Chapter thru 7-4 A Planetary Overview Courtesy of The International Astronomical Union/Martin Kornmesser

1. In 2006, the IAU defined a planet to be a celestial body that (a) orbits the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly-round shape, and (c) has cleared the neighborhood around its orbit. 2. Pluto has been demoted to a dwarf planet, an example of an object in the Kuiper belt (a disk-shaped region beyond Neptune’s orbit, 30 to 1000 AU from the Sun). Courtesy of NASA, ESA, and A. Feild (STScI)

7-1 Sizes and Distances in the Solar System Astronomical Unit: A unit of distance equal to the average distance between the Earth and the Sun. 1. Diameter of Sun (1.39  106 km) is about 110 times that of Earth (1.3  104 km). 2. Jupiter’s diameter is about 11 times that of Earth. 3. Pluto’s diameter is about 1/5 that of Earth.

Figure 7.01: The Sun, planets, and a few of the large moons drawn to scale.

Measuring Distances in the Solar System 1.Copernicus used geometry to determine relative distances to the planets, while today we measure them using radar. 2. An outgoing radar signal is usually a burst of 400 kilowatts (4  10 5 watts), but the returning signal is only watt. 3. Though Mars is about 1.5 AU from the Sun, the distance from Earth to Mars varies from about 0.5 AU to 2.5 AU.

Advancing the Model: The Titius-Bode Law The Titius-Bode “law” is an empirical relationship that allows us to approximate the distances to the planets. It is not based on any theoretical framework.

Question 1 (7-1 thru 7-4 PPT Questions) What do you notice about the layout of the solar system?

7-2 Measuring Mass and Average Density 1. Newton reformulated Kepler’s third law to include masses: a 3 /P 2 = K  (m 1 + m 2 ), where K = G/(4  2 ) 2. If one of the objects is the Sun and the other is a planet, the sum of their two masses is essentially equal to the mass of the Sun: therefore, a 3 /P 2 = K  M Sun 3. Newton’s reformulation of Kepler’s third law allows us to calculate the Sun’s mass.

4. The masses of 6 of the 8 known planets can be calculated based on the distances and periods of revolution of these planets’ natural satellites. 5. For Mercury and Venus, which do not possess any natural satellites, accurate determinations of their respective masses had to await orbiting or flyby space probes. 6. The Sun contains 99.85% of the mass of the solar system. The eight planets and their satellites account for about 0.135% of the total solar system mass.

Calculating Average Density 1. In calculating average density, we assume that the object approximately spherical: average density = mass/volume. 2. If we know the average density of an object we can gain reasonable insights into its makeup.

7-3 Planetary Motions 1. All planetary orbits are ellipses, but all (except Pluto’s) are nearly circular. 2. Each of the planets revolves around the Sun in a counterclockwise direction as viewed from far above the Earth’s North Pole. 3. All planets—except Venus, Uranus, and Pluto— rotate in a counterclockwise direction, as viewed from far above the Earth’s North Pole.

4. Most of the satellites revolving around planets also move in a counterclockwise direction as viewed from far above the Earth’s North Pole, though there are some exceptions.

5. The elliptical paths of all the planets are very nearly in the same plane, though Mercury’s orbit is inclined at 7° and Pluto’s at 17°. 6. The inclination of a planet’s orbit is the angle between the plane of a planet’s orbit and the ecliptic plane. Figure 7.04

Question 2 (7-1 thru 7-4 PPT Questions) What do you think is significant about all the planets sitting roughly in the same plane and revolving the Sun all in the same direction?

7-4 Classifying the Planets The eight planets fit into two groups: the inner (terrestrial) planets and the outer (Jovian) planets. Size, Mass, and Density 1. The Jovian planets have much bigger diameters and even larger masses than the terrestrial planets. 2. The terrestrial planets are denser than the Jovian planets.

Satellites and Rings 1. The Jovian planets have more satellites than the terrestrial planets. The four Jovian planets have a total of 150 satellites compared to only 3 satellites for the four terrestrial planets. 2. Pluto has 3 satellites. 3. Each Jovian planet has a ring or ring system. None of the terrestrial planets do.

Rotations 1. Solar day is the amount of time that elapses between successive passages of the Sun across the meridian. 2. Meridian is an imaginary line that runs from north to south, passing through the observer’s zenith. 3. Sidereal day is the amount of time that passes between successive passages of a given star across the meridian. 4. The Earth’s solar day and sidereal day differ by about 4 minutes. 5. All the Jovian planets rotate faster than any of the terrestrial planets.

Figure 7.05: The tilt between a planet’s axis of rotation and its orbital plane varies among the planets in our solar system.

Question 3 (7-1 thru 7-4 PPT Questions) What are some key differences between terrestrial and jovian planets?