The Solar System consists of the Sun and the other celestial objects gravitationally bound to it: the eight planets, their moons, five currently identified dwarf planets and their seven known moons, and billions of small bodies. This last category includes asteroids, Kuiper belt objects, comets, meteoroids and interplanetary dust. In broad terms, the charted regions of the Solar System consist of the Sun, four terrestrial inner planets, an asteroid belt composed of small rocky bodies, four gas giant outer planets, and a second belt, called the Kuiper belt, composed of icy objects. Beyond the Kuiper belt lies the scattered disc, the heliopause, and ultimately the hypothetical Oort cloud. In order of their distances from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Six of the eight planets are in turn orbited by natural satellites, usually termed "moons" after Earth's Moon, and each of the outer planets is encircled by planetary rings of dust and other particles. All the planets except Earth are named after gods and goddesses from Greco-Roman mythology. The five dwarf planets are Pluto, Makemake, and Haumea, the three largest known Kuiper belt objects; Ceres, the largest object in the asteroid belt; and Eris, the largest known object in the scattered disc.
is the best-known of the short-period comets
, and is visible from Earth every 75 to 76 years. Halley is the only short-period comet that is clearly visible to the naked eye, and thus, the only naked-eye comet that might appear twice in a human lifetime. Other naked-eye comets may be brighter and more spectacular, but will appear only once in thousands of years. Halley's returns to the inner solar system
have been observed by astronomers since at least 240 BC, and recorded by Chinese, Babylonian, and mediaeval European chroniclers, but were not recognised as reappearances of the same object. The comet's periodicity was first determined in 1705 by English astronomer Edmond Halley
, after whom it is now named. It last appeared in the inner Solar System
in 1986 and will next appear in mid-2061. During its 1986 apparition, Halley's Comet became the first to be observed in detail by spacecraft, providing the first observational data on the structure of the comet nucleus
and the mechanism of coma and tail formation. These observations supported a number of longstanding hypotheses about comet construction, particularly Fred Whipple
's "dirty snowball
" model, which correctly surmised that Halley would be composed of a mixture of volatile ices
, such as water
, carbon dioxide
, and dust. However, the missions also provided data which substantially reformed and reconfigured these ideas. (more...
False-color detail of Jupiter's atmosphere, imaged by Voyager 1, showing the Great Red Spot and a passing white oval. The wavy cloud pattern to the left of the Red Spot is a region of extraordinarily complex and variable wave motion. To give a sense of Jupiter's scale, the white oval storm directly below the Great Red Spot is approximately the same diameter as Earth.
- ...that the passing of the Great Comet of 1577 (pictured) caused almost century-long debate, during which Galileo argued that comets were merely optical illusions?