The Outer Planets

The Inner Planets to Scale

3,031 mi 7,521 mi 7,926 mi 4,222 mi

5.4 g/cm3 5.2 g/cm3 5.5 g/cm3 3.9 g/cm^3

The Outer Planets to Scale

88,700 mi 75,000 mi 31,700 mi 30,200 mi 1.3 g/cm3 0.7 g/cm3 1.3 g/cm3 1.6 g/cm3

Outer Planet Similarities

• All are tens of thousands of miles across – Much larger than the inner planets

• All are made of gas - hydrogen and helium – No rocky surface like the inner planets

• All are not very dense (b/c made of gas) • Why do the outer planets share these

similarities? • Why don’t the outer planets have more in

common with the inner planets?

Jupiter

• Largest planet • Colorful cloud bands • Rapid rotation

(9h50m) • -250°F at cloud tops • Great Red Spot

Great Red Spot • Hurricane-like storm,

fueled by heat inside of Jupiter

• Large enough to cover 3 Earths

• Discovered by Galileo nearly 400 years ago – Why can it last so long

compared with hurricanes on the Earth?

Why has Jupiter’s red spot lasted so long?

• No one knows why it has lasted for more than 300 years or how long it will continue to exist.

• Despite numerous scientific

studies.

Saturn

• Least dense planet - Would float on water!

• Bulges because of rapid rotation (10h30m)

• Large, broad rings

Saturn’s Rings

• Made of countless small chunks of ice and rock – Why don’t the chunks

combine to form a moon?

– Why doesn’t Earth have rings?

• 170,000 miles wide, <1 mile thick

Why don’t the chunks in Saturn’s rings combine to form Moons?

• They are too close to Saturn. • If they try to combine, they are pulled apart

again by Saturn’s gravity.

Why doesn’t the Earth have rings? • May have had rings in the past. • May have them in the future if asteroids or

comets break up and remain in pull of Earth’s gravity.

Uranus • Discovered by

accident (1781) • Blue-green color from

methane in its atmosphere

• -350°F at cloud tops • Almost featureless;

little weather – Why might Uranus

lack distinctive weather patterns?

Why does Uranus lack weather patterns?

• Low internal heat.

• Recent more detailed studies using infrared images: – Popcorn like clouds at poles – Scaffolded clouds at equator

Uranus’ Axis

• Axis tilted 98° from its orbit • Orbits Sun every 84y

What’s Wrong with Uranus?

• Following its discovery, astronomers calculated its orbit

• Uranus mysteriously speeded up, then slowed down – Why might Uranus

have deviated from its calculated orbit?

Why did Uranus deviate from its calculated orbit?

• Due to the pull of gravity from Pluto.

• This is actually how Pluto was discovered.

Neptune • Discovered at its

predicted position (1846)

• Blue color from methane • -370°F at cloud tops • Great Dark Spot in

1989, but not 1994 – How is this different from

the Great Red Spot? – Why is its weather

different from Uranus? Jupiter?

Great dark spot vs. Great red spot

• Neptune’s great dark spot is similar to Jupiter’s great red spot in that they are both storms.

• Neptune’s spot contained white “cirrus” like clouds of frozen methane.

• It disappeared in 1994…did not last nearly as long as Jupiter’s spot.

Neptune’s weather

• Probably different from Uranus’ weather because Uranus is tilted on its side.

• Similar to Jupiter, but faster winds. Probably due to coldness of atmosphere producing less friction.

The S.S. Family Portait

Pluto Kuiper Belt Oort cloud

Is Pluto a planet?

Here’s Pluto!

• Size: 1,480 mile (2,400 km) across • Length of day: about 6 earth days • Average Distance from Sun: 3.73

billion miles • Average surface temperature: -243 o

F • Number of known moons: 3 • Weight of 100- pound Earthling: 7

pounds

“Pluto: The Wonder Years” Discovered in 1930 by Clyde Tombaugh, Or was it? Was originally studied because of

gravitational influence on Neptune and Uranus starting in 1905.

Percival Lowell spent years searching for Planet X, and may have discovered it before his death in 1916.

What makes a Planet?

In 2006 the International Astronomical Union (IAU) stated that a planet is a celestial body that:

• is in orbit around the Sun • has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape • has "cleared the neighborhood” around its orbit.

The Problem with Pluto Size…..Smaller than our moon! Orbit is elliptical: Pluto's highly elliptical orbit is off-center in

relation to the sun Pluto’s Orbit Overlaps Neptune’s orbit, which

caused it to be the 8th planet from the sun during 1979-1999 February 11, 1999 Pluto crossed Neptune’s

path again and became the ninth planet again

Kuiper Belt • A vast swarm of icy bodies beyond

Neptune. • So far, Pluto is one of the largest icy

bodies in the Kuiper Belt. • There are many other Kupier belt Objects

(KBOs) that share Plutos orbital characteristics.

• So if Pluto is a Planet then there are other objects that may also be planets within the Kuiper Belt.

The Great Pluto War… Which side are you on?

R.I.P. Planet Pluto

Some may argue that Pluto is not a planet because:

Pluto is too small to be a planet! Pluto’s orbit is too elliptical! Pluto’s moon Charon is only half the size of

Pluto, while other planets are far larger than their moons.

Pluto doesn’t “sweep up” asteroids, comets, and other debris as other planets dominate their areas

If Pluto is a planet than there are too many KBOs that could be planets too!

Save Planet Pluto! Some may argue that Pluto is a planet

because: It has an atmosphere! It has moons! It does orbit the sun! Pluto is spherical! Only 424 of 10,000 astronomers voted on

Pluto’s planet status! Other planets such as Earth, Mars,

Jupiter, and Neptune share their orbits with asteroids.

Pluto Demoted to “Dwarf Planet” On August 24, 2006 Pluto was officially

renamed as a Dwarf Planet. IAU defines a dwarf planet as:

Planet that orbits the sun Enough mass for self-gravity and almost

spherical shape has not cleared the neighborhood around its

orbit Is not a satellite Pluto 1930-2006

First Graders’ Thoughts

Pluto is a planet because: It’s in space just like the other planets. Just because it’s small doesn’t mean it isn’t as good as the

other planets. It’s better to have 9 planets instead of 8 planets. Pluto was already a planet so you can’t take it away now.

Pluto is not a planet because: It’s too far away. It’s messy. It doesn’t circle the sun like the other planets.

Kuiper Belt

• Discovered in 1992 • Named for Dutch Astronomer Gerard

Kuiper, who had PREDICTED its existence in 1951.

Kuiper Belt

• Region beyond Neptune; 30 to over 50AU

• At least 70 000 small, icy, slow-moving, objects

Kuiper Belt

• Very faint objects because small, far and reflect light from Sun

• A million times fainter than faintest we can see with naked-eye

Kuiper Belt

• History of solar system

Oort Cloud

• Hypothetical spherical cloud. No direct observations.

• Proposed by Ernst Opik (1932), Jan Henrik Oort (1950)

Oort Cloud

• 50 000AU; defines gravitational boundary of solar system

• Source of long-period Comets

Oort Cloud

• Objects composed of ices (water, methane, and ammonia)

• 2 regions: outer sphere, inner disc

Oort Cloud

• Comets can not have formed on their current orbit

• Must be held in an outer reservoir

Solar System Debris

Asteroids, Comets, Meteors, Oh My!

Asteroids • Asteroids are small,

irregular, rocky bodies that orbit the sun

• Most are located between the orbits of Mars and Jupiter in the asteroid belt

• Some are located in the same orbit as Jupiter, but are separated by 60º - Trojans

• Some asteroids are in highly elliptical orbits that pass earth – Earth-crossing asteroids, Near-earth asteroids (NEAs),

What do asteroids look like? • Too small to be seen with the naked eye

– need large telescopes • Appear as streaks because of their

motion • Three fly-bys of asteroids:

– Gaspra by Galileo in 1998 – Ida by Galileo in ???? – Mathilde by NEAR in 1999

What are asteroids made of? • Asteroids are classified into a number of types according

to their spectra (and hence their chemical composition) and albedo: 1. C-type, includes more than 75% of known asteroids:

– extremely dark (albedo 0.03) – approximately the same chemical

composition as the Sun minus hydrogen, helium and other volatiles

2. S-type, 17%

– relatively bright (albedo .10-.22) – metallic nickel-iron mixed with iron- and

magnesium-silicates 3. M-type, most of the rest

– bright (albedo .10-.18) – pure nickel-iron

Asteroids • How many are there?

– Some 5000 with well-determined orbits

– Probably >500,000 larger than 1 km

• How big are they? – Largest (Ceres) is 940 km in diameter – Three larger than 500 km – About a dozen larger than 250 km – Number increases rapidly with decreasing

size • Total mass of all asteroids ~ 5%

Comets • Comets are small

bodies of ice and dust that form tails as they near the sun.

• What way is this comet moving? • Recall Kepler’s second

law. Where do comets spend most of their time?

What are comets made of?

• When they are near the Sun and active, comets have several distinct parts: – nucleus: relatively solid and stable,

mostly ice and gas with a small amount of dust and other solids;

– coma: dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus;

– hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen;

– dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eye;

– ion tail: as much as several hundred million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind.

Comet Parts

Where do comets live?

• Short period comets come from the Kuiper belt

• Long period comets from the Oort cloud

What happens when comets loose their tails?

• Meteor showers

Meteorites Three main types: • Stony

– Look like ordinary earth rocks at first glance. If you were to look closely, you would see that the outsides of the rocks look melted. Most meteorites (93%) striking the earth are stony

• Irons – These dark grey, metallic, and very

dense meteorites are unlike any earth rocks that you have ever seen. Five percent of meteorites striking the earth are irons

• Stony-Irons – These meteorites have clumps of both iron

and stone

So what’s the difference?

Finding Meteorites • Finds

– Found on earth, often long after they landed – usually noticed because of their unusual appearance or their

unusual location

• Falls – These meteorites are seen falling, making a fireball in the

sky. By following the trail, the landing site and meteorite can be found. Most of these are stony meteorites

Peekskill, NY 1994

Impacts • What happens when a meteor hits the

ground? – Usually

nothing – Occasionally,

large pieces make it to the ground and create impact craters

Impact Hazards • No person has ever been killed by a

meteorite hitting earth • However, a large impact killed the

dinosaurs • A smaller impact in remote Russia in

1908 knocked down trees and could have caused severe damage in a populated area

Earth Sterilizing Impact

Mass Extinction Impact Civilization Threatening Impact

Terrestrial Impact Frequency year

century

million yr.

billion yr.

ten thousand yr.

100 million million 10,000 100 1 0.01

Hiroshima

Tunguska

K/T

TNT equivalent yield (MT)

Global catastrophe

Tsunami danger