Читать книгу Universe: The story of the Universe, from earliest times to our continuing discoveries - Peter Grego - Страница 16

Our immediate planetary neighbours – Mercury, Venus and Mars – are solid worlds like the Earth. Between Mars and the outer planets of the Solar System lies a zone occupied by countless chunks of rock. These remnants of the Solar System’s formation range from the size of houses to the size of Iceland and are known as asteroids. Four giant planets preside over the outer reaches of the Solar System. Jupiter, Saturn, Uranus and Neptune are all swathed in thick layers of mainly hydrogen gas. Jupiter, the largest gas giant, is so big that a thousand Earths could comfortably fit inside its vast volume. Pluto, the outermost planet, is a diminutive world, smaller in fact than our own Moon. More than four light hours from the Sun, Pluto is one of a number of icy worlds at the cold fringes of the observable Solar System. Far beyond the planets, clinging on to the Sun’s gravity to a distance almost half way to the nearest stars, lies an unseen realm of comets known as the Oort Cloud. Cometary visitors from this distant region occasionally speed through the inner Solar System; warmed by the Sun, their icy nuclei emit large amounts of gas and dust, producing celestial spectacles like the magnificent Comet Hyakutake of 1997.