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What Are the Planets in the Solar System?

Using Slooh’s Online Telescope and integrated Quest learning activities, you can capture your own images of the Sun, Moon, and planets in the Solar System. Solar System Explorer is one of 60+ curriculum-aligned STEM Quest learning activities on Slooh for students 4th grade to college.

Slooh’s Online Telescope:


What Are the Planets in the Solar System?

What Is a Planet?

Planets are the smaller round bodies created from the debris disk left over from star formation (read on to learn more about their formation). They are usually found orbiting their original parent star. There are three requirements that a body must meet to be considered a planet in our Solar System: it must be in orbit around the Sun, have sufficient mass for its self-gravity to overcome rigid body forces and assume a hydrostatic equilibrium shape, and have cleared the neighborhood around its orbit.

Let’s break these conditions down further. The first requirement means that the planet itself must be orbiting the Sun; it can’t simply orbit another object that in turn orbits the Sun, like the Moon does. The second condition states that the body in question has to be roughly spherical. Small bodies remain whatever shape they took during formation, but, if an object has enough mass, its gravity will pull it into a spherical shape. The last requirement means that a planet must be the most massive object along its orbital path by a significant amount, making it gravitationally dominant. Other objects can only be present if they are orbiting the planet or are very small.

The Solar System's Planets

There are eight planets orbiting the Sun, including Earth. In order of increasing distance from the Sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. We can separate the planets into three groups based on their physical characteristics.

Terrestrial Planets:

First, we have the four terrestrial planets: Mercury, Venus, Earth, and Mars. The terrestrial planets are so named because they have rocky and metallic cores surrounded by a relatively thin atmospheric layer (except for Mercury, which lacks an atmosphere owing to its proximity to the Sun). They formed without much gas or ice because of their proximity to the Sun, where the Sun’s radiation blew away the gas and melted the ice.

Many of the rocks on Mars are rich in iron, which, when oxidized, turns a reddish color, giving the planet its red tint. Venus, on the other hand, has an atmosphere that is thick enough to completely obscure the Venusian surface. Its atmosphere is highly reflective, making the planet appear bright white through Slooh's telescopes. Mercury also tends to appear as a bright pinpoint of light because it reflects the glare of the nearby Sun.

All of the terrestrial planets have experienced volcanic activity, but only Earth appears to have active volcanism today. While Mercury and Mars have signs of past activity, they are small enough that their interiors have cooled and largely solidified. No volcanic activity has been observed on Venus, but it could still be active since its atmosphere makes it nearly impossible to see the surface.

Because all of the terrestrial planets are relatively close to Earth, each of them has been the target of multiple exploration missions, with Mars being the most popular destination. Mercury and Venus are more difficult to explore because of their extreme temperatures and Venus’ hostile atmosphere, but orbiting spacecraft have been able to map the surfaces of each planet. Mars is particularly interesting to scientists because of the potential to find evidence of life.


Gas Giants:

Jupiter and Saturn are called gas giants. Unlike the terrestrial planets, they have thick atmospheres which are many times the diameter of their rocky cores. Their atmospheres are primarily made up of hydrogen and helium and have persisted since their origin simply because of their distance from the Sun, their colder temperatures keeping the atmospheric gases gravitationally bound. Beneath the gas giants’ massive atmospheres, the pressures are high enough that hydrogen can exist as a liquid, which only happens at incredibly low temperatures on Earth!

Jupiter is characterized by parallel bands (light-colored ones called zones and dark-colored ones called belts), while Saturn is characterized by its rings. Jupiter, Uranus, and Neptune also have rings, but Saturn’s are far brighter, making them more prominent. Both planets have over 50 moons, including Ganymede (Jupiter) and Titan (Saturn), the two largest moons in the solar system.


Ice Giants:

The Solar System’s outer planets are Uranus and Neptune. They are called ice giants because of their different yet enormous atmospheres which contain a large fraction of heavier elements and molecules, including water, oxygen, carbon, nitrogen, ammonia, and methane. The methane is what gives them both their distinctive blue coloring. Their atmospheres are believed to have formed from the accretion of icy planetesimals, which are small ice-containing chunks.

Some scientists expect that the balance between temperature and pressure beneath the atmospheres could mean that the ice giants have planet-wide water oceans on their surface. However, if this is true, the water would behave strangely, like a mixture of liquid and gas!


What About Pluto?

Although Pluto was long considered the ninth planet in the solar system, it was reclassified as a dwarf planet in 2006. Remember the criteria that a full-sized planet in our solar system must meet: it needs to be in orbit around the Sun, have a nearly round shape, and have cleared the neighborhood around its orbit. Pluto meets the first two criteria but not the third. Because Pluto shares an orbital neighborhood with a variety of other objects near to its size, it is now classified as a dwarf planet rather than the furthest planet in the Solar System from the Sun.

How Were the Planets Formed?

Now we know what each of the planets is, but how did they come to exist? The solar nebula theory is the leading explanation for how the solar system formed from a vast interstellar cloud of gas and dust approximately 4.5 billion years ago.

During the first stage of formation, the cloud collapsed into an enormous disk-shaped region called a protoplanetary disk. This disk rotated around a hot object, known as a protostar, at its center. As time passed, the hydrogen gas in the protostar became hot enough to develop into the Sun, while the outer parts of the disk coalesced into the planets.

This theory gives explanations for the following features of the solar system:

  1. The planets all orbit in the same direction because they all formed from the same circumstellar disk that was, in turn, rotating around the Sun.

  2. The Sun spins in the same direction as the planets’ orbits since the Sun also formed from that same rotating disk.

  3. The planets’ orbits lie in the same plane because the circumstellar disk they formed from was flattened.

  4. The chemical makeup of the inner planets is more similar to the Sun, while Uranus and Neptune contain a higher percentage of water in their atmospheres because they were formed from the edge of the circumstellar disk.


More About Slooh's Solar System Explorer Quest

Explore the solar system in this easy, starter-level quest. Capture your very own images of the Sun, the Moon, and a planet while you learn fascinating details about them. At the end of the quest, you will create a poster of your captured objects to share.

Learning Objectives

By the end of this Quest, students will be able to answer the following questions:

  • What is the Sun, and what is the Moon?

  • What is the leading theory for how the Solar System formed?

  • What are planets, and what are the three types of planets orbiting the Sun?

Vocabulary Words

Standards Addressed


RST.9-10.1, RST.9-10.2, RST.9-10.3, RST.9-10.4, RST.9-10.5, RST.9-10.7, RST.9-10.10, WHST.9-10.2.D, WHST.9-10.2.E

Related Slooh Quests

  1. Stars Like Ours

  2. Mystery of the Changing Moon

  3. In the Footsteps of Kepler - Planetary Motions

About Slooh's Astronomy NGSS Aligned Learning Activities

Slooh’s Online Telescope is a learning platform designed to support any educator in teaching astronomy to meet NGSS requirements by collecting and analyzing real-world phenomena. No previous experience with telescopes is necessary to quickly learn how to use Slooh to explore space with your students.

You can join today to access Slooh's Online Telescope and all 60+ Quest learning activities if you are able to make astronomy a core subject of study for the semester or year. If you only have a few weeks to study astronomy, we also have a curriculum designed to fit your busy academic schedule and budgetary limitations. To learn more about our offers, click here.


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