Intro to the Solar System

Scaling

• Often one is interested in how quantities change when an object or a system is enlarged or shortened

• Different quantities will change by different factors!

• Typical example: how does the circumference, surface, volume of a sphere change when its radius changes?

How does it scale?

• Properties of objects scale like the perimeter, the area or the volume– Mass scales like the volume (“more of the same

stuff”)– A roof will collect rain water proportional to its

surface area

Example: Newton’s Law of Gravity

Note that in order to compute a "factor of change" you can ask: by what factor do I have to multiply the original quantity in order to get the desired quantity? Example: Q: By what factor does the circumference of a circle change, if its diameter is halved? A: It changes by a factor 1/2 = 0.5, i.e. (new circumference) = 0.5 * (original circumference), regardless of the value of the original circumference. 

•If the mass of the Sun was bigger by a factor 2.7, by what factor would the force of gravity change?    scales linear with mass same factor

• If the mass of the Earth was bigger by a factor 2.2, by what factor would the force of gravity change?  scales linear with mass same factor

•If the distance between the Earth and the Sun was bigger by a factor 1.2, by what factor would the force of gravity change?   falls off like the area factor 1/ f 2 = 1/1.44 = 0.694

From Phrase to Equation

• Important skill: translate a relation into an equation, and vice versa

• Most people have problems with this arithmetical reasoning

The Solar System

Contents of the Solar System

• Sun• Planets – 9 known (now: 8)

– Mercury, Venus, Earth, Mars (“Terrestrials”)– Jupiter, Saturn, Uranus, Neptune (“Jovians”)– Pluto (a Kuiper Belt object?)

• Natural satellites (moons) – over a hundred• Asteroids and Meteoroids

– 6 known that are larger than 300 km across– Largest, Ceres, is about 940 km in diameter

• Comets• Rings• Dust

Size matters: radii of the Planets

Sun: Jupiter: Earth: Moon =110:11:1:1/4

The Astronomical Unit

• A convenient unit of length for discussing the solar system is the Astronomical Unit (A.U.)

• One A.U. is the average distance between the Earth and Sun– About 1.5 108 km or 8 light-minutes

• Entire solar system is about 80 A.U. across

The Terrestrial Planets• Small, dense and rocky

Mercury

Venus

Earth

Mars

The Jovian Planets

• Large, made out of gas, and low density

Jupiter

Uranus

Saturn

Neptune

Asteroids, Comets and Meteors

Debris in the Solar System

Asteroids

Asteroid Discovery

• First (and largest) Asteroid Ceres discovered New Year’s 1801 by G. Piazzi, fitting exactly into Bode’s law: a=2.8 A.U.

• Today more than 100,000 asteroids known

• Largest diameter 960 km, smallest: few km

• Most of them are named

• about 20 of them are visible with binoculars

Comets - Traveling Dirty Snowballs• Small icy bodies, “dirty snowballs”

• Develops a “tail” as it approaches the Sun

Comet Anatomy

• Tail may be up to 1 A.U. long

Comet Tail

• Two kinds of tails:

• Dust

• Ion (charged particles)

Shapes

Comet Giacobini-Zinner (1959)

• Ion tail 500,000 km long

• Coma: 70,000 km across

Comet Hale-Bopp (1997)

• Tail 40° long as seen from earth

Halley’s Comet – a typical Comet

Meteor Showers – caused by comets

Radiant DurationQuadrantids (QUA) Dec. 28-Jan. 7Lyrids (LYR) Apr. 16-25Eta Aquarids Apr. 21-May 12Beta Taurids June 30Delta Aquarids July 25-31Perseids (PER) Aug. 10-14Draconids Oct. 6-10Orionids (ORI) Oct. 15-29Taurids Oct.12- Dec 2Leonids (LEO) Nov. 14-20Geminids (GEM)Dec. 6-19

Meteors, Meteroids and Meteorites

• A Meteor is a sudden strike of light in the night sky

• A Meteoroid is a small asteroid, less than 100 m in diameter

• A Meteorite is any piece of interplanetary matter that survives the passage through Earth’s atmosphere and lands on Earth’s surface

Meteors and Meteorites• Small particles that strike the atmosphere• Come from fragments of asteroids, Moon, Mars,

comets• Strike the earth all the time (“meteorites”)

– High speed means lots of energy released on impact

Meteorites – the Remains of Meteors

Impact Craters

• Quebec's Manicouagan Reservoir. Large meteorite landed about 200 million years ago. The lake, 45 miles in diameter, now fills the ring.

• Barringer Crater, AZ 0.8 mi diameter, 200 yd deep; produced by impact about 25,000 years ago

Tunguska

• ~30 m body struck Siberia in 1908

• Energy equal to that of a 10 Megaton bomb!

• Detonation above ground; several craters

Frequency of Impact Events

Formation of the Solar System• Features to explain:

– planets are far apart, not bunched together– orbits of planets are nearly circular – orbits of planets lie mostly in a single plane– directions of revolution of planets about Sun is the same, and is the

same as the direction of the Sun's rotation– directions of rotation of planets about their axes is also mostly in the

same direction as the Sun's (exceptions: Venus, Uranus, Pluto)– most moons revolve around their planets in the same direction as the

rotation of the planets– differentiation between inner (terrestrial) and outer (Jovian) planets– existence and properties of the asteroids– existence and properties of the comets

Formation of the Solar System

• Condenses from a rotating cloud of gas and dust– Conservation of angular

momentum flattens it

• Dust helps cool the nebula and acts as seeds for the clumping of matter

Formation of Planets

• Orbiting dust – planitesimals

• Planitesimals collide

• Different elements form in different regions due to temperature

• Asteroids

• Remaining gas

Structure of the Planets explained

Temperature and density of materials drop with distance to sun

Cleaning up the Solar System

• Small objects are forced out of the inner Solar System by gravitational pull of bigger planets

• Small planetesimals collide and form planets

-- or are thrown out!