Planets, moons, asteroids and more


Through Robert Hazen, Ph.D., George Mason University

Our solar system is littered with planets, moons, comets, asteroids and more. It includes all objects gravitationally related to the Sun. Space scientists are collecting information about the composition of these objects, their distribution, dynamics, etc., and this data provides solid clues as to the origin of our planetary system. Let’s explore.

A 3D illustration of a meteor falling into 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 Earth, that orbit the Sun. There are four inner planets; these are called terrestrial planets. They are little rocky worlds. They include Mercury, the planet closest to the Sun; Venus, which is enveloped in a cloudy atmosphere; Earth, the planet of life; and Mars, the red planet.

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

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Moons and asteroids of the solar system

In addition to the planets, there are many other objects related to the Sun. For example, most planets have one or more moons. Earth has one very large moon. Mars has two very small moons, then Jupiter and Saturn have over a dozen moons each.

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

All gas giant planets have rings – beautiful rings made of tiny particles that have been swept in a very thin band around these planets.

Then there are the asteroids; they are rocky objects, generally quite irregular in shape and varying in size from small rocks to objects hundreds of kilometers in diameter. There are many asteroids, and most of them are found in a belt between Mars and Jupiter; this is called the asteroid belt.

There are also a growing number of known asteroids that actually have highly elliptical orbits that intersect the orbit of Earth. These asteroid orbits passing through the Earth are very interesting because some of these asteroids could in fact hit the Earth one day.

Learn more about the topography of the Earth.

Comets: dirty snowballs of the solar system

Then there are the comets. Comets are like dirty snowballs in space. They are rich in volatile substances: water and methane, and other volatile matter which boils if they are brought close enough to the Sun.

Far from the Sun, they are only condensed snowballs; As they get closer to the Sun, these volatiles evaporate, forming a flow of gaseous particles, a flow that is pushed back from the Sun by the solar wind. So you see the comet’s large tail sweeping around the Sun as a comet returns 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, in fact, have orbits of tens of thousands of years and extend well beyond the most distant planet. Some of these comets could have originated from another object in the solar system, a hypothetical belt called the Oort cloud, which is thought to lie well 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 sometimes we have a physical sample of objects outside of our own Earth to study. These are important data for understanding the origin of the solar system.

Meteorites are of the utmost importance. Meteorites are rocks that fall to Earth from space. Meteorites fall on all parts of the Earth. They mostly fall into the oceans, of course, because three-quarters of the Earth’s surface are oceans; but many meteorites also fall to earth, and sometimes they are recovered.

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

Learn more about matter and forces in the universe.

Scientifically valuable meteorites in Antarctica

Scientists are particularly interested in a new set of meteorites that have been found in Antarctica. Meteorites on the glacial plains of Antarctica are a unique and valuable discovery. 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 surface of ice, it just stays on the surface. He is not buried; it does not erode or deteriorate.

However, these meteorites are very important because they give us a random sampling of objects in space. You find irons, you find stones, you find stony iron meteorites, where you have a mixture of metal and other minerals together. This 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 gas planets orbiting the Sun. The first four planets include Mercury, Venus, Earth, and Mars, and the last four planets are Jupiter, Saturn, Uranus, and Neptune.

Q: What are asteroids?

Asteroids are rocky objects, usually quite irregular in shape and varying in size from small boulders to objects hundreds of kilometers in diameter. 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 mainly an iron meteorite, a piece of iron whose surface is corroded and pockmarked due to flight through the atmosphere.

Keep reading
Newton’s mechanical view of the universe and its relationship to the force of gravity
Isaac Newton’s Discovery of Gravity and the Laws of Motion
The gravitational constant in Newton’s gravity equation


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