diameter, the distance from Earth to the Sun is roughly 10,000 Earths. The Sun also makes up 99.8% of the solar system’s mass; everything else in the solar system is a tiny ant trying to stay out of the Sun’s way.

 

The solar system is typically separated into inner and outer regions based on the type of planet found in each. The inner region of the solar system contains 4 planets: Mercury, Venus, Earth, and Mars. They are all extremely small compared to the Sun. Earth is the largest inner planet, and Venus is almost as large, while Mars is only 50% of Earth’s diameter, and Mercury is only 40% of it. These inner planets are called terrestrial planets. They are primarily composed of 2 types of materials: rocks (like what forms from cooling lava or sediments of sand) and metals (mostly iron).

The outer solar system contains 4 giant planets: Jupiter, Saturn, Uranus, and Neptune. Jupiter is the biggest of all the planets, and it’s more massive than all the other planets combined. The giant planets are not only much bigger than the terrestrial planets, but what they are primarily made of is also quite different.

 

The 2 innermost giant planets, Jupiter and Saturn, are gas giant planets. This means they are primarily made of hydrogen and helium—not just in the atmosphere, but deep into the planet as well.

 

The outermost planets, Uranus and Neptune, are often called ice giants because they are primarily composed of ices. They each have vastly more water locked away than is found on Earth.

 

So there appears to be an organized architecture to the solar system. Terrestrial planets, which are closest to the Sun, are dense with heavy materials, starting with a substantial iron core. That core is then surrounded by silicate rocks, typically built from silicon, oxygen, magnesium, other metals, and many trace elements.

 

Farther from the Sun are the gas giants, made of the lightest atoms, hydrogen and helium. Farthest from the Sun of all are the ice giants, built mostly with molecules comprised of carbon, oxygen, and nitrogen, all mixed with hydrogen.

 

This pattern isn’t a coincidence. The locations of planets are related to the temperatures in the solar system when and where they formed.

 

When planets were forming, the inner solar system temperatures were so hot that only metals and rocks could condense from a gaseous disk to make the building blocks of the planets. In the outer solar system, temperatures were cooler, so ices could condense as well as rocks and metals. This meant that there was a wider range of material to build planets in the outer solar system. And Jupiter and Saturn grew so big that their icy-rocky cores could also attract vast amounts of hydrogen and helium—the most common atoms in the universe—eventually becoming gas giant planets.

 

But our solar system doesn’t end at the farthest planet, Neptune. In fact, Neptune’s orbital distance is less than 0.1% of the distance to the farthest solar system inhabitants. The farthest objects are the small icy bodies in the Oort cloud, which are gravitationally bound to the Sun but orbit it 50,000 times farther than Earth’s orbit around the Sun.

 

 

Water in the Solar System

 

There are certain overarching phenomena that occur throughout the solar system. The first is the surprising prevalence of water in the solar system. Water is so important because liquid water is one requirement needed for a planet to be habitable, or able to support life—at least life as we know it.

 

On Earth, water takes many forms: fog, rain, snow, streams, lakes, underground aquifers, oceans, etc. Earth is the only place in the solar system that has standing bodies of liquid water on its surface, but there are many places in the solar system with frozen water ice on the surface.

 

Almost all of the moons in the outer solar system are covered in shells of frozen water ice. Mars also has frozen water. It’s concentrated in the polar ice caps. Mars has a north pole ice cap that’s around 1000 km, or 620 mi, across and about 1/2 the volume of the Greenland Ice Sheet.

 

Frozen water ice is even found in the most unexpected places in the solar system. Mercury’s surface has temperatures that can reach 800°F (427°C), yet there is frozen water hiding in permanently shadowed craters near the poles. That’s because Mercury doesn’t have an atmosphere, so these shadowed regions can stay cold enough for the ice to not sublimate away.

 

And there are liquid water oceans under the surfaces of some other worlds. For example, 3 of Jupiter’s moons—Europa, Ganymede, and Callisto—and 2 of Saturn’s moons—Titan and Enceladus—have subsurface water oceans. But these oceans are far below the surface, whether several kilometers or hundreds of kilometers.

 

The subsurface oceans in the outer solar system may be global in scale, creating an entire shell of liquid water below the icy surfaces of the moons The solar system also gives us water in forms we’ve never seen on Earth.

 

This is a new phase of water that occurs under the extreme pressures and temperatures in the deep interiors of Uranus and Neptune. The pressure reaches more than a million bars about halfway deep into Uranus and Neptune, and the temperatures at these depths reach several thousands of degrees. Here, the hydrogen and oxygen that make up the water molecule (H2O) rearrange themselves into a new structure. The oxygen atoms bond into a lattice, while the hydrogen atoms flow freely through the cage-like lattice structure. That’s water unlike any we’ve experienced.

 

Cooling

A second overarching phenomenon that is encountered across the solar system is the role of cooling. All planetary objects are cooling to outer space because their interiors are hot. And that cooling manifests in a variety of ways that you might not consider cooling.

 

Atmospheric storms, for example, occur in the layers of the atmosphere where hot parcels of air rise in updrafts, causing turbulent mixing of the atmosphere. This can cause strong winds, cyclones, and hurricanes.

 

The most impressive storms happen on the gas giant planets. Jupiter, which is covered in storms, has the Great Red Spot, a giant oval-shaped hurricane that has lasted since at least 1830. It’s larger in horizontal extent than the entire planet Earth and is about 200 mi deep. Its wind speeds reach more than 250 mph—which is much faster than the Category 5 winds of the most devastating hurricanes on Earth.

Storms on Saturn behave quite differently from Jupiter’s storms. The Great White Spot on Saturn formed in 2010, broadened out in longitude until it encircled the whole planet, ate its tail, and then died within a year. And similar great storms are seen on Saturn about every 30 Earth years. This is similar to Saturn’s year, so the planetwide storms on Saturn seem to be seasonal, like our hurricanes. Saturn’s polar storms are also bizarre. For example, the winds surrounding Saturn’s north pole form a giant hexagon, each side of which is bigger than the diameter of Earth.

 

But the most severe planetary storms we know of don’t happen in our solar system. Some planets around other stars orbit much closer to their parent stars than Mercury orbits our Sun. Traveling so close to a star means the planet’s surface temperatures can reach thousands of degrees. The superhot temperatures mean the “clouds” on that planet are made of silicates—sand—basically the building blocks of rocks. So precipitation may be in the form of molten-glass rain.

 

Another aspect of cooling on bodies across the solar system is the formation of volcanoes. The solar system is filled with volcanoes, many of which tower over Earth’s largest volcano: Mauna Loa on Hawaii.

 

There are places in the solar system that have a much larger number of volcanoes than Earth. For example, Venus is covered in volcanoes. More than 500 volcanoes on Venus are greater than 20 km, or 12 mi, wide, and more than 160 of the volcanoes on Venus are similar in size to, or bigger than, Mauna Loa.

 

Although Venus is covered in volcanoes, they may be taking a very long break. For current eruptions, you’ll want to visit Jupiter’s moon Io, which is constantly flexed by gravitational tidal forces from Jupiter, heating its interior. That heat manifests as volcanic activity at the surface.

 

Collisions

 

A third phenomenon that helps us understand the solar system is collisions. The solar system can be a chaotic place. Many objects are flying around at fast speeds, and they’re always feeling gravitational pulls from all the other objects in the solar system. Sometimes objects get close and collide. There is evidence of collisions everywhere in the form of impact craters—the remnants of violent collisions that occur as meteors hit a planet’s surface.

 

Impacts occur all the time. The solar system is full of debris—small asteroid-sized objects floating around in space. There are almost 20,000 known asteroids orbiting near Earth. We keep a careful watch over them to see if any pose threats of hitting the Earth in the future.

 

And there are many impact craters in the solar system that dwarf those on Earth. Just look up in the sky at the Moon. All of those circular features on the Moon are impact craters. For example, the dark spots on the near side of the moon are impact craters that were filled in with lava billions of years ago.

 

And the existence of the Moon itself is now understood to be a sign of an even larger impact that occurred early in solar system history. About 4.5 billion years ago, a Mars-sized object referred to as Theia hit the proto-Earth at an angle, causing a combination of material from Earth and Theia to be ejected into orbit around the Earth. Material in orbit from that gigantic collision eventually accreted back together to form the Moon.