Lecture 15: The Solar System. The Solar System. The Solar System

Transcription

1 Lecture 15: All planets orbit the Sun in the same general direction, which is counterclockwise as viewed from the north celestial pole Most planets orbit the Sun in nearly the same plane as the Earth (the ecliptic plane) Mercury and Pluto are the exceptions Jupiter Pluto Venus Earth Uranus Mercury Mars Saturn Neptune Asteroid Belt Tilt of Mercury s orbit is 7 degrees Tilt of Pluto s orbit is 17 degrees Most of the planetary orbits are near-perfect circles centered on the Sun The eccentricity e determines how far from circular (how elliptical) the orbit is 1

2 Semi-major axis a The eccentricity is related to the minimum and maximum distances R min and R max We see that R min = a (1-e) R max = a (1+e) a = 0.5 (R min +R max ) Mercury and Pluto have the most eccentric (elliptical) orbits Mercury has an eccentricity e = 0.21 and Pluto has an eccentricity e =

3 The distance between the Earth and the Sun is one astronomical unit (AU), which is about 93,000,000 miles, or 150,000,000 km The distance between the Sun and Pluto is 40 AU Light takes about 8 minutes to travel from the Sun to the Earth Light takes about 320 minutes (over 5 hours) to travel from the Sun to Pluto The distance to Pluto is about light-years The nearest star is thousands of times farther away than Pluto Empty Space Sun Solar System Nearest Star 3

4 Each planet is unique Comparative Planetology The planets have very different sizes: Comparative Planetology The planets have very different appearances: Venus Saturn Earth 4

5 Comparative Planetology The planets have very different internal structures: Comparative Planetology The planets have very different atmospheric structures: Venus Saturn Earth Comparative Planetology The planets have very different obliquities (tilt angles): 5

6 We see that the planets have different Distances from the Sun Masses Eccentricities Orbital inclinations Obliquities (tilt angles) Spin Periods They also have different Atmospheric temperatures Surface pressures Surface gravities Compositions Formation histories Satellites (moons) 6

7 First we focus on radius, mass, and average density: Planet Mass Radius Density Mercury Venus Earth Mars (Ceres) Jupiter Saturn Uranus Neptune Pluto (where we have used Earth units) Next we focus on distance, orbital period, and spin period: Planet Distance Orbit P Spin P Mercury Venus Earth Mars (asteroid) Jupiter Saturn Uranus Neptune Pluto (where we have used Earth units) There are two classes of planets in the Solar System: The Terrestrial planets are small and have high densities: Mercury Earth Venus Mars The Jovian planets are large and have low densities: Jupiter Saturn Uranus Neptune Pluto doesn t really fit into either category very well! 7

8 Comparative Planetology The properties also differ somewhat for planets in the same group Atmospheres Densities Length of Day Earth is the only planet with free oxygen in its atmosphere, and with liquid water on its surface! The length of the day on Earth and Mars is 24 hours, but Mercury and Venus take months to rotate just once Earth and Mars have moons, but Mercury and Venus do not Nonetheless, the Terrestrial planets seem quite similar to each other when compared with the Jovian planets Comparative Planetology The general characteristics of planets within the Terrestrial and Jovian groups are quite different: Terrestrial Jovian Close together in Widely spaced in outer inner solar system solar system Small, dense, rocky Large and gaseous Solid surfaces No solid surfaces Weak magnetic fields Strong magnetic fields Only three moons Lots of moons The Sun is the dominant object in the Solar System It contains over 1,000 times the mass of Jupiter The Sun contains 99.9% of the material in the Solar System 8

9 The Sun is about 10 times larger (radius) than Jupiter Jupiter is about 10 times larger (radius) than the Earth Interplanetary debris Leftover from the formation of the Solar System Ranges from large asteroids and comets to microscopic dust Rocky material resembles the outer layers of the terrestrial planets Interplanetary debris The total mass of the debris is less than that of the Moon These objects are nearly unchanged since the formation of the Solar System 9

10 Meteorites allow us to study material from the early Solar System here on Earth Meteorites come in two groups, made mainly of either carbon or silicon Comets are made of ice and rock, similar to the moons of the outer planets they are also composed of ancient material Earth Comet Comet s orbit Sun Comets fall into two major groups, depending on the period of their orbits (short- and long-period comets) 10

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12 Views of Mars 12

13 Solar System Satellites Jupiter and its Satellites Jupiter s Moons The moons of Jupiter form something like a miniature Solar system around Jupiter The properties of Jupiter s moons vary with the distance from the planet Io Europa Ganymede Callisto The densities of the moons decrease with increasing distance from Jupiter This is called differentiation and it is similar to what we find in the progression of planetary properties in the Solar System 13

14 Jupiter s Moons Jupiter s Moons Voyager 14

15 Solar System family portrait, February 14, 1990, by Voyager 1 Cassini s Path 15

16 Gravitational encounter Voyagers 1 & 2 Voyagers 1 & 2 Voyager 1 & Voyager 2 are the most distant man-made objects from Earth They are currently at distances of 88 AU and 70 AU from the Sun, respectively and still sending and receiving signals! 16

17 Voyagers 1 & 2 Voyager 1 & Voyager 2 are traveling at speeds of 3.6 AU/year and 3.3 AU/year, respectively At this rate, they will reach the Heliopause the edge of the solar system in about 2017 Voyagers 1 & 2 The Heliopause is where the pressure of the Solar Wind equals the pressure of interstellar space 17