GEOLOGY | THE SOLAR SYSTEM
THE
PLANETS
There are eight planets in our solar system. Each of the planets circles (orbits) the Sun. Most of the planetary bodies of the Solar System orbit the Sun in nearly the same plane, known as the plane of the solar system. This is likely due to the way in which the Solar System formed from a protoplanetary disk.
In order of increasing distance from the Sun, the planets are Mercury (0.4 AU), Venus (0.7 AU), Earth (1 AU), Mars (1.5 AU), Jupiter (5.2 AU), Saturn (9.5 AU), Uranus (19.2 AU), and Neptune (30 AU).
You can see the correct order of the planets from the Sun (we've ignored relative distance) in Picture 1.
How was it all formed?
The amazing diversity expressed by the various planetary bodies and the structure of the Solar System itself, can be explained by the widely accepted origin theory: the nebula hypothesis.
The theory suggests that the Solar System was formed from a huge rotating disc of hydrogen and helium gas, along with a smattering of lesser materials. As the majority of the material was drawn to the centre of the disc it triggered nuclear reactions which resulted in the birth of a relatively bright, G-type star - The Sun.
The remaining 10% of material in the protoplanetary disc started to collide and stick together by a process called accretion. The rotation of the disc prevented all the material forming a single object. The rocky material formed the inner terrestrial planets, where it was too hot for ices to accrete. Further out from the Sun, where it wasn't so hot, ices formed, giving rise to the outer gas giant planets.
INNER PLANETS
The four proximal planets are Mercury, Venus, Earth and Mars.
They are known as the Terrestrial (Earth-like) Planets, as they are formed of materials similar to those of Earth.
They have densities ranging from
3.9 - 5.5 g per cm cubed.
THE OUTER PLANETS
The distal planets, Jupiter, Saturn, Uranus and Neptune are collectively described as the gas giants.
They have radii greater than 24 000 km.
Densities close to:
1 g per cm cubed.
They are predominantly or wholly fluid.
Astronomical Units (AU)
The Earth is one astronomical unit from the Sun. Therefore by definition 149.6 x 10^6 km
Absolute Temperature
The SI Unit for absolute temperature is Kelvin (K).
0 K = -273°C
273 K = 0°C
Density
Is the measure of mass per unit of a substance:
density =
mass (kg) / volume (m^3)
The SI Unit is therefore kilograms per cubic metre:
kg m^-3
EARTH'S MOON
The rocks brought back by Apollo missions have been dated at 4400 Ma.
The Moon has a solid crust, mantle and core. It lacks the volatile
materials found on Earth.
The surface is made of the dark-coloured Maria: basalt lava flows and the lighter highlands formed of anorthosite.
The Solar System describes the relationships between the Sun and the planets, satellites/moons, dwarf planets, comets and asteroids.
The Sun is not a planet, it is a star. It's a huge ball of gas (mainly Hydrogen and Helium). At its centre nuclear reactions release huge amounts of energy. The Sun is therefore very hot.
The temperature of the sun at its core is 15 000 000 K and at its surface is 5770 K
The Sun is the largest body in the Solar System and accounts for 99.8% of the total mass.
A planet is a sizeable celestial body that has sufficient gravity to clear its own orbit. It's all down to size! All dwarf planets in the Solar System are smaller than our Moon.
Asteroids are rocky objects which failed to form a planet.
A comet is composed of ice and silicate material (rock/dust). The outer crust evaporates as it approaches the Sun. The orbit of a comet tends to be elongated. Most originate from the Kuiper Belt or Oort Cloud in the outer reaches of the Solar System, beyond Neptune (30 AU).
TWIN PEAKS on MARS (NASA PATHFINDER)
Swales, boulders and hummocky ridges - flood debris.
THE SURFACE OF VENUS
Captured by the Soviet U 'Venera 13 Probe' (1981)
What ever happened to Pluto?
The rise of the Dwarf Planets.
Following the discovery of Eris, in the distant Kuiper Belt, it became apparent that there were likely numerous planetary bodies like Pluto still to be discovered. Debate raged in the scientific community regarding the classification of planets. eventually in 2006 The International Astronomical Union agreed on a new classification: The Dwarf Planet.
The key difference between a planet and a dwarf planet is that the latter does not dominate its region in space around the Sun. Pluto, for example, has not cleared its orbit of similar objects while Earth or Jupiter have no similarly-sized planetary bodies on the same path around the Sun. A dwarf planet has to be massive enough to be plastic and have a gravity that can maintain a hydrostatically equilibrious (roughly spheroid) shape - Haumea is shaped like a rugby ball. A dwarf planet may not be a satellite of another planetary body, it has to directly orbit the Sun in its own right. There are likely thousands of dwarf planets waiting to be discovered beyond Neptune.
The International Astronomical Union have recognised these planetary bodies as dwarf planets because they are massive, round, and orbit the Sun, but haven't cleared their orbital path: Ceres, Pluto, Haumea, Makemake & Eris.
Except for Ceres, which lies in the main asteroid belt (between Mars and Jupiter), these dwarf planets are located in the Kuiper Belt (in the outer solar system, beyond Neptune).
