The Planets

We Have 8 Planets

• The solar system is made up of 4 inner (terrestrial) planets, and 4 outer (Jovian) planets (sometimes called the gas giants).

• The inner planets are relatively small, dense and rocky.

• The outer planets are larger, have relatively low densities, and are made primarily of gases and ice.

• The asteroid belt separates the inner and outer planets.

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The Planets (and Dwarf Planets) (Sizes are to Scale, Distances are NOT)

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The Outer Planets in Size Scale to Our Sun

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The Planets

• The distances of the planets from the Sun are to scale.

• The size of the planets relative to each other are to scale.

• The distances and sizes are not the same scale.

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The Planets (Everything's to scale, so the planets are all too small to see!)

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The Inner Planets

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Mercury ☿

• Mercury has the shortest period of all the planets. It takes only 88 days to orbit the sun. (This should make sense because of Kepler's 3rd Law).

• It is the smallest planet in the solar system (although it’s radius is twice Pluto’s) and has the weakest gravity.

• Its surface is heavily cratered (like our moon).

• One Mercury day (the time it takes to rotate once, not 24 hours) is 59 Earth days!

• This means that 1 Mercury year is only 1.5 Mercury days long! it goes through nearly 3 season between every sunrise!

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• The slow rotation means that day and night temperatures are extreme. 400oC during the day, and -200oC at night.

• The temperature can rise and drop rapidly since Mercury has no atmosphere.

• Despite its high temperatures, there may be water on Mercury in craters near the poles.

• Mercury has no volcanic activity. It’s core has lost all of its original heat from formation.

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Mercury's Cratered Surface

Mercury Compared to the Earth

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What's this scar?

• The "scar" is actually just a portion of the surface of Mercury that was never mapped by satellite.

• Scientists don't want to "photoshop" out the missing area to make it look like the rest of the planet, as this could lead to false discoveries made by the artificial area.

• Of course some videos and pictures for the general public have had this band removed.

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• Roman: Mercury

• Greek: Hermes

• Messenger of the Gods

Mythology

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Venus ♀

• Venus is the closest planet to Earth in both size and proximity. The similarities end there.

• It is the brightest object in the sky other than the Sun and Moon of course.

• Venus has no (or a VERY weak) magnetic field.

• Venus rotates in the opposite direction to Earth - it is the only planet to do this (although Uranus spins on its back).

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• Venus is covered with thick, yellow clouds that prevent us from visibly seeing its surface. We only know what Venus looks like from radio pictures taken by Magellan.

• The atmosphere is mostly carbon dioxide with clouds of concentrated sulfuric acid.

• The CO2 has caused Venus to undergo a runaway greenhouse effect.

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• Surface temperatures are almost 500oC... the hottest planet in the solar system. This is hot enough to melt lead.

• At the surface, the atmosphere is gaseous, but nearly thick enough that we could swim through it (and yes that sounds really weird). The atmospheric pressure is over 90 times that of Earth. This is enough to crush people ... and most submarines!

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• Venus's atmospheric pressure is equivalent to an ocean depth of about 1000 m.

The picture shows

the approximate

"crush depths"

of various crafts

and animals.

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Note that the scale isn't linear!

Being on Venus's surface would feel like being here!

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Visible Light Spectrum Image (little detail visible)

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Radio Wave Spectrum Image (much better surface detail)

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False colour image showing differences in altitude.

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Earth / Venus Size Comparison

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Venus: Brightest "star" in the night sky.

Of course it isn't a start, but most people assume it is!

The Birth of Venus by Sandro Botticelli

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Mythology

• Roman: Venus

• Greek: Aphrodite

• Goddess of Love & Beauty

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Mars ♂

• Mars has a radius just over half that of Earth. • The gravity on Mars is only about 40% that of

Earth. You could life 2.5 times as much! • The "Red Planet" gets its colour from high

amounts of iron oxide (rust). • Temperatures vary from as high around 20oC

(which is perfect for us) to as low as -140oC (which is far less pleasant).

• Mars has a very thin atmosphere made mostly (95%) of carbon dioxide. Although Earth's atmosphere is only 0.04% CO2, we still have more because our atmosphere is so much thicker.

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• Mars has the largest (inactive) volcano in the solar system - Olympus Mons, which is over 500 km across and 26 km high. Mount Everest is just under 9 km.

• Water once flowed on Mars. Proof of this can be seen in features such as riverbeds and areas with water erosion. Mars still has water, mostly under its polar caps.

• There is still a lot of debate as to whether life ever existed on Mars, or if it still does. To be clear, scientists aren't talking about finding animals... more likely bacteria.

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Polar Cap

View From the Mars Pathfinder

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Note that the soil seems more brown than red...

• As it turns out, the first colour pictures we got back from the Spirit Rover were based the software's "auto colour enhancing" which assumed the sky should be blue, like Earth.

• Because the atmosphere has a different composition, it turns out Mars has a red atmosphere!

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Spirit Rover (Landed in Mars in 2004)

Olympus Mons

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The size of Olympus Mons can truly be appreciated once viewed on the Mars horizon.

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• Mars is the only inner planet other than Earth to have a moon.

• It actually has 2 moons... Phobos (left) and Deimos (right).

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15 km

27 km

Mythology

• Roman: Mars

• Greek: Ares

• God of War

• Phobos: "Fear"

• Deimos: "Terror"

Who better to hang out

with the God of War?

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The Asteroid Belt

• The asteroid belt consists of countless chunks of material (mostly rock & metal) from the size of dust up the dwarf planets Ceres (950 km in diameter).

• Over half of this mass in the belt is contained in the 4 largest objects (including Ceres).

• All of the mass together is only equal to about 4% of the Moon.

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The Outer Planets

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• The outer planets all have a similar structure, made up of light elements (mostly hydrogen and helium), arranged in 3 layers.

1. A gaseous outer layer

2. A liquid layer (becoming metallic hydrogen in Jupiter and Saturn)

3. A solid, rocky core

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• These layers aren't distinct - they gradual change from one to the next.

• It's hard to explain how they go from one layer to the next... there isn't an equivalent on Earth. They just blend in from one layer to the next.

• Here's an analogy... – The top bar shows colours going from colour to the

next distinctly, like air, water and ground on Earth.

– The bottom rainbow shows a gradual change from one colour to the next with no clear breaking point, like the gaseous, liquid and solid layers of the outer planets.

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Don't get too concerned about what metallic hydrogen is... it's a little confusing.

• All the outer planets have multiple moons and ring systems.

• The rings are made up of countless chunks of material (mostly ice) that orbit over the planet's equator.

• All the major moons of each planet are beyond its rings.

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Jupiter ♃

• Jupiter is the largest of the planets, with over twice the mass of all the other planets combined.

• It has the fastest rotation, with its day just under 10 hours long.

• Jupiter has the strongest magnetic field of all planets, extending to over 30 times its own radius.

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A visual representation of Jupiter's magnetic field.

• Jupiter actually radiates twice as much heat into space as it receives from the Sun. This is likely left over heat from its original creation.

• The Great Red Spot (one of the most recognizable features of any planet) is actually a hurricane. It is twice the size of Earth and at least 300 years old. It's most likely older. It was first noticed 300 years ago, but could have been there long before that.

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Great Red Spot

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How big is it?

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That big!

• Jupiter has 66 known moons

– many are quite small... only 1 to 5 km in diameter.

– 46 were discovered between 2001 and 2003.

– 1 of was just discovered in 2010, and 2 in 2011!

– The 4 largest are called the "Galilean Satellites".

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Earth, the Moon and Ganymede

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More Asteroids

• There are 2 large groups of asteroids that share Jupiter’s orbit known as the "Trojan Asteroids".

• The "Greeks" orbit in front of Jupiter, while the "Trojans" orbit behind.

The points labelled with "L" are called "Lagrangian Points". We won't worry about these in this course.

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• Each dot represents a known asteroid: – white: main belt asteroids – green: Trojans – red: Hildas

• The Hildas are another (less common) group of

asteroids caused by Jupiter's Legrangian points.

Asteroids in the Solar System

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Mythology

• Roman: Jupiter

• Greek: Zeus

• King of the Gods

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Saturn ♄

• Saturn is best known for having the most visible rings in the solar system.

• It also has the lowest density - lower than that of water.

• Saturn also radiates more heat than it receives, and has a magnetic field (weaker than Jupiter's but much stronger than Earth's).

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Saturn casting a shadow on its rings.

A couple of Saturn's moons.

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Saturn's Rings

• Saturn... the only planet light enough to float!

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Hexagonal Storm on Saturn

• Viewed by Voyager 1 and 2 (launched 1977, Saturn flyby in 1980) as well as the Cassini orbiter (launched 1997, Saturn orbit 2004)

• The hexagon is 4 times as wide as Earth.

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• While not fully understood, scientists have been able to create similar air flow patterns in the lab (shown in green).

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Saturn has 62 moons - the largest by far is Titan

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Mythology

• Roman: Saturn

• Greek: Cronus

• God of the Harvest

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Uranus ♅

• Uranus orbits the Sun "on its back", with an orbital tilt of 98o (compared to Earth's 23.5o).

• This is thought to have been caused by a collision with an Earth sized body.

• Uranus has a strange magnetic field.

– Although the planet is tilted over, the magnetic field isn't.

– It's also not going through the planets centre.

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• The North and South poles are named based on the planet's rotation (to keep it consistent with other planets, although the North pole is actually below the ecliptic.

• These are technically the North and South "rotational poles".

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Uranus is too far from the Sun to get much energy, so there's much less "weather" than the other planets.

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The seldom seen rings of Uranus.

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An artist's conception of Uranus and the Sun, as viewed from within its rings.

• Uranus has 27 moons.

• The largest 5 are...

Titania

Oberon

Umbriel

Ariel

Miranda

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Unlike the other planets so far, the moon of Uranus were not given Greek / Roman names, but names based on the works of Shakespeare. Keep in mind that the Greeks / Roman never discovered Uranus, which was recognized as a planet in 1783.

Titania (Fairy Queen)

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Oberon (Fairy King)

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Umbriel (A gnome in Pope’s "Rape of the Loch")

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Ariel (Spirit in "The Tempest")

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Miranda (Heroine in "The Tempest")

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Mythology

• Roman: Caelus

• Greek: Uranus

• Sky God and First Ruler

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Neptune ♆

• The most distant planet, Neptune takes 165 years to orbit the Sun.

• Like Uranus, it has an odd magnetic field... 47o from its axis of rotation.

• The magnetic field is also offset from the centre of the planet by 13,500 km (half its radius).

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• Neptune has 13 moons.

• The only one of notable size is Triton (with a radius more than 6 times larger than the next biggest).

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In Greek mythology, Triton is the son of Poseidon (Neptune). He's also the king from "The Little Mermaid"!

Mythology

• Roman: Neptune

• Greek: Poseidon

• God of the Sea

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Pluto: ♇ The Little Planet that Couldn't

• Pluto is no longer considered a planet, but is now a dwarf planet. There are many reasons for this. Size is not the only one!

• Pluto is less than half the radius of Mercury (the smallest planet) and almost 30 times lighter.

• All planets (except Mercury) have eccentricities less than 0.1. Pluto's is 0.25

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• It rotates backwards. (Venus is the only planet to do this).

• It has a density significantly higher than the outer planets and significantly lower than the inner planets.

• All other outer planets have atmospheres made of hydrogen and helium. Pluto's is methane.

• Most planets have an orbit that it within a couple of degrees of the ecliptic. Pluto's is 17o.

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• Pluto has 3 moons.

• Charon is the largest moon in the solar system relative to its planet.

• Nix and Hydra are very small and both were just discovered in 2005.

Pluto Charon

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• Pluto was originally thought to be much larger than it is now currently known to be.

• Because of poor resolution photos, the light produced by Pluto and Charon appeared to be 2 bright spots on the same object.

• The black & white photo shows a similar poor resolution photo to the original one for Pluto. – Can you tell if there's 1 object in the middle of the

photo, or 2?

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Mythology

• Roman: Pluto

• Greek: Hades

• God of Wealth and the Underworld

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Dwarf Planets

• In 2006, the International Astronomical Union (IAU) developed a new classification system for planets and objects in the solar system.

• The three classes created were the planets, dwarf planets and small solar system bodies (SSSBs).

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• The IAU classification of objects orbiting the Sun:

– A "planet" is a celestial body that... • is in orbit around the Sun

• has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape

• has cleared the neighbourhood around its orbit

– A "dwarf planet" is a celestial body that... • is in orbit around the Sun

• has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape

• has not cleared the neighbourhood around its orbit

• is not a satellite (moon)

– All other objects except satellites (moons) orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies", or "SSSBs".

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• As of January, 2012 there are 5 dwarf planets. – There are 4 other "candidates": Orcus, Quaoar, 2007 OR10

and Sedna. These may be made dwarf planets when their characteristics are more accurately determined.

• Pluto was formerly considered the smallest of the "nine traditional planets", until being reclassified as a dwarf planet.

• Ceres, the largest asteroid in the asteroid belt, has enough mass to produce a hydrostatic equilibrium (spherical shape), one of the main differences between dwarf planets and asteroids / comets.

• Pluto, Eris, Makemake and Haumea are icy bodies discovered much farther away, in the Kuiper belt (except Eris, which is in the scattered disc beyond the Kuiper belt).

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• Astronomers estimate that with better telescopes and better funding for research, more than 200 more dwarf planets may be proven to exist in the outer reaches of the solar system.

• This is exactly why scientists purposely redefined planets in such a way to eliminate Pluto.

• If Pluto were to remain a planets, then we could soon have to introduce hundreds more.

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Comparison of Dwarf Planet Sizes Note: This picture only includes the 3 dwarf planets that were recognized in 2006.

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More Mythology

• For those who are interested...

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The Sun ☉

• Roman: Apollo

• Greek: Helios

• Sun God, God of Musicians and Poets

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Earth

• Roman: Terra Mater (Mother Earth)

• Greek: Gaea

• Earth Goddess

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Moon ☾

• Roman: Diana or Luna

• Greek: Artemis

• Goddess of the Moon, Animals and Hunting

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