The Solar System: Planets, Moons, Asteroids, and More


By Robert Hazen, Ph.D.George Mason University

Our solar system is littered with planets, moons, comets, asteroids, and much more. It includes all the objects that are gravitationally bound to the Sun. Space scientists gather information about the composition of these objects, their distribution, their dynamics, etc., and that data provides strong hints about the origin of our planetary system. Let’s explore.

A 3D illustration of a meteor falling in the ocean.
Meteorites are used as physical samples to understand the origin of the solar system. (Image: Alexyz3d/Shutterstock)

Our Solar System

With telescopes, we can observe that there are eight planets, including the Earth, that orbit the Sun. There are four inner planets; these are called terrestrial planets. They are small, rocky worlds. They include Mercury, the planet closest to the Sun; Venus, which is enshrouded in a cloudy atmosphere; Earth, the planet of life; and Mars, the red planet. 

These four terrestrial planets are small worlds compared to the next four planets, which are giant gas planets composed primarily of hydrogen and helium. They include Jupiter, the largest of all the planets; Saturn, a planet of magnificent rings; and then Uranus and Neptune, two planets that can only be observed with telescopes. These four planets are also called the Jovian planets.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

Moons and Asteroids of the Solar System

In addition to the planets, there are many other objects that are bound to the Sun. For example, most of the planets have one or more moons. The Earth has a single, very large moon. Mars has two very tiny moons, and then Jupiter and Saturn have more than a dozen moons each.

An illustration of the asteroid belt
Asteroids are mostly found in the asteroid belt between Mars and Jupiter. (Image: Jurik Peter/Shutterstock)

All the gas giant planets have rings – magnificent rings made of tiny particles that have been swept out into a very thin band around those planets. 

Then there are asteroids; these are rocky objects, usually quite irregular in shape and ranging in size from small boulders to objects that are hundreds of kilometers across. There are many asteroids, and most of them are found in a belt between Mars and Jupiter; that’s called the asteroid belt. 

There’s also a growing number of known asteroids that actually have highly elliptical orbits that cross the orbit of the Earth. These Earth-crossing orbits of asteroids are quite of interest because some of these asteroids might actually hit the Earth someday.

Learn more about the Earth’s topography.

Comets: Dirty Snowballs of the Solar System

Then there are comets. Comets are like dirty snowballs in space. They’re rich in volatiles—water and methane, and other volatile materials that boil if they’re brought close enough to the Sun.

Far out from the Sun, they’re just condensed snowballs; as they come closer and closer to the Sun, those volatiles evaporate, forming a stream of gaseous particles, a stream that’s pushed away from the Sun by the solar wind. So you see the great comet tail sweeping around the Sun as a comet winds its way back to the depths of space. 

An illustration of a comet in space
Comets have extremely elongated orbits. (Image: muratart/Shutterstock)

Comets have extremely elongated orbits; some of them, indeed, have orbits of tens of thousands of years and extend far, far beyond the farthest planet. Some of these comets may originate in another object of the solar system, a hypothetical belt called the Oort Cloud, which is supposedly lying far beyond Pluto.

Meteorites and Their Importance

Most of our observations and most of our knowledge of the solar system come from telescopes and space probes that actually photograph and sample these objects, but occasionally we have a physical sample of objects outside our own Earth to study. This is important data for understanding the origin of the solar system. 

Of greatest importance are the meteorites. Meteorites are rocks that fall to Earth from space. Meteorites fall on every part of the Earth. They fall primarily in the oceans, of course, because three-quarters of the Earth’s surface is oceans; but many meteorites fall on land as well, and occasionally these are recovered. 

In most places, when a meteorite is found, it’s an iron meteorite: it’s a chunk of iron that has a corroded and pockmarked surface from the flight through the atmosphere. These are very distinctive rocks, and they stand out from the other rocks on the surface of the Earth. But many times, meteorites are also stony, and those are much harder to spot on the ground. 

Learn more about the matter and forces in the universe.

Scientifically Valuable Meteorites in Antarctica

Scientists are especially interested in a new set of meteorites that have been found in Antarctica. The meteorites on the ice plains of Antarctica are a unique and valuable find. In this situation, we have plains of blue ice, old ice, that have been there for thousands of years, and when a meteorite falls on that ice surface, it just stays on the surface. It doesn’t get buried; it doesn’t get eroded or weathered. 

Now, these meteorites are very important because they give us a random sampling of objects from space. You find some irons, you find some stones, you find some stony-iron meteorites, where you have a mixture of metal and other minerals together. That random sampling gives us the best clues about the distribution of mass and matter in the solar system.

Common Questions about Planets, Moons, Asteroids, and More

Q: What planets are there in the solar system?

The solar system has four terrestrial planets and four gaseous planets orbiting the Sun. The former four planets include Mercury, Venus, Earth, and Mars, and the latter four planets are Jupiter, Saturn, Uranus, and Neptune.

Q: What are asteroids?

Asteroids are rocky objects, usually quite irregular in shape and ranging in size from small boulders to objects that are hundreds of kilometers across. Most asteroids are found in the asteroid belt between Mars and Jupiter.

Q: What are meteorites?

Meteorites are rocks that fall to Earth from space. When a meteorite is found, it is mostly an iron meteorite, a chunk of iron that has a corroded and pockmarked surface from the flight through the atmosphere.

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