Sun with planets and some of the dwarf planets of the solar system.  Sizes may be roughly to scale, but not the distances.

Our ancestors long ago recognized the motions of the planets through the sky, but it has been only a few hundred years since we learned that we are part of a solar system centered on the Sun. Even then, we knew little about the other planets until the advent of large telescopes. More recently, our understanding of other worlds has exploded with the dawn of space exploration. We’ve lived in this solar system all along, but only now are we getting to know it. In this chapter, we’ll explore our solar system like newcomers to the neighborhood. We’ll first look at the broad patterns we observe in the solar system and the general characteristics of the objects within it. We’ll then take a brief tour of the individual worlds, starting from the Sun and moving outward through the planets. Finally, we’ll discuss the use of spacecraft to explore the solar system, examining how we are coming to learn so much more about our neighbors.



Section 7-1: Layout and Structure


The Solar System or solar system comprises the Sun and the retinue of celestial objects gravitationally bound to it: the eight planets, their known moons, three currently identified dwarf planets and their four known moons, and thousands of small bodies. This last category includes asteroids, meteoroids, comets, and interplanetary dust.  In order of their distances from the Sun, the planets are Mercury Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Six of the eight planets are in turn orbited by natural satellites (usually termed “moons” after Earth’s Moon) and every planet past the asteroid belt is encircled by planetary rings of dust and other particles. The planets other than Earth are named after gods and goddesses from Greco-Roman mythology.  The planets all have nearly circular orbits and all orbit counterclockwise around the Sun (from the vantage point of looking down on the North Pole of the Earth).  In fact, large moons also generally orbit their host planets in the CCW direction.  The planets also lie very near the ecliptic plane, the plane defined by the Earth’s orbit around the Sun.  The axes of rotation of the planets is roughly perpendicular to the plane of their motion (diagrams below). From 1930 to 2006, Pluto, the largest known Kuiper belt object, was considered the Solar System’s ninth planet. However, in 2006 the International Astronomical Union (IAU) created an official definition of the term “planet”.  Under this definition, Pluto is reclassified as a dwarf planet, and there are eight planets in the Solar System.

The ecliptic viewed in sunlight from behind the Moon in this Clementine image. From left to right: Mercury, Mars, Saturn

Astronomers most often measure distances within the solar system in astronomical units or AU. One AU is the average distance between the Earth and the Sun or roughly 149 598 000 km (93,000,000 mi). Pluto is roughly 39 AU from the Sun while Jupiter lies at roughly 5.2 AU. Informally, the Solar System is sometimes divided into separate regions. The inner Solar System includes the four terrestrial planets and the main asteroid belt.  The next region contains the four gas giant planets or Jovian planets.  The next region, just past the orbit of Neptune, is the Kuiper belt; a region of icy objects, some as bit as large moons called trans-Neptunian objects.  The outermost region is called the Oort cloud, believed to be composed of icy objects, some of which become visible as comets if they happen to fall into the inner part of the solar system and swing past the Sun.  Nobody know the actual extent of the Kuiper belt.  Estimates put it from Neptune to a distance of 30-50 AU from the Sun.  The extent of the Oort cloud may be as much as 100,000 AU in diameter.  Like planets, objects in both the asteroid belt and in the Kuiper belt generally orbit the Sun in a CCW direction, fairly concentrated near the ecliptic plane.

Kuiper Belt and Oort Cloud (left) and the main asteroid belt (right)

One common misconception is that the orbits of the major objects within the Solar System (planets, Pluto and asteroids) are equidistant. To cope with the vast distances involved, many representations of the Solar System simplify these orbits by showing them the same distance apart. However, in reality, with a few exceptions, the Solar System is arranged so that the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33 AU farther out than Mercury while Jupiter is 1.9 AU from the farthest extent of the asteroid belt and Neptune’s orbit is roughly 20 AU farther out than that of Uranus. Attempts have been made to determine a correlation between these distances (see Bode’s Law) but to date there is no accepted theory that explains the orbital distances.

Planets, dwarf planets, and small solar system bodies

In a decision passed by the International Astronomical Union General Assembly on August 24, 2006, the objects in the Solar System other than the Sun and natural satellites were divided into three separate groups: planets, dwarf planets and small solar system bodies. Under this classification, a planet

  1. is a body in orbit around the Sun that
  2. has enough mass to form itself into a spherical shape and
  3. has cleared its immediate neighborhood of all smaller objects. Eight objects in the Solar System currently meet this definition; they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Dwarf planet was a second and new classification. The key difference between planets and dwarf planets is that while both are required to orbit the Sun and be of large enough mass that their own gravity pulls them into a nearly round shape, dwarf planets are not required to clear their neighborhood of other celestial bodies. Five objects in the solar system are currently included in this category:

  1. Ceres – discovered on January 1, 1801 (45 years before Neptune), considered a planet for half a century before reclassification as an asteroid. Classified as a dwarf planet on September 13, 2006.
  2. Pluto – discovered on February 18, 1930, classified as a planet for 76 years. Classified as a dwarf planet on August 24, 2006.
  3. Eris – discovered on October 21, 2003. Unofficially referred to as the “tenth planet” in media reports. Classified as a dwarf planet on September 13, 2006.
  4. Makemake – discovered on March 31, 2005. Classified as a dwarf planet on July 11, 2008.
  5. Haumea – discovered on 2004 December 28. Classified as a dwarf planet on September 17, 2008.

The IAU is evaluating other known objects to see if they fit within the definition of dwarf planets. The most likely candidates are some of the larger asteroids and several trans-Neptunian objects such as Sedna, Orcus, and Quaoar. In June 2008, the IAU announced a sub-classification of dwarf planets called plutoids.  A plutoid is basically a dwarf planet that is farther than Neptune.  Pluto, Makemake, Haumea and Eris are considered plutoids, but Ceres, as it seems to be a one-of-a-kind object, has no sub-classification. The remainder of the objects in the Solar System were classified as small solar system bodies (SSSBs). As the IAU noted in its resolution: These currently include most of the Solar System asteroids, most trans-Neptunian objects (TNOs), comets, and other small bodies.

Some dwarf Planets of the solar system compared with Earth, our Moon, and Mars.

Section 7-2: Sun The Sun is the Solar System’s parent star, and far and away its chief component.  It contains 99.86% of the system’s known mass and dominates it gravitationally.  Jupiter and Saturn, the Sun’s two largest orbiting bodies, account for more than 90% of the system’s remaining mass. (The currently hypothetical Oort cloud would also hold a substantial percentage were its existence confirmed).  Its large mass gives it an interior density high enough to sustain nuclear fusion, releasing enormous amounts of energy, most of which is radiated into space in the form of electromagnetic radiation including visible light.  It rotates on its axis in the same direction that the planets revolve around it – CCW. It is classed as a moderately large yellow dwarf; however, this name is misleading, as on the scale of stars in our galaxy, the Sun is rather large and bright. The Sun is in the “prime of life” for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion, and been forced, as older red giants must, to fuse more inefficient elements such as helium and carbon. The Sun is growing increasingly bright as it ages. Early in its history, it was roughly 75 percent as bright as it is today.  Calculations of the ratios of hydrogen and helium within the Sun suggest it is roughly halfway through its life cycle, and will eventually begin moving off the main sequence, becoming larger and brighter but also cooler and redder, until, about five billion years from now, it too will become a red giant. The Sun is a population I star, meaning that it is fairly new in galactic terms, having been born in the later stages of the universe’s evolution. As such, it contains more elements heavier than hydrogen and helium (“metals” in astronomical parlance) than older population II stars such as those found in globular clusters.  Since elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, the first generation of stars had to die before the universe could be enriched with them. For this reason, the very oldest stars contain very few metals, while stars born later have more. This high metallicity is thought to have been crucial in the Sun’s developing a planetary system, because planets form from accretion of metals.

 Interplanetary medium

The Sun radiates a continuous stream of charged particles, a plasma known as solar wind, ejecting it outwards at speeds greater than 2 million kilometers per hour, creating a very tenuous atmosphere (the heliosphere), that permeates the solar system for at least 100 AU. This environment is known as the interplanetary medium. The influence of the Sun’s rotating magnetic field on the interplanetary medium creates the largest structure in the solar system, the heliospheric current sheet. Earth’s magnetic field protects its atmosphere from interacting with the solar wind. However, Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space. The interplanetary medium is home to at least two disc-like regions of cosmic dust. The first, which lies in the inner solar system, is known as the zodiacal dust cloud and is responsible for the phenomenon of zodiacal light. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets.  The second, which extends from about 10 AU to about 40 AU, was probably created by similar collisions within the Kuiper belt.

Section 7-3: Comparing Terrestrial and Jovian Planets

Densities An important property of a planet that tells what a planet is made of is its density. A planet’s density is how much material it has in the space the planet occupies: density = mass/volume. Planets can have a wide range of sizes and masses but planets made of the same material will have the same density regardless of their size and mass. For example, a huge, massive planet can have the same density as a small, low-mass planet if they are made of the same material. I will specify the density relative to the density of pure water because it has an easy density to remember: 1 gram/centimeter3 or 1000 kilograms/meter3. The four planets closest to the Sun (Mercury, Venus, Earth, Mars) are called the terrestrial planets because they are like the Earth: small rocky worlds with relatively thin atmospheres.

The terrestrial planets to the same scale (using images from NASA and JPL). From top left and proceeding clockwise: Earth, Venus, Mercury, Mars (at bottom left).


Terrestrial (Earth-like) planets have overall densities = 4-5 (relative to the density of water) with silicate rocks on the surface. Silicate rock has density = 3 (less than the average density of a terrestrial planet) and iron has a density = 7.8 (more than the average density of a terrestrial planet). The four inner or terrestrial planets are characterized by their dense, rocky composition, few or no moons, and lack of ring systems. They are composed largely of minerals with high melting points, such as the silicates which form their solid crusts and semi-liquid mantles, and metals such as iron and nickel, which form their cores. Terrestrial planets also have longer rotational periods than the larger Jovian planets and partly as a consequence of their slower rotation, they have weaker magnetic fields than their Jovian counterparts.  Three of the four inner planets (Venus, Earth and Mars) have substantial atmospheres; all have impact craters and possess tectonic surface features such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets which are closer to the Sun than the Earth is (i.e. Mercury and Venus). The Jovian planets to the same scale (using images from NASA and JPL). From top left and proceeding clockwise: Jupiter, Uranus, Neptune, Saturn (at bottom right). The Earth is also included to the same scale at center. The four giant planets beyond Mars (Jupiter, Saturn, Uranus, Neptune) are called the Jovian planets because they are like Jupiter: large, mostly liquid worlds with thick atmospheres. Jovian (Jupiter-like) planets have overall densities = 0.7-1.7 (relative to the density of water) with light gases visible on top. The four Jovian planets, or gas giants, (sometimes called outer planets) are so large they collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn are true giants, at 318 and 95 Earth masses, respectively, and composed largely of hydrogen and helium. Uranus and Neptune are both substantially smaller, being only 14 and 17 Earth masses, respectively. Their atmospheres contain a smaller percentage of hydrogen and helium, and a higher percentage of “ices”, such as water, ammonia and methane. For this reason some astronomers suggested that they belong in their own category, “ice giants.” All four of the gas giants exhibit orbital debris rings, although only the ring system of Saturn is easily observable from Earth. The term outer planet should not be confused with superior planet, which designates those planets which lie outside Earth’s orbit (thus consisting of the outer planets plus Mars).   As well as all having ring systems, Jovian planets  are all characterized by rotational periods much shorter than their terrestrial counterparts as well as having much stronger magnetic fields.  Jovian planets also have many moons.  A summary comparing Jovian and Terrestrial planets is given below.

Comparison of the Terrestrial and Jovian Planets

close to the Sun far from the Sun
closely spaced orbits widely spaced orbits
small masses large masses
small radii large radii
predominantly rocky predominantly gaseous
solid surface no solid surface
high density low density
slower rotation faster rotation
weak magnetic fields strong magnetic fields
few moons many moons
no rings many rings

Section 7-4: The Inner Solar System

The inner planets. From left to right: Mercury, Venus, Earth, and Mars (sizes to scale)


Image:MESSENGER first photo of unseen side of mercury.jpg  Mercury (0.4 AU), the closest planet to the Sun, is also the smallest of the planets, at only 0.055 Earth masses. Mercury is very different from the other terrestrial planets; it has no natural satellite, and its only known geological features besides impact craters are “wrinkle ridges” probably produced by a period of contraction early in its history.  Its almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.  Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Sun’s energy.   Click the thumbnail on the left to view a larger image.

Click picture to view video.


Image:Venus-real.jpg  Venus (0.7 AU) is of comparable mass to the Earth (0.815 Earth masses), and, like Earth, possesses a thick silicate mantle around an iron core, as well as a substantial atmosphere and evidence of internal geological activity, such as volcanoes. However, it is much drier than Earth and its atmosphere is 90 times as dense. Venus has no natural satellite. It is the hottest planet, with surface temperatures over 400 °C, most likely due to the amount of greenhouse gases in the atmosphere.  Although no definitive evidence of current geological activity has yet been detected on Venus, its substantial atmosphere and lack of a magnetic field to protect it from depletion by the solar wind suggest that it must be regularly replenished by volcanic eruptions.  Click the thumbnail on the left to view a larger image.

Click picture to view video.


Image:The Earth seen from Apollo 17.jpg The largest and densest of the inner planets, Earth (1 AU) is also the only one to demonstrate unequivocal evidence of current geological activity. Earth is the only planet known to have life. Its liquid hydrosphere, unique among the terrestrial planets, is probably the reason Earth is also the only planet where plate tectonics has been observed, because water acts as a lubricant for subduction.  Its atmosphere is radically different from the other terrestrial planets, having been altered by the presence of life to contain 21 percent free oxygen.  It has one satellite, the Moon; the only large satellite of a terrestrial planet in the Solar System.  Click the thumbnail on the left to view a larger image.


Image:Mars Hubble.jpg  Mars (1.5 AU), at only 0.107 Earth masses, is smaller than Earth and Venus. It possesses a tenuous atmosphere of carbon dioxide. Its surface, peppered with vast volcanoes and rift valleys such as Valles Marineris, shows that it was once geologically active and recent evidence suggests this may have been true until very recently. Mars possesses two tiny moons (Deimos and Phobos) thought to be captured asteroids.  Click the thumbnail on the left to view a larger image.”

Click picture to view video.

Asteroid Belt

d8a2a89f  Asteroids are mostly small solar system bodies that are composed in significant part of rocky and metallic non-volatile minerals.   Like planets, asteroids also go counterclockwise around the Sun.  Asteroids in the asteroid belt are in roughly-circular orbits, while asteroids found in other regions of the solar system can have very eccentric orbits.  Click the thumbnail on the left to view a larger image. The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be the remnants from the Solar System’s formation that failed to coalesce because of the gravitational interference of Jupiter. Asteroids range in size from hundreds of kilometers to as small as dust. All asteroids save the largest, Ceres, are classified as small solar system bodies; however, a number of other asteroids, such as Vesta and Hygieia, could potentially be reclassified as dwarf planets if it can be conclusively shown that they have achieved hydrostatic equilibrium. The asteroid belt contains tens of thousands – and potentially millions – of objects over one kilometer in diameter.  However, despite their large numbers, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth.  In contrast to its various depictions in science fiction, the main belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with diameters between 10 and 10-4 m are called meteoroids.


7fe317fd  Ceres (2.77 AU) is the largest astronomical body in the asteroid belt and the only known dwarf planet in this region. It has a diameter of slightly under 1000 km, large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the nineteenth century, but was reclassified as an asteroid as further observation revealed additional asteroids.  It has since been again reclassified as a dwarf planet.  Click the thumbnail on the left to view a larger image.

Section 7-5: Jovian Planets

From left to right: Jupiter, Saturn, Uranus and Neptune.


58d4a723  Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. Its composition of largely hydrogen and helium is not very different from that of the Sun. Jupiter’s strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. The four largest of its 63 satellites, Ganymede, Callisto, Io, and Europa (the Galilean satellites) share elements in common with the terrestrial planets, such as volcanism and internal heating. Ganymede, the largest satellite in the Solar System, has a diameter larger than Mercury. Click the thumbnail on the left to view a larger image.

Click picture to view video.


Image:Saturn (planet) large.jpg  Saturn (9.5 AU), famous for its extensive ring system, has many qualities in common with Jupiter, including its atmospheric composition, though it is far less massive, being only 95 Earth masses. Two of its 56 moons, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan, like Ganymede, is larger than Mercury; it is also the only satellite in the solar system with a substantial atmosphere.  Click the thumbnail on the left to view a larger image.

Click picture to view video.


Image:Uranus.jpg  Uranus (19.6 AU) at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt lies at over ninety degrees to the ecliptic. Its core is remarkably cold compared with the other gas giants, and radiates very little heat into space. Uranus has 27 satellites, the largest being Titania, Oberon, Umbriel, Ariel and Miranda.  Click the thumbnail on the left to view a larger image.

Click picture to view video.


Image:Neptune.jpg  Neptune (30 AU), though slightly smaller than Uranus, is denser and slightly more massive, at 17 Earth masses, and radiates more internal heat than Uranus, but not as much as Jupiter or Saturn. Neptune has 13 moons. The largest, Triton, is geologically active, with geysers of liquid nitrogen, and is the only large satellite to revolve around its host planet in a retrograde motion. Neptune possesses a number of Trojan asteroids.  Click the thumbnail on the left to view a larger image. solarsystemproperties.swf

Section 7-6: Into the Kuiper Belt


 the   Comets are small solar system bodies, usually only a few kilometers across, composed largely of volatile ices and possessing highly eccentric orbits; generally having a perihelion within the orbit of the inner planets and an aphelion far beyond Pluto. When a comet approaches the Sun, its icy surface begins to sublimate, or boil away, creating a coma; a long tail of gas and dust which is often visible with the naked eye.   Click the thumbnail on the left to view a larger image. There are two basic types of comet: short-period comets, with orbits less than 200 years, and long-period comets, with orbits lasting thousands of years. Short-period comets, such as Halley’s Comet, are believed to originate in the Kuiper belt, while long period comets, such as Hale-Bopp (pictured), are believed to originate in the Oort Cloud. Some comets with hyperbolic orbits may originate outside the solar system.  Old comets that have had most of their volatiles driven out by solar warming are often categorized as asteroids.


The Centaurs, which roughly extend from 9 to 30 AU, are icy comet-like bodies that orbit in the region between Jupiter and Neptune. The largest known Centaur, 10199 Chariklo, has a diameter of between 200 and 250 km. The first centaur to be discovered, 2060 Chiron, has been called a comet since it has been shown to develop a coma just as comets do when they approach the sun. Some astronomers class Centaurs as scattered Kuiper belt objects along with the residents of the scattered disc; merely Kuiper belt objects scattered inward, rather than outward

Kuiper Belt

The area beyond Neptune,  or simply the “trans-Neptunian region”, is still largely unexplored.  Kuiper belt objects also seem to generally orbit the Sun in a counterclockwise direction somewhat near the ecliptic plane.  Click the thumbnail on the left to view a larger image. This region’s first formation is the Kuiper belt, a great ring of debris, similar to the asteroid belt but composed mainly of ice and far greater in extent, which lies between 30 and 50 AU from the Sun. This region is thought to be the place of origin for short-period comets, such as Halley’s comet. Though it is composed mainly of small solar system bodies, many of the largest Kuiper belt objects, such as Quaoar, Varuna, 2003 EL61, 2005 FY9 and Orcus, could soon be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km; however, the total mass of the Kuiper belt is relatively low, perhaps barely equaling the mass of the Earth.  Many Kuiper belt objects have multiple satellites and most have orbits that take them outside the plane of the ecliptic. The Kuiper belt can be roughly divided into two regions: the “resonant” belt, consisting of objects whose orbits are in some way linked to that of Neptune (orbiting, for instance, three times for every two Neptune orbits, or twice for every one), which actually begins within the orbit of Neptune itself, and the “classical” belt, consisting of objects that don’t have any resonance with Neptune, and which extends from roughly 39.4 AU to 47.7 AU.  Members of the classical Kuiper belt are classified as Cubewanos, after the first of their kind to be discovered, 1992 QB1.

Pluto and Charon

Pluto, and its three known moons Pluto, and its three known moons   Pluto (39 AU average), is the largest known object in the Kuiper belt and was previously accepted as the smallest planet in the Solar System. In 2006, it was reclassified as a dwarf planet by the Astronomers Congress organized by the International Astronomers Union (IAU).  Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion. Prior to the 2006 redefinitions, Charon was considered a moon of Pluto, but in light of the redefinition it is unclear whether Charon will continue to be classified as a moon of Pluto or as a dwarf planet itself. Charon does not exactly orbit Pluto in a traditional sense; Charon is about one-tenth the mass of Pluto and the center of gravity of the pair is not within Pluto. Both bodies orbit a barycenter of gravity above the surface of Pluto (in empty space), making Pluto-Charon a binary system. Two much smaller moons, Nix and Hydra, orbit Pluto and Charon. Pluto lies in the resonant belt, having a 3:2 resonance with Neptune (i.e., it orbits three times round the Sun for every two Neptune orbits). Those Kuiper belt objects which share this orbit with Pluto are called Plutinos. Overlapping the Kuiper belt but extending much further outwards is the scattered disc. Scattered disc objects are believed to have been originally native to the Kuiper belt, but were ejected into erratic orbits in the outer fringes by the gravitational influence of Neptune’s outward migration (see Formation and evolution of the Solar System). Most scattered disc objects have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. Their orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers, such as Kuiper belt co-discoverer David Jewitt, consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as “scattered Kuiper belt objects.”


Eris and its moon Dysnomia (left) and its orbital path

Eris (68 AU average) is the largest known scattered disc object and was the cause of the most recent debate about what constitutes a planet since it is at least 5% larger than Pluto with an estimated diameter of 2400 km (1500 mi). It is now the largest of the known dwarf planets.  Like Pluto, Eris is also classified as a plutoid.  It has one moon, Dysnomia. The object has many similarities with Pluto: its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto’s distance from the Sun) and an aphelion of 97.6 AU, and is steeply inclined to the ecliptic plane, at 44 degrees, and is believed to consist largely of rock and ice.

Section 7-7: Farthest Regions

The point at which the solar system ends and interstellar space begins is not precisely defined, since its outer boundaries are delineated by two separate forces: the solar wind and the Sun’s gravity. The solar wind extends to a point roughly 130 AU from the Sun, whereupon it surrenders to the surrounding environment of the interstellar medium. The Sun’s gravity however, holds sway to almost halfway to the next star system. The vast majority of the solar system therefore, is completely unknown; however, recent observations of both the solar system and other star systems have led to an increased understanding of what is or may be lying at its outer edge.


An artist's conception of Sedna An artist's conception of Sedna  Sedna is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper Belt as it has too distant a perihelion to have been affected by Neptune’s migration. He and other astronomers consider it to be the first in an entirely new population, one which also may include the object 2000 CR105, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years.  Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.

Oort Cloud

The Oort cloud, currently only hypothetical, is a great mass of up to a trillion icy objects that is believed to be the source for all long-period comets and to surround the solar system like a shell from 50,000 to 100,000 AU beyond the Sun. It is believed to be composed of comets which were ejected from the inward Solar System by gravitational interactions with the outer planets. Because the Sun’s gravitational hold on them is so weak, Oort cloud objects move only very slowly, though they can be perturbed by such rare events as collisions, or the gravitational effects of a passing star or the galactic tides. Their orbits, unlike the material in Kuiper Belt, are believed to be randomly oriented (shown below.)

Section 7-8: Major Solar System Patterns

Any model invented to explain the formation of our Solar System needs to explain major patterns of construction and motion.  It should explain the general layout of our Solar System. Planetary orbits follow noticeable patterns rather than being randomly strewn about. The most important patterns of motion in our solar system include:

  1. All planetary orbits are nearly circular and lie nearly in the same plane.
  2. All planets orbit the Sun in the same direction–counterclockwise as viewed from high above Earth’s North Pole.
  3. Terrestrial planets are found close to the Sun and Jovian planets farther away.
  4. Asteroids are found in the Asteroid Belt and orbit close to the Sun in nearly circular orbits and roughly on the ecliptic plane.
  5. Comets orbit much further away, generally in the Kuiper Belt and in the Oort cloud.
  6. Comets in the Kuiper Belt orbit near the ecliptic plane (but can be found farther away from it that planets) and go counterclockwise around the Sun.
  7. Most planets rotate in the same direction in which they orbit (counterclockwise as viewed from above the North Pole), with fairly small axis tilts. The Sun also rotates in this same direction.
  8. Most of the solar system’s large moons exhibit similar properties in their orbits around their planets–for example, orbiting in their planet’s equatorial plane in the same direction that the planet rotates.


These orderly patterns are no accident. Rather, they are consequences of the fact that our entire solar system formed from the gravitational collapse of a single cloud of gas and dust. In Chapter 8, we will discuss how this collapse led to these orderly motions.  There are exceptions to the overall patterns and any explanation of the solar system’s formation must allow for such and provide some mechanism for them to occur.  For example, while most of the planets rotate in the same direction that they orbit, Uranus and Pluto rotate nearly on their sides, and Venus rotates “backward”–clockwise, rather than counterclockwise, as viewed from high above Earth’s North Pole. Similarly, while most large moons orbit their planets in the same direction that their planets rotate, Neptune has a large moon (called Triton) that goes in the opposite direction. One of the most interesting exceptions concerns our own Moon. While the other terrestrial planets have either no moons (Mercury and Venus) or very tiny moons (Mars, with two moons), Earth has one of the largest moons in the solar system. Just as exceptional people make the world a more interesting place, the exceptions in our solar system make it a more interesting subject of study.

Section 7-9: Measuring Planetary Size

Angular Measurement

The physical size of a planet can be found from measurements of its angular size and its distance. How large something appears to be is its angular size or angular diameter—the angle between two lines of sight along each side of the object. How big something appears to be obviously depends on its distance from us—it appears bigger when it is closer to us. Every time you drive a car or ride a bicycle, you use another car’s or bicycle’s angular size to judge how far away it is from you. You assume that you are not looking at some toy model. The planets are close enough to the Earth that you can see a round disk and, therefore, they have a measurable angular size. All of the stars (except the Sun) are so far away that they appear as mere points in even the largest telescopes, even though they are actually much larger than the planets. If you know how far away a planet is from you, you can determine its linear diameter D. The diameter of a planet D = 2p × (distance to the planet) × (the planet’s angular size in degrees)/360°, where the symbol p is a number approximately equal to 3.14 (your calculator may say 3.141592653…). The figure above explains where this formula comes from. This technique is used to find the actual diameters of other objects as well, like moons, star clusters, and even entire galaxies.

Example: As the planets orbit the Sun, their distance from us changes. At “opposition” (when they are in the direct opposite direction from the Sun in our sky) a planet gets closest to us. These are the best times to study a planet in detail. The planet Mars reaches opposition every 780 days. Because of their elliptical orbits around the Sun, some oppositions are more favorable than others. Every 15–17 years Mars is at a favorable opposition and approaches within 55 million kilometers to the Earth. At that time its angular size across its equator is 25.5 arc seconds. In degrees this is 25.5 arc seconds × (1 degree/3600 arc seconds) = 0.00708 degrees, cancelling out arc seconds top and bottom.

Its actual diameter = (2p × 55,000,000 km × 0.00708°)/360° = 6800 kilometers. Notice that you need to convert arc seconds to degrees to use the angular size formula.

Eclipsing Systems

Little Pluto is so small and far away that its angular diameter is very hard to measure. Only a large telescope above the Earth atmosphere (like the Hubble Space Telescope) can resolve its tiny disk. However, the discovery in 1978 of a moon, called Charon, orbiting Pluto gave another way to measure Pluto’s diameter. Every 124 years, the orientation of Charon’s orbit as seen from the Earth is almost edge-on, so you can see it pass in front of Pluto and then behind Pluto. This favorable orientation lasts about 5 years and, fortunately for us, it occurred from 1985 to 1990. determining the diameter from eclipse time interval When Pluto and Charon pass in front of each other, the total light from the Pluto-Charon system decreases. The length of time it takes for the eclipse to happen and the speed that Charon orbits Pluto can be used to calculate their linear diameters. Recall that the distance travelled = speed × (time it takes). Pluto’s diameter is only about 2270 kilometers (about 65% the size of our Moon!) and Charon is about 1170 kilometers across. This eclipsing technique is also used to find the diameters of the very far away stars in a later chapter. Pluto’s small size and low mass (see the previous section) have some astronomers calling it an “overgrown comet” instead of a planet and it was recently re-classified as a “dwarf planet”.


Another way to specify a planet’s size is to use how much space it occupies, i.e., its volume. Volume is important because it and the planet’s composition determine how much heat energy a planet retains after its formation billions of years ago. Also, in order to find the important characteristic of density, you must know the planet’s volume. Planets are nearly perfect spheres. Gravity compresses the planets to the most compact shape possible, a sphere, but the rapidly-spinning ones bulge slightly at the equator. This is because the inertia of a planet’s material moves it away from the planet’s rotation axis and this effect is strongest at the equator where the rotation is fastest (Jupiter and Saturn have easily noticeable equatorial bulges). Since planets are nearly perfect spheres, a planet’s volume can be found from volume = (p/6) × diameter3. Notice that the diameter is cubed. Even though Jupiter has “only” 11 times the diameter of the Earth, over 1300 Earths could fit inside Jupiter! On the other end of the scale, little Pluto has a diameter of just a little more than 1/6th the diameter of the Earth, so almost 176 Plutos could fit inside the Earth.  The average density of a planet is then calculated by dividing the planet’s mass by its volume.

Section 7-10: Exploration of the Solar System

How have we learned so much about the solar system? Much of our knowledge comes from telescopic observations, using both ground-based telescopes and telescopes in Earth orbit such as the Hubble Space Telescope. In one case–our Moon–we have learned a lot by sending astronauts to explore the terrain and bring back rocks for laboratory study. The recent revolution in our understanding of the solar system has come largely through robotic spacecraft that actually visit the worlds we wish to study. In this section, we’ll briefly investigate the types of robotic spacecraft used in exploring the solar system.

Discovering the Outermost Planets

The planets Mercury, Venus, Mars, Jupiter, and Saturn were all known to ancient people. Each can be seen with the naked eye and each clearly wanders among the fixed stars of the constellations. In contrast, the three planets beyond Saturn are all relatively recent discoveries in human history. Uranus was the first “discovered” planet. Although faintly visible to the naked eye, Uranus is so faint and moves so slowly in its 84-year orbit of the Sun that ancient people did not recognize it as a planet. Uranus even appeared as a star on some sky charts prior to its “discovery” as a planet in 1781. English astronomer William Herschel discovered Uranus with the help of his sister, Caroline Herschel, who helped build his telescopes and assisted with much of the observing. William Herschel originally suggested naming the planet Georgium Sidus, Latin for George’s star, in honor of his patron, King George III. Fortunately, the idea of “Planet George” never caught on. Instead, many eighteenth- and nineteenth-century astronomers referred to the new planet as Herschel. The modern name Uranus–after the mythological father of Saturn–was first suggested by one of Herschel’s contemporaries, astronomer Johann Bode. It was generally accepted by the mid–nineteenth century. Neptune’s discovery followed next, and it represented an important triumph for Newton’s law of universal gravitation and the young science of astrophysics. By the mid-1800s, careful observations of Uranus had shown its orbit to be slightly inconsistent with that predicted by Newton’s law of gravity–at least if it was being influenced only by the Sun and the other known planets. In the early 1840s in England, a student named John Adams suggested that the inconsistency could be explained by a previously unseen “eighth planet” orbiting the Sun beyond Uranus. According to the official story, he used Newton’s theory to predict the location of the planet but was unable to convince British astronomers to carry out a telescopic search. However, recently discovered documents suggest his prediction may not have been as precise as the official history suggests. Meanwhile, in the summer of 1846, French astronomer Urbain Leverrier made very precise calculations independently. He sent a letter to Johann Galle of the Berlin Observatory suggesting a search for the eighth planet. On the night of September 23, 1846, Galle pointed his telescope to the position suggested by Leverrier. There, within 1° of its predicted position, he saw the planet Neptune. Hence, Neptune’s discovery truly was made by mathematics and physics and was only confirmed with a telescope. As a side note to this story, Leverrier had such faith in Newton’s universal law of gravitation that he also suggested a second unseen planet, this one orbiting closer to the Sun than Mercury. He got this idea because other astronomers had identified slight discrepancies between Mercury’s actual orbit and the orbit predicted by Newton’s theory. He assumed that the planet, which he called Vulcan, had not yet been seen because it was so close to the Sun. Leverrier died in 1877, still believing that Vulcan would someday be discovered. In fact, Vulcan does not exist. Mercury’s actual orbit does not match the orbit predicted by Newton’s law of gravity because Newton’s theory is not the whole story of gravity. About 40 years after Leverrier’s death, Einstein showed that Newton’s theory is only an approximation of a broader theory of gravity, known today as Einstein’s general theory of relativity. Einstein’s theory predicts an orbit for Mercury that matches its actual orbit. This match was one of the first key pieces of evidence in favor of Einstein’s theory. Pluto was discovered in 1930 by American astronomer Clyde Tombaugh, culminating a search that began when astronomers analyzed the orbit of Neptune. The story of Pluto’s discovery at first seemed much the same as that of Neptune’s. Just as discrepancies between the predicted orbit and the actual orbit of Uranus led to the prediction that Neptune must exist, apparent discrepancies between the predicted orbit and the actual orbit of Neptune suggested the existence of an even more distant “ninth planet.” Tombaugh found Pluto just 6° from the position in the sky where this ninth planet had been predicted to lie, so it seemed that the search had been successful. However, while initial estimates suggested that Pluto was much larger than Earth, we now know that Pluto has a radius of only 1,160 kilometers and a mass of just 0.002 Earth mass–making it far too small to affect the orbit of Neptune. In retrospect, the supposed orbital irregularities of Neptune appear to have been errors in measurement. No “ninth planet” is needed to explain the orbit of Neptune. The discovery of Pluto was just a happy coincidence!

Space Mission Types

The spacecraft we send to explore the planets are robots suited for long space journeys. They carry specialized equipment for scientific study. All spacecraft have control computers, power sources such as solar cells, propulsion systems, and devices to point cameras and other instruments precisely at their targets. Robotic spacecraft operate primarily with preprogrammed instructions. They carry radios for communication, allowing them to receive additional instructions from Earth and to send home the data they collect. Most robotic spacecraft make one-way trips from Earth, never physically returning but sending their data back from space in the same way we send radio and television signals around the world. Broadly speaking, the robotic missions we send to explore other worlds fall into one of four major categories:

  • Flyby: A spacecraft on a flyby goes past a world just once and then continues on its way.
  • Orbiter: An orbiter is a spacecraft that orbits the world it is studying, allowing longer-term study during its repeated orbits.
  • Lander or probe: These spacecraft are designed to land on a planet’s surface or to probe a planet’s atmosphere by flying through it.
  • Sample return mission: A sample return mission involves a spacecraft designed to return to Earth carrying a sample of the world it has studied.

The choice of spacecraft type depends on both scientific objectives and cost. In general, a flyby is the lowest-cost way to visit another planet, and some flybys gain more “bang for the buck” by visiting multiple planets. For example, Voyager 2 flew past Jupiter, Saturn, Uranus, and Neptune before continuing on its way out of our solar system.


Flybys tend to be cheaper than other missions because they are generally less expensive to launch into space. Launch costs depend largely on weight, and onboard fuel is a significant part of the weight of a spacecraft heading to another planet. Once a spacecraft is on its way, the lack of friction or air drag in space means that it can maintain its orbital trajectory through the solar system without using any fuel at all. Fuel is needed only when the spacecraft needs to change from one trajectory (orbit) to another. Moreover, with careful planning, some trajectory changes can be made by taking advantage of the gravity of other planets. Look closely at the Voyager 2 trajectory in Figure below. You’ll see that it made significant trajectory changes as it passed by Jupiter and Saturn. In effect, it made these changes for free by using gravity to bend its path rather than by burning fuel. (This technique is known as a “gravitational slingshot.”)

Trajectories of Voyagers I and II

Although a flyby offers only a relatively short period of close-up study, it can provide valuable scientific information. Flybys generally carry small telescopes, cameras, and spectrographs. Because these instruments are brought within a few tens of thousands of kilometers of other worlds (or closer), they can obtain much higher resolution images and spectra than even the largest current telescopes viewing these worlds from Earth. In addition, flybys sometimes give us information that would be very difficult to obtain from Earth. For example, Voyager 2 helped us discover Jupiter’s rings and learn about the rings of Saturn, Uranus, and Neptune through views in which the rings were backlit by the Sun. Such views are possible only from beyond each planet’s orbit.  Flybys may also carry instruments to measure local magnetic field strength or to sample interplanetary dust. The gravitational effects of the planets and their moons on the spacecraft itself provide information about object masses and densities. Like the backlit views of the rings, these types of data cannot be gathered from Earth. Indeed, most of what we know about the masses and compositions of moons comes from data gathered by spacecraft that have flown past them.


An orbiter can study another world for a much longer period of time than a flyby. Like the spacecraft used for flybys, orbiters often carry cameras, spectrographs, and instruments for measuring the strength of magnetic fields. Some missions to Venus and Mars have used radar to make precise altitude measurements of surface features. In the case of Venus, radar observations give us our only clear look at the surface, because we cannot otherwise see through the planet’s thick cloud cover (figure below).

Magellan Venus Orbiter

An orbiter is generally more expensive than a flyby for an equivalent weight of scientific instruments, primarily because it must carry added fuel to change from an interplanetary trajectory to a path that puts it in orbit around another world. Careful planning can minimize the added expense. Two recent Mars orbiters, Mars Global Surveyor and Mars Odyssey, saved on fuel costs by carrying only enough fuel to enter highly elliptical orbits around Mars. In each case, the spacecraft settled into the smaller, more circular orbit needed for scientific observations by skimming the Martian atmosphere at the low point of every elliptical orbit. Atmospheric drag slowed the spacecraft with each orbit and, over several months, circularized the spacecraft orbit. (This technique of using the atmosphere to slow the spacecraft and change its orbit is called aerobraking.)


Landers and Probes

The most “up close and personal” study of other worlds comes from spacecraft that send probes into the atmospheres or landers to the surfaces. For example, in 1995, the Galileo spacecraft dropped a probe into Jupiter’s atmosphere. The probe collected temperature, pressure, composition, and radiation measurements for about an hour as it descended before being destroyed by Jupiter’s high interior pressures and temperatures. On planets with solid surfaces, a lander can offer close-up surface views, local weather monitoring, and the ability to carry out automated experiments. Some landers, such as the Pathfinder lander that arrived on Mars in 1997, carry robotic rovers able to venture across the surface. Landers typically require fuel to slow their descent to a planetary surface, but clever techniques can reduce cost. For example, Pathfinder hit the surface of Mars at crash-landing speed but was protected by a cocoon of air bags deployed on the way down. These air bags allowed the lander to bounce along the surface of Mars for more than a kilometer before it finally came to rest (Figure below). The same strategy was employed for the Spirit and Opportunity landers that followed years later.

Artist conception of the landing of Spirit rover.  The image on the right shows the rover shrouded and padded with inflatable balls.

Sample Return Missions

While probes and landers can carry out experiments on surface rock or atmospheric samples, the experiments must be designed in advance and must fit inside the spacecraft. These limitations make scientists long for missions that will scoop up samples from other worlds and return them to Earth for more detailed study. To date, the only sample return missions have been to the Moon. Astronauts collected samples during the Apollo missions, and the Soviet Union sent robotic spacecraft to collect rocks from the Moon in the early 1970s. Many scientists are working toward a sample return mission to Mars. They hope to begin such a mission within the next decade or so. (A slight variation on the theme of a sample return mission is the Stardust mission, currently en route to collect a sample of comet dust and return it to Earth in 2006.)

Combination Spacecraft

Many missions combine elements of more than one type of spacecraft. For example, the Galileo mission to Jupiter included an orbiter that studied Jupiter and its moons as well as the probe that entered Jupiter’s atmosphere. The Cassini mission, which arrived at Saturn in 2004, included flybys of Venus, Earth, and Jupiter. It involved both an orbiter to study Saturn and its moons and a probe, called the Huygens probe, that descended through the atmosphere of Saturn’s moon Titan (Figure below). This link takes you to a Wikipedia timeline for past, current and planned planetary missions

Section 7-11: Interplanetary Trips

The simplest way to travel between the planets is to let the Sun’s gravity do the work and take advantage of Kepler’s laws of orbital motion. A fuel efficient way to travel is to put the spacecraft in orbit around the Sun with the Earth at one end of the orbit at launch and the other planet at the opposite end at arrival. These orbits are called “Hohmann orbits” after Walter Hohmann who developed the theory for transfer orbits. The spacecraft requires only an acceleration at the beginning of the trip and a deceleration at the end of the trip to put it in orbit around the other planet. Let’s go to Mars! The relative positions of Earth and Mars must be just right at launch so that Mars will be at the right position to greet the spacecraft when it arrives several months later. These good positionings happen once every 780 days (the synodic period of Mars). The spacecraft must be launched within a time interval called the “launch window” that is just few of weeks long to use a Hohmann orbit for the spacecraft’s path. The Earth is at the perihelion (point closest to the Sun) of the spacecraft orbit (here, 1.0 A.U.) and Mars is at the aphelion (point farthest from the Sun—here, 1.52 A.U.). Kepler’s third law relates the semi-major axis of the orbit to its sidereal period. The major axis is the total length of the long axis of the elliptical orbit (from perihelion to aphelion). For the Mars journey, the major axis = 1.52 + 1.0 A.U. = 2.52 A.U. The semi-major axis is one-half of the major axis, so divide the major axis by two: 2.52/2 = 1.26 A.U. Now apply Kepler’s third law to find the orbital period of the spacecraft = 1.263/2 = 1.41 years. This is the period for a full orbit (Earth to Mars and back to Earth), but you want to go only half-way (just Earth to Mars). Travelling from Earth to Mars along this path will take (1.41 / 2) years = 0.71 years or about 8.5 months. When the craft is launched, it already has the Earth’s orbital velocity of about 30 kilometers/second. Since this is the speed for a circular orbit around the Sun at 1.0 A.U., a reduction in the spacecraft’s speed would make it fall closer to the Sun and the Hohmann orbit would be inside the Earth’s orbit. Since you want to go beyond the Earth’s orbit, the spacecraft needs an increase in its speed to put it in an orbit that is outside the Earth’s orbit. It will slow down gradually as it nears aphelion. At aphelion the spacecraft will not be travelling fast enough to be in a circular orbit at Mars’ distance (1.52 A.U.) so it will need to arrive at aphelion slightly before Mars does. Mars will then catch up to it. But the spacecraft will be moving much too fast to be in a circular orbit around Mars, so it will need to slow down to go in orbit around Mars. On its journey to Mars, the spacecraft’s distance from the Sun is continuously monitored to be sure the craft is on the correct orbit. Though the spacecraft responds mostly to the Sun’s gravity, the nine planets’ gravitational pulls on the spacecraft can affect the spacecraft’s path as it travels to Mars, so occasional minor firings of on-board thrusters may be required to keep the craft exactly on track.

Astronomy Cast Astronomy Casttakes a fact based journey through the cosmos as it offers listeners weekly discussions on astronomical topics ranging from planets to cosmology. Hosted by Fraser Cain (Universe Today) and Dr. Pamela L. Gay (SIUE), this show brings the questions of an avid astronomy lover direct to an astronomer. Together Fraser and Pamela explore what is known and being discovered about the universe around us. Transcripts for many shows are available at the Astronomy Cast site for the hearing-impaired and anyone else who is interested


Getting Around the Solar System: Have you ever wondered what it takes to get a spacecraft off the Earth and into space. And how managers at NASA can actually navigate a spacecraft to another planet? And how does a gravity assist work? And how do they get them into orbit? And how do they land? So many questions… Rockets: To move around in space, you need some kind of propulsion system. And for now, that means rockets. Let’s learn the underlying science of rockets, and how they work. And learn why a rocket will never let us reach the speed of light.
Space Elevators: If you want to travel into the Solar System, you have to get off the Earth. Traditionally, that meant blasting off in a rocket. But there’s another strategy for escaping the Earth’s gravity. Climb to the top of an extremely tall tower, and just jump away. That’s the idea behind space elevators. Theoretically possible, but practically unfeasible, space elevators have gotten new life thanks to new, super strong materials. Advanced Propulsion Systems: We’re going to look at the future of propulsion systems. From the ion engines that are already working to explore the Solar System to the prototype solar sails to futuristic technologies like magnetic sails, and bussard ramjets. This is how we’ll travel to other stars. Mercury: What mysteries is it hiding from us? How similar is Mercury to the other rocky planets? How much do we really know about this first rock from the Sun? Venus: It’s the brightest object in the sky, the hottest object in the solar system, and it’s probably one of the most deadly places to go and visit. Earth: This time we talk about our own home world: Earth. You might think you know the planet beneath your feet, but it’s actually one of the most interesting and dynamic places in the Solar System. Learn about our planet’s formation, weather, its changing climate, and life. Mars: Apart from the Earth, it’s the most explored planet in our Solar System. Even now there are rovers crawling the surface, orbiters overhead, and a lander on its way. It’s a cold, dry desert, so why does this planet hold such fascination? Pluto and the Icy Outer Solar System: We’re going to talk about Pluto, its moons, the Kuiper belt, and the other icy objects that inhabit the outer Solar System. Pluto’s Planetary Identity: Pluto. It’s a planet, then it’s not. This week we review Pluto’s history, from discovery to demotion by the International Astronomical Union. Learn the 3 characteristics that make up a planet, and why Pluto now fails to make the grade. Dwarf Planets: In 2006, the International Astronomical Union demoted Pluto out of the planet club. But they also started up a whole new dwarf planet club, with Pluto, Eris and the asteroid Ceres as charter members. Let’s find out what it takes to be a dwarf planet, and discuss the current membership.

These notes were assembled from Nick

Strobel’s Astronomy Notes(with his permission), sections of Wikipedia articles, and original work.