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The Solar System

The Solar System is the assembly formed by the Sun, eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune), their moons and other minor planets.

The solar system. Nine planets orbiting.The Sun and planets of the solar system orbiting. © iStockphoto/Baris Simsek

Mercury

The planet Mercury
The planet Mercury. © Image taken by the Messenger probe (NASA/JHU Applied Physics Laboratory/Carnegie Institution)

Mercury is named for the Greco-Roman messenger of the gods. He was very fast, and Mercury has the shortest and fastest orbit around the sun. Mercury orbits the Sun roughly every 88 Earth days. It rotates every 58.6 days, giving it one full day and a fraction of a rotation every Mercury year.

Mercury is small. Its mass and volume are about five percent that of Earth. Mercury becomes incredibly hot during its day, reaching temperatures up to 870°F. However, Mercury gets very cold on its night side, since there is no atmosphere to retain the warmth. At night, the surface gets as low as -300°F.

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Venus

The planet Venus
The planet Venus. © NASA/Ricardo Nunes

Venus is the second planet from our sun. Venus was named for the Roman goddess of love. Because it is so close to the Sun and has a shorter orbital time, it is always visible from Earth, usually around sunrise or sunset. Venus is sometimes called Earth’s twin. Its mass is about 80% that of Earth and it has about 85% of Earth’s volume, making it a little less dense than Earth. The surface gravity is similar to that of Earth. Venus orbits the Sun once every 224.7 Earth days and rotates every 243 days.

This means that a day on Venus is longer than its year. Unlike Earth, Venus does not have a moon. This may or may not be the reason it has such a thick atmosphere. While Earth’s atmosphere is mostly nitrogen and oxygen, Venus has an atmosphere mostly comprised of carbon dioxide, nitrogen and sulfur compounds. The Venus surface temperature can reach 870°F, as hot as that of Mercury, due to the greenhouse effect. While Venus has an atmosphere and lightening has been photographed by probes to that planet, it has no rain. Venus has almost no water vapor in its atmosphere and is too hot for liquid water.

Venus’ core may be completely solid. Venus has no magnetic field; these magnetic fields are caused by a rotating liquid iron core. Venus has had volcanic activity in the past. There is evidence of this in hundreds of volcanoes identified on its surface through radar mapping. Venus is also pock marked by meteor craters. Some of these craters have lava flows over their edges, showing that the volcanoes did erupt and spill lava after the crater’s formation. While Venus has very fast winds up to 220 miles per hour, it shows few signs of erosion.

Venus’ surface is an enigma. Based on Monte Carlo simulations of the number of asteroid strikes and estimated ages of the craters, scientists believe that the surface of Venus is about 300 to 500 million years old. Venus itself is as old as Earth, about 4.5 billion years old. This implies that Venus reformed its entire surface several hundred million years ago.

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Earth

The planet Earth
This spectacular “blue marble” image is the most detailed true-color image of the entire Earth to date. © NASA Goddard Space Flight Center Image by Reto Stöckli

Earth is the third planet from the sun. Earth is the only planet known to harbor life. About 70% of Earth is covered by water. Earth is the only planet known to currently have active plate tectonics. Earth has one moon, sometimes called Luna. Earth’s moon is the largest moon of an inner planet.

The distance of Earth from the sun called one astronomical unit or AU. The distance of other planets, whether around the Sun or other solar systems, are measured in AU.

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Mars

The planet Mars
The planet Mars. © NASA/USGS

Mars is the second smallest planet in our solar system since the demotion of Pluto from planet-hood. Mars is named for the Greco-Roman god of war. Mars has two moons, Phobos and Diemos. Both of these moons are thought to be captured asteroids from the asteroid belt between Jupiter and Mars. Mars does not have liquid water on its surface, but it has deeply etched canyon systems that show that it did once have liquid water.

The Valles Marineris canyon system is larger than most rivers on Earth. Olympus Mons on Mars is the largest volcano in the solar system. It is thought to be the remains of a large hot spot similar to the hot spot that created the Hawaiian Islands. However, Mars shows no signs of tectonic activity. This is probably why Olympus Mons is so large. The volcanic hot spot under it continued to erupt lava and create a large volcano, instead of tectonic activity shifting a continental plate and a new volcano forming.

Mars has a very thin atmosphere of mostly carbon dioxide. Even the ice caps are primarily made of frozen carbon dioxide. Mars has received a number of space craft in an ongoing search for life. Mars does not have a magnetic field, though it clearly did at one time. It is thought that the loss of the protective magnetic field, the Martian equivalent of the Magellan belts around Earth, caused Mars to lose its atmosphere. Without a magnetic field to deflect the solar winds, solar radiation was able to blow away the Martian atmosphere. It also ionized the water and ice on the Martian surface. The water molecules broke up. Oxygen remained and became chemically bound with the iron rocks, creating the red hue we associate with Mars today. The hydrogen was then lost to space. The diameter of Mars is about half that of Earth. However, it is much less dense than Earth, meaning that its gravity is only 40% that of Earth’s. Mars is considered the last of the inner planets.

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The Asteroid Belt

This artist's conception shows the closest known planetary system to our own, called Epsilon Eridani. It has two asteroid belts.This artist's conception shows the closest known planetary system to our own, called Epsilon Eridani. It has two asteroid belts. © NASA/JPL-Caltech

The objects in the asteroid belt between Mars and Jupiter have never been considered planets. They may have been debris that could have become a planet if not for Jupiter’s gravitational pull. The largest objects in the asteroid belt are Ceres, Pallas and Vesta. Ceres is large enough to be classified as a dwarf planet, since it is large enough for its gravity to cause it to collapse into a sphere.

Jupiter

The planet Jupiter (The Great Red Spot)
This true-color simulated view of Jupiter is composed of four images taken by NASA's Cassini spacecraft. Jupiter's moon Europa is casting the shadow on the planet. © NASA/JPL/University of Arizona

Jupiter is the largest planet in the solar system, and it was named for the king of the Roman gods. If you combined all of the other planets in the solar system together, Jupiter would still have 2½ times their mass. Jupiter is the closest gas giant to the sun.

Jupiter is called a gas giant, but may have a rocky or solid hydrogen core beneath the never ending storm clouds. The largest storm on Jupiter is the Great Red Spot. Probes sent to Jupiter have documented lightening in its thunderstorms.

Jupiter has dozens of moons, 67 at last count. As our telescopes have improved, the number of moons we’ve attributed to Jupiter has risen from the first four identified by Galileo.

Jupiter’s moon Io was the first body on which we discovered active volcanoes aside from those on Earth. The moon Europa looks like a giant snow ball, but there are signs of an ocean under the ice. The constant gravitational pull between Jupiter and the other Jovian moons creates heat that kept Europa from freezing through to the rocky core. Jupiter also has a faint ring.

Jupiter has recently gained credit as one reason why Earth may harbor complex life. Astronomers have documented many comet strikes on Jupiter, the first major impact recorded being Shoemaker-Levy 9 in 1994. Subsequent strikes in 2009 and 2010 suggest that Jupiter routinely collects interplanetary debris. Those objects not caught in orbit to become a moon crash into the planet. These objects then cannot threaten the inner planets. Without Jupiter, more asteroids would have struck the Earth during its development. Jupiter may have protected Earth from repeated asteroid strikes like the one 65 million years ago that wiped out the dinosaurs. If mega-extinctions had been more common, complex life like ours might not exist on Earth.

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Saturn

The planet Saturn
Voyager 2 spacecraft took this photograph of Saturn on July 21, 1981, when the spacecraft was 33.9 million kilometers (21 million miles) from the planet. Two bright, presumably convective cloud patterns are visible in the mid-northern hemisphere and several dark spoke-like features can be seen in the broad B-ring (left of planet). The moons Rhea and dione appear as blue dots to the south and southeast of Saturn, respectively. © NASA

Saturn is the next most massive planet in the solar system after Jupiter. Saturn’s days are just under eleven hours long. However, its year is twenty nine and a half Earth years long.

Saturn is perhaps most famous for its rings. Its rings are held in formation due to the orbits of two small moons. These two moons are said to shepherd the ring material. The gravitational tug of war between the two moons prevents it from coalescing into another moon. Saturn is mostly comprised of hydrogen with some helium. If you calculated the density of Saturn, you would find that it is less dense than water. Saturn’s largest moon is Titan. Titan is the only moon in the solar system with an atmosphere. Not only does Titan have an atmosphere, but it has liquid on its surface. The Cassini probe found lakes of liquid methane and ethane on Titan.

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Uranus

The planet Uranus
Uranus' moon Ariel (white dot) and its shadow (black dot) were caught crossing the face of Uranus in this Hubble Space Telescope image. © NASA/Space Telescope Science Institute

The planet Uranus is named for the father of the god Jupiter. Uranus orbits the sun every 84 Earth years. Uranus is mostly composed of hydrogen with a large percentage of helium. The 2% of its atmosphere that is methane gives it the blue color we see. Uranus rotates on its side relative to the rest of the solar system. The planet’s pole is not parallel to the poles of the Sun and other planets – Uranus’ pole actually points toward the Sun. This may have been due to another planet or planetoid hitting Uranus early in the solar system’s formation, but there is no longer evidence of this except for the planet’s unique angle.

The largest moons of Uranus are Titania and Oberon, though there are at least twenty other moons around Uranus. Uranus also has rings; these were discovered by the Voyager probes in the 1970s.

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Neptune

The planet Neptune
Picture of Neptune taken by Voyager 2 spacecraft. It shows the Great Dark Spot and its companion bright smudge; on the west limb the fast moving bright feature called Scooter and the little dark spot are visible. These clouds were seen to persist for as long as Voyager's cameras could resolve them. North of these, a bright cloud band similar to the south polar streak may be seen. © NASA

Neptune is the last of the official planets in our solar system and named for the ancient Greco-Roman god of the sea. Methane in Neptune’s atmosphere gives it its deep, dark blue color. Neptune has a blue Great Dark Spot similar to Jupiter’s Great Red Spot. Voyager recorded the fastest winds in the solar system on Neptune. Neptune’s winds near the Great Dark Spot approach 1,200 miles per hour. Neptune’s core is probably a mixture of rock, water, methane and other chemicals.

Neptune orbits the sun every 165 Earth years. Each of its days is about 16 Earth hours long.

To date, thirteen moons have been found around Neptune, though smaller ones may yet remain undiscovered. Triton is the largest moon in orbit around Neptune. Moons like Psamathe are in a distant orbit from Neptune, suggesting that they are captured comets instead of rocky bodies that formed when Neptune did.

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The Kuiper Belt

The Kuiper Belt refers to all objects past Neptune’s orbit. The Kuiper Belt is a belt similar to the asteroid belt between Jupiter and Mars but at a distance of thirty to fifty times the distance of Earth to the Sun. While the asteroid belt is mostly comprised of rock, Kuiper Belt objects are generally made of ice. The Kuiper Belt is thought to be the source of most short-period comets. The Plutoids are the largest of the Kuiper objects, also called trans-Neptunian objects.

An illustration of the Kuiper Belt and Oort Cloud in relation to our solar system.An illustration of the Kuiper Belt and Oort Cloud in relation to our solar system. © NASA

The Oort Cloud

The Oort cloud is a sphere of debris though to encircle the Sun. The Oort cloud is thought to be the source of all comets. The Oort cloud may be debris left over from the formation of the solar system, material that wasn’t pulled in and absorbed into the growing planets. It is thought that the passage of nearby stars may pull comets from the Oort cloud in toward the inner planets. Objects in the Oort cloud often follow orbits at radical angles from the primary plane of the solar system. The Oort cloud is probably the source of all long period comets.

Pluto and the Plutoids

The dwarf planet Pluto
3D model of the dwarf planet Pluto. © NASA

Pluto was predicted before it was first spotted based on its gravitational effect on Neptune. It was quickly classified as the ninth planet. In the early 2000s, the discovery of a number of dwarf-planets almost as close to the Sun as Pluto caused a major rift in the field of astronomy.

If Pluto was a planet, then dwarf planets such as the newly discovered Eris, Sedna, Quaoar, Makemake and Haumea would also have to be considered planets. For example, Makemake, discovered in 2005, is two thirds the size of Pluto. Eris, discovered in 2005, is actually larger than Pluto.

These dwarf planets are called Plutoids after the former-planet Pluto. However, since Greek and Roman mythology has been thoroughly mined for celestial names, plutoids like Makemake were given names from Hawaiian mythology while Sedna is an Inuit goddess. Many of these dwarf planets have moons of their own. Charon orbits Pluto. Eris’ moon is called Dysnomia.

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