Open Course Info


Introduction to Astronomy


Lecture 21: Planetary Systems


The optimist proclaims that we live in the best of all possible worlds; the pessimist fears this is true.

-- James Branch Cabell

21.1 Planetary Systems

(Discovering the Universe, 5th ed., §II.0)

  • In Lecture 16 we saw that the formation of stars begins with the gravitational compression of gas and dust into cocoon nebulae.

    Cocoon nebulae develop one or more hot protostars in their centers, which may eventually become main sequence stars.

    Many such cocoon nebulae have been observed in recent years, such as the one at the right being pushed back by a new-born star.

  • There is increasing evidence that cocoon nebulae commonly give rise to many smaller condensations of matter that orbit the protostar(s) and subsequent new-born stars.

    These objects eventually become planets, and together with the star they orbit they form a planetary system.

    Besides the obvious example of our own Solar System, numerous planetary systems around other stars have also been detected in recent years.

  • Within our Solar System we find that, in addition to the planets, many smaller objects orbit the Sun and planets, from small grains of dust on up to "minor planets" hundreds of kilometers in diameter.

    The presence of minor planets beyond the orbit of Neptune is also a recent discovery that has expanded the boundaries of our Solar System.

21.2 An Overview of the Solar System

(Discovering the Universe, 5th ed., §II.4, §II.5)
  • To understand how our Solar System came into existence, it is first helpful to study the basic characteristics of the objects that inhabit it: average distance from the Sun (semimajor axis), diameter, density, eccentricity, and orbital inclination.

    These characteristics help us categorize these Solar System objects into a few small groups.

  • Astronomers early on determined the semimajor axis of solar system objects.

    With telescopes, the diameters of the larger objects are visible and can also be measured.

    For smaller objects, diameter is often estimated based on their measured brightness and assumptions about the fraction of sunlight they reflect, known as their albedo.

    By comparing these two characteristics, semimajor axis and diameter, we can identify five basic groups of objects in the solar system:

  • The Jovian planets are the largest objects, on the order of 10 times the size of the Earth, and are so-called because they all have basic similarities to Jupiter (also known as Jove).

    The Jovian planets are, inclusively, Jupiter, Saturn, Uranus, and Neptune.

    The region where the Jovian planets orbit, between 5 A.U. and 30 A.U., is called the Jovian Belt.

    • The terrestrial planets are so-called because they all have basic similarities to the Earth.

      The terrestrial planets are, inclusively, Mercury, Venus, the Earth, the Moon, and Mars.

      The region where the terrestrial planets orbit, between 0 A.U. and 1.5  A.U., is called the Terrestrial Belt.

    • The asteroids are significantly smaller than the terrestrial planets, on the order of 1/10 to 1/100 of the size of the Earth, and are therefore called minor planets.

      The name asteroid comes from the fact that they are so small that they didn't show a disk in early telescopes, and were therefore "star-like".

      The three largest asteroids are Ceres, Pallas, and Vesta.

      More than 10,000 other asteroids have been identified.

      The region where most of the asteroids orbit, between 1.7 A.U. and 3.5 A.U., is called the Asteroid Belt.
    • The distant icy asteroids are similar in size to the asteroids, and are also called minor planets.

      Note that the smallest "planet", Pluto, is categorized here as an icy asteroid, though so far as we know it's the largest of them.

      Extra: this classification of Pluto is not "official"; read the International Astronomical Union's press release on the subject.

      Discovered in 2004, Sedna is the second largest icy asteroid known, at roughly two thirds the diameter of Pluto.

      Sedna is followed closely in size by Quaoar and then by Pluto's own moon Charon.

    Except for Pluto (1930) and Charon (1978), all of the more than 500 icy asteroids known have been discovered since 1992.
    The region where most of the icy asteroids orbit, between 30 A.U.and 1000 A.U., is called the Kuiper Belt, after Gerard Kuiper, the astronomer who proposed its existence in 1951.

    The icy asteroids are commonly known amongst astronomers as Kuiper Belt objects (KBOs) or trans-Neptunian objects (TNOs).


    • Comets are the smallest objects, on the order of 1/1000 to a 1/10,000 of the size of the Earth, though they sometimes approach the Earth and become very bright.

      Famous examples of comets include Halley, Hale-Bopp, and Shoemaker-Levy 9.

      The region where most of the comets orbit, between 1000 A.U. and 100,000 A.U., is called the Oort Cloud, after Jan Oort, the astronomer who proposed its existence in 1950.

      A signficant number of comets can also be found in the Kuiper Belt, though as we will see they are physically distinct from the icy asteroids.

    • We can gain more understanding and distinction of these classes by considering their mass, or more usefully, density.

      Applying Kepler's and Newton's Laws allow the determination of planetary mass, and by combining with diameter, their density:

      Distinct grouping are again visible on this chart.

      It's useful to compare these numbers with the density of various materials:
      Iron and other dense metals such as nickel, lead, etc.: >~ 8 g/cm3
      Rock and light metals such as aluminum: ~ 3 glcm3
      Water and other liquids/ices such as methane and ammonia: ~ 1 glcm3
      Compressed gas such as hydrogen, helium, etc. (highly variable): <~ 0.1 g/cm3

      The terrestrial planets are the densest objects, with densities suggesting they are mixtures of iron or other metals and rock.

      The asteroids have similar or smaller densities; the densest appear to be pure metal, the least dense appear to be loose piles of rock (they are not large enough for their gravity to hold onto any gas).

      The icy asteroids have lower densities than the other asteroids, suggesting they may be rock covered with ice (hence their designation).

      The comets have the lowest densities, indicating they are mostly ice.

      The Jovian planets also have very low densities, suggesting a large liquid and gaseous composition.
    • Spectroscopic measurements of sunlight reflected by these objects have helped to confirm these compositions.

      If a dark line appears in the reflected light that isn't in the Sun's spectrum (usually because it is molecular), it must be due to absorption by the planet's atmosphere or surface.

      Question: how else might we determine the composition of these objects? To which does that apply?

    • Eccentricity again helps to distinguish these objects from each other:

      Eccentricity is relatively small for the terrestrial and Jovian worlds.

      It is typically much larger for the asteroids and icy asteroids.

      It is generally the maximum for the comets (possibly because these are only the ones we can observe).

    • Orbital inclination is another distinguishing characterisitic:

      An angle less than 90° indicates motion around the Sun in the same direction as the Earth.

      An angle greater than 90° indicates motion in the opposite direction to the Earth, which is called a retrograde orbit.

      Orbital inclination is relatively small for the terrestrial and Jovian worlds, all being relatively close to the ecliptic and in the same direction as the Earth.

      It is typically larger for the asteroids and icy asteroids, but still less than about 30°, and again in the same direction as the Earth.

      It is every possible angle for the comets, often opposite to the direction of the Earth, a random distribution of orbits.

    • We can summarize these characteristics as follows:
      Object Semimajor Axis Diameters Densities Eccentricities Inclinations
      Terrestrial Planets 0 A.U. to
      1.5  A.U.
      Medium High Small Small
      Asteroids Mostly between
      1.7 A.U. to
      about 3.5 A.U.
      Small Mostly Medium Small to
      Small to
      Jovian Planets About 5 A.U. to
      about 32 A.U.
      Large Very
      Small Small
      Icy Asteroids Mostly between
      32 A.U. to
      about 1000 A.U.
      Small Mostly
      Small to
      Small to
      Comets Mostly between
      32 A.U. to
      about 100,000 A.U.
      Large Small to


    The star chart background was produced on a Macintosh with the Voyager II program, and are ©1988-93 Carina Software, 830 Williams St., San Leandro, CA 94577, (510) 352-7328. Used under license.
    ©1996-2004 Scott R. Anderson
    Last update: 2004 September 10
    Please send questions, comments, suggestions, or corrections to
    The material on this website may be reused as described under the Open Course License.

    The Gateway to Educational Materials (GEM) is the key to one-stop, any-stop access to thousands of high quality lesson plans, curriculum units and other education resources on the Internet! GEM is a project of the U.S. Department of Education. The Introduction to Astronomy Webbook is catalogued in the Gateway, and Scott R. Anderson is a member of the GEM Consortium.