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 a trans-Neptunian object
currently about three times as far from the Sun as Neptune
. For the majority of its orbit it is the most distant known object in the Solar System
other than long-period comets
. Roughly two-thirds the size of Pluto
, Sedna is hypothetically large enough to be rounded by its own gravity, and thus would qualify as a dwarf planet
under current definitions. However, its distance makes determining its shape difficult. Spectroscopy
has revealed that Sedna's surface composition is similar to that of some other trans-Neptunian objects, being largely a mixture of water, methane
, and nitrogen ices
. Its surface is one of the reddest in the Solar System. Its exceptionally long and elongated orbit, taking approximately 12,000 years to complete, and distant point of closest approach to the Sun have led to much speculation as to its origin. Astronomer Mike Brown
, who co-discovered Sedna in 2003, believes it to be the most scientifically important trans-Neptunian object found to date, as understanding its peculiar orbit is likely to yield valuable information about the origin and early evolution of the Solar System.
Mars, the fourth planet from the Sun, is named after the Roman god of war because of its blood red color. Mars has two small, oddly-shaped moons, Phobos and Deimos, named after the sons of the Greek god Ares. At some point in the future Phobos will be broken up by gravitational forces. The atmosphere on Mars is 95% carbon dioxide. In 2003 methane was also discovered in the atmosphere. Since methane is an unstable gas, this indicates that there must be (or have been within the last few hundred years) a source of the gas on the planet.
- ...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?