Learn about the Solar System

The words “solar system” refer to the Sun or a star and all of the objects that travel around it. These objects include planets, natural satellites such as the Moon, the asteroid belt, comets, and meteoroids. Our solar system has an elliptical shape and is part of a galaxy known as the Milky Way. The Sun is the center of the solar system. It contains 99.8% of all of the mass in our solar system. Consequently, it exerts a tremendous gravitational pull on planets, satellites, asteroids, comets, and meteoroids.

The Inner Solar System

The Sun

The diameter of our closest star, the Sun, is 1,392,000 kilometers. The Sun is a medium size star known as a yellow dwarf. It is a star in the Milky Way galaxy and the temperature in its core is estimated to be over 15,000,000 degrees Celsius. In the Sun’s core, hydrogen is being fused to form helium by a nuclear fusion process. The energy created by this process radiates up to the visible boundary of the Sun and then off into space. It radiates into space in the form of heat and light. Because the Sun is so massive, it exerts a powerful gravitational pull on everything in our solar system. It is because of the Sun’s gravitational pull that Earth orbits the Sun in the manner that it does. More information about the Sun and solar features can be found here.


A planet is a large space body which reflects the light of a star around which it revolves. The planets in our solar system are classified as inner planets and outer planets. The inner planets, the closest to the Sun, are solid spheres of rock and include Mercury, Venus, Earth, and Mars. You will find craters of varying sizes on the inner planets and their satellites. The outer planets, with the exception of Pluto, are large gaseous spheres with rings and include Jupiter, Saturn, Uranus, and Neptune. Between the inner and outer planets is an asteroid belt. Every planet, except for Earth, was named for an ancient Roman god or goddess. Some of the planets have naturally occurring satellites, or moons, while others do not. All nine planets orbit the Sun in their own unique way.


Mercury is only about one-third the size of the Earth. It is smaller than any other planet except Pluto. Mercury is very close to the Sun and has no substantial atmosphere. These factors contribute to the fact that the surface of Mercury has the greatest temperature range of any planet or natural satellite in our solar system. The surface temperature on the side of Mercury closest to the Sun reaches 427 degrees Celsius, a temperature hot enough to melt tin. On the side facing away from the Sun, or the night side, the temperature drops to -183 degrees Celsius. Scientists have detected a magnetic field surrounding Mercury, though it is not as strong as the field around the Earth. Scientists theorize that Mercury’s field is due to an iron-bearing core or possibly to the solar winds. Mercury’s atmosphere is very thin and is composed of helium and sodium. The surface of Mercury has been shaped by three processes: impact cratering where large objects struck the surface resulting in crater formation, volcanism where lava flooded the surface, and tectonic activity where the planet’s crust moved in order to adjust to the planetary cooling and contracting. Mercury does not have any naturally occurring satellites. Because Mercury is closer to the Sun than Earth, we sometimes see rare solar transits when the planet passes directly in front of the Sun from our vantage point.


Venus and Earth are similar in size, composition, and mass. They differ in that Venus does not have oceans or human life, and its temperature during the day reaches 484 degrees Celsius. The daytime temperature is so hot it could melt lead. The dense atmosphere is composed of carbon dioxide and sulfuric acid which acts as a greenhouse and traps the heat. Venus revolves around the Sun in a circular orbit once every 225 Earth days. Venus rotates slowly on its axis in a clockwise direction, which is referred to as a “retrograde” rotation because it is the opposite of the other eight planets. A rotation takes 243 Earth days, so a Venusian day is longer than a Venusian year. As with the other inner planets, the surface of Venus has been shaped by impact craters, tectonic activity, and volcanoes which scientists believe to be ongoing. The volcanic activity is believed to be the source of the sulfur found in the atmosphere. Venus does not have any naturally occurring satellites.


Earth’s amazing gaseous atmosphere is responsible for making life possible on this, the third planet from the Sun. Our atmosphere contains water vapour which helps to moderate our daily temperatures. Our atmosphere contains 21% oxygen, which is necessary for us to breathe, 78% nitrogen, and .9% argon. The other 0.1% consists of water vapor, carbon dioxide, neon, methane, krypton, helium, xenon, hydrogen, nitrous oxide, carbon monoxide, nitrogen dioxide, sulfur dioxide, and ozone. These latter elements are important to have because they help to absorb harmful solar radiation before it can reach the surface of the Earth. If present in larger amounts, most of these latter elements would be poisonous to humans. The atmosphere protects us from meteors as well. Due to the friction generated between a meteor and the atmospheric gases, most meteors burn up before hitting Earth’s surface as a meteorite.

Earth rotates on an imaginary axis which is tilted at a 23.5 degree angle. The rotation is what causes the change from day to night. The tilt is what determines our change in seasons. If the Earth was not tilted, we would have the same season all year long. Earth has a core of molten iron-nickel. The rapid spin of the Earth along with the liquid, hot metallic core causes a magnetic field to surround the Earth. This magnetic field traps the charged particles which are hurled at the Earth by the Sun during solar wind activity. When these charged particles react with the gases in our atmosphere, the gases begin to glow. These aurorae, or glowing gases, are mostly seen in the Arctic Circle and the Antarctic Circle but can travel as far south as 50 degrees Latitude. As with all inner planets, the Earth’s surface has been affected by volcanism, tectonic activity, and to a lesser degree, meteorite impacts. Earth has one naturally occurring satellite, the Moon.

The Moon

The Moon travels around Earth in an oval orbit at 3680 kilometers per hour. The Moon does not have an atmosphere, so temperatures range from -184 degrees Celsius during its night to 214 degrees Celsius during its day except at the poles where the temperature is a constant -96 degrees Celsius. The Moon is actually a little lopsided due to the lunar crust being thicker on one side than the other. When you look at the Moon, you will see dark and light areas. The dark areas are young plains called maria and are composed of basalt. The light areas are the highlands. The lunar surface is covered by a fine-grained soil called “regolith” which results from the constant bombardment of the lunar rocks by small meteorites. The gravitational pull of the Moon on the Earth affects the ocean tides on Earth. The closer the Moon is to Earth, the greater the effect. The time between high tides is about 12 hours and 25 minutes.

The phases, or changing appearance, of the Moon depend on its position relative to the position of the Sun. When the Moon is between the Sun and the Earth, the side of the Moon facing the Earth is dark. This is called a “new moon”. As the Moon travels eastward in its orbit, more of its sunlit side becomes visible to Earth and the Moon is said to be “waxing”. More specifically, the phase after a new moon is called a “waxing crescent” because we can see no more than a quarter of the Moon at this point. As the Moon continues eastward, the Sun, Moon, and Earth form a 90 degree angle and the Moon appears half dark and half light to us here on Earth. This is a “first quarter” phase. After the first quarter phase, more than a quarter of the Moon is visible to us, so it is now in a “waxing gibbous” phase. As the Moon continues its revolution around Earth, the Sun, Earth, and Moon align with the Earth in the middle. The side of the Moon facing Earth is now fully lit. This is called a “full moon” phase. As the Moon travels further around in its orbit, the lit portion of the Moon visible to Earth becomes smaller, so the Moon is now said to be “waning” as it enters the next phase. After the “waning gibbous” phase, the Moon enters the “third quarter” phase where once again the Moon appears half dark and half light from Earth. As it completes its revolution around Earth, the Moon enters a “waning crescent” phase just prior to starting the cycle again as a new moon.


The orbit of Mars around the Sun is extremely elliptical. Because the distance between the Sun and Mars varies, temperatures range from -125 degrees Celsius in the Martian winter to 22 degrees Celsius in the Martian summer. The Martian atmosphere is composed of over 95% carbon dioxide. Solar winds carry the thin, weak atmosphere away because Mars has a weak gravitational and magnetic field. At the Martian poles are polar ice caps which shrink in size during the Martian spring and summer. From data gathered by the Viking 1 and 2 probes, we know that the Martian surface is covered by various rocks and a soil which is rich in an iron-laden clay. The presence of iron explains the planet’s reddish-orange appearance. Mars contains highlands which occur in the southern hemisphere and are composed of the most heavily cratered crustal material. Mars also contains lowlands which are found in the northern hemisphere. The extremely weak magnetic field of Mars suggests that its iron core is no longer fluid and circulating.

The surface of Mars has not only been affected by meteorite impacts, but also by volcanic and tectonic activity. In fact, Mars has some of the largest volcanoes in the solar system; Olympus Mons is over 600 kilometers wide and 26 kilometers high! Tectonic activity is in evidence at the tremendous Valles Marineris canyon system, which is over 8 kilometers deep and 4500 kilometers long. Mars has two small natural satellites, Phobos and Deimos.

Asteriods and the Asteriod Belt

An asteroid is a rocky body in space which may be only a few hundred feet wide or it may be several hundred miles wide. Many asteroids orbit the Sun in a region between Mars and Jupiter. This “belt” of asteroids follows a slightly elliptical path as it orbits the Sun in the same direction as the planets. It takes anywhere from three to six Earth years for a complete revolution around the Sun. The largest asteriod found in the asteriod belt is called Ceres and is approximately the size of the US State of Texas. The gas gaint planet, Jupiter, protects the inner solar system planets from constant bombardment by these asteriods by exerting its gravitational force on the asteroids in the belt. The presence of Jupiter actually protects Mercury, Venus, Earth, and Mars from repeated asteroid collisions!

The Outer Solar System


Jupiter is a large gas planet whose rapid rotation causes the planet to flatten at the poles and bulge at the equator. Jupiter emits twice as much heat as it absorbs from the Sun, which indicates it has its own internal heat source. Astronomers estimate the core temperature at 20,000 degrees Celsius, approximately three times greater than the temperature of the Earth’s core. The planet’s powerful magnetic field is thought to be generated by the electric currents produced by pressurized hydrogen in the mantle. Jupiter’s atmosphere is thought to be composed of hydrogen, helium, sulfur, and nitrogen. Clouds in the atmosphere move in alternating bands from east to west or west to east. Lightning, more powerful than any that has been experienced on Earth, has been noted in Jupiter’s atmosphere. Also in Jupiter’s atmosphere are oval features which are thought to be circular winds. The most prominent of these is the Great Red Spot, a hurricane-like storm that has been seen in Jupiter’s southern hemisphere since Jupiter was first discovered. Jupiter has at least sixteen natural satellites (and may have over 28!) . One of these satellites, Io, is volcanically active. Instruments aboard the space probe Galileo have detected surface temperatures on Io higher than any other planetary body in our Solar System. Voyager 2, also a space probe, has confirmed that Jupiter is surrounded by a system of thin rings. The majority of the rings are made up of very small particles thought to be debris from meteoroid collisions.


Saturn is a large gas planet with an atmosphere composed of hydrogen and helium. Saturn’s rapid spin tends to flatten out the poles while causing a bulge at its equator. The winds in Saturn’s atmosphere reach speeds up to 1800 kilometers per hour! Astronomers see large white spots (or clouds) on Saturn which they believe are storms. Just like Jupiter, Saturn emits twice as much heat as it absorbs from the Sun indicating it also has an internal heat source. Saturn has an extensive ring system which is formed by a thousand individual rings. The rings contain water ice and dust. The thickness of the rings ranges from 10 to 100 meters and the rings vary in brightness. There are gaps between some rings, while other rings appear to be braided together. The particles in the rings closer to the planet, orbit at a faster speed than the particles in the rings farther from the planet. There are satellites within the rings which result in the gaps that are present between some rings. As with Jupiter, the pressurized hydrogen in Saturn’s mantle produces electric currents which create a strong magnetic field around the planet. Saturn has at least 30 naturally occurring satellites.


Uranus is unique in our solar system because it is tilted 98 degrees. When viewed from Earth, it appears to rotate on its side! At different times throughout its orbit, we can actually view one of the planet’s poles head-on. The atmosphere is composed of hydrogen, helium, and methane. The temperature in the upper atmosphere is so cold that the methane condenses and forms a thin cloud layer which gives the planet its blue-green appearance. The winds on Uranus blow mainly to the east and can reach speeds up to 600 kilometers per hour. The rapid spin of Uranus influences the winds in the atmosphere. Uranus has a very strong magnetic field. This planet has a system of rings which was not discovered until 1977. The ring system contains eleven dark rings composed of varying sized particles. Satellites embedded in the rings create gaps between the rings. Uranus has 21 known natural satellites (and may have at least 27), both within the rings and outside of the rings


Voyager 2, a space probe, passed within 4900 kilometers of Neptune in 1989. From the data collected, we know that Uranus and Neptune are very similar in composition. Neptune has a mantle of liquid hydrogen while the atmosphere is a combination of ammonia, helium, and methane. In the upper atmosphere, methane freezes and forms an ice cloud which casts a shadow on the clouds below. Neptune has bands in its atmosphere where wind speeds may reach 2000 kilometers per hour! Neptune has large, dark ovals on its surface which astronomers believe are hurricane-like storms. Neptune generates more heat than it absorbs from the Sun, indicating it has its own internal heat source. Neptune has a very strong magnetic field. It also has a ring system consisting of four rings; two thin and two thick. The rings are composed of dark particles which vary in size. Neptune has at least eight natural satellites, four of which orbit within the rings. The largest satellite is Triton. Triton has a retrograde orbit abd is thought to be a combination of rock and ice. Its surface temperature is -245 degrees Celsius, and it has a thin atmosphere of nitrogen and methane.

Trans-Neptunian Objects

TNOs are small bodies orbiting the sun beyond Neptune. There are around 350 known “trans-Neptunians” with diameters larger than 100 km extending outwards from the orbit of Neptune (at 30 AU) to 50 AU. Current astronomical trends classify these objects incorrectly as Kuiper Belt objects though the Kuiper Belt, described by cosmologists as “a hypothetical massive flattened disc of billions of icy planetesimals beyond the orbit of Neptune”, has never been observed or detected.


Pluto is tilted 122.5 degrees on its axis. It has an extreme elliptical orbit. Because of the shape of Pluto’s orbit, it actually slips inside of Neptune’s orbit once every 248 Earth years for a period of twenty years. Pluto has one natural satellite, Charon, which is half the size of Pluto. Because Pluto and Charon are comparable in size, many scientists consider them to be a double planet (but many scientists don’t consider Pluto a planet at all!). Studies conducted using a spectroscope have detected methane frost on Pluto and water frost on Charon. Like Triton, Neptune’s satellite, Pluto has an atmosphere of nitrogen and methane. Pluto’s atmosphere appears to extend out to include Charon, which suggests that they may share an atmosphere. Through the Hubble Space Telescope, Charon appears to be more blue in color than Pluto. During the time in its orbit when Pluto is farthest from the Sun, its atmosphere condenses and falls to the surface as frost.


A meteoroid is a piece of stony or metallic debris which travels in outer space. Meteoroids travel around the Sun in a variety of orbits and at various speeds. The fastest meteoroids move at about 42 kilometers per second. Most meteoroids are about the size of a pebble. When one of these pieces of debris enters the Earth’s atmosphere, friction between the debris and atmospheric gases heats it to the point that it glows and becomes visible to our eyes. This streak of light in the sky is known as a meteor. Most meteors glow for only a few seconds prior to burning up before hitting the Earth’s surface. On most dark nights, meteors can be seen. The chance of seeing a meteor with the unaided eye increases after midnight. People often refer to meteors as “falling” or “shooting” stars. The brightest of the meteors are called fireballs. Sonic booms often follow the appearance of a fireball just as thunder often follows lightning. At certain times of the year, more meteors than normal can be seen. When the Earth passes through an orbiting stream of debris from a comet that has broken up, what’s known as a meteor shower occurs. Meteor showers take place on about the same dates each year.

If the meteor does not burn up completely, the remaining portion hits the Earth and is then called a meteorite. Over 100 meteorites hit the Earth each year. Fortunately, most of them are very small. There has only been one report of a “HBM” (hit by meteorite), and that occurred in 1954. Ann Hodges of Sylacauga, Alabama was slightly injured when a 19.84 kilogram meteorite crashed through the roof of her home. The larger meteorites are believed to have originated in the asteroid belt. Some of the smaller meteorites have been identified as moon rock, while still others have been identified as pieces of Mars. Large meteorites that crashed onto the Earth long ago made craters like those found on the Moon. The Barringer Meteorite Crater near Winslow, Arizona is believed to have been formed about 49,000 years ago by the impact of a 300,000 ton meteorite. The Hoba iron meteorite is the largest single meteorite known. Its present weight is estimated at 66 tons. Part of the Hoba meteorite has rusted away, therefore it’s original weight may have been as much as 100 tons! It has never been removed from its landing sight in Namibia. The largest single meteorite found in the United States is the fifteen ton Willamette (Oregon) iron meteorite found in 1902.


A comet has a distinct center called a nucleus. Most astronomers think the nucleus is made of frozen water and gases mixed with dust and rocky material. Comet nuclei are described as dirty snowballs. A hazy cloud called a coma surrounds the nucleus. The coma and the nucleus together form the comet’s head.

Comets follow a regular orbit around the Sun. If the comet nucleus is pulled into an orbit which carries it close to the Sun, the solar heat will cause the outer layers of the icy nucleus to evaporate. During this process, dust and gases which form the coma around the nucleus are released. As the comet gets closer to the Sun, the coma grows. The solar winds push the dust and gas away from the coma causing them to stream off into space to form the comet’s tail. The solar winds cause the comet’s tail to point away from the Sun. The tails of comets can reach 150 million kilometers in length! Each time the comet passes close to the Sun, it loses some of its material. Over time, it will break up and disappear completely.

Many comets enter an elliptical orbit and repeatedly return to the inner solar system where they can be viewed from Earth at specific times. Short period comets, of which Halley’s Comet is the most famous, reappear within a 200 year time frame. Halley’s makes an appearance once every 76 years. The comet was named after Sir Edmond Halley.

A comet has no light of its own. We are able to see a comet because of the reflection of the Sun’s light off of the comet and because of the gas molecules in the coma releasing energy absorbed from the Sun’s rays.