types of planets
terrestial planets
There more than one type of planet out there,
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There are two basic types of planets, Earth-like (“terrestrial”) planets and Jupiter-like (“Jovian”) planets. The terrestrial planets—Mercury, Venus, Earth and Mars—are small, dense, rocky worlds. A terrestrial planet, telluric planet, or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun, i.e. Mercury, Venus, Earth, and Mars. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth (Terra and Tellus), as these planets are, in terms of composition, "Earth-like".
Terrestrial planets have a solid planetary surface, making them substantially different from the larger giant planets, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states. |
mercury
Mercury is the smallest and innermost planet in the Solar System. Its orbital period (about 88 Earth days) is less than any other planet in the Solar System. Seen from Earth, it appears to move around its orbit in about 116 days. It has no known natural satellites. It is named after the Roman deity Mercury, the messenger to the gods.
It has a very tenuous atmosphere, which is only a little more substantial than a vacuum. Sunlight heats up the surface of the planet to high temperatures during the day, up to 450oC (840oF). At night, the surface cools off rapidly, and the temperatures can drop down to -180oC (-300oF). This daily temperature variation is the largest of all of the planets. However, Mercury’s day is much longer than Earth’s. Due to Mercury’s closeness to the Sun and its slow rotation, the length of one day on Mercury is equal to 176 Earth days; that is, the time from one sunrise to another on the surface of Mercury is 176 Earth days (while on Earth, this is equal to one day, or 24 hours.) |
The Romans believed that gods and goddesses were in charge of everything on Earth. Mercury is named after the messenger for their gods. The Roman Mercury had wings on his helmet and shoes. He could travel very quickly from place to place. The planet Mercury moves quickly around the sun. That is how it got its name.
Mercury is a little bigger than Earth's moon. It is made of heavier materials, like iron. But if you could weigh Mercury and the moon, Mercury would weigh a lot more. Mercury is heavy, but it is small. It would take more than 18 Mercurys to be as big as Earth.
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The surface of Mercury looks like Earth's moon. It is covered with holes. The holes are called impact craters. The craters were made by rocks falling from space. The rocks are going very fast when they hit Mercury. A hole is made where the rock hits. Earth has a blanket of air around it. Mercury does not. The blanket is what helps keep Earth from getting too hot or cold. Because it is so close to the sun, Mercury can be very hot. At night, Mercury gets very cold. We could not live on Mercury!
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venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. It has the longest rotation period (243 days) of any planet in the Solar System and rotates in the opposite direction to most other planets. It has no natural satellite. It is named after the Roman goddess of love and beauty.
Venus is a terrestrial planet and is sometimes called Earth's "sister planet" because of their similar size, mass, proximity to the Sun, and bulk composition. It is radically different from Earth in other respects. It has the densest atmosphere of the four terrestrial planets, consisting of more than 96% carbon dioxide. |
The atmospheric pressure at the planet's surface is 92 times that of Earth. The thick carbon dioxide atmosphere trap heat from the Sun during the day and does not let the surface cool at night; as a result, temperatures on the Venusian surface are over 464oC (867o F).
The high temperature and un-breathable thick atmosphere would make the planet very inhospitable to human visitors. None of the robotic spacecraft (called Venera) sent to land on the planet’s surface by the Soviet Union in the 1970s and 1980s were known to last more than a little over two hours under the harsh Venusian conditions.
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During its evolution, ultraviolet rays from the sun evaporated water quickly, keeping it in a prolonged molten state. There is no liquid water on its surface today because the scorching heat created by its ozone-filled atmosphere would cause any to boil away. Roughly two-thirds of the Venusian surface is covered by flat, smooth plains that are marred by thousands of volcanoes, some which are still active today, ranging from about 0.5 to 150 miles (0.8 to 240 kilometers) wide, with lava flows carving long, winding canals up to more than 3,000 miles (5,000 km) in length, longer than on any other planet.
Six mountainous regions make up about one-third of the Venusian surface. One mountain range, called Maxwell, is about 540 miles (870 km) long and reaches up to some 7 miles (11.3 km) high, making it the highest feature on the planet |
Venus was the first planet to have its motions plotted across the sky, as early as the second millennium BC, and was a prime target for early interplanetary exploration as the closest planet to Earth. It was the first planet beyond Earth visited by a spacecraft in 1962, and the first to be successfully landed on in 1970. Venus's thick clouds render observation of its surface impossible in visible light, and the first detailed maps did not emerge until the arrival of the Magellan orbiter in 1991. Plans have been proposed for rovers or more complex missions, but they are hindered by Venus's hostile surface conditions.
mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury. Mars is about half the size of Earth in diameter. This makes the surface of Mars equal in area to all the land area on Earth. Named after the Roman god of war, it is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance.
Mars has a carbon dioxide atmosphere, but it is extremely thin, only about one percent as thick as Earth’s atmosphere. The thin air does not retain heat well, and surface temperatures range from a frigid -130oC (-200oF) on a cold winter night to 27oC (80oF) at the equator on a hot summer day |
Mars has polar ice caps, made of water ice and carbon dioxide ice. There may be ice under the surface of Mars at lower latitudes, as well. The Martian surface has features that look like dry streambeds, leading many researchers to surmise that at some time in the distant past, Mars may have had liquid water owing on its surface.
The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature. Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Mars trojan.
NASA has used both spacecraft and robots to learn more about Mars. In 1965, Mariner 4 was the first NASA spacecraft to get a close look at the planet. In 1976, Viking 1 and Viking 2 were the first NASA spacecraft to land on Mars. They took pictures and explored the planet’s surface. Since then, more spacecraft have flown near or landed on Mars.>
NASA's Spirit and Opportunity rovers landed on Mars in January 2004. They found evidence that water once flowed on Mars. Living things need water to survive. So, any sign of water on Mars would mean that there could be, or could have been, life on the planet. |
Today, three spacecraft are circling above, or orbiting, Mars. The spacecraft are using scientific tools to measure the volcanoes, canyons, craters, temperature and the kinds of minerals on Mars. They also are taking pictures and searching for water.
Two robots that move, called rovers, are exploring Mars’ surface. Their names are Opportunity and Curiosity. The rovers travel around taking pictures and looking closely at the planet’s soil and rocks. NASA uses pictures and information from the spacecraft and the rovers to learn more about Mars. |
Until the first successful Mars flyby in 1965 by Mariner 4, many speculated about the presence of liquid water on the planet's surface. This was based on observed periodic variations in light and dark patches, particularly in the polar latitudes, which appeared to be seas and continents; long, dark striations were interpreted by some as irrigation channels for liquid water. These straight line features were later explained as optical illusions, though geological evidence gathered by unscrewed missions suggests that Mars once had large-scale water coverage on its surface at an earlier stage of its existence.
In 2005, radar data revealed the presence of large quantities of water ice at the poles and at mid-latitudes. The Mars rover Spirit sampled chemical compounds containing water molecules in March 2007. The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008. On September 28, 2015, NASA announced the presence of briny flowing salt water on the Martian surface.
In 2005, radar data revealed the presence of large quantities of water ice at the poles and at mid-latitudes. The Mars rover Spirit sampled chemical compounds containing water molecules in March 2007. The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008. On September 28, 2015, NASA announced the presence of briny flowing salt water on the Martian surface.
jovian planets
So what is the second type of planet?
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The Jovian planets—Jupiter, Saturn, Uranus and Neptune—are large planets located in the outer part of the planetary realm of the Solar System. The Jovian planets are gas giants—large objects made mostly of hydrogen and helium.
Beyond our Solar System’s “Frost Line” – the region where volatiles like water, ammonia and methane begin to freeze – four massive planets reside. Though these planets – Jupiter, Saturn, Uranus and Neptune – vary in terms of size, mass, and composition, they all share certain characteristics that cause them to differ greatly from the terrestrial planets located in the inner Solar System |
They are much larger than terrestrial planets; for example, eleven Earths could fit across Jupiter’s equator. They are rapidly rotating objects: they rotate once around their axis in less than a day while terrestrial planets take anywhere from 24 hours (1 day) to months to rotate once. They all have rings, and extensive families of moons. They have no solid surface on which to stand, and the apparent visible surfaces are just the top layers of clouds in their atmospheres. Deeper in their atmospheres, the gases get thicker and thicker, until nally they turn into a liquid. At their centers, they may have a solid, rocky core a few times the size of Earth.
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The term Jovian is derived from Jupiter, the largest of the Outer Planets and the first to be observed using a telescope – by Galileo Galilei in 1610. Taking its name from the Roman king of the gods – Jupiter, or Jove – the adjective Jovian has come to mean anything associated with Jupiter; and by extension, a Jupiter-like planet.
jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a giant planet with a mass one-thousandth that of the Sun, but two and a half times that of all the other planets in the Solar System combined. Jupiter is a gas giant, along with Saturn, with the other two giant planets, Uranus and Neptune, being ice giants. Jupiter was known to astronomers of ancient times.
Jupiter is the largest planet in the solar system. It is approximately 143,000 kilometers (about 89,000 miles) wide at its equator. Jupiter is so large that all of the other planets in the solar system could fit inside it. More than 1,300 Earths would fit inside Jupiter. Jupiter is like a star in composition. If Jupiter had been about 80 times more massive, it would have become a star rather than a planet. |
When viewed from Earth, Jupiter is usually the second brightest planet in the night sky, after Venus. The planet is named after Jupiter, the king of the Roman gods in mythology. When viewed from Earth, Jupiter can reach an apparent magnitude of −2.94, bright enough for its reflected light to cast shadows, and making it on average the third-brightest object in the night sky after the Moon and Venus.
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Jupiter has 53 named moons. Scientists have discovered 14 more. But those 14 moons do not have official names. Scientists now think Jupiter has a total of 67 moons.
The most recent moons were discovered in 2011. The planet’s four largest moons are Ganymede (GAN-i-meed), Callisto (kuh-LIS-toe), Io (eye-OH), and Europa (yur-O-puh). These four moons are called the Galilean satellites. Italian astronomer Galileo Galilei discovered these moons in 1610. The largest of Jupiter’s moons is Ganymede. It is the largest moon in the solar system. Ganymede is larger than the planet Mercury and three-fourths the size of Mars. Ganymede is the only moon in the solar system known to have its own magnetic field. Ganymede and Callisto have many craters and appear to be made of ice and rocky material. Io has many active volcanoes. The volcanoes produce gases containing sulfur. The yellow-orange surface of Io is most likely made of sulfur from the volcanic eruptions. Europa is the smallest of the Galilean satellites. Europa's surface is mostly water ice. Beneath the ice may be an ocean of water or slushy ice. Europa is thought to have twice as much water as Earth. |
saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius about nine times that of Earth. Saturn is just a little smaller than Jupiter (its diameter is about 85% of Jupiter’s) but a lot lighter (its mass is about a third of Jupiter’s). This means that it has a very low density. In fact, its density is the lowest of all the planets and less than the density of water. This leads to the popular description that in a bathtub lled with water (assuming the tub is big enough to hold a planet) Saturn would oat. Still, in composition and internal structure, the planet is thought to be fairly similar to its larger sibling, Jupiter.
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Perhaps Saturn’s most striking property is its exquisite ring system. All Jovian planets are surrounded by a complex ring system made of icy particles. Saturn’s ring system is, by far, the most beautiful—an extensive, complex system of billions of tiny particles orbiting the planet above its equator. The rings of the other Jovian planets are much thinner and fainter. Scientists are still trying to determine the origin of the ring particles; the most commonly accepted suggestion is that they are bits of dust blown off the planets’ moons by asteroid or meteoroid impacts.
Saturn is a gas giant made up mostly of hydrogen and helium. Saturn is big enough to hold more than 760 Earths, and is more massive than any other planet except Jupiter, roughly 95 times Earth's mass. However, Saturn has the lowest density of all the planets, and is the only one less dense than water — if there were a bathtub big enough to hold it, Saturn would float.
The yellow and gold bands seen in Saturn's atmosphere are the result of super-fast winds in the upper atmosphere, which can reach up to 1,100 mph (1,800 km/h) around its equator, combined with heat rising from the planet's interior.
Saturn spins faster than any other planet except Jupiter, completing a rotation roughly every 10-and-a-half hours. This rapid spinning causes Saturn to bulge at its equator and flatten at its poles — the planet is 8,000 miles (13,000 kilometers) wider at its equator than between the poles. |
Saturn's most recent curiosity may be the giant hexagon circling its north pole, with each of its sides nearly 7,500 miles (12,500 km) across — big enough to fit nearly four Earths inside. Thermal images show it reaches some 60 miles (100 km) down into the planet's atmosphere. It remains uncertain what causes it.
Other titanic storms appear in Saturn’s atmosphere once every Saturn year (approximately 30 Earth-years), disrupting the temperature and winds of the planet’s skies. Six such storms have been observed on the planet since 1876, but in 2011, NASA’s Cassini spacecraft became the first orbiter to observe one. Like other giant planets, Saturn also has northern and southern lights, caused by particles from the sun. |
uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both have different bulk chemical composition from that of the larger gas giants Jupiter and Saturn. For this reason, scientists often classify Uranus and Neptune as "ice giants" to distinguish them from the gas giants.
Uranus’s unique feature is that it appears to have been knocked over sometime in the past. Most planets orbit around the Sun spinning upright; that is, their rotational axes are almost perpendicular with respect to their orbit (with small deviations, like the Earth’s 23.5o tilt). Uranus’s rotation axis, however, is almost lying within its orbital plane. The cause of this unique feature is not certain, but it has been sug- gested that it was caused by an impact of a large object, such as a large asteroid or moon. Giant impacts like this were common during the early history of the Solar System; a similar impact is thought to have created the Earth’s Moon |
Uranus is the only planet whose name is derived from a figure from Greek mythology, from the Latinised version of the Greek god of the sky Ouranos. Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration among those of the planets because its axis of rotation is tilted sideways, nearly into the plane of its solar orbit. Its north and south poles, therefore, lie where most other planets have their equators.
In 1986, images from Voyager 2 showed Uranus as an almost featureless planet in visible light, without the cloud bands or storms associated with the other giant planets. Observations from Earth have shown seasonal change and increased weather activity as Uranus approached its equinox in 2007. Wind speeds can reach 250 metres per second (900 km/h, 560 mph). |
- Uranus was officially discovered by Sir William Herschel in 1781.
- It is too dim to have been seen by the ancients. At first Herschel thought it was a comet, but several years later it was confirmed as a planet. Herscal tried to have his discovery named “Georgian Sidus” after King George III. The name Uranus was suggested by astronomer Johann Bode. The name comes from the ancient Greek deity Ouranos.
- Uranus turns on its axis once every 17 hours, 14 minutes.
- The planet rotates in a retrograde direction, opposite to the way Earth and most other planets turn.
- Uranus makes one trip around the Sun every 84 Earth years.
- During some parts of its orbit one or the other of its poles point directly at the Sun and get about 42 years of direct sunlight. The rest of the time they are in darkness.
- Uranus is often referred to as an “ice giant” planet.
- Like the other gas giants, it has a hydrogen upper layer, which has helium mixed in. Below that is an icy “mantle, which surrounds a rock and ice core. The upper atmosphere is made of water, ammonia and the methane ice crystals that give the planet its pale blue colour.
- Uranus hits the coldest temperatures of any planet.
- With minimum atmospheric temperature of -224°C Uranus is nearly coldest planet in the solar system. While Neptune doesn’t get as cold as Uranus it is on average colder. The upper atmosphere of Uranus is covered by a methane haze which hides the storms that take place in the cloud decks.
- Uranus has two sets of very thin dark coloured rings.
neptune
Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth and slightly larger than Neptune. Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 astronomical units (4.50×109 km). It is named after the Roman god of the sea and has the astronomical symbol of Neptune’s trident.
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Neptune is similar in size to Uranus (and both are smaller than Jupiter and Saturn). Giant storm centers can be seen on its visible surface, similar to those on the other Jovian planets. The atmosphere features great wind patterns; its winds are the fastest in the Solar System, reaching speeds of 2,000 km/hour (or 1,200 miles/hour). When the Voyager 2 spacecraft (the only spacecraft to visit this remote planet) flew by in 1989, one of the most distinguishing features of the planet was the Great Dark Spot, a storm similar to Jupiter’s Great Red Spot (but only about half its size). Later observations of the planet made with the Hubble Space Telescope showed the Spot to have disappeared (or masked by other atmospheric phenomena), and follow-up observations revealed the appearance of another dark spot elsewhere. All these features indicate that Neptune has a very active and rapidly changing atmosphere.
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Neptune's composition is similar to that of Uranus and unlike those of the larger gas giants, Jupiter and Saturn. Like Jupiter's and Saturn's, Neptune's atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, but contains a higher proportion of "ices" such as water, ammonia, and methane. However, its interior, like that of Uranus, is primarily composed of ices and rock, which is why Uranus and Neptune are normally considered "ice giants" to emphasise this distinction. Traces of methane in the outermost regions in part account for the planet's blue appearance.
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- Neptune was not known to the ancients.
- It is not visible to the naked eye and was first observed in 1846. Its position was determined using mathematical predictions. It was named after the Roman god of the sea.
- Neptune spins on its axis very rapidly.
- Its equatorial clouds take 18 hours to make one rotation. This is because Neptune is not solid body.
- Neptune is the smallest of the ice giants.
- Despite being smaller than Uranus, Neptune has a greater mass. Below its heavy atmosphere, Uranus is made of layers of hydrogen, helium, and methane gases. They enclose a layer of water, ammonia and methane ice. The inner core of the planet is made of rock.
- The atmosphere of Neptune is made of hydrogen and helium, with some methane.
- The methane absorbs red light, which makes the planet appear a lovely blue. High, thin clouds drift in the upper atmosphere.
- Neptune has a very active climate.
- Large storms whirl through its upper atmosphere, and high-speed winds track around the planet at up 600 meters per second. One of the largest storms ever seen was recorded in 1989. It was called the Great Dark Spot. It lasted about five years.
- Neptune has a very thin collection of rings.
- They are likely made up of ice particles mixed with dust grains and possibly coated with a carbon-based substance.
- Neptune has 14 moons.
- The most interesting moon is Triton, a frozen world that is spewing nitrogen ice and dust particles out from below its surface. It was likely captured by the gravitational pull of Neptune. It is probably the coldest world in the solar system.
- Only one spacecraft has flown by Neptune.
- In 1989, the Voyager 2 spacecraft swept past the planet. It returned the first close-up images of the Neptune system. The NASA/ESA Hubble Space Telescope has also studied this planet, as have a number of ground-based telescopes.
dwarf planets
There is one type of planet left!
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A dwarf planet is a planetary-mass object that is neither a planet nor a natural satellite. That is, it is in direct orbit of the Sun, and is massive enough for its gravity to crush it into a hydrostatic equilibrium shape, but has not cleared the neighborhood of other material around its orbit.
The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of objects farther away from the Sun than Neptune that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris.
The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; it was said to be the "right decision" by astronomer Mike Brown, who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern, who had coined the term dwarf planet in April 1991. The International Astronomical Union (IAU) currently recognizes five dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris. |
pluto
There is one type of planet left!
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Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered. It is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris, a dwarf planet in the scattered disc.
Like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume. Pluto is not very big. It is only half as wide as the United States. Pluto is smaller than Earth's moon. This dwarf planet takes 248 Earth years to go around the sun. If you lived on Pluto, you would have to wait 248 Earth years to celebrate your first birthday. One day on Pluto is about 6 1/2 days on Earth. Pluto is about 40 times farther from the sun than Earth is. Pluto is in an area of space called the Kuiper (KY-per) Belt. Thousands of small, icy objects like Pluto but smaller are in the Kuiper Belt.
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Pluto was discovered by Clyde Tombaugh in 1930, and was originally considered the ninth planet from the Sun. After 1992, its planethood was questioned following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, which is 27% more massive than Pluto, was discovered, which led the International Astronomical Union (IAU) to define the term "planet" formally for the first time the following year. This definition excluded Pluto and reclassified it as a member of the new "dwarf planet" category.
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Pluto has five known moons: Charon (the largest), Styx, Nix, Kerberos, and Hydra.
Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body. The IAU has not formalized a definition for binary dwarf planets, and Charon is officially classified as a moon of Pluto. In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be ingredients for the emergence of life, and produced from methane, nitrogen and related gases released from the atmosphere of Pluto and transferred over about 19,000 km (12,000 mi) distance to the orbiting moon. |