Of the eight planets in our solar system Jupiter Saturn Uranus and Neptune are called the outer planets They are much larger than the inner planets and have deep gaseous atmospheres Because of this they’re sometimes nicknamed the gas giants Since Jupiter being the largest is the outstanding representative of this group the four outer planets are also known as the Jovian or Jupiter-like planets The outer planets have no solid surfaces Jupiter Jupiter and Saturn consist mainly of liquid and gaseous hydrogen and helium Uranus and Neptune have melted ices and molten rock as well as hydrogen and helium All the outer planets are less than twice as dense as water In fact Saturn’s density is so low that it would float if put in water The inner planets Mercury Venus Earth and Mars formed at temperatures too high to allow the abundant volatile substances those with comparatively comparatively low freezing temperatures such as water carbon dioxide and ammonia to condense to their ices They therefore remained small rocky bodies In contrast the large low-density gas-rich outer planets formed at distances beyond what astronomers astronomers have dubbed the snow line the minimum radius from the Sun at which water ice could have 6
IntroductIon condensed With the outer planets the accretion of icy planetesimals produced objects with masses 10 times that of Earth sufficient to cause the gravitational collapse of the surrounding gas and dust in the solar nebula These planets grew so large that their composition approached that of the Sun itself with hydrogen and helium the dominant dominant elements Each of the outer planets has a magnetic field a ring system and many known moons with more likely to be discovered Most of the known moons move around their respective planets in the same direction that the planets orbit the Sun The moons are extremely diverse representing a wide range of environments Jupiter is orbited by Io a body wracked by intense volcanism while Saturn’s largest moon Titan a body larger than Mercury exhibits a primitive atmosphere denser than that of Earth Triton moves in a retrograde orbit around Neptune that is opposite to the direction of the planet’s orbit around the Sun and features plumes of material rising through its tenuous atmosphere This book explores the characteristics of each of the outer planets and their moons Jupiter is the most massive massive of the solar system more than 1,300 Earths would fit inside it and the fifth in distance from the Sun It is a world of extreme and complex weather patterns including an orange-red oval called the Great Red Spot a long-lasting 7
the outer Planets storm system that is bigger across than Earth Jupiter is one of the brightest objects in the night sky and only the Moon Venus and sometimes Mars are more brilliant Saturn is easily visible to the unaided eye as a nontwinkling point of light It is a planet girded by extraordinary rings composed of dusty chunks of ice and are more complex than the rings of any other planet in our solar system Saturn is arguably the most sublime object in the solar system Uranus is the seventh planet in distance from the Sun and the least massive of the solar system’s Jovian planets It is a dazzling blue-green world with an atmosphere of hydrogen helium and methane and winds that are several times stronger than Earth’s Neptune the eighth planet from the Sun is the smallest in diameter of the four outer planets It is a stormy windswept world with a vivid blue hue Its highly active atmosphere is surprising since it receives so little sunlight to power its weather systems Each of the outer planets is a unique and exotic world As scientists gain the ability to further explore these distant distant neighbours we will have a better understanding of the history and formation of our solar system and of our own planet 8
CHAPTER 1 Giant Jupiter Jupiter is the most massive planet of the solar system When ancient astronomers named the planet Jupiter for the Roman ruler of the gods and heavens also known as Jove they had no idea of the planet’s true dimensions but the name is appropriate for Jupiter is larger than all the other planets combined It takes nearly 12 Earth years to orbit the Sun and it rotates once about every 10 hours more than twice as fast as Earth its colourful cloud bands can be seen with even a small telescope It has a narrow system system of rings and almost 70 known moons one larger than the planet Mercury and three larger than Earth’s Moon Some astronomers speculate that Jupiter’s moon Europa may be hiding an ocean of warm water and possibly even some kind of life beneath an icy crust Given Jupiter’s large proportion of hydrogen and its huge mass it has been traditional to assume that the planet formed by condensation from the primordial solar nebula nebula This hypothesis implies that the elements should all 9
