Origins and Characteristics of the Solar System

The solar system comprises the sun, the planets, their moons, and all of the asteroids and comets that are held in place by the sun’s gravity, Figure 6.1(a, b). It formed from an enormous interstellar cloud of gas and dust left over from the Big Bang. Around 4.5 billion years ago, the dust cloud collapsed into a flattish disc and separated into the sun and individual planets, with the sun at its center. Its age has been determined through measurements of the radioactive decay of elements within the earth, along with separate measurements for the age of the sun. Before and during its collapse, the dust cloud was slowly rotating in space, so that when each of the solar system components formed, they were also all moving in the same direction around the dust cloud’s center. This process explains why the solar system is “flat,” with all of the planetary orbits lying essentially in the same plane, Figure 6.1(a). This also explains why the planets all orbit in the same direction, and why they generally all rotate about their axes in the same direction.¹ In turn, the flatness of the orbital planes of the planets explains why they form a nearly straight line across the sky whenever several are visible at the same time, e.g. Figure 15.19.

Terrestrial Planets vs. Gas Giants

The systematic difference in size between the solar system’s eight planets is a striking characteristic, Figure 6.1(b). The four, innermost terrestrial planets are all relatively small and rocky. They have a relatively thin, gaseous atmosphere or none at all, as in the case of Mercury. In contrast, the four outermost gas giants are all four to ten times larger, and contain a thick gaseous atmosphere in both liquid, solid, and vapor form, above what is believed to be a rocky core. The large size of the gas giants, particularly Jupiter and Saturn, allows us to see their reflected sunlight at night without the aid of telescopes or binoculars. We are able to see the reflected sunlight from Mars, Venus, and Mercury since they are relatively close to us, despite their smaller sizes.

The differences in physical structure between the terrestrial planets and the gas giants can actually be explained quite easily based solely on their proximity to the sun. The terrestrial planets simply receive much higher intensity heat from the sun then the gas giants. For the case of Mercury, the heat from the sun is so intense it has completely vaporized its atmosphere, just like a kettle that has boiled dry! For Venus, Earth, and Mars, the intensity of heat from the sun is low enough that the planetary atmospheres remain in place, fortunately, in both liquid and vapor form. For Jupiter, Saturn, Uranus, and Neptune, however, their distance from the sun causes the sun’s heat intensity to be so low that their enormous, gaseous atmospheres have remained more or less intact, and largely frozen, above their suspected rocky cores.

Motion of the Planets

The planets appear to move relative to the stars as well as to each other, partly as the result of their proximity to Earth and partly due to their movements around the sun. These relative movements gave rise to their name, “planets,” which is rooted in ancient Greek for “wandering star.” Furthermore, while all the planets in the solar system orbit the sun in the same direction, they all have very different orbital periods, or times required to complete a full trip around the sun. The gas giants take much, much longer than the terrestrial planets. For example, Saturn takes almost thirty Earth years to complete a single orbit, whereas Mercury completes over four orbits just within a single Earth year!

6.2 Simulated positions of Venus, Mars, and Jupiter on (a) October 21, 2015, and (b) October 28, 2015; 7 days apart. First, notice how all three planets have moved relative to the (stationary) stars. Second, note how Mars and Venus have moved much more than Jupiter. This is the result of their relative proximity to Earth and their greater orbital velocities than Jupiter’s.

Source: Distant Suns

The differences in the orbital periods of the planets coupled with their different orbital velocities combine to produce significant differences in their movements as viewed from Earth. Whereas Saturn and Jupiter’s night-to-night movement is barely perceptible, the positions of Mercury, Venus, and Mars all change visibly between successive nights. These night-to-night differences may be seen in Figure 6.2, where the simulated positions of Venus, Mars, and Jupiter are shown only 7 days apart on (a) October 21, 2015, and (b) October 28, 2015. Notice how all three planets have moved relative to the stars, but that the change in Jupiter’s position is far less than that of either Mars or Venus.

Finally, planetary conjunctions occur when two or more planets move into position relatively close to one another, as viewed from Earth, for example, Figures 6.2 and 6.3. Such conjunctions provide a wonderful opportunity to gain often difficult to grasp insights into the geometry of the solar system, as well as being aesthetically pleasing. When three or more planets are simultaneously visible, they form a nearly straight line, for example, as seen in Figure 15.19. When this occurs, it is a magnificent demonstration of the flat geometry of our solar system.

6.3 When two or more planets are very close together, they form a planetary conjunction, as seen here also with the waxing crescent moon over Mt. Whitney, California. Earthshine is also visible from the shadowed side of the moon. Note that while Venus lies between the earth and the sun, Mars is on the opposite side of the sun from the earth.

6.4 The zodiacal light (left) and the Milky Way (right) both rise upwards from the horizon approximately an hour after the end of astronomical twilight over Maui, Hawai’i.

Zodiacal Light

The zodiacal light is a relatively rare phenomenon that is beautiful to behold, Figure 6.4. It is only visible under completely dark skies for an hour or so around when astronomical twilight begins or ends. The zodiacal light is a cone of diffuse, white light emanating upwards from the horizon above where the sun has set or will rise. The zodiacal light, also known as the “false dawn,” originates from sunlight reflected from vast fields of dust that lie along the midplane of the solar system, or the ecliptic. These dust fields are thought to be the remnants of comets that originate from far beyond the outskirts of the solar system, rather than leftover remnants of asteroids within the solar system.

The zodiacal light is best seen during the spring equinox during early astronomical twilight in the evening, and during the autumnal equinox during late astronomical twilight in the morning. The zodiacal light is more prominent nearer the equator owing to the nearly perpendicular orientation of the ecliptic at tropical and lower latitudes. It is best photographed with a wide-angle or fisheye lens under extremely dark skies, far from city lights.

Solar and Lunar Analemmas

A wonderful, but challenging, landscape astrophotography image is that of a solar analemma, Figure 6.5. Combining photographs of the sun taken at the same time on regularly spaced, clear days throughout an entire year allows you to create a solar analemma. They are the result of two completely independent features of the earth’s motion around the sun: (i) its 23 ½° tilt about its rotational axis; and (ii) the ellipticity of its orbit, Figure 6.5(c). The earth’s tilt results in the figure-eight shape of the analemma, while the ellipticity of its orbit causes the asymmetry in the figure eight.

The similar tilt of the moon and ellipticity of its orbit allows the creation of a lunar analemma as well, Figure 6.5(b). The main difference between a solar and lunar analemma is that the lunar analemma must be created, on average, 51 minutes later each day during one lunation, or lunar month, to result in the moon arriving at equivalent positions, whereas a solar analemma must be created at precisely the same time of day. This is the result of the fact that the moon rises approximately 51 minutes later on successive days.

6.5 (a) A solar analemma created over Trinity School in Indiana. The image was created by combining photographs of the sun, taken at the same time (within 5 seconds) on regularly spaced, clear days throughout an entire year, with a separately exposed photograph of the foreground taken near sunset. (b) A lunar analemma created during one lunation, or lunar month. (c) Schematic of solar analemma illustrating how the tilt of the earth’s axis of rotation results in the figure eight shape of the analemma while the ellipticity of its orbit causes the asymmetry in the analemma.

Source: (a) Craig Lent/www3.nd.edu/~lent/Astro /The World At Night; (b) György Soponyai/The World At Night