ast 111 lecture 14. my rough definition: the 8 large, unique celestial bodies orbiting the sun
TRANSCRIPT
THE SOLAR SYSTEMAST 111 Lecture 14
The Planets
My rough definition:
The 8 large, unique celestial bodies orbiting the Sun
The Planets
Formal Definition
A planet is a celestial body that is:
In orbit around the Sun
Has sufficient mass for its self-gravity to overcome its rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
Not massive enough to produce fusion reactions at the core
Has cleared the neighborhood around its orbit
Pluto fails!
Which of the above criteria does Pluto fail to meet?
Planetary Motion
All planetary orbits are nearly circular and lie nearly in the same plane
Planetary Motion
ALL planets orbit the Sun counterclockwise when viewed looking at Earth’s north pole
Planetary Motion
Most planets rotate in the same direction as they orbit
Small axis tilts
The Sun rotates in the same direction
Planetary Motion
Large moons tend to orbit in planets’ equatorial planes
Same direction as planet rotates
Moons
Terrestrial planets have few moons
Look “out of place”
Jovian planets have many moons
Larger Jovian moons follow parent planet’s rotation
Exceptions
Uranus rotates practically on its side Its axis of rotation is nearly in the plane of its
orbit
Exceptions
Venus rotates clockwise about its own axis (viewed from high above Earth’s north pole)
Exceptions
Terrestrial planets besides Earth either have no moons or small moons
Earth’s Moon is very large Almost a quarter of
the mass of Mercury
Exceptions
Neptune’s moon Triton orbits Neptune clockwise
Neptune spins counter-clockwise
Inner and Outer Planets
What differentiates the “inner planets” from the “outer planets”?
Two Types of Planets
Two Types of Planets
Terrestrial Planets:
Mercury, Venus, Earth, Mars
Small and dense
Rocky exteriors, metal interiors Solid surface
Few moons
No rings
Jovian Planets:
Jupiter, Saturn, Uranus, Neptune
Large!
Lower average density (“gas giants”) No solid surface
Rings
Many moons
H, He, and H compounds
Solar System Geometry
Conjunction: Planet lines up with the Sun as seen from Earth, looking toward the Sun!
Solar System Geometry
Opposition: Planet lines up with the Sun as seen from Earth, looking away from the Sun!
Solar System Geometry
Greatest Eastern Elongation: An inner planet is as far east of the Sun as it can get as viewed from Earth
Solar System Geometry
Perihelion: point along orbit where closest to the Sun
Aphelion: the point along orbit where farthest from the Sun
The Mars Hoax!
In 2003, Mars had a better-than-usual opposition (in terms of being close) Consider elliptical orbits
It was claimed that Mars would be the size of the full moon Would Mars be at conjunction or opposition for
it to be as large as possible in the sky?
Was only 25% closer than usual closest-approach
Exploring the Solar System
We have sent spacecraft to: All terrestrial and
jovian planets Moons Asteroids Comets
Pioneer 10
Launched in 1972
First encounter with Jupiter
Lost contact with it in 2003 (7 billion miles from Earth)
Could reach Aldebaran in 2 million years
Pioneer 11
Launched in April 1973
Passed by Jupiter in December 1974
Passed Saturn in September 1979
Voyager 1
Launched in 1977
Visited Jupiter and Saturn Opted to study Titan
instead of Pluto; flung out of Solar System
Voyager 1 is 0.002 light years (11 billion miles) from the Sun Will pass by a star in
40,000 years 10 miles per second
Voyager 2
Launched in 1979
Voyager 2 visited all four Jovian planets
Used a chain of gravitational slingshots
Planets were lined up just right for the “Grand Tour”
Exploring the Solar System
Notable Orbiters
Galileo Orbited and probed
Jupiter and its moons
Cassini Orbits and probes
Saturn and its moons
Exploring the Solar System
Mars Rovers (Sojourner, Spirit, and Opportunity) Analyze chemistry and geology of Mars
Searching for past and present signs of water
Home, sweet home…