The planets, asteroids, and comets in the solar system are loose particles left over from the formation of the Sun. Originally the gas and dust that would become the Sun was the core of a cloud much larger than the solar system, probably several light-years across One light-year is approximately 10 trillion (10,000,000,000,000) km, or 6 trillion miles. The core was slowly rotating at first, but as the cloud collapsed it spun faster, like a spinning ice skater pulling in his arms. The rotation prevented the material at the core's equator from collapsing as fast as the material at the poles, so the core became a spinning disk.

Gas and dust in the disk spiraled gradually in to the center, where it accumulated to form the Sun. But because dust is denser than gas, some of the dust settled to the mid-plane of the disk. These dust particles stuck together to make clumps, then clumps stuck together to make rocks, then rocks collided to make planets. In the case of the "gas giant" planets (Jupiter, Saturn, Uranus, and Neptune), the rocky cores were massive enough to also attract appreciable amounts of gas. The outer layers of Jupiter and Saturn are made up of hydrogen and other gases. Uranus and Neptune are also gas giant planets, but they were built up mainly from ice chunks.

The Sun, then, is the collapsed core of an interstellar gas cloud. The planets, asteroids, and comets are small lumps of dust or ice chunks that stayed in orbit instead of spiraling into the Sun. The planets all formed within a very short period — probably a few million years — about 5 billion years ago.

The solar system is about 4.5 billion years old.

There are no physical boundaries in space. The traditional view of the solar system is that it consists of eight planets orbiting around one star: the Sun. Neptune, the farthest planet from the Sun, orbits at approximately 30 astronomical units (AU) from the Sun. An astronomical unit is a unit of length used by astronomers. One astronomical unit equals the average distance from Earth to the Sun — about 93 million miles (150 million km). The solar system also includes the Kuiper Belt, a comet-rich area that begins near Neptune's orbit and stretches far beyond it, to about 50 AU from the Sun. Part of Pluto's elliptical orbit extends far into the Kuiper Belt. Beyond Pluto's orbit is another region of icy objects in our solar system, called the Oort Cloud, which extends approximately 50,000 AU from the Sun.

Planets come in different sizes and colors. The four planets closer to the Sun are called rocky, or terrestrial, planets. They are small in size and similar to Earth in composition. They have no rings, and only two of them (Earth and Mars) have moons.

The four outer planets, called gas giants, are much larger than the rocky planets. They all have rings and many moons. The gas giants are made up mostly of hydrogen, helium, frozen water, ammonia, methane, and carbon monoxide.

There is a discussion over whether Pluto is a planet. Some astronomers think that Pluto might be little more than a giant comet (see question 15). Its composition is similar to that of comets, and its orbit is quite different from that of the other planets. Astronomers agree that Pluto is part of the Kuiper Belt of comets because its composition and orbit fit neatly within that group. However, some argue that it deserves planetary status as well.

The asteroid belt is a zone between the orbits of Mars and Jupiter. Millions of asteroids inhabit the asteroid belt, with many more scattered throughout the solar system. It is believed that the asteroids in the asteroid belt never formed a planet because the gravity of nearby Jupiter kept pulling them apart.

Comets are giant snowballs of ice and rock that formed in the outer solar system, the regions we call the Kuiper Belt and the Oort Cloud. When the gravity of a large planet disturbs such an iceball, its orbit can change to pass through the inner solar system. If it passes close enough to the Sun, the ices melt and produce the coma and tail of a comet.

Short-period comets — comets that return to the solar system about once every 100 years — probably originate from the Kuiper Belt. This belt is located within the solar system's ecliptic plane, beyond the orbit of Neptune. Since 1992, thousands of objects have been discovered in the Kuiper Belt. These objects are small compared with planets. Their sizes range from 10 to 2500 kilometers in diameter. Earth's diameter, by comparison, is 14,000 kilometers. Astronomers estimate that this belt contains at least 200 million comets.

Long-period comets — comets that we see rarely (once every few thousand years) — are thought to originate from a vast, spherical cloud of frozen bodies called the Oort Cloud, named for the Dutch astronomer Jan Hendrik Oort. This cloud of comets, which also orbits the Sun, resides in the farthest region of the solar system, beyond Neptune and Pluto. Occasionally, a gravitational disturbance caused by a passing star or an interstellar cloud causes one of the frozen bodies in the Oort Cloud to begin a journey toward the inner solar system, where it makes a passing rendezvous with our Sun.

Yes. Some planets can indeed be seen with the unaided eye, which is how they were discovered by the ancient civilizations. They are Mercury, Venus, Mars, Jupiter, and Saturn. The other two planets — Uranus and Neptune — were discovered using a telescope, as was Pluto.

Mercury, Venus, Earth and Mars are called rocky, or terrestrial, planets. They are similar to Earth in composition. Heat from the Sun evaporated lightweight elements like hydrogen and helium into interplanetary space. Mostly rock and metal were left in this zone. Eventually, it clumped together to form the inner terrestrial planets. Mercury, Venus, Earth, and Mars are called the rocky, or terrestrial, planets. They are the planets closest to the Sun. Their composition is similar to Earth's composition, containing mostly the heavy materials of rock and metal.  Because they formed closer to the heat of Sun, the lighter gases and ices could not condense during their formation, as they did in the outer parts of the solar system.

Jupiter, Saturn, Uranus, and Neptune are called the gas giants. Jupiter and Saturn contain the largest percentages of hydrogen and helium, while Uranus and Neptune contain the largest shares of ices — frozen water, ammonia, methane, and carbon monoxide.

The four gas giants — Jupiter, Saturn, Uranus and Neptune — all have rings.

No. Mercury and the Earth are the only two planets that the Hubble Space Telescope has not observed for astronomical purposes. Mercury is too close to the Sun, which is too bright for Hubble to look at. The Earth's surface, only 380 miles from Hubble, is too close for Hubble to observe.

No. The Sun is too bright for the Hubble Space Telescope to observe. Its bright light can damage the telescope's sensitive detectors.

Pluto was called a planet from its discovery in 1930 until it was re-classified as a "dwarf planet" in 2006. The change in status stems from the fact that, since 1993, astronomers have discovered thousands of objects similar to Pluto in size and composition, in the region of Pluto’s orbit. This region is called the Kuiper Belt and the objects are called Kuiper Belt objects (KBOs). Pluto fits with the objects in the Kuiper Belt, and does not fit with either the rocky planets (Mercury, Venus, Earth, and Mars) or the gas giant planets (Jupiter, Saturn, Uranus, and Neptune). Factors that distinguish Pluto from the eight planets include its composition, atmosphere, small size, a comparatively large moon, and the shape of its orbit around the Sun.

Composition: Pluto is composed of ice, rock, and frozen gases, similar to the composition of the comet-like objects in the Kuiper Belt (the region beyond Neptune's orbit).

Atmosphere: Pluto's atmosphere grows as it moves closer to the Sun and recedes as it moves away. This is similar to the comet-like objects found in its vicinity. The frozen gases sublimate (turn from a solid to a gas) as Pluto moves closer to the Sun and then condense on the surface of Pluto as it moves away from the Sun. In contrast, the atmospheres of the planets do not appear or disappear during their orbits.

Small size: Pluto is relatively tiny, having a mass about 1/500th that of Earth (in contrast, Mercury's mass is 1/20th that of Earth). In diameter, Pluto is 1/5th the size of Earth. Although puny on a planetary scale, Pluto is one of the largest objects inhabiting the Kuiper belt.

Large moon: Pluto's largest moon, Charon, is a little more than one-half the radius of Pluto and one-eighth its mass. Pluto and Charon are in a "synchronous" orbit: they always show the same face to each other as they orbit. Charon's orbit is only 20,000 km away from Pluto. For comparison, Earth's Moon has about 1/80th the mass of Earth and orbits 400,000 km away. Many astronomers consider Pluto and Charon to be a binary Kuiper Belt object since they orbit a common center. Pluto’s two smaller moons, Nix and Hydra, are considerably smaller and more distant from Pluto than is Charon.

Orbit around the Sun: Pluto's orbit is more elliptical than that of any of the planets (it actually crosses Neptune's orbit). It is tilted at an inclination of 17.15 degrees relative to the plane of Earth's orbit. The planets all have much smaller inclinations, while the Kuiper Belt objects can have large inclinations like Pluto.

A dwarf planet is a celestial body within the solar system that shares the characteristics of planets. It orbits the Sun, is not a moon, and has a spherical or nearly spherical shape. Unlike a planet, however, a dwarf planet is not massive enough to clear away any loose cosmic rubble from its orbit. Dwarf planets include Ceres, Pluto, and Eris.