Planetary System


Our Solar System

Our solar system is made up of:

8 Planets - Mercury, Venus, Earth, Mars = Terrestrial (rocky) planets; Jupiter, Saturn, Uranus, Neptune = Gas Giants

160 Moons are known 

Ceres, Pluto and Eris = Dwarf Planets

Asteroids (minor planets) mostly only a few hundred kilometres in size but larger than 10m. Most of these orbit in the asteroid belt or main belt between Mars and Jupiter.  Include Vesta,(the brightest) and Pallas

Comets - Nuclei of Ice, dust and rock which develop a coma and gaseous tail when near the sun.

Centaurs - Similar to both asteroid and comets, they generally orbit  the sun between Jupiter and Neptune.  Include Chiron, Hidalgo and Asbolus

Trans-Neptunian Objects (TNO's) objects orbiting the sun beyond Neptune

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Our Solar System - 2

2006 extraordinary decisions of the International Astronomical Union (IAU)

Ceres was promoted from Asteroid to Dwarf Planet

Pluto was demoted from Planet to Dwarf Planet

Definition of a Planet by IAU:-

Is in orbit around the Sun

Is large enough to be spherical

Has "cleared its orbit" of other objects

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Astronomical Units (AU)

A convenient unit in which to quote distances within the solar system.

1AU is defined as 150million km 

This is the mean distance between the Earth and the Sun.

Mercury = .38AU

Venus = .72AU

Earth = 1AU

Mars = 1.5AU

Jupiter = 5.2AU

Saturn = 9.5AU

Uranus = 19.1AU

Neptune = 10AU

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Mercury - Heavily cratered & moon-like

Mean Distance / AU = 0.38

Orbital Period / Earth Years = 0.24

Average Temperature = 170C

Diameter / 1000km = 4.9

Rotation Period / Earth Days = 59

Heavily Cratered

Contains Highlands

Contains Lava Filled Basins

Many parts of its surface appear almost identicle to that of the moon

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Venus - backward spinning and sulphorous

Mean Distance / AU = 0.72

Orbital Period / Earth Years = 0.62

Average Temperature = 470C

Diameter / 1000km = 12.1

Rotation Period / Earth Days = 243 (retrograde)

Backwards Spinning

Clouds of sulphuric acid

Surface pressure 90 times greater than on Earth

Dense atmosphere containing carbon dioxide - This prevents infra-red radiation from leaving Venus making the surface and lower atmosphere extremely hot = Global Warming gone mad

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Mars - Red Planet

Mean Distance / AU = 1.5

Orbital Period / Earth Years = 1.9

Average Temperature = -50C

Diameter / 1000km = 6.8

Rotation Period / Earth Days = 1.0

Red Planet

Iron-rich rocks

Seasonal ice caps

450 km  lond water-carved canyon = Velles Marineris (Mariner Valley) east to west

Highest volcano in the solar system = Olympus Mons

Violent dust storms

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Jupiter - Great Red Spot (GRS)

Mean Distance / AU = 5.2

Orbital Period / Earth Years = 11.9

Average Temperature = -150C

Diameter / 1000km = 143

Rotation Period / Earth Days = 0.41

Rotates on its axis in only 10 hours = equatorial bulge and causes dynamic wind systems that split the atmosphere into a series of red-brown and yellow-white zones.

The Great Red Spot is an anticyclone weather system larger than the planet earth!

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Saturn - Majestic rings

Mean Distance / AU = 9.5

Orbital Period / Earth Years = 29.5

Average Temperature = -180C

Diameter / 1000km = 121

Rotation Period / Earth Days = 0.43

Similar to Jupiter

LesHas beautiful rings 

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Uranus - Sometimes referred to as twin to Neptune


 Mean Distance / AU = 19.1 

Orbital Period / Earth Years = 84

Average Temperature = -210C 

Diameter / 1000km = 51 

Rotation Period / Earth Days = 0.72 

Cold gas giants of similar size made up of Hydrogen, Helium, Methane and Ammonia

Spins on its side 

Almost Featureless 

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Neptune - Sometimes referred to as twin to Uranus

Mean Distance / AU = 30

Orbital Period / Earth Years = 165

Average Temperature = -220C

Diameter / 1000km = 50

 Rotation Period / Earth Days = 0.67

Shows many surface markings

Great Dark Spot

Dark Banded features

Cirrus-like clouds of frozen methane at high altitude

Cold gas giants of similar size made up of Hydrogen, Helium, Methane and Ammonia

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Understanding of planets

Most information gathered using scientific instruments (cameras, spectrometers and magnetometers) flown on unmanned space probes.

No manned space missions so far beyond the moon because of costs and problems for astronauts:-

Space Adaptation Syndrome = no up or down causes headaches, nausea, vomitting and poor concentration

Zero Gravity over-exposure = brittle bones, muscle fatigues and reduced red blood cell counts

Communication delays = radio waves travel at the same speed as light therefore slow communication the further away the astronauts are

Radiation Risk = Without earth's protective shiels, they would be exposed to harmful ionising radiation from the sun = cancer

Psychological Problems = confined living for long periods of time causes irritability, fatigue and low motivation

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Most planets have systems of natural satellites or moonds in orbit around them.

Mars: 2 small moons - Deimos and Phobos

Both are irregular in shape, only 10s of km in size and heavily cratered.  Their density is half that of Mars and they are made up of similar materials to asteroids.  It is thought they were captured from the asteroid belt.

Neptune: many moons - Triton, Porteus, Nereid and others

Triton is the largest - revolves around Neptune in the opposite sense to that in which Neptune spins. (Unique to the moons in the solar system).  Large size and mass suggests that Triton is a captured body but this may be as a result of a collision with Neptune or another of its moons. Possible that the small inner moons were formed from left-over debris from this collision.

Porteus is the second largest moon and largest irregular satellite in the solar system - Orbits Neptune in the plane of Neptune's Equator and in the same sense as Neptune. Suggests is was created at the same time.

Nereid - Highly eccentric orbit.  Largest orbit of any moon in the solar system.  Takes 360 days to orbit Neptune.Probably captured object from the Kuiper Belt.

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Ring System

Saturns rings are the most prominent but all the gas giants have ring systems above their equators.

Each system is devided in to several distinct rings made up of billions of particles of ice, rock and dust.

Jupiter - very dusty, dark rings

Saturn - Icy and highly reflective

Ring systems are generally extremely wide (100s of km across) they are extremely thin (less than 1.5km thick).

There are gaps in the ring systems

Saturn's Casini division are incomplete

Uranus and Neptune have ring arcs

It is thought that the ring systems are relatively short-lived features. It is thought they may have been formed from the debris left over after the planet's formation.

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Planetary Ring Facts

Galileo Galilei is credited to have discovered Saturn's rings in 1610.  He did not understand what he saw.

Dutch astronomer Christiaan Huygens described their nature almost 50 years later.  He also discovered Saturn's large moon Titan.

The Structure of many rings and gaps between them are maintained gravitationally by small "shepherd" moons that orbit the host planet close to the rings or gaps.

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Orbits of Planets

Planets orbit the Sun in slightly squashed circles or ELLIPSES

The plane of the Earth's orbit is called the ECPIPTIC and the planes of all the other planets are inclined by only a few degrees to this.

Because of this planets appear to move through a narrow band of sky called the ZODIACAL BAND

The relative position of a planet and the Earth determine how well the planet can be observed.

This is different for the two inner (INFERIOR) planets - Mercury and Venus and the outer (SUPERIOR) planets - Mars to Neptune.

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Orbits of Inferior Planets

Because the inferior planets are physically close to the sun they always appear close to the sun in the sky.  Especially Mercury

Mercury and Venus are best observed when they are furthest from the sun in the sky.  These positions in their orbits are known as the GREATEST EASTERN ELONGATION (GEE) and GREATEST WESTERN ELONGATION (GWE).  At these positions the angle between the lines  planet - Earth and Planet - Sun is 90degrees.

At GEE an inferior planet may be visible shortly after sunset in the west and at GWE just before dawn in the east.

Venus has such a highly reflective surface that it is often referred to as the Morning or Evening Star.

An inferior planet lies at INFERIOR CONJUNCTION when its angle is 0degrees.  At this time the planet may pass in front of the solar disc ie a TRANSIT.  This is very rare.  Last transit occured in 2004 when Venus passed infront of the Sun.  Transits of Mercury are more common as it is closer to the sun.

When the planet disappears behind the Sun = Superior Conjunction - it is known as an Occultation.

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Orbits of Superior Planets

Superior planets such as Mars and Jupiter are best observed when they are at OPPOSITION.  At this position they are closest to the earth (offering best resolution) directly opposite the sun (giving full illumination). 

The orbital motion (DIRECT MOTION) of the planets around the sun makes their position change nightly.

Generally they appear to move from west to east (more distant planets move more slowly).

if observed over a long period of time they appear to Loop-the-loop for a period of a few weeks.  This is called RETROGRADE MOTION.  

This is because the planets orbit the Sun at different speeds - both angular and actual - with the speed decreasing as the distance from the Sun increases.  We therefore see planets from different viewpoints sometimes "overtakign" and "undertaking" the inner and outer planets rescpectively and making them appear to move backwards.

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