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The Solar System

Our solar system includes the Sun together with the eight planets and their moons as well as all other celestial bodies that orbit the Sun.

Ancient astronomers observed points of light that appeared to move among the stars. They called these objects planets, meaning wanderers, and named them after Roman deities - Jupiter, king of the gods, Mars, the god of war, Mercury, messenger of the gods, Venus, the goddess of love and beauty, and Saturn, father of Jupiter and god of agriculture. The stargazers also observed comets with sparkling tails, and meteors - or shooting stars apparently falling from the sky.

Since the invention of the telescope, three more planets have been discovered in our solar system: Uranus (1781), Neptune (1846) and Pluto (1930). Pluto was reclassified as a dwarf planet in 2006. In addition, our solar system is populated by thousands of small bodies such as asteroids and comets. Most of the asteroids orbit in a region between the orbits of Mars and Jupiter, while the home of comets lies far beyond the orbit of the dwarf planet Pluto, in the Oort Cloud.

The four planets closest to the sun - Mercury, Venus, Earth, and Mars - are called the terrestrial (or telluric) planets because they have solid rocky surfaces. The four large planets beyond the orbit of Mars - Jupiter, Saturn, Uranus, and Neptune - are called the gas giants.

Beyond Neptune, on the edge of the Kuiper Belt, tiny, distant, dwarf planet Pluto has a solid but icier surface than the terrestrial planets.

Nearly every planet - and some moons - has an atmosphere. Earth's atmosphere is primarily nitrogen and oxygen. Venus has a thick atmosphere of carbon dioxide, with traces of poisonous gases such as sulphur dioxide. Mars' carbon dioxide atmosphere is extremely thin. Jupiter, Saturn, Uranus, and Neptune are primarily hydrogen and helium. When Pluto is near the sun, it has a thin atmosphere, but when it travels to the outer regions of its orbit, the atmosphere freezes and collapses to the planet's surface. In that way, Pluto acts like a comet.

There are 146 known natural satellites (also called moons) in orbit around the planets in our solar system, ranging from bodies larger than our own Moon to small pieces of debris. Many of these were discovered by planetary spacecraft. Currently, another 25 moons are awaiting final approval before being added to our solar system's moon count.

Some moons have atmospheres (Saturn's Titan) while others even have magnetic fields (Jupiter's Ganymede). Jupiter's moon Io is the most volcanically active body in the solar system. An ocean may lie beneath the frozen crust of Jupiter's moon Europa, while images of Jupiter's moon Ganymede show historical motion of icy crustal plates. Some moons may actually be asteroids that were captured by a planet's gravity. The captured asteroids presently counted as moons may include Mars' Phobos and Deimos, several satellites of Jupiter, Saturn's Phoebe, many of Uranus' new satellites, and possibly Neptune's Nereid.

From 1610 to 1977, Saturn was thought to be the only planet with rings. We now know that Jupiter, Uranus and Neptune also have ring systems, although Saturn's is by far the largest. Particles in these ring systems range in size from dust to boulders to house sized, and may be rocky and/or icy.

Most of the planets also have magnetic fields which extend into space and form a magnetosphere around each planet. These magnetospheres rotate with the planet, sweeping charged particles with them. The sun has a magnetic field, the heliosphere, which envelops our entire solar system.

Did you know...?

Jupiter's Great Red Spot

The Great Red Spot is a great anti-cyclonic (high pressure) storm akin to a hurricane on Earth, but it is enormous (three Earths would fit within its boundaries) and it has persisted for at least the 400 years that humans have observed it through telescopes. Since it is anti-cyclonic in Jupiter's Southern hemisphere, the rotation is counterclockwise, with a period of about 6 days. (A hurricane in Earth's Southern hemisphere rotates clockwise because it is a low pressure system.) The clouds associated with the Spot appear to be about 8 km above neighboring cloud tops. The following two figures show a recent Galileo view of the Great Red Spot, and a closeup of the turbulence in its vicinity.

The Coriolis effects that are responsible for cyclones and anti-cyclones on Earth are greatly magnified on Jupiter, which has a rotational frequency about 2 1/2 times that of Earth, but this alone would not account for the persistence and size of the Great Red Spot. There are other features similar to the Great Red Spot on the surface (note the white spots in the above images) but none are as large as the Great Red Spot.

Presumably the persistence of the Great Red Spot is related to the fact that it never comes over land, as in the case of a hurricane on Earth, and that it is driven by Jupiter's internal heat source. Computer simulations suggest that such large disturbances may be stable on Jupiter, and that stronger disturbances tend to absorb weaker ones, which may explain the size of the Great Red Spot. Furthermore, as for the clouds in general, we do not understand fully the reason for the coloring. It has been suggested that certain compounds of phosphorous are responsible for the reddish-brown hue, but this remains somewhat speculative. Thus, we understand the broad properties, but not all the detailed features of this remarkable phenomenon.