2.6: The Wanderers

The Solar System

The Sun is not the only object that moves among the fixed stars. The Moon and each of the planets that are visible to the unaided eye—Mercury, Venus, Mars, Jupiter, Saturn, and Uranus (although just barely)—also change their positions slowly from day to day. During a single day, the Moon and planets all rise and set as Earth turns, just as the Sun and stars do. But like the Sun, they have independent motions among the stars, superimposed on the daily rotation of the celestial sphere. Noticing these motions, the Greeks of 2000 years ago distinguished between what they called the fixed stars—those that maintain fixed patterns among themselves through many generations—and the wandering stars, or planets. The word “planet,” in fact, means “wanderer” in ancient Greek.

Fixed and Wandering Stars

Today, we do not regard the Sun and Moon as planets, but the ancients applied the term to all seven of the moving objects in the sky. Much of ancient astronomy was devoted to observing and predicting the motions of these celestial wanderers. They even dedicated a unit of time, the week, to the seven objects that move on their own; that's why there are 7 days in a week. The Moon, being Earth's nearest celestial neighbor, has the fastest apparent motion; it completes a trip around the sky in about 1 month. To do this, the Moon moves about 12°, or 24 times its own apparent width on the sky, each day.

The individual paths of the Moon and planets in the sky all lie close to the ecliptic, although not exactly on it. This is because the paths of the planets about the Sun, and of the Moon about Earth, are all in nearly the same plane, as if they were circles on a huge sheet of paper. The planets, the Sun, and the Moon are thus always found in the sky within a narrow 18-degree-wide belt, centered on the ecliptic, called the zodiac. How the planets appear to move in the sky as the months pass is a combination of their actual motions plus the motion of Earth about the Sun; consequently, their paths are somewhat complex. As we will see, this complexity has fascinated and challenged astronomers for centuries.

The generic term for a group of planets and other bodies circling a star is planetary system. Ours is called the Solar System because our Sun is sometimes called Sol. The Solar System consists of the Sun and many smaller objects: the planets, their moons and rings, and such debris as asteroids, comets, and dust. Decades of observation and spacecraft exploration have revealed that most of these objects formed together with the Sun about 4.5 billion years ago. They represent clumps of material that condensed from an enormous cloud of gas and dust. The central part of this cloud became the Sun, and a small fraction of the material in the outer parts eventually formed the other objects.

During the past 50 years, we have learned more about the Solar System than anyone imagined before the space age. In addition to gathering information with powerful new telescopes, we have sent spacecraft directly to many members of the planetary system. Humans have set foot on the Moon and returned samples of its surface soil for laboratory analysis Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\) : Astronauts on the Moon. This photograph from the Apollo 15 mission shows the lunar lander, rover, and an astronaut saluting the flag on the Moon's surface, the only place beyond Earth that humans have explored directly. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{1}\).

An Inventory of the Solar System

Table \(\PageIndex{1}\) compares the mass of objects in the Solar System. The Sun contains over 99% of the total mass of the Solar System. The next most massive object is Jupiter, a giant planet, but with only 0.1% of the Solar System's mass. All other planets together are only 0.04% of the total mass. All other objects in the Solar System, including asteroids, comets, moons, and dust, are tiny fractions of the total mass.

The Sun is a star that is brighter than about 80% of the stars in the Galaxy. It 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. The Sun will be discussed in later chapters as our first, and best-studied, example of a star.

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

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)

Table \(\PageIndex{1}\) also shows that most of the material of the planets is actually concentrated in the largest one, Jupiter, which is more massive than all the rest of the planets combined. Astronomers were able to determine the masses of the planets centuries ago using Kepler's laws of planetary motion and Newton's law of gravity to measure the planets' gravitational effects on one another or on moons that orbit them. Today, we make even more precise measurements of their masses by tracking their gravitational effects on the motion of spacecraft that pass near them.

Besides Earth, five other planets were known to the ancients—Mercury, Venus, Mars, Jupiter, and Saturn—and two were discovered after the invention of the telescope: Uranus and Neptune. The eight planets all orbit in the same direction around the Sun in approximately the same plane. Each planet follows a nearly circular orbit about the Sun. Besides these planets, we have also been discovering smaller worlds beyond Neptune that are called trans-Neptunian objects or TNOs. The first to be found, in 1930, was Pluto, but others have been discovered during the twenty-first century. One of them, Eris, is about the same size as Pluto and has at least one moon. The largest TNOs are also classed as dwarf planets, as is the largest asteroid, Ceres. To date, more than 2600 of these TNOs have been discovered, and one, called Arrokoth, was explored by the New Horizons spacecraft. Figure \(\PageIndex{2}\) includes the orbits of all of the objects mentioned in this paragraph. While the blue orbits of the planets are in the same plane, the red orbits of the dwarf planets are tipped out of the orbital plane of the planets. The orbits of the dwarf planets have higher inclinations than the planets.

Figure \(\PageIndex{2}\) : Orbits of the Planets. This diagram shows the nearly circular, coplanar orbits of the eight major planets and Ceres around the Sun, compared with the more inclined, elongated orbits of the distant dwarf planets Pluto, Haumea, Makemake, and Eris. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{2}\).

Each of the planets and dwarf planets also rotates about an axis running through it, and in most cases the direction of rotation is the same as the direction of revolution about the Sun. The exceptions are Venus, which rotates backward, or retrograde, very slowly, and Uranus and Pluto, which also have strange rotations, each spinning about an axis tipped nearly on its side. We do not yet know the spin orientations of Eris, Haumea, and Makemake.

The four planets closest to the Sun are called the inner or terrestrial planets. Often, the Moon is also discussed as a part of this group, bringing the total of terrestrial objects to five. The terrestrial planets are relatively small worlds, composed primarily of rock and metal. All of them have solid surfaces that hold the records of their geological history in the forms of craters, mountains, and volcanoes. Figure \(\PageIndex{3}\) is an image of Mercury, that is very similar to the surface of the Moon, mostly gray with many craters. Since Earth's surface is covered with water, and its crust is constantly changed by geological processes, it looks very differnt comapred to the heavily cratered surface of Mercury and the other inner planets.

Figure \(\PageIndex{3}\) : Mercury. This grayscale Mariner 10 image reveals Mercury's ancient, heavily cratered surface, more typical of the rocky inner planets than Earth's water-covered one. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{3}\).

The next four planets are much larger and are composed primarily of lighter ices, liquids, and gases. We call these four the outer planets, also called giant or jovian planets. About 1,300 Earths could fit inside Jupiter, for example. These planets do not have solid surfaces on which future explorers might land. They are more like vast, spherical oceans with much smaller, dense cores. Figure \(\PageIndex{4}\) contains all four outer planets with Earth to demonstrate the scale. Jupiter and Saturn have yellowish orange atmospheres, while Uranus and Neptune are blue-green. Jupiter is the largest of the outer planets, followed by Saturn, with Neptune and Uranus both significantly smaller thant the other two outer planets. Saturn is the only planet with large distinctive rings. The other planets have rings, but they are very small and very hard to detect.

Figure \(\PageIndex{4}\) : Outer Planets. This montage compares the four giant outer planets, Jupiter, Saturn, Uranus, and Neptune, to scale, with Earth shown below for size comparison. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{4}\).

Near the outer edge of the system lies Pluto, which was the first of the distant icy worlds to be discovered beyond Neptune. Pluto was visited by a spacecraft, the NASA New Horizons mission, in 2015. Figure \(\PageIndex{5}\) is one of a handful of images collected by New Horizons as it flew by Pluto. Its surface is covered in ices made of nitrogen with small amounts of carbon dioxide and methane. Its surface has many different regions of geologic structures including craters, mountains, and smooth plains. The surface of the broad white area, Sputnik Plain, is possibly less than 10 million years old. Some changes in the surface are caused by seasonal condensation and sublimation of Pluto's thin atmosphere.

Figure \(\PageIndex{5}\) : Pluto. This New Horizons image of Pluto reveals the smooth, icy expanse of the Sputnik Plain bordering rugged, heavily cratered terrain, showing a surface far more complex than expected for a small, distant world. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{5}\).

Table \(\PageIndex{2}\) summarizes some of the main facts about the planets. An AU (or astronomical unit) is the distance from Earth to the Sun. The inner planets are all relatively close to the Sun, within 2 AU. From there the outer planets are much further spread apart with Jupiter near 5 AU, and Neptune at 30 AU. The orbital periods of the planets are all scaled to Earth years. Note that Neptune's year is significantly longer than the life span of a human being. Mercury is the smallest planet while Jupiter is the largest. The inner planets are all less than 10,000 km in diameter. Earth and Venus are nearly the same size. Saturn and Jupiter are over 100,000 km in diameter while Uranus and Neptune are nearly the same size at about 50,000 km. The outer planets are all more massive than the inner planets. We give densities in units where the density of water is 1 g/cm³. Because the outer planets are mostly made of lighter elements like hydrogen and helium, they all have very low densities, close to the value of water. In fact, since Saturn's density is less than water, if you had a large enough tub, it would float in water. The inner planets are made of mostly rock and metal, giving them densities close to 5. Mars has a density more similar to the Moon.

Table \(\PageIndex{2}\): The Planets

Name	Distance from Sun (AU)	Orbital Period (y)	Diameter (km)	Mass (10^23 kg)	Density (g/cm^3)
Mercury	0.39	0.24	4,878	3.3	5.4
Venus	0.72	0.62	12,120	48.7	5.2
Earth	1.00	1.00	12,756	59.8	5.5
Mars	1.52	1.88	6,787	6.4	3.9
Jupiter	5.20	11.86	142,984	18,991	1.3
Saturn	9.54	29.46	120,536	5686	0.7
Uranus	19.18	84.07	51,118	866	1.3
Neptune	30.06	164.82	49,660	1030	1.6

Smaller Objects

Most of the planets are accompanied by one or more moons; only Mercury and Venus move through space alone. There are more than 210 known moons orbiting planets and dwarf planets (see Appendix G for a listing of the larger ones), and undoubtedly many other small ones remain undiscovered. The largest of the moons are as big as small planets and just as interesting. In addition to our Moon, they include the four largest moons of Jupiter (called the Galilean moons, after their discoverer) and the largest moons of Saturn and Neptune (confusingly named Titan and Triton).

Each of the giant planets also has rings made up of countless small bodies ranging in size from mountains to mere grains of dust, all in orbit about the equator of the planet. Figure \(\PageIndex{6}\) is an image of the bright rings of Saturn, which are, by far, the easiest to observe. The rings are not uniform, with changes in color and gaps between different groups. 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.

Figure \(\PageIndex{6}\) : Saturn with Rings. This Cassini spacecraft image captures Saturn's ring system nearly face-on, showing a thin ring shadow on the planet's cloud tops and Saturn's own shadow falling across part of the rings. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{6}\).

The Solar System has many other less-conspicuous members. Another group is the asteroids, rocky bodies that orbit the Sun like miniature planets, mostly in the space between Mars and Jupiter. Figure \(\PageIndex{7}\) is an image of the asteroid Eros, collected by the NEAR-Shoemaker spacecraft. Unlike planets, most asteroids are not spherical, but come in all different types of shapes, many of them similar to potatoes. Most asteroids are remnants of the initial population of the Solar System that existed before the planets themselves formed. Some of the smallest moons of the planets, such as the moons of Mars, are very likely captured asteroids.

Figure \(\PageIndex{7}\) : Asteroid Eros. This NEAR-Shoemaker spacecraft image shows the irregular, elongated shape and heavily cratered surface typical of asteroids, in this case the near-Earth asteroid Eros. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{7}\).

Another class of small bodies is composed mostly of ice, made of frozen gases such as water, carbon dioxide, and carbon monoxide. These objects are called comets. Figure \(\PageIndex{8}\) is an image of comet Churyumov-Gerasimenko from the Rosetta spacecraft. Comets spend most of their existence far from the Sun. However, when their orbits bring them near the Sun, the ices on their surface begin to sublime, or transform from solid to gas. This leads to long tails of gas and dust that can sometimes be seen from Earth. This comet is active, with jets of gas escaping from its surface. Comets also are remnants from the formation of the Solar System, but they were formed and continue (with rare exceptions) to orbit the Sun in distant, cooler regions—stored in a sort of cosmic deep freeze.

Figure \(\PageIndex{8}\) : Comet Churyumov-Gerasimenko. This Rosetta spacecraft image shows the double-lobed nucleus of Comet 67P/Churyumov-Gerasimenko, with bright jets of gas and dust streaming away from the sunlit surface as the comet neared the Sun in 2015. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{8}\).

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 (as millions do each day) they burn up, producing a brief flash of light in the night sky known as a meteor (meteors are often referred to as shooting stars). 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 known as a meteorite.

A Scale Model of the Solar System

Astronomy often deals with dimensions and distances that far exceed our ordinary experience. What does 1.4 billion kilometers—the distance from the Sun to Saturn—really mean to anyone? It can be helpful to visualize such large systems in terms of a scale model.

In our imaginations, let us build a scale model of the Solar System, adopting a scale factor of 1 billion (10⁹)—that is, reducing the actual Solar System by dividing every dimension by a factor of 10⁹. Earth, then, has a diameter of 1.3 centimeters, about the size of a grape. The Moon is a pea orbiting this at a distance of 40 centimeters, or a little more than a foot away. The Earth-Moon system fits into a standard backpack.

In this model, the Sun is nearly 1.5 meters in diameter, about the average height of an adult, and our Earth is at a distance of 150 meters—about one city block—from the Sun. Jupiter is five blocks away from the Sun, and its diameter is 15 centimeters, about the size of a very large grapefruit. Saturn is 10 blocks from the Sun; Uranus, 20 blocks; and Neptune, 30 blocks. Pluto, with a distance that varies quite a bit during its 249-year orbit, is currently just beyond 30 blocks and getting farther with time. Most of the moons of the outer Solar System are the sizes of various kinds of seeds orbiting the grapefruit, oranges, and lemons that represent the outer planets.

In our scale model, a human is reduced to the dimensions of a single atom, and cars and spacecraft to the size of molecules. Sending the Voyager spacecraft to Neptune involves navigating a single molecule from the Earth–grape toward a lemon 5 kilometers away with an accuracy equivalent to the width of a thread in a spider's web.

If that model represents the Solar System, where would the nearest stars be? If we keep the same scale, the closest stars would be tens of thousands of kilometers away. If you built this scale model in the city where you live, you would have to place the representations of these stars on the other side of Earth or beyond.

By the way, model Solar Systems like the one we just presented have been built in cities throughout the world. In Sweden, for example, Stockholm's huge Globe Arena has become a model for the Sun, and Pluto is represented by a 12-centimeter sculpture in the small town of Delsbo, 300 kilometers away. Another model Solar System is in Washington on the Mall between the White House and Congress.

Examples and Exercises

Example: Comparing Planet Densities

Let's compare the densities of several members of the Solar System. The density of an object equals its mass divided by its volume. The volume (V) of a sphere (like a planet) is calculated using the equation

\[V = \frac{4}{3} \pi R^3 \nonumber\]

where π (the Greek letter pi) has a value of approximately 3.14. Although planets are not perfect spheres, this equation works well enough. The masses and diameters of the planets are given in Table 7.2. For data on selected moons, see Appendix G. Let's use Saturn's moon Mimas as our example, with a mass of \(4 \times 10^{19}\) kg and a diameter of approximately 400 km (radius, \(200 \text{ km} = 2 \times 10^5 \text{ m}\)).

Solution

The volume of Mimas is

\[\frac{4}{3} \times 3.14 \times (2 \times 10^5 \text{ m})^3 = 3.3 \times 10^{16} \text{ m}^3. \nonumber\]

Density is mass divided by volume:

\[\frac{4 \times 10^{19} \text{ kg}}{3.3 \times 10^{16} \text{ m}^3} = 1.2 \times 10^3 \text{ kg/m}^3. \nonumber\]

Note that the density of water in these units is 1000 kg/m³, so Mimas must be made mainly of ice, not rock.

Exercise: Comparing Planet Densities

Calculate the average density of our own planet, Earth. Show your work. How does it compare to the density of an ice moon like Mimas? See Table \(\PageIndex{2}\) for data.

Answer

 

For a sphere, \(\text{density} = \dfrac{\text{mass}}{\left(\frac{4}{3}\pi R^3\right)} \text{ kg/m}^3.\) For Earth, then, \(\text{density} = \dfrac{6 \times 10^{24} \text{ kg}}{4.2 \times 2.6 \times 10^{20} \text{ m}^3} = 5.5 \times 10^3 \text{ kg/m}^3.\) This density is four to five times greater than Mimas'. In fact, Earth is the densest of the planets.

Further Exploration

• View this gallery of NASA images of the Apollo mission .
• Learn more about NASA's mission to Pluto and see high-resolution images of Pluto and its moon Charon.
• Watch two videos that provide a tour of the Solar System objects. Shane Gellert's I Need Some Space uses NASA photography and models to show the various worlds with which we share our system. In the more science fiction-oriented Wanderers video, we see some of the planets and moons as tourist destinations for future explorers, with commentary taken from recordings by Carl Sagan.
• This model of the Solar System has all orbits and sizes to scale, and it lets you fly between the planets at an enhanced speed.