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 Uranus. 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.

How Strong Is Jupiter’s Gravity?

Jupiter is the most massive planet in our Solar System and; therefore, the gravity of Jupiter is the most intense in the Solar System. The gravity of Jupiter is 2.5 times what it is here on Earth.

In the 1990s Jupiter’s gravity tore apart Comet P/Shoemaker-Levy 9 and pulled the broken pieces into the to planet. This marked the first time that humans had direct observation of two extraterrestrial Solar System bodies colliding. Jupiter had actually captured the asteroid between 20 and 30 years prior to impact and it had been orbiting the planet since. In 1992, the asteroid entered Jupiter’s Roche limit and was broken apart by the planet’s tidal forces. The asteroid resembled a string of pearls until its fragments impacted the surface July 16-22 of 1994. The fragments were as large as 2 km each and hit the surface at 60 km/s. The impacts allowed astronomers to make several new discoveries about Jupiter.

Astronomers found several chemicals within the Jovian atmosphere that had not been seen prior to the impacts. Diatomic sulfur and carbon disulfide were the most interesting. This was only the second time that diatomic sulfur had been detected in any astronomical object. Ammonia and hydrogen sulfide were detected for the first time on Jupiter. You can read up on other discoveries made during and shortly after these impacts by reading this article and this pdf from C.A. Olano.

Some scientists, including Jacques Laskar of the Paris Observatory, as well as Konstantin Batygin and Gregory Laughlin of the University of California, Santa Cruz believe that Jupiter’s gravity may lead to the destruction of Mercury. After running some simulations the group found that Jupiter is perturbing Mercury’s already eccentric orbit. They arrived at four possible end results: Mercury will crash into the Sun, Mercury will be ejected from the solar system altogether, Mercury will crash into Venus, or Mercury will crash into Earth. None is pleasant for Mercury and the last would be even less pleasant for humans. Not to fear though, none of these possible outcomes will happen in the next 5-7 billion years anyway.

The gravity of Jupiter affects every planet to one degree or another. It is strong enough to tear asteroids apart and capture 64 moons at least. Some scientist think that Jupiter destroyed many celestial objects in the ancient past as well as prevented other planets from forming. How’s that for a powerful neighbor?

Here’s an article from Universe Today about how Jupiter’s gravity might actually wreck the Solar System, and here’s an article about how big planets like Jupiter could get.

Use this site to calculate your weight on other worlds, and here’s more information about Comet P/Shoemaker Levy 9.

We’ve also recorded an entire show just on Jupiter for Astronomy Cast. Listen to it here, Episode 56: Jupiter, and Episode 57: Jupiter’s Moons.

Sources:
http://www2.jpl.nasa.gov/sl9/
http://adsabs.harvard.edu/full/1996EM%26P…73..147H

What are Temperatures Like on Jupiter?

Jupiter, which takes its name from the father of the gods in ancient Roman mythology, is the largest planet in our Solar System. It also has the most moon’s of any solar planet – with 50 accounted for and another 17 awaiting confirmation. It has the most intense surface activity, with storms up to 600 km/h occurring in certain areas, and a persistent anticyclonic storm that is even larger than planet Earth.

And when it comes to temperature, Jupiter maintains this reputation for extremity, ranging from extreme cold to extreme hot. But since the planet has no surface to speak of, being a gas giant, it’s temperature cannot be accurately measured in one place – and varies greatly between its upper atmosphere and core.

Currently, scientists do not have exact numbers for the what temperatures are like within the planet, and measuring closer to the interior is difficult, given the extreme pressure of the planet’s atmosphere. However, scientists have obtained readings on what the temperature is at the upper edge of the cloud cover: approximately -145 degrees C.

Because of this extremely cold temperature, the atmosphere at this level is composed primarily of ammonia crystals and possibly ammonium hydrosulfide – another crystallized solid that can only exist where conditions are cold enough.

However, if one were to descend a little deeper into the atmosphere, the pressure would increases to a point where it is ten times what it is here on Earth. At this altitude, the temperature is thought to increase to a comfortable 21 °C, the equivalent to what we call “room temperature” here on Earth.

Descend further and the hydrogen in the atmosphere becomes hot enough to turn into a liquid and the temperature is thought to be over 9,700 C. Meanwhile, at the core of the planet, which is believed to be composed of rock and even metallic hydrogen, the temperature may reach as high as 35,700°C – hotter than even the surface of the Sun.

Interestingly enough, it may be this very temperature differential that leads to the intense storms that have been observed on Jupiter. Here on Earth, storms are generated by cool air mixing with warm air. Scientists believe the same holds true on Jupiter.

A close-up of Jupiter's great red spot. Credit: NASA/JPL-Caltech/ Space Science Institute
A close-up of Jupiter’s great red spot, an anticyclonic storm that is larger than Earth. Credit: NASA/JPL-Caltech/ Space Science Institute

One difference is that the jet streams that drive storms and winds on Earth are caused by the Sun heating the atmosphere. On Jupiter it seems that the jet streams are driven by the planets’ own heat, which are the result of its intense atmospheric pressure and gravity.

During its orbit around the planet, the Galileo spacecraft observed winds in excess of 600 kph using a probe it deployed into the upper atmosphere. However, even at a distance, Jupiter’s massive storms can be seen to be humungous in nature, with some having been observed to grow to more than 2000 km in diameter in a single day.

And by far, the greatest of Jupiter’s storms is known as the Great Red Spot, a persistent anticyclonic storm that has been raging for hundreds of years. At 24–40,000 km in diameter and 12–14,000 km in height, it is the largest storm in our Solar System. In fact, it is so big that Earth could fit inside it four to seven times over.

Given its size, internal heat, pressure, and the prevalence of hydrogen in its composition, there are some who wonder if Jupiter could collapse under its own mass and trigger a fusion reaction, becoming a second star in our Solar System. There are a few reasons why this has not happened, much to the chagrin of science fiction fans everywhere!

This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of liquid metallic hydrogen. Credit: Kelvinsong/Wikimedia Commons
This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of liquid metallic hydrogen. Credit: Kelvinsong/Wikimedia Commons

For starters, despite its mass, gravity and the intense heat it is believed to generate near its core, Jupiter is not nearly massive or hot enough to trigger a nuclear reaction. In terms of the former, Jupiter would have to multiply its current mass by a factor of 80 in order to become massive enough to ignite a fusion reaction.

With that amount of mass, Jupiter would experience what is known as gravitational compression (i.e. it would collapse in on itself) and become hot enough to fuse hydrogen into helium. That is not going to happen any time soon since, outside of the Sun, there isn’t even that much available mass in our Solar System.

Of course, others have expressed concern about the planet being “ignited” by a meteorite or a probe crashing into it – as the Galileo probe was back in 2003. Here too, the right conditions simply don’t exist (mercifully) for Jupiter to become a massive fireball.

While hydrogen is combustible, Jupiter’s atmosphere could not be set aflame without sufficient oxygen for it to burn in. Since no oxygen exists in the atmosphere, there is no chance of igniting the hydrogen, accidentally or otherwise, and turning the planet into a tiny star.

Scientists are striving to better understand the temperature of Jupiter in hopes that they will eventually be able to understand the planet itself. The Galileo probe helped and data from New Horizons went even further. NASA and other space agencies are planning future missions that should bring new data to light.

To learn more about Jupiter, check out this article on how weather storms on Jupiter form quickly. Here’s Hubblesite’s News Releases about Jupiter, and NASA’s Solar System Explorer.

We’ve also recorded an entire show just on Jupiter for Astronomy Cast. Listen to it here, Episode 56: Jupiter, and Episode 57: Jupiter’s Moons.

Sources:
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jupiter&Display=OverviewLong
http://www.jpl.nasa.gov/news/news.cfm?release=2008-013

How Long is a Year on Jupiter

The answer to ”how long is a year on Jupiter” is 11.86 Earth years. There is so much more to know about the Jovian system, that we can not just leave you with one fact, so here are some more interesting facts about Jupiter.

At perihelion Jupiter is 741 million km from the Sun(4.95 AU). At aphelion it is 817 million km from the Sun(5.46 AU). That gives Jupiter a semi-major axis of 778,340,821 km. Jupiter’s orbit varies by 76 million km, but it has one of the least eccentric orbits in the Solar System.

Jupiter has 2.5 times the mass of all of the other objects in the Solar System except the Sun. It is so massive that if it gained any more mass it would shrink. Gravitational compression would take over making the planet more dense instead of larger.

There are some conspiracy theorists who like to propose that Jupiter will become a star and destroy Earth. That can never happen. Jupiter would have to accrete about 80 times more mass than it has now and experience a huge increase in temperature in order to ignite fusion. The planet has the hydrogen it needs, but not the wherewithal to fuse it into helium and become a star.

Earth’s magnetic field is generated by its core through a dynamo effect. Scientist are not even sure that Jupiter has a rocky/metallic core, yet the planet has a magnetic field that is 14 times stronger than Earth’s. Astronomers think the magnetic field is generated by the churning of metallic hydrogen near the center of Jupiter. This magnetic field traps ionized particles from the solar wind and accelerates them to nearly the speed of light.

One of the most well known aspects of Jupiter is the Great Red Spot. Astronomers have been documenting it for nearly 350 years. It seems to grow and shrink over time. It is actually a giant storm that would totally engulf the Earth. At one time the storm covered an area that was 40,000 km long. It is slowly getting smaller, but astronomers do not know if it will ever disappear.

Knowing the answer to ”how long is a year on Jupiter” is just one minor detail about the planet. The others above are just a few facts that do not even scratch the surface of the Jovian mystery. None of Jupiter’s 67 moons or it ring system have been mentioned. Imagine the stories yet to be told.

Here’s a great image of Jupiter, captured by amateur astronomer Mike Salway, and an interesting hypothetical article about how Jupiter’s orbit could mess up the Solar System.

Here’s some general information on Jupiter from the Nine Planets, and more information from Solar Views.

We’ve also recorded an entire show just on Jupiter for Astronomy Cast. Listen to it here, Episode 56: Jupiter, and Episode 57: Jupiter’s Moons.

Sources:
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jupiter&Display=OverviewLong
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jupiter&Display=Facts