Solar System Overview

Part I

The Sun and Jupiter dominate the solar system.  Jupiter has more than twice the mass of all the other planets combined.  We can divide the system up into two classes: the giants and the dwarfs.  The giants include the Sun, Jupiter, Saturn, Uranus, and Neptune.  The dwarfs include the Earth, Mercury, Venus, Mars, Pluto, and all of the moons.  The Earth is the largest member of the dwarfs.  The inner four planets are small and cling close to the Sun.  The giants are farther out and stretch out to great distances away from the Sun.

There are only two types of planets in the solar system:

                    Terrestrial               Jovian

                      Mercury                Jupiter

                      Venus                   Saturn

                      Earth                     Uranus

                      Mars                     Neptune

Notice that Pluto is missing.  It more closely resembles a large asteroid and will be discussed separately.

The planet system has several properties, which are the direct result of the formation mechanism.

Recall that as the Sun formed the inner portion of the solar nebula became hot.  Much of the light element content of the nebula was pushed back farther away from the protosun.  The heavier elements remained in place.  But heavy elements make up only a small fraction of the mass of the universe.  So the planets that formed close to the Sun formed in a hot region composed mostly of heavy elements.  They could not get very large.  The planets forming farther away, where most of the hydrogen and helium had been pushed, had a composition more like the Sun's and could grow much larger, since hydrogen and helium are very abundant.

The two planet types have a number of properties that make them distinct:

                                                               Terrestrial                           Jovian

            Distance from the Sun               Close                                  Far

            Diameter                                     Small                                  Large

            Mass                                            Small                                  Large

            Density (Mass/Volume)             Large  (0.4-5.5)                 Small (0.7-1.7)

            Composition                               Rocky                                 Solar

            Rotation Rate                             Slow                                    Rapid

Concentrate on how the properties of the planets are discovered, not what these properties are.  Also look over the indices of planetary properties, not to memorize the numbers, but to look for trends.

The first property we want to measure is mass.  There are a number of techniques to determine the mass, but they all rely on observing the effect the mass of the planet has on nearby objects.

Part II

The next thing we would like to know is radius.  Measuring the angular diameter of the planet in the telescope would work for any planet that shows a disk, but we then need the distance to the planet in order to convert that angular size into a physical diameter or radius.

Some planets (Uranus, Neptune, and Pluto) are so far away that they have either very small or zero angular diameters.  A useful technique here is Stellar Occultation.  Since planets wander across the starry background, a planet will often cover up (occult) a star.  From Newton's Laws we can compute the orbital speed of the planet.  If we simply measure the time of the occultation, we can compute the diameter of the planet.

Stellar occultations have been very useful to astronomers is ways other than measuring the diameter of the planet.  In 1977 an occultation by Uranus lead to the detection of the rings of the planet.  Minor dips in the light equally spaced on either side of the main occultation event are just what we expect if thin rings are encircling the equator.  Here luck played a role in that the tilt (inclination) of the rotational pole of Uranus is 98° and the rings are presented to us full face quite often.

Occultations have also played a role in learning the mass of Pluto.  A short history of the outer solar system is in order.  In the year 1781 William Hershel discovered Uranus, the first planet ever "discovered."  The orbit of the new planet was irregular - it had perturbations.  The perturbations must be caused by the gravity of another object - another planet.  We can correct for the known planets, but still some residual perturbations exist.  Now gravity depends on the mass and distance of the unseen planet.  Using the trends in the masses of the outer planets (Jupiter - 318 MÅ; Saturn - 95 MÅ; Uranus - 14.5 MÅ), we can guess that the next planet would be a small, Jovian planet.  To be conservative people generally assumed that the new planet's mass would be equal to that of Uranus.

The distance was another problem.  Since the time of Kepler, people have been fascinated by possible rules for the spacing of the planets.  In the 18th century the Bode-Titius Law came into prominence.  The scheme is:

                                           0      3      6      12      24      48      96      192

                   Add 4              4     7     10     16      28      52     100     196

           Divide by 10         0.4   0.7   1.0   1.6    2.8     5.2    10.0    19.6

                                           D   E   Å   G          H      V     F

We produce a sequence of numbers (the last line) that very closely matches the distances of the planets from the Sun in astronomical units.  The idea of Bode's Law was bolstered with the discovery of the first asteroid on Jan. 1, 1801 (the first day of the 19th century).  In short order several more asteroids were found.  The average distance from the Sun of the asteroids is 2.8 AU.  Extending Bode's Law by one place yields a distance to the unseen planet of 38.8 AU.  Using the estimated mass and distance, the position of the new planet in the sky could now be computed.  On the first night of observation in 1846, Neptune was found is about the predicted position.

By 1847 two moons had been found for Neptune.  The mass could now be accurately determined and it was found to be 17 MÅ.  Astronomers could also compute the distance of the planet from the Sun.  Here they got 30 AU, not the 39 AU predicted from Bode's Law.  Perhaps Bode's number sequence wasn't a Law at all.  The key to the discovery was luck.  In 1846 Neptune would have be in about the same position whether its distance were 30 or 39 AU.

The search for Neptune was conducted to account for the perturbations in the orbit of Uranus.  By the turn of the 20th century is was clear that Neptune did not account for all of the perturbations, and worse yet, new perturbations were discovered in the orbit of Neptune.  The search was on again, this time for Planet IX.  Bode's Law had lost all credibility and the best guesses as to the mass (based on the size of the perturbations) was about 6.6 MÅ.  The search was conducted by Percival Lowell at his new Lowell Observatory in Flagstaff, Arizona.  He hired Clyde Tombaugh to perform the search over the band of the ecliptic.  Based on the expectations of a small, Jovian planet (and the density commiserate with it), Tombaugh computed that the unknown planet would be about magnitude 10.  He finally found Pluto in 1930 at magnitude 14.

Part III

The mass history of the planet Pluto includes stellar occultation observations.  As I mentioned above the pre-discovery estimate of the mass was 6.6 MÅ, based on the assumption of a Jovian planet.  The first real chance to modify this concept was a predicted occultation in 1968.  The fact that this event did not occur meant that Pluto could not be as large as predicted.  The size was re-estimated, the density approximated, and a new mass computed.  The mass of Pluto following the 1968 occultation stood at 0.91 MÅ.  The next opportunity to learn the mass came in 1976 with the prediction of another occultation, this one much closer to the center of the planet.  This occultation also failed to occur.  Again astronomers could get a new estimate of the diameter, estimate the density, and compute the mass.  The mass of Pluto then stood at 0.11 MÅ.  Clearly Pluto was not a Jovian planet.

In 1978 Pluto dipped within the orbit of Neptune due to the large eccentricity of Pluto's orbit.  From then until 2000 Pluto is slightly closer to the Sun than Neptune.  Now is the best time for Pluto studies.  A new photographic study revealed that Pluto has a moon later named Charon.  Of course, the presence of the moon allowed astronomers to compute an accurate value for the mass (0.002 MÅ).  Pluto is a bit smaller than our Moon and would have not been found at all if the surface were not covered in methane ice, raising the reflectivity.

The search for Pluto commenced in order to find a cause for the residual perturbations in the orbits of Uranus and Neptune.  Pluto is far too small to be that cause and the search for yet another planet (Planet X) began in the 1980's.  But a reexamination of the Voyager data reveals that the perturbations can be accounted for with the known planets.  And Clyde Tombaugh completed his survey of the ecliptic band to reveal that no Neptune-sized planet exists within 270 AU of the Sun.  Could we have missed planets closer to the Sun than Mercury?  Yes, even Mercury is difficult to observe.  Should such a planet be found, it would be named Vulcan, the god of fire.  But careful searches of the vicinity of the Sun during solar eclipses shows no hint of Vulcan.  The list of planets is probably complete.

So far we have listed

as methods of determining the radius of a planet.  To this list we now add radar.  Radar was developed during WWII as a reconnaissance tool, but has been used by planetary astronomers since the 1960's to measure planetary distances, radii, and rotation rates.  We can consider radar as a radio telescope used in both directions, to send as well as receive light.  To measure the radius, we send out very short bursts of radio light (typically one nanosecond in duration).  The beam will hit the closest section of the planet first and begin its return to Earth.  Then a ring of the planet surface a little farther away will be hit and the beam returns.  This reflection continues until the beam reaches the limb of the planet.  Because of the physical size of the planet the beam, which started out one nanosecond in length, will return stretched out in time.

Next we would like to measure the rotation rate of the planet.  The first technique is also radar.  Here we send out a beam of a single wavelength and take advantage of the Doppler Effect.  Some sections of the planet are moving at us and will "blue shift" the beam.  Other sections of the planet are moving away from us and will induce a "red shift" into the radar beam.  The success of this technique relies on measuring the spread of wavelengths of the radar beam.

Several interesting discoveries have been made using radar imaging.  Mercury is a very difficult planet to observe from ground-base because it is always close to the Sun.  Early observers thought they had seen faint markings on the surface and concluded that Mercury was a synchronous rotator, i.e., it rotated once per orbit.  This would make Mercury both the hottest and coldest place in the solar system.  But the radar experiments showed that the rotational period was 2/3 the orbital period.  This yields an unusual coupling between these two periods that has a profound effect on the length of the day on Mercury.  One day (one complete rotation) lasts for two orbits!

Venus is another difficult planet to observe because it has a permanent cloud cover.  But the radar beam is radio light and can penetrate the cloud layers.  Radar observations are then our only means of producing a map of the surface of Venus.  But the radar also returned the rotation period, which is retrograde, i.e., the planet rotates in the opposite sense of the other planets.  This is so unusual that astronomers feel a catastrophic event must have altered the rotation of the planet early in the history of the solar system.  We will see that such catastrophes were common in the early solar system.

Part IV

Jupiter has been observed by radar, but the beam failed to return.  Jupiter has no solid surface.  Eventually Saturn was also imaged with radar.  Again the beam failed to return from the planet, but did return from the rings.  The nature of the signal confirmed that the rings are individual particles orbiting the planet like billions of individual moons.

The most obvious technique for measuring rotation rate is to watch surface features.  By simply timing the feature and correcting for the motions of the planet and the Earth, we can measure the rotational period of the planet.  Only Mars, however, has features on the surface that allows the technique to be successful.

The third technique to measure rotation rate is to measure the slant of the spectral lines in the spectrum of the planet.  You have seen this technique in the lab in determining the rotational period of Saturn.  By review we are using the Doppler Effect to note that the light reflected from the approaching limb of the planet will be slightly blue-shifted, whereas the light reflecting from the receding limb is red-shifted.  If we place the slit of the spectrograph along the equator of the planet, we can measure the same spectral feature at a variety of Doppler shifts and the line looks slanted.  This technique works well for planets that rotate rapidly, i.e., the Jovian planets.

Finally, a technique that works best on asteroids is to measure the light variation of the object.  Because asteroids are irregularly shaped, they present different surface areas to us as they rotate, changing the apparent brightness.  I list this technique here for planets because the rotational period of Pluto was established in this way.  Pluto is large enough for gravity to make it spherical, so we are not observing an irregular shape to the planet.  Rather the light variations are caused by strong differences in the reflectivity of the surface.  One hemisphere is very bright while the other is quite dark.

To summarize for rotation rate:

The reflectivity of the planet's surface will give us some general clues as to the composition.  The percent of sunlight reflected from the surface is called the albedo.  Several factors affect the determination of the albedo:

Combining all of these factors gives us the albedo.  Albedo for a good reflector is close to 100% and for a poor reflector near 0%.  I listed the albedos of the planets to look for trends and to see what influences the albedo.  Clearly the planets having cloud cover (Venus, Jupiter, Saturn, Uranus, Neptune) have rather high albedos.  Clouds elevate the albedo as anyone who has flown above the clouds can testify to.  Ice caps also elevate the albedo.  Bare surfaces with no ice, clouds, or atmosphere give very low albedos.