The Solar System

The Solar System consists of:

1 star, the Sun
9 planets
65 moons (number known as of June 1998)
asteroids (thousands known)
comets (less than 1,000 recorded, possibly more than a trillion trapped beyond Pluto)
dust

Photo of the Sun The Sun is the largest body in the Solar System, with more than 1,000 times the mass of everything else added together. It's the Sun's huge gravity that keeps all of the nine planets, their moons, the asteroids, the comets, and the dust between the planets all orbiting the Sun. It would take more than 100 Earths placed side-by-side to go from one edge of the Sun to the other. The Sun is a star, and shines because it generates light and heat by nuclear reactions in its core.

The Solar System is so big that it's easiest to express distances using a special measuring stick called the astronomical unit, instead of using kilometers or miles. The Earth's average distance from the Sun is equal to 1 astronomical unit or AU. 1AU = 93 million miles, or about 150 million kilometers. The farthest planet, Pluto, lies almost 40 AU from the Sun. Comets are stored in even more distant regions of space, called the Kuiper belt and the Oort cloud regions, which extend to 1,000 AU and perhaps 30,000 AU respectively from the Sun.

The planets are much smaller than the Sun and shine by reflecting the Sun's light. In order of increasing average distance from the Sun, the planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Pluto's orbit, however, is eccentric causing it to overlap with Neptune's orbit, and for several years out of its 248-year period it is closer to the Sun than Neptune. All of the planets travel in the same direction around the Sun in approximately circular paths. As the planets orbit, their paths are nearly in the same plane as the Earth's orbital path, which is called the ecliptic plane.

The planets fall into two general composition types: terrestrial and gas. The terrestrial planets are Mercury, Venus, Earth, and Mars. Generally speaking they are small, rocky bodies (lots or materials made of the elements silicon, oxygen, aluminum, magnesium, sulfur and iron) with thin atmospheres. The gas planets Jupiter, Saturn, Uranus, and Neptune are large, gas and liquid bodies with thick deep hydrogen-rich atmospheres and probably no real "surface". The outer planet Pluto is a small icy body.

Here are a few facts about each planet:

Mercury is the smallest terrestrial-type planet. It orbits the Sun in 88 Earth days moving faster than any other planet. The day on Mercury is 59 Earth days long! On Mercury you would experience the most extremes in temperature; a sizzling midday temperature near 27 C (800 F), is quite a contrast with a nippy - 173 C (- 280 F) nighttime temperature. Mercury's surface is covered with craters and looks similar to the Earth's moon. Photo of Mercury

Venus is most like the Earth in size. It has thick clouds that cover the entire planet and astronomers can't see its surface unless they use radar mapping. This picture of Venus is what you would see if you had "radar eyes". The pressure of Venus' atmosphere at the surface is 90 atmospheres (about the same as the pressure at a depth of 1,000 m (3,280 feet) in Earth's oceans). This dense atmosphere causes a run-away greenhouse effect on Venus. The daytime temperature is a roasting 482 C (900 F), hot enough to melt lead. Photo of Venus

Earth is the only planet with liquid water on its surface, and an atmosphere of mostly nitrogen and oxygen. There is a small amount of carbon dioxide in the atmosphere which helps maintain the temperature of the Earth through the greenhouse effect. There are many diverse climates and ecosystems. It is the only planet in the solar system with intelligent life. Photo of Earth

Mars is about 1/2 the diameter of the Earth, has nearly a 24 hour day, and a year equal to 2 Earth years. Mars is a reddish colored planet with white polar caps made mostly of dry ice (carbon dioxide). On a warm day, the temperature at the equator could be a balmy 10 C (50 F), but nighttime temperatures near - 129 C (- 200 F) are found at the poles. Its varied landscape includes craters, canyons, mountains and plains. The largest mountain in the solar system, Olympus Mons, is on Mars. Compared to the northeastern United States, the base of this mountain would reach from Boston to Washington, D.C.; its peak would rise 255 km (78,000 ft.). Mars has a very thin atmosphere which is mostly carbon dioxide. Photo of Mars

Jupiter is the largest planet in the Solar system. It would take about 12 Earths side by side to go from one edge to the other. It has 28 known satellites. The picture of Jupiter shows its different colored cloud belts and its red spot, a cyclone-like area. Complex molecules are probably responsible for the different cloud colors. Jupiter doesn't have a surface that an astronaut could land on. Instead it is made mostly of hydrogen and helium (like the Sun) with traces of methane and ammonia. Photo of Jupiter

Saturn has bright spectacular rings made primarily of water ice and rocky material. The planet is mostly hydrogen and helium like the Sun with traces of water, methane, ammonia, and "rock", like Jupiter. It is also a gas planet with no accessible surface. Amazing as it seems, it is less dense than ice and would float in a tub of water. It has eighteen named satellites, more than any other planet. Photo of Saturn

Uranus looks blue because methane gas in its atmosphere absorbs most of the red light from the Sun. But, the bulk of its atmosphere is still hydrogen and helium. It is about four times as large as the Earth and is tilted so far that its poles are almost in the same plane as its orbital path. If the Earth was tilted as much as Uranus, the north pole could point toward the Sun and north would be in the direction of the Sun! Photo of Uranus

Neptune is similar to Uranus; it is nearly the same size and has the same gases in its atmosphere. Neptune's atmosphere, however, has features such as bright clouds and a dark spot which can be seen in this photograph. It is possible that there are various ices and rock somewhere under the blue layers on both Neptune and Uranus. An astronaut attempting to land on the ices would be blasted by winds of 2,200 kilometers/hour (about 1,300 miles/hour). Neptune is the last of the large gas planets. Photo of Neptune

Pluto is the smallest planet (smaller than our Moon), and usually the most distant planet from the Sun. Astronomers estimate its surface temperature to be an extra nippy - 233 C (- 387 F). Pluto is made of ice and rock material which makes it very different from the large gas planets. Like Uranus, it is tilted so its poles are near the plane of its orbital path. This photograph of Pluto includes its large moon Charon (about half the size of Pluto). Astronomers find features of water ice on Charon's surface, whereas Pluto has nitrogen ice mixed with some methane and carbon monoxide. Photo of Pluto

Here is a table with additional information on each planet.

- Marla H. Moore (Code 691)

Laboratory Branches participating in this area of research:
Code 691, 692, 693, 694, 695, and 696

This page last updated April 21, 2003