Eight planets have been discovered in our solar system. Mercury,Venus,Earth, andMars are the planets closest to the Sun. They are called the inner planets. The inner planets are made up mostly of rock.

The outer planets areJupiter,Saturn,Uranus,Neptune, andPluto. Jupiter,Saturn,Uranus, andNeptune are large balls of gases with rings around them. Pluto was the farthest planet from the Sun, but is no longer officially defined as a planet. It is coated with ice.

A Brief History

The Solar System began forming about 4.6 billion years ago when a swirling cloud of gas began to spin and collapse inwards, flattening into a central mass with a surrounding 'accretion' disk. Dust and gases in the disk formed smaller clouds each rotating about its own centre. These clouds collided with each other and stuck together, forming larger and larger lumps. These swirling, gaseous clouds became protoplanets, eventually cooling to form the planets we see today.

Gravitation condensed and heated the central mass. Density increased dramatically and nuclear fusion began. Energy was released and our Sun ignited into existence. Scientists believe that the accretion disk took 100 million years to collapse from the moment the Sun ignited.

The solar wind of the newly ignited Sun blew away leftover dust and gas in the vicinity of the inner condensations, leaving the rocky inner planets: Mercury, Venus, Earth and Mars. In the outer regions of the disk, the solar wind was weaker. The remaining dust and gas condensed into the larger gaseous planets: Jupiter, Saturn, Uranus and Neptune. It is believed that the 'left over' material became comets.

The Sun

The Sun is the star at the centre of the Solar System, and is the only body within it that emits light of its own. The Sun's enormous gravity holds the whole family of planets together.

It plays a very important rôle in our everyday lives. It provides light and heat, which sustains life on the Earth. The sun furnishes directly or indirectly all of the energy supporting life on earth, because all foods and fuels are derived ultimately from plants using the energy of sunlight ('photosynthesis'), and is the source of all food and fossil fuels. It causes seasons, climate, currents in oceans, circulation in air, and the weather in our atmosphere.

The sun is an intensely hot, self-luminous body of gases, comprising about 75% hydrogen, 25% helium, and less than 1% oxygen and other elements. It is considered to be a medium-sized star, has a diameter of about 1.35 million km (100x the diameter of Earth), and is about 333,400x more massive than the Earth. It contains about 99.86% of all the matter in the entire solar system.

Planet Classifications

All eight planets travel around the Sun in a different orbit (see applet below). Smaller bodies that also have the sun as their primary (that is, are not satellites of a planet) are called asteroids or planetoids.

The asteroids are sometimes called minor planets. The major planets are classified either as inferior, with an orbit between the sun and the orbit of the earth (Mercury and Venus), or as superior, with an orbit beyond that of the earth (Mars, Jupiter, Saturn, Uranus, Neptune).

The terrestrial planets - Mercury, Venus, Earth, and Mars - resemble the earth in size, chemical composition, and density.

The Jovian planets - Jupiter, Saturn, Uranus, and Neptune - are much larger in size and have thick, gaseous atmospheres (much thicker in the cases of Jupiter and Saturn) and lower densities.

Planetary Orbits

Note: The applet above shows the orbits and positions of the planets right now. Click Next to see next slide. Click and drag your mouse to tilt or rotate the solar system. Shift+Click zooms in and Ctrl+Click zooms out. The orbits of the planets are shown to scale, but the sun and planets themselves are much larger than scale so you can see them easily.

The inner planets (Mercury, Venus, Earth, and Mars), are relatively small and made primarily of rock and iron. The asteroids orbit the sun in a belt beyond the orbit of Mars, tumbling and sometimes colliding with one another. Made mostly of rock and iron, the asteroids may be the remnants of a planet that never formed. The outer planets, are much larger and made mainly of hydrogen, helium, and ice.

The planets move around the sun in fairly circular orbits (i.e., they are elliptical orbits with low eccentricity). Use your mouse to click and drag on the solar system above until you are looking straight down on the planets' orbits. Now zoom in and out to see how all the orbits are very circular.

All planets orbit the sun in almost the same plane. Using your mouse again, drag the solar system image until you are looking at the orbits edgewise. You can see how well they line up.

Applet and portions of text courtesy NASA.

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Beyond the Solar System

The average distance of the earth to the sun is used as a standard for measuring distances in the solar system and is called an astronomical unit (AU). One AU is about 93 million miles or 150 million kilometers. Mercury, the planet closest to the sun, is at about 0.387 AU. The boundary between the solar system and interstellar space is called the heliopause, and is about 100 AU from the sun. Some comets have highly eccentric orbits, and can travel out as far as 50,000 AU from the sun.

In 1992 radio astronomers reported evidence that strongly suggested the existence of two to three planets orbiting a pulsar in the Milky Way about 1,500 light-years from earth. Since then, astronomers have discovered many more planets outside our solar system - theextrasolar planets. These are planets orbiting around a star other than the Sun. Extrasolar planets were first discovered in the 1990s as a result of improved telescope technology, CCD and computer-based image processing which allowed far more accurate measurements of stellar motions.

These discoveries, and recent advances in astrobiology biochemistry applied to astronomy), are leading more and more scientists to speculate that life on other planets is not only possible, but even, perhaps, rather likely - stimulating enthusiasm for SETI.