While the inner four planets seem large, they are nothing compared to the four outer planets, which are also known as gas giants or Jovian planets. The four giant planets in our Solar System are Jupiter, Saturn, Uranus, and Neptune.
Jupiter is the largest planet in our Solar System, and it truly is a giant planet. Jupiter is so large that you could fit 1321 Earths inside the planet. It is a gas giant, which means that it is comprised almost entirely of gas with a liquid core of heavy metals. Since none of the gas giants has a solid surface, you cannot stand on any of these planets, nor can spacecraft land on them. Another common characteristic of the giant planets is that they all have dozens of moons. In fact, Jupiter has 63 moons that have been discovered so far.
All of the giant planets in our Solar System have rings, but Saturn’s rings are by far the most famous of any. This planet’s ring system is composed of rock, dust, and other particles. The other planetary ring systems are made of similar elements.
Uranus and Neptune are also gas giants, but instead of just helium and hydrogen, they also have significant amounts of ices in their atmospheres. These ices include water, methane, and ammonia. It is the methane in the atmospheres of Uranus and Neptune that give the planets their blue color. Uranus and Neptune are also known as ice giants because of the proportion of ices in their atmospheres.
Giant planets are not limited to our Solar System either. In fact, astronomers have discovered many Jupiter-like planets in other solar systems. For example, in 2007, a group of British astronomers discovered three gas giants that are heavier than Jupiter is. These gas giants are much closer to their star than our Solar System’s gas giants are to the Sun. Scientists think that this may be one reason why these extrasolar planets are heavier, suggesting that only heavier planets can survive closer to a star. Because these planets are so much closer to their sun, they are much hotter than Jupiter and our Solar System’s other gas giants are.
These are just a handful of the gas giants discovered in different solar systems. Astronomers have discovered other extrasolar planets much bigger than Jupiter. Since all of the first extrasolar planets found were gas giants similar to Jupiter, astronomers began to despair of ever finding Earth-like planets that could support life. Recently though, astronomers have discovered different types of extrasolar planets, raising their hopes of finding life on other planets.
This is a picture of the sequence of the eight planets and three of the dwarf planets. Image courtesy of IAU.
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Since 2006, due to a controversial decision by the International Astronomical Union (IAU) that demoted Pluto to a dwarf planet, we have had eight planets.
Mercury is a small planet, which can reach extreme temperatures. Since the planet is the closest one to the Sun, it can reach temperatures of 450°C. However, because the planet has almost no atmosphere due to very little gravity, the surface also drops to low temperatures of -170°C.
Venus is farther from the Sun than Mercury is, but it stays hotter due to its thick, toxic atmosphere. The main compound in Venus’ atmosphere is carbon dioxide, which creates the strongest greenhouse effect of any planet.
Undoubtedly, you already know a lot about Earth, but you may not know that our planet is the only one in our Solar System that has plate tectonics. The Earth’s outer crust is broken up into various sections called plates, which can move. These plates also take carbon out of the atmosphere and recycle it. This prevents a greenhouse effect like Venus’ and keeps the Earth from getting too hot. This is just one feature of our unique planet that helps support life.
Mars is the only inner planet, except for Earth, that has moons. Its two moons are called Phobos and Deimos. In Greek mythology, Phobos is a son of Ares (the equivalent of Mars) and Deimos is a figure that represents dread.
Jupiter is the model for gas giants as well as being the largest planet in our Solar System. It was named after the Roman king of the gods who was also the god of the sky and of thunder, which is fitting considering its size. Jupiter has 63 moons – more than any other planet in our Solar System.
Saturn is the only planet in our Solar System that has an average density less than water. Its core is actually denser than water, but its gas atmosphere balances the heavier core. You may consider floating Saturn in water, but even if you found a planet with a large enough body of water, the gases that make up Saturn’s atmosphere would simply merge with the other planet’s atmosphere.
Uranus and Neptune both belong to a class of gas giants called ice giants because they contain higher amounts of “ices” in their atmosphere. These ices include water, ammonia, and methane.
Neptune is an ice giant with the fastest winds of any planets. These winds can reach speeds of 2,100 kilometers per hour. The planet was discovered with mathematical predictions when astronomers noticed discrepancies in Uranus’ orbit.
The symbols of the eight planets, and Pluto, Credit: insightastrology.net
In our long history of staring up at the stars, human beings have assigned various qualities, names, and symbols for all the objects they have found there. Determined to find patterns in the heavens that might shed light on life here on Earth, many of these designations ascribed behavior to the celestial bodies.
When it comes to assigning signs to the planets, astrologists and astronomers – which were entwined disciplines in the past -made sure that these particular symbols were linked to the planets’ names or their history in some way.
Consider the planet Mercury, named after the Roman god who was himself the messenger of the gods, noted for his speed and swiftness. The name was assigned to this body largely because it is the planet closest to the Sun, and which therefore has the fastest rotation period. Hence, the symbol is meant to represent Mercury’s helmet and caduceus – a herald’s staff with snakes and wings intertwined.
Mercury, as imaged by the MESSENGER spacecraft, which was named after the messenger of the gods because it has the fastest orbit around the Sun. Image Credit: NASA/JHU/Carnegie Institution.
Venus:
Venus’ symbol has more than one meaning. Not only is it the sign for “female”, but it also represents the goddess Venus’ hand mirror. This representation of femininity makes sense considering Venus was the goddess of love and beauty. The symbol is also the chemical sign for copper; since copper was used to make mirrors in ancient times.
Earth:
Earth’s sign also has a variety of meanings, although it does not refer to a mythological god. The most popular view is that the circle with a cross in the middle represents the four main compass points. It has also been interpreted as the Globus Cruciger, an old Christian symbol for Christ’s reign on Earth.
This symbol is not just limited to Christianity though, and has been used in various culture around the world. These include, but are not limited to, Norse mythology (where it appears as the Solar or Odin’s Cross), Native American cultures (where it typically represented the four spirits of direction and the four sacred elements), the Celtic Cross, the Greek Cross, and the Egyptian Ankh.
In fact, perhaps owing to the simplicity of the design, cross-shaped incisions have made appearances as petroglyphs in European cult caves dating all the way back to the beginning of the Upper Paleolithic, and throughout prehistory to the Iron Age.
Mars, as photographed with the Mars Global Surveyor, is identified with the Roman god of war. Credit: NASA
Mars:
Mars is named after the Roman god of war, owing perhaps to the planet’s reddish hue, which gives it the color of blood. For this reason, the symbol associated with Mars represents the god of wars’ shield and spear. Additionally, it is the same sign as the one used to represent “male”, and hence is associated with self-assertion, aggression, sexuality, energy, strength, ambition and impulsiveness.
Jupiter:
Jupiter’s sign, which looks like an ornate, oddly shaped “four,” also stands for a number of symbols. It has been said to represent an eagle, which is Jupiter’s bird. Additionally, the symbol can stand for a “Z,” which is the first letter of Zeus – who was Jupiter’s Greek counterpart.
The line through the symbol is consistent with this, since it would indicate that it was an abbreviation for Zeus’ name. And last, but not least, there is the addition of the swirled line which is believed to represent a lighting bolt – which just happens to Jupiter’s (and Zeus’) weapon of choice.
Saturn:
Like Jupiter, Saturn resembles another recognizable character – this time, it’s an “h.” However, this symbol is actually supposed to represent Saturn’s scythe or sickle, because Saturn is named after the Roman god of agriculture.
Jupiter, the largest planet in the Solar System, is appropriately named after the Roman father of the gods. Credit: NASA/ESA/A. Simon (Goddard Space Flight Center)
Uranus:
The sign for Uranus is a combination of two other signs – Mars’ sign and the symbol of the Sun – because the planet is connected to these two in mythology. Uranus represented heaven in Roman mythology, and this ancient civilization believed that the Sun’s light and Mars’ power ruled the heavens.
Neptune:
Neptune’s sign is linked to the sea god Neptune, who the planet was named after. Appropriately, the symbol represents this planet is in the shape of the sea god’s trident.
Pluto:
Although Pluto was demoted to a dwarf planet, it still has a symbol. Pluto’s sign is a combination of a “P” and a “L,” which are the first two letters in Pluto as well as the initials of Percival Lowell, the astronomer who discovered the planet.
Other Objects:
The Moon is represented by a crescent shape, which is a clear allusion to how the Moon appears in the night sky more often than not. Since the Moon is also tied to people’s perceptions, moods, and emotional make-up, the symbol has also come to represents the mind’s receptivity.
A full moon captured July 18, 2008. Credit: NASA/Sean Smith
And then there’s the sun, which is represented by a circle with a dot in the middle. In the case of the Sun, this symbol represents the divine spirit (circle) surrounding the seed of potential, which is a direct association with ancient Sun worship and the central role Sun god’s played in ancient pantheons.
The planets have played an important role in the culture and astrological systems of every human culture. Because of this, the symbols, names, and terms that denote them continue to hold special significance in our hearts and minds.
Take a look at the Solar System from above, and you can see that the planets make nice circular orbits around the Sun. But dwarf planet’s Pluto’s orbit is very different. It’s highly elliptical, traveling around the Sun in a squashed circle. And Pluto’s orbit is highly inclined, traveling at an angle of 17-degrees. This strange orbit gives Pluto some unusual characteristics, sometimes bringing it within the orbit of Neptune. Credit: NASA
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Centuries ago, people believed that the Earth was the center of the Solar System. Slowly, that view was replaced with the heliocentric view. With that change came the realization that the planets orbit the Sun.
When Pluto was reclassified as a dwarf planet, Mercury became the planet with the most eccentric orbit. The eccentricity of an orbit is a measurement of how much the orbit deviates from a circular shape. If an orbit is a perfect circle, it has an eccentricity of zero, and that number increases with an increase in eccentricity. Mercury has an eccentricity of .21. Its orbit ranges from 46 million kilometers at the closest point to the Sun to 70 million kilometers at the farthest point. The closest point to the Sun in an orbit is called the perihelion, and the farthest point is the aphelion. Mercury is the fastest planet to orbit the Sun at approximately Earth 88 days.
Venus has the least eccentricity of any planet in our Solar System – eccentricity of .007 – with a nearly perfect circular orbit. Venus’ orbit ranges from 107 million kilometers at the perihelion to 109 million kilometers from the Sun. It takes 224.7 of our days to orbit the Sun. A day on Venus is actually longer than a year because the planet rotates so slowly. Seen from the Sun’s north pole, all of the planets rotate counter-clockwise, but Venus actually rotates clockwise; it is the only planet to do that.
Earth also has a very low eccentricity of .017. On average, the planet is about 150 million kilometers from the Sun, but it can range from 147 million kilometers to 152 million kilometers. It takes our planet roughly 365.256 days to orbit the Sun, which is the reason for leap years.
Mars has an eccentricity of .093 making it one of the most eccentric orbits in our Solar System. Mars perihelion is 207 million kilometers and its aphelion is 249 million kilometers from the Sun. Over time, Mars’ orbit has become more eccentric. It takes 687 Earth days to orbit the Sun.
Jupiter has an eccentricity of .048 with a perihelion of 741 million kilometers and an aphelion of 778 million kilometers. It takes 4331 Earth days – 11.86 of our years – for Jupiter to orbit the Sun.
Saturn has an eccentricity of .056. At its closest point, Saturn is 1.35 billion kilometers from the Sun, and 1.51 billion kilometers away at its farthest point. Depending on what position it is in its orbit, Saturn’s rings are fully visible or almost invisible. The planet takes 29.7 years to orbit the Sun. In fact, since it was discovered in 1610, Saturn has only orbited approximately 13 times. Earth has orbited the Sun almost 400 times since then.
Uranus has a perihelion of 2.75 billion kilometers and an aphelion of 3 billion kilometers from the Sun. Its eccentricity is .047. It takes Uranus 84.3 Earth years to orbit the Sun. Uranus is unique because it actually rotates on its side with an axial tilt of almost 99°.
Neptune’s eccentricity is .009, almost as low as Venus’. The planet has a perihelion of 4.45 billion kilometers and an aphelion of 4.55 billion kilometers. Since Pluto was reclassified as a dwarf planet, Neptune is the planet with an orbit farthest from the Sun.
Universe Today has articles on orbits of all the planets including Mercury and Mars.
It is often difficult to grasp just how large the planets actually are. There are a number of ways to measure a planet, including diameter, volume, and surface area.
Mercury is the smallest planet in our Solar System since Pluto was demoted to a dwarf planet. It has a diameter of 4,879 km, and a surface area of 17.48 x 107 km2, which is only about 11% of Earth’s surface area. Mercury’s volume is even smaller in comparison at 6.083 x 1010 km3, which is only 5.4% the volume of Earth.
Venus is similar in size to Earth, which earned it the title of Earth’s twin. Venus has a diameter of 12,100 km and a surface area of 4.6 x 108 km2. These measurements are 95% and 90% of Earth’s diameter and surface area respectively. With a volume of 9.38 x 1011 km3, Venus’ volume is 86% of Earth’s.
Earth has a diameter of 12,742 km and a surface area of 5.1 x 108 km2. Its volume of 1.08 x 1012 km3 gives the planet the largest volume of any of the terrestrial planets.
Mars is also a small planet, the second smallest in our Solar System. Mars’ diameter is 6,792 km, only about 53% of Earth’s diameter. At only 28% of Earth’s surface area, Mars has a very small surface area of 1.45 x 108 km2. Mars’ volume of 1.63 x 1011 km3 is only 15% of Earth’s volume.
All of the gas giants are larger in size than the four inner planets. Jupiter is the largest planet in our Solar System. It has a diameter of 143,000 km, which is more than 11 times the size of Earth’s diameter. The numbers only get larger from there. Jupiter has a surface area of 6.22 x 1010 km2. That is 122 times greater than Earth’s surface area. Jupiter’s volume of 1.43 x 1015 km3 is an incredible number. You can fit 1321 Earths inside Jupiter.
Saturn is the second largest planet in our Solar System. It has a diameter of 120,536 km across the equator, and a surface area of 4.27 x 1010 km2. With a volume of 8.27 x 1014 km3, Saturn can hold 764 Earths inside.
Uranus has a diameter of 51,118 km and a surface area of 8.1 x 109 km2. Although Uranus is much smaller than Jupiter, it is still large. With a volume of 6.83 x 1013 km3, you could fit 63 Earths inside the gas giant.
Neptune is slightly smaller than Uranus, but still very large. The planet has a diameter of 49,500 km. You could fit 57.7 Earths inside Neptune, which has a volume of 6.25 x1013 km3. Neptune has a surface area of 7.64 x 109 km2, which is 15 times Earth’s surface area.
Planets and other objects in our Solar System. Credit: NASA.
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Many children, and even adults, dream of visiting other planets and wonder what it would be like to stand on another planet. For one thing, your weight would be different on another planet, depending on a number of factors including the mass of the planet and how far you are away from the center of the planet.
Before we start, it’s important to understand that the kilogram is actually a measurement of your mass. And your mass doesn’t change when you go anywhere in the Universe and experience different amounts of gravity. Your weight is best measured in newtons. But since your bathroom doesn’t measure your weight in newtons, we’ll use kilograms. This is what your bathroom scale would say if you stepped on another world.
Mercury is the smallest planet in our Solar System, but it is dense. Because Mercury is so small, it has very little gravity. If you weighed 68 kg on Earth, you would only weigh 25.7 kg on Mercury.
Venus is very close to Earth in size and mass. Venus’ mass is roughly 90% of the mass of the Earth. Thus, it is no surprise that someone would weigh a similar amount on Venus. Someone who weighed 68 kg on Earth would weigh 61.6 kg on Venus.
Mars is quite a bit smaller than Earth with only 11% of our planet’s mass. Mars is larger than Mercury, but it is not as dense as the smaller planet. If you weighed 68 kg on Earth then you would weigh 25.6 kg on Mars. Since Pluto was demoted to a dwarf planet, Mars became the planet where you would weigh the least.
Jupiter is the largest planet in our Solar System with the most mass. Because of Jupiter’s mass, you would weigh more on that planet than on any other one in our Solar System. If you weighed 68 kg on Earth then you would weigh 160.7 kg on Jupiter, over twice your normal weight. That is if you could actually stand on Jupiter’s surface, which is impossible because it is a gas giant, and gas giants do not have solid surfaces.
Saturn is a gas giant best known for its planetary rings system. It is also the second biggest planet in our Solar System. Despite its mass though, the planet has a very low density and a lower gravity than Earth. If you weighed 68 kg on Earth, you would weigh 72.3 kg on Saturn.
Uranus is a gas giant without a solid surface. Although Uranus is larger in size than Neptune, it has less mass and therefore less gravity. You would only weigh 60.4 kg on Uranus, if you weighed 68 kg on Earth.
Neptune, the last planet in our Solar System, is a gas giant. If you weighed 68 kg on Earth, then you would weigh 76.5 kg on Neptune if you could stand on the planet’s surface.
Although the Moon is not a planet, it is one of the few objects that astronauts have actually visited. Because the Moon is so small, it has a low density and low gravity. If you weighed 68 kg on Earth, then you would only weigh 11.2 kg on the Moon.
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We actually only know the exact date of when a few of the planets were discovered. Five of the planets, not including Earth, have been known to exist for thousands of years – Mercury, Mars, Venus, Jupiter, and Saturn. The Ancient Greeks and Romans wrote about the planets many centuries ago. Because the planets look like stars to the naked eye, that is where the term planets comes from. Because the planets move in the sky, they were termed wandering stars. The term planet comes from the Greek word for wanderer, “planetes.” Many ancient people thought that the planets were gods, so they gave them the names of their gods. All of the planets, except Earth have names of Roman deities.
The other three planets – Uranus, Neptune, and Pluto – were not discovered until at least the 1700’s. Pluto is no longer a planet since it was reclassified as a dwarf planet in 2006. It was known as the ninth planet for 70 years though, so its discovery will be included here. Uranus was discovered in 1781 by the famous astronomer Sir William Herschel, although that was not the first sighting of it. The planet had been sighted as early as 1690 by the English astronomer John Flamsteed. It was also sighted by Pierre Lemonier in the mid 1700’s. Sir Herschel at first thought that Uranus was a comet, but he noticed the irregularities early on and compared it to a planet in his notes.
Because Neptune cannot be seen without the help of a telescope, it was not discovered until after 1610, when Galileo created the telescope. Alexis Bouvard, a mathematician, saw that another planet had to be affecting Uranus’ orbit, so astronomers started looking for it. Two astronomers, John Couch Adams and Urbain Le Verrier, discovered Neptune independently or rather made the calculations and determined where Neptune could be found. The planet turned out to be 1° from Verrier’s calculations and 12° from Adams’. There was a dispute between France and England over who discovered the new planet because Adams and Verrier are from England and France respectively.
Pluto was the last planet discovered, although that distinction returned to Neptune when Pluto was reclassified as a dwarf planet. Pluto was discovered in 1930 by the astronomer Clyde Tombaugh. Many people had been searching for a ninth planet – the elusive planet X – for quite a while. Since Pluto was discovered near the calculated location of planet X, they thought the two planets were one and the same. Later, astronomers realized that there was no such planet X.
Planets and other objects in our Solar System. Credit: NASA.
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You may recognize the names of the planets from your high school literature course or a history class. That is because many of the planets were first discovered by ancient civilizations, and so planets are named after their gods.
The Romans named Mercury after the messenger of the gods because it appears to move so quickly.
Venus was named after the Roman goddess of love because of its shining presence. The planet is the brightest object in the sky beside the Moon and the Sun. A number of other cultures also named Venus after their own gods or goddesses of love and war.
Earth is the only planet not named after a god. The name is based on Germanic and Old English words for “ground.”
Mars was named after the Roman god of war because of its red color, which reminded people of blood. Other civilizations also had names for the planet based on its color. The Egyptians called it “Her Desher,” which means “the red one.”
Jupiter was named after the king of the gods – Zeus by the Greeks and Jupiter by the Romans. Ancient civilizations most likely named this planet after the most powerful god because of its size. Jupiter is the largest and most massive planet in our Solar System.
Saturn was named after the father of the king of gods as well as being the god of agriculture and harvest. In mythology, Saturn had taken the position of king of the gods from his own father, Uranus, and then Jupiter overthrew him. Saturn is the last planet that can be seen from Earth without the aid of a telescope.
Uranus was not discovered until 1781 by Sir William Herschel, so it was not necessarily going to be named after a Roman god. In fact, Herschel named the planet “Georgium Sidus” in honor of George III who was King of England at the time. Others called the planet Herschel in honor of the astronomer who had discovered it. The name Uranus, which is the name of the Roman god who is the father of Jupiter, was suggested by the astronomer Johann Bode. That name was widely accepted in the mid 1800’s, and it fit in with the other planets, which all had names from mythology.
Neptune had been observed by a number of astronomers, but they believed it was a star. Two people, John Couch Adams and Urban Le Verrier, calculated the planet’s location. Johann Galle, the astronomer who discovered the planet using Verrier’s calculations, wanted to name the planet after Verrier. Many astronomers objected though, so it was named after Neptune the Roman god of the sea. The name was very fitting because the planet is a bright sea blue.
The Jovian planets of the Solar System. Credit: bork.hampshire.edu
Astronomers have divided the eight planets of our solar system into the inner planets and the outer planets. The 4 inner planets are the closest to the Sun, and the outer planets are the other four – Jupiter, Saturn, Uranus, and Neptune. The outer planets are also called the Jovian planets or gas giants. Like the inner planets, the outer planets have similar characteristics to one another.
The outer planets are so much larger than the inner planets that they make up 99 percent of the mass of the celestial bodies that orbit the Sun. Although mainly composed of gas, the outer planets also have other ingredients. Somewhere at the center is what scientist refer to as a rocky core, although it is actually composed of liquid heavy metals. While the inner planets have few or no moons, the outer planets have dozens each. The inner and outer planets are separated by the asteroid belt.
Jupiter is the largest planet in our Solar System with a mass more than three hundred times Earth’s mass. Jupiter is not only the largest planet, it also has the most moons – 63 identified so far. Jupiter is one of the brightest objects in the sky and has a very stormy atmosphere. One major storm, the Great Red Spot, is as big as the Earth.
Saturn is hard to miss with its distinctive, large rings. Although all of the outer planets have rings, Saturn’s are the most visible ones. Astronomers did not know that there any other planets with rings until 1977 when Uranus’ rings were discovered. Soon after that, astronomers discovered rings around both Jupiter and Neptune.
Uranus has the smallest mass of the outer planets, although in size it’s a little bigger than Neptune. It is the only planet to rotate on its side. Scientists are unsure why it rotates that way, although there are a couple theories. One suggests it suffered a major collision and another hypothesizes that smaller shifts during the planets’ formation caused its unusual rotation.
Neptune is the final outer planet in the solar system. Neptune’s winds are the fastest of any planets in the Solar System and can reach more than 1,200 miles per hour. While all of the outer planets’ atmospheres contain hydrogen and helium, Neptune and Uranus contain significant amounts of what astronomers call ices. These ices include water, ammonia, and methane. The methane in Neptune and Uranus is what gives the planets their blue color.
Check out these other articles from Universe Today on the outer solar system and the inner solar system.
If you are searching for more information, NASA has many articles, including one on the planets and Science Daily is also a rich resource for information on the outer planets.
Astronomy Cast has a number of episodes on the Jovian planets including this one on Jupiter.
Uranus as seen through the automated eyes of Voyager 2 in 1986. (Credit: NASA/JPL).
The gas (and ice) giant known as Uranus is a fascinating place. The seventh planet from out Sun, Uranus is the third-largest in terms of size, the fourth-largest in terms of mass, and one of the least dense objects in our Solar System. And interestingly enough, it is the only planet in the Solar System that takes it name from Greek (rather than Roman) mythology.
But these basic facts really only begin to scratch the surface. When you get right down to it, Uranus is chock full of interesting and surprising details – from its many moons, to its ring system, and the composition of its aqua atmosphere. Here are just ten things about this gas/ice giant, and we guarantee that at least one of them will surprise you.
