4.1: The Sun and Planets

An Inventory

The Solar System consists of the Sun, the planets, and their moons and rings, and smaller objects such as dwarf planets, asteroids, comets, and dust. Table \(\PageIndex{1}\) lists the objects in the Solar System in order of percentage of the total mass of the Solar System. While it is not possible to place the entire Solar System on scale to measure its mass, scientists can calculate the mass of objects using physics and math. We will discuss how this is done when we discuss the orbits of the planets.  For now, let's review the different types of objects in the Solar System.

In Table \(\PageIndex{1}\), the Sun takes up 99.8% of the total matter in our planetary system and is by far the most massive object in the Solar System. Jupiter contains only 0.1% of the total mass of the Solar System, while all of the other planets and dwarf planets add up to 0.04%. The remaining 0.06% includes all of the remaining objects in the Solar System: moons, asteroids, comets, and dust. Astronomers can only estimate the total mass of these objects since there are probably many small objects that have not been discovered.   

Moons are objects (>10 m radius) that orbit planets. Except for Mercury and Venus, all of the planets are accompanied by one or more moons. There are more than 210 known moons orbiting planets and dwarf planets. This number continues to grow as astronomers regularly discover new, small moons.

Each of the four outer planets also has rings made up of countless small objects ranging in size from mountains to grains of dust, all in orbit about the equator of the planet. The bright rings of Saturn are the easiest to see. All four ring systems are interesting to scientists because of their complicated forms, influenced by the pull of the moons that also orbit these giant planets. 

Asteroids and comets are small objects that orbit the Sun like miniature planets, generally grouped together in belts, or in the Oort Cloud. Asteroids are mostly made of rock, while comets are mostly ice.

Finally, there are countless grains of broken rock, which we call cosmic dust, scattered throughout the Solar System. When these particles enter Earth’s atmosphere they burn up, producing a brief flash of light. The bright streak across the sky is known as a shooting star, but the scientific term is meteor. Occasionally, some larger chunk of rocky or metallic material survives its passage through the atmosphere and lands on Earth. Any piece that strikes the ground is called a meteorite. 

Figure \(\PageIndex{1}\): The Solar System. Solar System graphic by NASA of the Solar System includes the Sun, the planets, asteroids, and comets. The size of the objects and the distance between them are not to scale. (Public Domain; JPL/NASA via Wikimedia Commons)

Table \(\PageIndex{1}\): Mass of Solar System Objects

Object	Percentage of Total Mass of Solar System
Sun	99.80
Jupiter	0.10
Comets	0.0005–0.03 (estimate)
All other planets and dwarf planets	0.04
Moons and rings	0.00005
Asteroids	0.000002 (estimate)
Cosmic dust	0.0000001 (estimate)

The Sun, a Star

The Sun, our central star in the solar system, plays a major role in life on Earth. The sun is about 300,000 times closer than the next closest star, Proxima Centauri. It takes light about 8. 3 minutes to travel from the sun to Earth; whereas light from Proxima Centauri takes over four years. Size and temperature-wise, the sun is an “average” star, at best. The sun goes through constant changes: energy output, brightness, size; all of which affect Earth. The image of the Sun in Figure \(\PageIndex{2}\) was collected by NASA's Solar Dynamics Observatory. Near the center of the Sun is a large sunspot that is almost 128,000 kilometers across. Ten Earths could be laid across its length.

Studying the sun and how it works allows astronomers to understand other stars. This study of how stars work is called Astrophysics. We specifically study the sun’s fusion process.

The sun’s photosphere vibrates like ripples on a pond. This is due to the sun producing sound waves through the gases. Sound waves and ripples emanate from within the Sun at very low frequencies deep within the Sun and higher frequencies close to the Sun’s surface. Studying these ripples and sound waves allows astronomers to examine the sun’s interior.

Modern astronomy has shown us that the Sun is a star, similar to the other stars that we see at night. The Sun is, in astronomical terms, a rather ordinary star—not unusually hot or cold, old or young, large or small. Just as studies of Earth help us understand observations of the more distant planets, so too does the Sun serve as a guide to astronomers interpreting the messages contained in the light we receive from distant stars. The Sun is an enormous ball about 1.4 million kilometers in diameter, with surface layers of incandescent gas and an interior temperature of millions of degrees. 

As noted previously, the Sun contains over 99% of the mass in the Solar System.  At roughly 1.4 million km in diameter, it is also the largest object in the Solar System.  All of the other objects are significantly smaller than the Sun. The Earth has a diameter of 12,576 km, which means that one could line up almost 110 Earth's across the Sun's equator. Figure \(\PageIndex{3}\) is an image of the Sun with several sunspots, or darker regions, on its surface. The Earth could easily fit inside one of the smaller sunspots. 

The Sun is the source of all visible light in the Solar System. For most of history, we have observed all the other objects by the sunlight reflected off their surface. The Sun also warms the atmosphere of Earth and provides energy for life on Earth through the process of photosynthesis. We will learn more about the Sun in the next chapter.  

Figure \(\PageIndex{2}\): The Sun. This image of the Sun was collected by NASA's Solar Dynamics Observatory. Near the center of the Sun is a large sunspot that is almost 128,000 kilometers across. Ten Earths could be laid across its length. (NASA's SDO Observes Largest Sunspot of the Solar Cycle, Public Domain; Goddard Space Flight Center via NASA Image and Video Library)

Planets

There are eight planets in the Solar System, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Figure \(\PageIndex{3}\) includes images of the eight planets and Earth's moon. The image was created by combining individual images of the planets, arranged in positions that do not reflect their actual distances from each other.  The planets are also not to scale, meaning the relative size of each planet does not reflect its actual size compared to the others. For example, the Earth is not a little smaller than Jupiter. In reality it, would fit inside the Big Red Spot of Jupiter, shown on the bottom right side of the planet. The useful information from this image is that it shows the actual colors of the surfaces or atmospheres of these planets, demonstrating that they each have their own unique properties.  Mercury and the Moon have a similar grey and black surface color. Note that the image of Mercury only shows part of its surface and the Moon has large dark regions on its surface. The image of Venus is a simulated image from radar data of its surface, because it is covered in an atmosphere that is too thick to let through visible light.  The image of Earth shows both its bright white clouds and the blue of its oceans, while Mars shows a few thin clouds and its red, orange, and dark black surface.  For the next four planets, the only visible part is their outer atmospheres.  Jupiter and Saturn have bands of brown, orange, and white clouds.  Saturn has very large rings that are similar in color to its atmosphere.  Uranus has a mostly featureless green-blue atmosphere, while Neptune's atmosphere is blue with some storm clouds that are either dark blue or bright white.  

 Figure \(\PageIndex{3}\): Solar System Planets. This is a montage of planetary images taken by spacecraft managed by NASA’s Jet Propulsion Laboratory in Pasadena, CA. Included are, from top to bottom, images of Mercury, Venus, Earth and Moon, Mars, Jupiter, Saturn, Uranus, and Neptune. Images are not to scale. (Solar System Montage - High Resolution 2001 Version, Public Domain; JPL/NASA via NASA Image and Video Library)
Alternative Description

Definition of a Planet 

In the section about wandering stars, we reviewed how early astronomers noticed that certain objects moved in the sky relative to the pattern of fixed stars. The ancient Greeks called these objects wanderers or planetos in Greek, which later became the word planet in English. The early list of planets included the Moon and the Sun, but did not include the Earth. After the heliocentric model replaced the geocentric model, astronomers knew of five planets, Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Uranus, Neptune, and Pluto were discovered over time as telescopes became more powerful. Beginning in the 1990s, telescope technology had made even greater advances and astronomers began to find objects of similar small size and icy composition to Pluto. In 2005, a team of astronomers found an object that was larger than Pluto.  Due to these discoveries, the International Astronomical Union (IAU), an astronomical society that approves the naming of astronomical objects, decided to adjust the definition of a planet, while proposing a new term, dwarf planet, for Pluto and other objects of similar size. The IAU definition from 2006 is as follows:

A planet in the Solar System is a celestial body that: 

1. is in orbit around the Sun, 
2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
3. has cleared the neighborhood around its orbit.

The first thing to notice is that this definition is restricted to objects orbiting the Sun, so it only defines a planet in the Solar System.  We will discuss the definition of planets outside of our planetary system in the chapter about Exoplanets. This definition also requires that planets be round, which may seem like a strange requirement. This part of the definition separates planets from most smaller objects, such as asteroids and comets that can come in a variety of non-spherical shapes. We will discuss the meaning of hydrostatic equilibrium and how planets become spherical in the next section. The last part of the definition excludes any object that is in one of the Solar System's debris belts. For example, Pluto and it's neighbors are members of the Kuiper Belt. This makes Pluto and up to 9 other asteroids and Kuiper Belt objects all dwarf planets. 

The definition of a planet has changed in the past, and will probably continue to change in the future. Astronomers did not intend to cause frustration in the public or even to "downgrade" Pluto when they changed the definition. Instead, they were attempting to adjust the definition based on new observations.  As astronomers learn more about planets in and out of the Solar System, they will continue to make necessary adjustments.  These adjustments may seem arbitrary, but we will see as we learn more about different objects in the Solar System that it is important to know the differences between a planet that formed in its own orbit around the Sun compared to a dwarf planet that formed in a belt full of other, similar objects.

The International Astronomical Union (IAU) ruled in August 2006 that to be a planet an object must meet three criteria:

1. It must have enough mass and gravity to gather itself into a sphere
2. It must orbit the sun
3. It must reign supreme in its own orbit, having “cleared the neighborhood” of other competing bodies

Yes. This ruling demoted Pluto from a planet to a dwarf planet. According to the new rule, Pluto has not ‘reigned supreme in its own orbit, ‘clearing the neighborhood’ of other competing bodies.

The International Astronomical Union (IAU) has sanction over astronomical definitions and rules. Most of these are minor decisions, like adding leap seconds to a year. The decision – stating what a planet is – is a major decision. The Pluto Planet debate had been brewing for several years.

Yet this was not the IAU’s first vote on the subject. One week prior to the “final” planet definition vote, the IAU’s Planet Definition Committee endorsed the following:

1. It must have enough mass and gravity to gather itself into a sphere
2. It must orbit the sun

What were the implications of this first definition?

Pluto was still a planet, so was its moon Charon, the asteroid Ceres, and the recently discovered Eris. For about a week, the solar system would have had twelve planets instead of nine. The IAU rejected this definition and the now-infamous definition accepted.

What are the implications of the new planet definition?

There are now eight planets in our solar system, along with a new classification of objects called Dwarf Planets. Currently there are three Dwarf Planets: Pluto, Ceres (previously classified as an asteroid), and Eris. A subclass of Dwarf planets is called the Plutoids ; these ice dwarfs are trans-Neptunian (past the planet Neptune) planets that orbit the Sun and are large enough to be somewhat spherical in shape. It’s important to note that these rulings only apply to our solar system, not to other stellar systems with planets. 

Properties of Planets

To learn more about the planets, it is helpful to study their properties.  Table \(\PageIndex{1}\) lists several different properties of the planets.  The section below explains the meaning of those properties. 

Earth revolves around our star, the Sun, which is about 150 million kilometers away—approximately 400 times as far away from us as the Moon. We call the average Earth–Sun distance an astronomical unit (AU) because, in the early days of astronomy, it was the most important measuring standard. Table \(\PageIndex{1}\) includes the distance of all of the planets from the Sun in AU.

A planet’s orbital period is the time it takes a planet to travel once around the Sun. Table \(\PageIndex{1}\) includes the orbital period of all of the planets in Earth years.  

The diameter of a circle is a line that passes through the center of the circle and ends on the edge of the circle, as shown in Figure \(\PageIndex{1}\).  In math, the size of a circle is measured by its diameter, or its radius - the distance from the center to the edge of the circle. A spherical object, like a ball or star, is also measured by its diameter.

Figure \(\PageIndex{4}\): Diameter. This illustration shows the diameter of a circle. The line must pass through the center of the circle to measure its diameter. (Public Domain; Wilinckx via Wikimedia Commons)

Mass is a measurement of the amount of matter within an object. Matter is what the universe is made out of, including stars, planets, and humans. 

The volume of an object is a measurement of the physical space it occupies. Volume is measured in cubic units, such as cubic centimeters (cm³) or liters. 

A penny and an inflated balloon may both have the same mass, but they have very different volumes. The reason is that they also have very different densities, which is a measure of how much mass there is per unit volume. Specifically, density is the mass divided by the volume. The units of density that will be used in this book are grams per cubic centimeter (g/cm³). Familiar materials span a considerable range in density, from artificial materials such as plastic insulating foam (less than 0.1 g/cm³) to gold (19.3 g/cm³). Table \(\PageIndex{2}\) lists the densities of some familiar materials. 

When describing an object, mass is how much matter, volume is how much physical space an object occupies, and density is how tightly matter is packed into the volume of the object.

Table \(\PageIndex{2}\): Densities of Common Materials

Material	Density (g/cm^3)
Gold	19.3
Lead	11.3
Iron	7.9
Earth (bulk)	5.5
Rock (typical)	2.5
Water	1
Wood (typical)	0.8
Insulating foam	0.1
Silica gel	0.02

Comparative Planetology

To learn more about the planets, astronomers compare their properties, such as those listed in Table \(\PageIndex{3}\). This table includes the planetary properties described above and the units they are measured in. The distance from the Sun is measured in astronomical units (AU). The orbital period is measured in Earth years. This means that Mercury and Venus take less than one Earth year to complete an orbit around the Sun and the rest of the planets take longer than one Earth year. The diameter, or the size, of the planets is measured in kilometers.  If you are more familiar with Imperial units, 1 kilometer is equal to 0.62 miles. The mass of the planets is given as a ratio to Earth's mass of 5.97 x 10²⁴ kg. The value for each planet is calculated by taking the mass of that planet and dividing it by the mass of Earth. This makes it easier to see that Mercury, Venus and Mars all have masses smaller than Earth's, while the outer planets are all much more massive than Earth.  The last column of the table is density in units of g/cm³. Mercury has the highest density while Saturn has the lowest density.

Reading the table from top to bottom, or closest planet to the Sun to furthest, there is a sharp change in values between Mars and Jupiter.  The planets suddenly become larger, more massive, and much lower density.  Astronomers split the planets up by these properties into two groups.  Mercury, Venus, Earth, and Mars are the inner planets, while Jupiter, Saturn, Uranus, and Neptune are the outer planets. 

We will look at our Solar System and its worlds from a comparison perspective, called Comparative Planetology, rather than focus on a lot of numbers and data. To compare the planets, they will be grouped into two major types: the Rocky or Terrestrial (Earth-Like) planets, and the Gas Giants or Jovian (Jupiter-like) planets. Recall, the focus of this module will be on the Rocky Planets. The next module will focus on the Gas Giants.

The table, Rocky Planets Vs. Gas Giant Planets, provides a broad snapshot of the differences between the Rocky Planets and the Gas Giant Planets.

Table \(\PageIndex{3}\) : The Planets

Name	Distance from Sun (AU)^2	Orbital Period (Earth Years)	Diameter (km)	Mass (ratio to Earth mass)	Average Density (g/cm^3)
Mercury	0.39	0.24	4,878	0.055	5.4
Venus	0.72	0.62	12,120	0.815	5.2
Earth	1.00	1.00	12,756	1.00	5.5
Mars	1.52	1.88	6,787	0.107	3.9
Jupiter	5.20	11.86	142,984	317.8	1.3
Saturn	9.54	29.46	120,536	95.2	0.7
Uranus	19.18	84.07	51,118	14.5	1.3
Neptune	30.06	164.82	49,660	17.1	1.6

The Inner Planets

Figure \(\PageIndex{5}\) is a collection of individual images of Mercury, Venus, Earth, and Mars, from left to right. The planets are the correct size relative to each other and in "true color" which means that the images are how a human would see them with their eyes. Each inner planet has a unique appearance compared to the others. Mercury is overall grey in color, with a few bright white spots on its surface surrounded by regions of light to medium-dark grey. There are no visible clouds on Mercury. Venus is also grey, but we are not looking at the surface.  Venus has an atmosphere that covers the entire planet in a thick layer of clouds, making it impossible to see the surface from space. Unlike Jupiter and Saturn, Venus's atmosphere has no visible storms or color variations. The image of Earth depicts many white clouds, blue oceans, and a tan colored continent of Africa. Mars is mostly dark orange in color, with some regions that are dark brown and black. There are white, wispy clouds near both poles and a little bit of white ice on Mars' south pole. 

Figure \(\PageIndex{5}\): Inner Planets. This a montage of the inner planets. This includes images taken by NASA spacecraft that show an accurate comparison of diameter. Details in text. (File:4 Terrestrial Planets Size Comp True Color, Public Domain; Scooter20 via Wikimedia Commons)

Since humans live on the surface of the Earth, our observations might tell us that the Earth is made of mostly water, because that is what covers most of the surface. When astronomers study a planet, they consider the overall composition of the entire planet.  From this point of view, the Earth is mostly made of rock and metal with an extremely thin layer of lower density rock and water on the surface.  Mercury, Venus, Earth and Mars are also called the terrestrial planets because their overall composition is mostly rock and metal. The word terrestrial comes from the Latin word terra, which means both the entire planet Earth and the rocks on the surface of the Earth. In these planets, the most abundant rocks, called silicates, are made of silicon and oxygen, and the most common metal is iron. The density of Earth is 5.5 g/cm³, in between the values for rock and iron. 

The Outer Planets

Like the Rocky Planets, the Gas Giant planets – sometimes referred to as the Jovian Planets (Jupiter-like) – also have a number of general characteristics. Gas Giant planets are large in size when compared to the Rocky Planets. Figure \(\PageIndex{6}\) is an image of the 4 outer planets.

Gas Giant planets are made up of primarily Gases – thus their name – with basically the same compounds and elements, but differing percentages. These worlds are not primarily composed of rock or other solid matter as the Rocky Planets.

Some other Gas Giant characteristics include short revolution periods (when compared to the Rocky Planets), many moons, rings, and farther from the Sun in our solar system thus the longer periods of revolution.

The two largest planets, Jupiter and Saturn, have nearly the same chemical makeup as the Sun; they are composed primarily of the two elements hydrogen and helium, with 75% of their mass being hydrogen and 25% helium. On Earth, both hydrogen and helium are gases, so Jupiter and Saturn are sometimes called gas planets or gas giants due to their large size. But, this name is misleading. Jupiter and Saturn are so large that the gas is compressed in their interior until the hydrogen becomes a liquid. Because the bulk of both planets consists of compressed, liquefied hydrogen, we should really call them liquid planets.

Under the force of gravity, the heavier elements sink toward the inner parts of a liquid or gaseous planet. Both Jupiter and Saturn, therefore, have cores composed of heavier rock, metal, and ice, but we cannot see these regions directly. In fact, when we look down from above, all we see is the atmosphere with its swirling clouds. 

Uranus and Neptune are much smaller than Jupiter and Saturn, but each also has a core of rock, metal, and ice. Uranus and Neptune were less efficient at attracting hydrogen and helium gas, so they have much smaller atmospheres in proportion to their cores. Uranus and Neptune are sometimes called the ice giants.

Figure \(\PageIndex{6}\): Outer Planets. This is a montage of the outer planets. This includes images taken by NASA spacecraft that show an accurate comparison of diameter. (Gas planet size comparisons, Public Domain; Solar System Exploration, NASA via Wikimedia Commons)

Rings

One characteristic we will find with the Gas Giant planets is rings. A ring is a disk of dust or small object orbiting a planet or other body. It is hypothesized that rings can form in one of three ways.

1. Material from the solar system’s original protoplanetary disk that was within the planet’s Roche limit, thus not forming a moon.
   • The Roche limit is the minimum distance to which a satellite can approach its primary body without being torn apart by tidal forces due to gravity.
2. A moon that broke apart due to Roche limit tidal stresses.
3. Debris from the moon that broke up due to a large impact.

Different forces can influence rings and ring structures. These include other rings, satellites orbiting the planet, and the planet itself.

There are some objects that have rings but are not Gas Giants. For example, one asteroid has been discovered with rings; this is thought to be due to a collision or collisions. The Martian moon Deimos will eventually break up due to the gravitational tidal forces of Mars and Deimos’ orbit getting too close to Mars; one hypothesis is that a ring will be formed from Deimos around Mars at that time.

References

Barnett, A., Ed. (2025, Mar 12). What is a Planet?. National Aeronautics and Space Administration.

Barnett, A., Ed. (2025, May 28). About the Planets. NASA Science, National Aeronautics and Space Administration. 

Kazmierczak, J., Ed. (2025, May 2). Star Basics. NASA Science, National Aeronautics and Space Administration.

Further Exploration

You may try this density simulation to explore how the density of an object affects its ability to float in water. To float in water, an object must have a lower density than water, 1 g/cm³.