The Solar System

How Many Planets?• To the ancients, planets (wanderers) were objects that

moved, relative to the background stars (gods)• Greeks knew of 7 planets – one for each day of the week:Day of Week French Object Norse God

SundayMondayTuesdayWednesdayThursdayFridaySaturday

Dimanche SunLundi MoonMardi Mars Tiw’s DayMercredi Mercury Woden’s DayJeudi Jupiter Thor’s DayVendredi Venus Frig’s DaySamedi Saturn

Planets Today

• In 2006, the International Astronomical Union (IAU) defined a planet as an object that:– Orbits the Sun– Has enough mass to be spherical (in hydrostatic equilibrium)– Has cleared the neighborhood of its orbit, meaning there aren’t any similar-sized objects sharing

its orbital space

• By the above definition, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune

• There are 5 planets visible to the unaided eye, besides Earth: Mercury, Venus, Mars, Jupiter, and Saturn

What About Pluto?• Pluto is not a planet because it has NOT cleared its

neighborhood – it shares its orbital space with several objects similar in size or larger than it is

• Dwarf planets were defined as objects that:– Orbited the Sun– Were spherical in shape (in hydrostatic equilibrium)

• As of 2015, there are 5 dwarf planets: Ceres, which used to be called an asteroid and orbits within the asteroid belt, Pluto, Makemake, Haumea, and Eris

• Many more objects have been discovered that may eventually be classified as dwarf planets

Moons and Small Solar System Bodies

• Comets, asteroids, miscellaneous meteoroids, and other objects that orbit the sun, but do NOT have enough mass to be spherical are called small solar system bodies, but will continue to be called asteroids, comets, etc.

• Moons are objects that orbit planets

Observing the Planets• Observed motion depends on the time scale

Daily motion is essentially the same as for Sun, Moon, and stars – rise in the East, set in the West, moving with the stars

Long-term motion (over weeks to months)– Mercury and Venus switch back and forth from morning to

evening, always appearing relatively close to the Sun because their orbits are closer to the Sun than Earth is.

– Mars, Jupiter, and Saturn• MOST of the time, appear to move from West to East

“through” the background stars• Periodically stop and go “backwards” ( East to West) =

RETROGRADE MOTION

Retrograde Motion

Explaining Retrograde MotionHow to explain retrograde motion was a challengeLed to the birth of astronomy as a science, separate from

astrologyAncients were shackled by two ideas

– All movement in the heavens must be circular– Earth was located at the center of the universe (geocentric model)

• Ptolemy perfected the celestial sphere idea, with the planets attached to epicycles, which were attached to crystal spheres (deferents) with Earth at the center

• Retrograde motion occurred when rotation of epicycle carried planet backwards while rotation of deferent carried the planet and epicycle forward – it is much easier to show than it is to explain!

• Accepted for nearly 2000 years!

Ptolemy’s Explanation

• Watch Ptolemaic Model video in BlackBoard

A New IdeaCopernicus: 1473 – 1543• Proposed that the Sun, not the Earth was the center of

the solar system (heliocentric model)• Earth’s rotation caused the daily motion of the Sun,

Moon, planets and stars• Earth’s revolution around the Sun caused the retrograde

motion of the planets, as a faster-moving Earth passed up a slower-moving planet

• Fearing persecution by the Catholic Church, he did NOT publish his ideas

• An assistant finally published his work, as Copernicus was near death

Copernicus’ Explanation of Retrograde Motion

Watch Copernicus Retrograde Motion Video in BlackBoard

Copernicus and Retrograde Motion• Retrograde motion occurs as a faster-moving planet

passes up a slower-moving planet• Retrograde motion seen for any object farther from

the Sun than the observer• Copernicus’ ideas were NOT widely accepted

Tycho Brahe 1546 - 1601• Danish Nobleman• Gold nose to replace the original lost in a

duel over who was the better mathematician

• Genius at designing and building instruments for measuring the positions of stars and planets

• His work showed that the best reference books of the day were incorrect.

• Value of his work was his observations.• Able to measure angles to high precision• Had highly accurate observations of the

positions of almost 800 stars, and the Sun, Moon, and planets, for about 20 years

Johannes Kepler: 1571 - 1630• Brilliant mathematician• Thought the spacing between

the planets and the sizes of their orbits was related to the perfect solids of geometry

The Perfect Solids

Kepler

• Couldn’t make his model fit the observations of the planets that he had, so he thought the observations were wrong

• Went to work with Tycho, since Tycho had the best observations

Tycho and Kepler

• Did not get along at all – Tycho refused to share his data• Very different personalities• Tycho was a renowned partier• Kepler was very serious• Tycho eventually died– Maybe from a burst bladder– Probably from accidentally poisoning himself with his home

remedies (Mercury-based)

• Kepler got Tycho’s data after Tycho’s Death

Kepler’s Great Discovery• After many years calculations, in 1606, Kepler discovered the

orbit of Mars is an ellipse, not a circle• He also discovered that planets do not move at a uniform speed,

they move faster when they are close to the Sun, slower when they are farther away

Kepler’s First Law of Planetary Motion• The orbit of a planet is an ellipse, with the Sun at one

focus.

Kepler’s 2nd Law of Planetary Motion• a planet sweeps out equal areas in equal times• a planet moves fastest when it is closest to the Sun,

slowest when it is farthest from the Sun

Kepler’s 3rd Law of Planetary Motion• P^2 = a^3• The time a planet takes to orbit the Sun(in Earth

years) squared = the planet’s distance from the Sun, (in astronomical units), raised to the third power

• Means that planets closer to the Sun take less time to revolve around the Sun than planets farther away

• Now Copernicus’ model of the solar system predicted the positions of the planets perfectly!

Orbits• Shape is an ellipse• Perihelion – when planet is closest

to the Sun• Aphelion – when planet is farthest

from the Sun• Major axis – long diameter of the

orbit• Semimajor axis – half the major

axis, = the average distance the planet is from the Sun

• Minor axis – short diameter of the orbit

• Eccentricity – a number for how round or “flat” the ellipse is

• Eccentricity = distance between the focus points divided by the major axis– Always less than 1 and greater than

or equal to 0– Closer to 0 = more round– Closer to 1 = more “flat”

Galileo• Published a book announcing

three major discoveries following his observations of the sky using a telescope– the Moon was not perfect – it

had craters and mountains– the Milky Way was made up of

stars too faint to distinguish with the unaided eye

– there were four moons orbiting Jupiter

• showed that Ptolemy’s belief that everything orbited the Earth was incorrect – the first observational evidence that Ptolemy was not correct

GalileoTwo additional discoveries quickly followed:• The Sun had spots

and rotated• Venus went through

a phase cycle, proving that it revolved around the Sun, not the Earth

Got into trouble with the church for teaching Copernicus’s heliocentric theory

• Composed mostly of iron and silicate rocks

• Relatively small (mass less than or equal Earth’s mass

• Dense ~5 g/cm3• Few or no moons• Relatively close together

and to the Sun

Terrestrial Planets

Jovian Planets• Relatively large (mass 14x

to 320x Earth’s mass)• low density ( ~1 g/cc)• Lots of moons! • All have rings• Great distances from each

other and from Sun

NOT to scale!

Jupiter and Saturn• Both thought to

have very similar interiors

• Mostly hydrogen and helium

• Core most likely solid rock and iron, but look at the temperatures – 40,000 K = 72,000 F!

Uranus and Neptune• Thought to have very

similar interiors• Thick hydrogen and

helium atmosphere• Mantle with lots of

water – maybe slushy ice-water mixture

• slushy giants might be a better name than gas giants

Characteristic Properties of Solar System Give Clues About How it Formed

• Disk shape of the solar system– Orbits of objects, except comets, in nearly the same geometric plane

(ecliptic)– Objects mostly rotate and revolve in the same direction

(counterclockwise when viewed from above Sun’s north pole)

• Two planetary types– Terrestrial– Jovian

• Planetary ring systems for Jovian planets• Space Debris – asteroid, comets, and meteoroids

– Composition– Orbits

• Common age of about 4.6 billion years for Earth, the Moon, Mars, meteorites, and the Sun

Formation of the Planets- Overview• Solar Nebula Theory• About 5 billion years ago• Solar system grew out of a

cloud of hydrogen, helium and dust

• Disk of gas and dust forms around newborn star

• Dust grains clump together, grow

• Clumps collide, stick together continue to grow until a few dozen protoplanets remain

• Protoplanets combine to form planets

Planet Formation• Planets grow by two processes

– accretion, where gases condense to form solid grains of dust, which collide and stick together to form larger and larger particles

– gravitational collapse, where objects grow large enough and their gravity is strong enough to capture gases (hydrogen and helium) that allow them to grow very large, ie the Jovian planets

• Radiation from star blows the less dense material (gases) away

• Planets that form nearest a star should be made mostly of more dense material (dust) with relatively high melting points (Mercury, Venus, Earth, and Mars)

• Planets that form farther away from the star should be mostly made of less dense material (gases) with low melting points (Jupiter, Saturn, Uranus, Neptune)

• Icy objects like comets probably formed near Jupiter and were ejected to outer areas of solar system