Lecture 15: The Solar System
The Solar System •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 Venus
Pluto
Earth Uranus
Neptune
Mars
Mercury
Saturn
Asteroid Belt
The Solar System •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
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Semi-major axis
a
The Solar System •The eccentricity is related to the minimum and maximum distances Rmin and Rmax •We see that
Rmin= a (1-e)
Rmax= a (1+e)
a = 0.5 (Rmin + Rmax) •Mercury and Pluto have the most eccentric (elliptical) orbits •Mercury has an eccentricity e = 0.21 and Pluto has an eccentricity e = 0.25
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The Solar System
The Solar System •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
The Solar System •Light takes about 320 minutes (over 5 hours) to travel from the Sun to Pluto •The distance to Pluto is about 0.001 light-years •The nearest star is thousands of times farther away than Pluto
Empty Space Solar System
Nearest Star
Sun
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The Solar System
Each planet is unique…
Comparative Planetology •The planets have very different sizes:
Comparative Planetology •The planets have very different appearances:
Venus Saturn
Earth
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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):
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The Solar System •We see that the planets have different Distances from the Sun Masses Eccentricities Orbital inclinations Obliquities (tilt angles) Spin Periods
The Solar System
The Solar System •They also have different Atmospheric temperatures Surface pressures Surface gravities Compositions Formation histories Satellites (moons)
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The Solar System
•First we focus on radius, mass, and average density: Planet
Mass
Radius
Density
Mercury
0.06
0.38
0.98
Venus
0.8
0.95
0.95
Earth
1.0
1.0
1.00
Mars
0.11
0.53
0.71
(Ceres)
0.0002
0.07
0.5
Jupiter
318
11
0.24
Saturn
95
9
0.13
Uranus
15
4
0.23
Neptune
17
4
0.30
Pluto
0.002
0.18
0.37
(where we have used Earth units)
The Solar System
•Next we focus on distance, orbital period, and spin period: Planet
Distance
Orbit P
Spin P
Mercury
0.4
0.24
59
Venus
0.7
0.62
-243
Earth
1.0
1.0
1.0 1.03
Mars
1.5
1.9
(asteroid)
2.8
4.7
Jupiter
5.2
11.9
0.41
Saturn
9.5
29.5
0.44
Uranus
19.2
84.0
-0.72
Neptune
30.1
164.8
0.67
Pluto
39.5
248.6
-6.4
(where we have used Earth units)
The Solar System •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!
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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 inner solar system
Widely spaced in outer 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 Solar System •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
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The Solar System •The Sun is about 10 times larger (radius) than Jupiter •Jupiter is about 10 times larger (radius) than the Earth
The Solar System •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
The Solar System •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
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The Solar System •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 http://www.washingtonpost.com/wp-dyn/articles/A136982003Oct11.html
The Solar System •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)
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Views of Mars
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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
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Jupiter’s Moons
Jupiter’s Moons
Voyager
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Solar System family portrait, February 14, 1990, by Voyager 1
Cassini’s Path
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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!
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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
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