Discovery of the Planets

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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.

Universe Today has a number of articles on the planets including who discovered Neptune and the planets of our Solar System.

Check out these other articles including mathematical discovery of the planets and the planets.

Astronomy Cast has episodes on all of the planets including Mercury.

Names of the Planets

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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.

Universe Today has a number of articles on the planets including facts about the planets and the planets of the solar system.

If you are looking for more information on the planets take a look at the planets and interesting facts about the planets.

Astronomy Cast has episodes on all of the planets, so start with Mercury.

What are The Outer Planets of the Solar System?

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.

Vertical Structures Tower Above Saturn’s Rings

Cassini has imaged towering vertical structures in the planet’s otherwise flat rings that come from the gravitational effects of a small nearby moon. This is the first time these structures have been seen. They reach up over one kilometer high, and are visible now as the sun nears “high noon” directly overhead at the planet’s equator, as Saturn approaches its equinox.

The search for ring material extending above and below Saturn’s ring plane has been a major goal of the imaging team during Cassini’s “Equinox Mission,” the two-year period containing the exact equinox. This novel illumination geometry, which occurs every half-Saturn-year, or about 15 Earth years, lowers the sun’s angle to the ring plane and causes out-of-plane structures to cast long shadows across the rings, making them easy to detect.

Images taken in recent weeks have demonstrated how small moons in very narrow gaps can have considerable and complex effects on the edges of their gaps, and that such moons can be smaller than previously believed.

Looming vertical structures, seen here for the first time and created by Saturn's moon Daphnis, rise above the planet's otherwise flat, thin disk of rings to cast long shadows in this Cassini image.  Credit: CICLOPS
Looming vertical structures, seen here for the first time and created by Saturn's moon Daphnis, rise above the planet's otherwise flat, thin disk of rings to cast long shadows in this Cassini image. Credit: CICLOPS

The 8-kilometer-wide (5-mile) moon Daphnis orbits within the 42-kilometer-wide (26-mile) Keeler Gap in Saturn’s outer A ring, and its gravitational pull perturbs the orbits of the particles forming the gap’s edges. Earlier images have shown “waves” in the rings from Daphnis eccentric orbit.

But new images show the shadows of the vertical waves created by Daphnis cast onto the nearby ring. These characteristics match what was predicted by scientists.

Scientists have estimated, from the lengths of the shadows, wave heights that reach enormous distances above Saturn’s ring plane – as large as 1.5 kilometers (1 mile) — making these waves twice as high as previously known vertical ring structures, and as much as 150 times as high as the rings are thick. The main rings — named A, B and C — are only about 10 meters (30 feet) thick.

“We thought that this vertical structure was pretty neat when we first saw it in our simulations,” said John Weiss, lead author of a paper reporting on these images. “But it’s a million times cooler to have your theory supported by such gorgeous images. It makes you suspect you might be doing something right.”

Click here to watch a movie of the vertical structures and waves in motion.

Also presented in the paper is a refinement to a theory used since the Voyager missions of the 1980s to infer the mass of gap-embedded moons based on how much the moons affect the surrounding ring material. The authors conclude that an embedded moon in a very narrow gap can have a smaller mass than that inferred by earlier techniques. One of the prime future goals of the imaging team is to scour the remaining gaps and divisions within the rings to search for the moons expected to be there. “It is one of those questions that have been nagging us since getting into orbit: ‘Why haven’t we yet seen a moon in every gap?’” said Carolyn Porco, lead for the Cassini imaging team. “We now think they may actually be there, only a lot smaller than we expected.”

Source: CICLOPS

What is the Atmosphere Like on Saturn?

Like the rest of the planet, the atmosphere of Saturn is made up approximately 75% hydrogen and 25% helium, with trace amounts of other substances like water ice and methane.

From a distance, in visible light, Saturn’s atmosphere looks more boring than Jupiter; Saturn has cloud bands in its atmosphere, but they’re pale orange and faded. This orange color is because Saturn has more sulfur in its atmosphere. In addition to the sulfur in Saturn’s upper atmosphere, there are also quantities of nitrogen and oxygen. These atoms mix together into complex molecules we have here on Earth; you might know it as “smog”. Under different wavelengths of light, like the color-enhanced images returned by NASA’s Cassini spacecraft, Saturn’s atmosphere looks much more spectacular.

Saturn has some of the fastest winds in the Solar System. As NASA’s Voyager spacecraft was approaching Saturn, it clocked winds going as fast as 1800 km/hour at the planet’s equator. Large white storms can form within the bands that circle the planet, but unlike Jupiter, these storms only last a few months and are absorbed into the atmosphere again.

The part of Saturn that was can see is the visible cloud deck. The clouds are made of ammonia, and sit about 100 km below the top of Saturn’s troposphere (the tropopause), where temperatures dip down to -250 degrees C. Below this upper cloud deck is a lower cloud deck made of ammonium hydrosulphide clouds, located about 170 km below. Here the temperature is only -70 degrees C. The lowest cloud deck is made of water clouds, and located about 130 km below the tropopause. Temperatures here are 0 degrees; the freezing point of water.

Below the cloud decks pressures and temperatures increase with depth, and the hydrogen gas slowly changes to liquid. And below that, the helium forms a liquid as well.

We have written many articles about Saturn for Universe Today. Here’s an article about long-term patterns in Saturn’s atmosphere, and here’s an article about Saturn’s southern atmosphere.

Want more information on Saturn? Here’s a link to Hubblesite’s News Releases about Saturn, and here’s NASA’s Solar System Exploration Guide.

We have recorded a podcast just about Saturn for Astronomy Cast. Click here and listen to Episode 59: Saturn.

References:
NASA APOD
NASA Saturn Fun Facts

Ten Interesting Facts About Uranus

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.

Continue reading “Ten Interesting Facts About Uranus”

How Long Does it Take to Get to Saturn?

How long does it take to get to Saturn? That is a great question that happens to have several answers. Just as it can take different amounts of time to get to a destination here on Earth depending on what route you take, it can take different amounts of time to get to Saturn based on how you travel.

In the past spacecraft have taken greatly different amounts of time to make it to Saturn. Pioneer 11 took six and a half years to arrive. Voyager 1 took three years and two months, Voyager 2 took four years, and the Cassini spacecraft took six years and nine months to arrive. The New Horizons spacecraft took a short two years and four months to arrive on the scene. Why such huge differences in flight time?

The first factor to consider is whether the spacecraft is launched directly toward Saturn or if the spacecraft is sent toward other celestial objects to uses their gravity to slingshot itself to Saturn. Another factor is consider is the type of engine propelling the spacecraft, and a third factor to think about is that it takes a great deal of time to slow down, so if a spacecraft is simply going to flyby, it need to slowdown, but if it is to orbit, its trip to Saturn will take longer.

With those factors in mind, lets look a the mission mentioned above. Pioneer 11 and Cassini used the gravitational influence of different planets before making their way to Saturn. These flybys of other planets added years to their trip. Voyager 1 and 2 did not meander around the Solar System so much and made their appearances near Saturn much more quickly. The New Horizons spacecraft had several distinct advantages over all of the other spacecraft mentioned. The two main being that it has the fastest, most advanced engine available and it was launched on a single trajectory past Saturn on its way to Pluto.

As you can see, the answer to ”how long does it take to get to Saturn” lacks a straightforward answer. Even with New Horizons flying past in just over two years, scientist are hoping to improve upon that speed with better engines and more efficient flight patterns.

Just in case you were wondering, here’s how long it takes to fly to Mars, and how long it takes to get to the Moon.

Here’s the same question answered at NASA’s Starchild, and information about how long each of NASA’s spacecraft took to make the journey.

We have recorded two episodes of Astronomy Cast just about Saturn. The first is Episode 59: Saturn, and the second is Episode 61: Saturn’s Moons.

Source: NASA

How Far is Saturn from Earth?

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The answer to ”how far is Saturn from Earth” has a different answer every day. As the planets move along their orbital paths they move nearer and further in comparison to each other. For the sake of simplicity, Saturn is 1.2 billion km, roughly 7 AU, from the Earth when the two are at their closest approach to one another. They are 1.67 billion km, around 11 AU, from each other when they are at their most distant. Saturn and Earth are the closest to each other when they are on the same side of the Sun and at similar points in their orbits. The are the most distant when on opposite sides of the Sun.

Here are some of the other orbital and physical characteristics of Saturn compared to those of Earth.

Equatorial Diameter… 120,536 km, 9.44 times that of Earth
Polar Diameter… 108,728 km, 8.55 times that of Earth
Surface Area…4.27×1010 km2, 83.7 times that of Earth
Volume…8.2713×1014 km3, 763.6 times that of Earth
Mass…5.6846×1026 kg, 95.2 times that of Earth
Density… 0.687 g/cm3, one tenth that of Earth…Saturn could float in water.

Here are a few other interesting facts about Saturn that may interest you:

Saturn has 60 moons. That means that about 40% of the moons in our Solar System orbit around the planet. Many of these moons are very small and can not be seen from Earth. The last four were discovered by the Cassini spacecraft and scientist fully expect to find more as more spacecraft make their way toward Saturn.

Saturn is known for its amazing set of rings, but did you know that the occasionally disappear? Well, they disappear from our point of view anyway. The planet is tilted on its axis very similar to Earth. AS it makes its way along its 30 Earth year orbit of the Sun we sometimes see the rings full on and other time they are edge on from our perspective and disappear. This will next happen in 2024-2025.

While Saturn is too hostile for any form of life that we know, its moon Enceladus has ice geysers. That means that some mechanism is keeping the moon warm enough for liquid water to exist. As you know, here on Earth where ever there is liquid water there is life. Some scientist think that there is a chance for some type of life to exist on Enceladus.

Now that you know the answer to ”how far is Saturn from Earth”, we here at Universe Today hope that you will be inspired to find out more about the ringed planet.

Here’s an article that has photos of Earth seen from other worlds, including Saturn, and an article about how far each of the planets are from the Sun.

Here’s Hubblesite’s News Releases about Saturn, and more facts on Saturn from Kid Cosmos.

We have recorded two episodes of Astronomy Cast just about Saturn. The first is Episode 59: Saturn, and the second is Episode 61: Saturn’s Moons.

Source: NASA

Diameter of Saturn

Saturn has an equatorial diameter of 120,536 km, 9.44 times that of Earth. That makes it the second largest planet in our Solar System, trailing only Jupiter. Saturn, like all of the other planets, is an oblate spheriod. This means that its equatorial diameter is larger than is diameter measured through the poles. In the case of Saturn this distance is quite a bit different due to the planet’s high rotational speed. The polar diameter of Saturn is 108,728 km, meaning that it is flattened by a factor of 9.796%.

Scientist know that Saturn rotates very quickly, but the exact speed of that rotation has been hard to determine because of the thick clouds in the atmosphere. With terrestrial planets, scientists are able to find surface features and basically time how long it takes for that feature to reappear in the same position. This is a simplified description of how they determine rotational speed. The problem with Saturn is that the surface can not be observed. To make things even more difficult, the visible features of the planet’s atmosphere rotate at different speeds depending on their latitude.

The atmosphere of Saturn is broken down into systems. System I is the equatorial zone has a rotational period of 10 hours and 14 minutes. System II encompasses all other areas of Saturn and has a rotational speed of 10 hours 38 minutes and 25.4 seconds. System III is based on radio emissions and has mostly replaced the use of the term System II. It has a rotational speed of 10 hours 39 minutes and 22.4 seconds. Despite these numbers, the rotational speed of the planet’s interior is currently impossible to measure precisely. The Cassini spacecraft found the radio rotational speed of Saturn to be 10 hours 45 minutes and 45 seconds. In 2007, it was determined that the varying radio emissions from the planet did not match Saturn’s rotation rate. Some scientists think that the variance is due to geyser activity on the Saturnian moon Enceladus. The water vapor from these geysers enter Saturn’s orbit become charged, thus creating a drag effect on Saturn’s magnetic field. This slows the magnetic field’s rotation slightly compared to the rotation of the planet. The current estimate of Saturn’s rotation is based on various measurements from the Cassini, Voyager and Pioneer probes. That estimated speed is 10 hours 32 minutes and 35 seconds as of September 2007.

Again, the equatorial diameter of Saturn is 120,536 km and its polar diameter is 108,728 km. It is very important to understand why the difference in these diameters is so large, that is why so much detail is given on the rotational speed of the planet. You can take many of the same factors into account when thinking about all of the gas giants.

Here’s an article about how long a day is on Saturn, and another article about how the storms never end on Saturn.

Here’s Hubblesite’s News Releases about Saturn, and more information from Solar Views.

We have recorded two episodes of Astronomy Cast just about Saturn. The first is Episode 59: Saturn, and the second is Episode 61: Saturn’s Moons.

Source: NASA

What are Saturn’s Rings Made Of?

Saturn is sometimes called the ”Jewel of the Solar System” because its ring system looks like a crown. The rings are well known, but often the question ”what are Saturn’s rings made of” arises. Those rings are made up of dust, rock, and ice accumulated from passing comets, meteorite impacts on Saturn’s moons, and the planet’s gravity pulling material from the moons. Some of the material in the ring system are as small as grains of sand, others are larger than tall buildings, while a few are up to a kilometer across. Deepening the mystery about the moons is the fact that each ring orbits at a different speed around the planet.

Saturn is not the only planet with a ring system. All of the gas giants have rings, in fact. Saturn’s rings stand out because they are the largest and most vivid. The rings have a thickness of up to one kilometer and they span up to 482,000 km from the center of the planet.

The rings are named in alphabetical order according to when they were discovered. That makes it a little confusing when listing them in order from the planet. Below is a list of the main rings and gaps between them along with distances from the center of the planet and their widths.

  • The D ring is closest to the planet. It is at a distance of 66,970 – 74,490 km and has a width of 7,500 km.
  • C ring is at a distance of 74,490 – 91,980 km and has a width of 17,500 km.
  • Columbo Gap is at a distance of 77,800 km and has a width of 100 km.
  • Maxwell Gap is at a distance of 87,500 km and has a width of 270 km.
  • Bond Gap is at a distance of 88,690 – 88,720 km and has a width of 30 km.
  • Dawes Gap is at a distance of 90,200 – 90,220 km and has a width 20 km.
  • B ring is at a distance of 91,980 – 117,580 km with a width: 25,500 km.
  • The Cassini Division sits at a distance of 117,500 – 122,050 km and has a width of 4,700 km.
  • Huygens gap starts at 117,680 km and has a width of 285 km – 440 km.
  • The Herschel Gap is at a distance of 118,183 – 118,285 km with a width of 102 km.
  • Russell Gap is at a distance of 118,597 – 118,630 km and has a width of 33 km.
  • Jeffreys Gap sits at a distance of 118,931 – 118,969 km with a width of 38 km.
  • Kuiper Gap ranges from 119,403 -119,406 km giving it a width of 3 km.
  • Leplace Gap is at a distance of 119,848 – 120,086 km and a width of 238 km.
  • Bessel Gap is at 120,305 – 120,318 km with a width of 10 km.
  • Barnard Gap is at a distance of 120,305 – 120,318 km giving it a width of 3 km.
  • A ring is at a distance of 122,050 – 136,770 km with a width of 14,600 km.
  • Encke Gap sits between 133,570-133,895 km for a width of 325 km.
  • Keeler Gap is at a distance of 136,530-136,565 km with a width of 35 km.
  • The Roche Division is at 136,770 – 139,380 km for a width 2600 km.
  • F ring is begins at 140,224 km, but debate remains as to whether it is 30 or 500 km in width.
  • G ring is between 166,000 – 174,000 km and has a width of 8,000 km.
  • Finally, we get to the E ring. It is between 180,000 – 480,000 km giving it a width of 300,000 km.

As you can see, a great deal of observation has been dedicated to understanding and defining Saturn’s rings. Hopefully, having the answer to ”what are Saturn’s rings made of” will inspire you to look more deeply into the topic.

We have written many articles about Saturn for Universe Today. Here’s an article about the orbit of Saturn, and here’s an article about the temperature of Saturn.

If you’d like more info on Saturn, check out Hubblesite’s News Releases about Saturn. And here’s a link to the homepage of NASA’s Cassini spacecraft, which is orbiting Saturn.

We have recorded two episodes of Astronomy Cast just about Saturn. The first is Episode 59: Saturn, and the second is Episode 61: Saturn’s Moons.

Source: NASA