Posted on by Abby Cessna

Discovery of the Planets

Sir William Herschel

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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.

Posted on by Abby Cessna

Names of the Planets

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.

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.

Posted on by Abby Cessna

Planetary Alignment

Planets and other objects in our Solar System. Credit: NASA.

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Thousands of years ago, men looked to the planets to guide them. Planetary alignments foretold momentous events of good and evil. In those days, planning wedding,s or even assassinations, according to the planets and stars was common in many cultures. Many people still look to the planets to tell the future. But what really is a planetary alignment and how does it happen?

There are two different ways that the term alignment is used referring to the planets in our Solar System. The first way is if you were standing on the Sun, all the planets would appear to be lined up in a row. This is what most people think of when the they hear the term planetary alignment. The other kind of alignment is if all the planets follow a straight line. In other words, it is as if a straight line can be drawn through the center of all the planets.

It is impossible for an exact alignment as viewed from the Sun – where the planets are superimposed – because of the differences in axial tilts of the planets. What we actually see in the sky is planetary configuration where the planets are in the same quadrant. It is very rare for all the planets to be within the exact same quadrant – approximately 90° of each other. Having all the planets within about 90° of each other only occurs about every 200 years, which is a long time for humans, although it is nothing for the universe. The planets do however line up loosely more often than that.

In 2000, five of the planets – Mars, Saturn, Jupiter, Mercury, and Venus were within 50° of each other. You may have heard a lot about future planetary alignments, including rumors about 12/21/2012 when some say the world is going to end, and there will be a planetary alignment. That is all it is, a rumor. There will be no planetary alignment in 2012. Many of the planets will align in a rather straight line in 2010. On June 13, 2010, Uranus, Jupiter, and Mercury will be lined up on one side of the Sun while Venus, Mars, and Saturn will be lined up on the other side. Earth will be perpendicular to the Sun at this time. In September of the year 2040, four of the planets – Saturn, Venus, Jupiter and Mars – will be roughly aligned when seen from Earth. Check out when other planetary alignments will happen, and then search for them in the sky.

Check out this site about planetary alignments and this site, which shows you the position of the planets for any date you enter.

Universe Today has a number of articles to read including galactic alignment and planets line up.

Astronomy Cast has an episode about planetary alignment.

Posted on by Abby Cessna

What is Interplanetary Space?

The heliosphere Credit: NASA/Feimer)

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The region of space within our Solar System is called interplanetary space, also known as interplanetary medium. Most people are so fascinated by the planets, Sun, and other celestial objects that they do not pay any attention to space. After all, there is nothing in outer space right? A common misconception is that outer space is a perfect vacuum, but there are actually particles in space including dust, cosmic rays, and burning plasma spread by solar winds. Particles in interplanetary space have a very low density, approximately 5 particles per cubic centimeter around Earth and the density decreases further from the Sun. The density of these particles is also affected by other factors including magnetic fields. The temperature of interplanetary medium is about 99,727°C.

Interplanetary space extends to the edge of the Solar System where it hits interstellar space and forms the heliosphere, which is a kind of magnetic bubble around our Solar System. The boundary between interplanetary space and interstellar space is known as the heliopause and is believed to be approximately 110 to 160 astronomical units (AU) from the Sun. The solar winds that blow from the Sun, and are part of the material in interplanetary space, flow all the way to the edge of the Solar System where they hit interstellar space. The magnetic particles in these solar winds interact with interstellar space and form the protective sphere.

The way that interplanetary space interacts with the planets depends on the nature of the planets’ magnetic fields. The Moon has no magnetic field, so the solar winds can bombard the satellite. Astronomers study rocks from Earth’s Moon to learn more about the effects of solar winds. So many particles have hit the Moon that it emits faint radiation. Some planets, including Earth, have their own magnetospheres where the planets’ magnetic fields override the Sun’s.  The Earth’s magnetic field deflects dangerous cosmic rays that would otherwise damage or kill life on Earth. Material leaking from the solar winds is responsible for auroras in our atmosphere. The most famous aurora is the Aurora Borealis, which appears in the sky and is only visible in the Northern Hemisphere.

Interplanetary medium also causes a number of phenomena including the zodiacal light, which appear as a faint broad band of light only seen before sunrise or after sunset. This light, brightest near the horizon, occurs when light bounces off dust particles in the interstellar medium near Earth. In addition to interplanetary space, there is also interstellar space, which is the space in a galaxy in between stars.

Universe Today has a number of articles on space including the heliosphere and zodiacal light.

Check out these articles from NASA on the heliosphere and sunspots leaking plasma into interplanetary space.

Astronomy Cast has an episode on the heliosphere and interstellar medium.

References:
NASA: Heliosphere
NASA Voyager: Interstellar Mission
What’s It Like Where Voyager Is?

Posted on by Fraser Cain

What is Mercury Made Of?

Interior of Mercury
Interior of Mercury

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Mercury is the closest planet to the Sun, orbiting at only 57.9 million km. It’s classified as one of the “rocky” or terrestrial planets of the Solar System. But what is Mercury made of? How does Mercury’s composition compare to the other planets in the Solar System.

Scientists believe that Mercury has an interior composition similar to Earth. Mercury has a large core of liquid metal surrounded by a mantle of silica and a solid outer crust. In the case of Mercury, though, the core accounts for 42% of the planet, while Earth’s core is only 17%.

It’s difficult to know the actual interior composition of Mercury. Scientists know that it has a density of 5.427 grams per cubic centimeter. This is only a little less than our own planet. That means that Mercury has an interior metal core, and the rest is rock. In fact, Mercury’s core is thought to have a higher iron content than any other planet in the Solar System. It’s believed that Mercury used to be similar in composition to a chondrite meteorite, but then it was struck by a planetesimal early in its history that stripped away its outer crust and mantle. The remaining planet was much denser.

Astronomers were unsure if Mercury’s core was solid or liquid until recently. They bounced radio signals off the surface of the planet and measured how long they took to return. They determined that Mercury was wobbling at a rate you would expect if its core was liquid and not solid.

Mercury’s surface is similar in appearance to the Moon, with large basins and impact craters. The largest crater is Caloris Basin, measuring 1,550 km across. The impact that created Caloris Basin sent shock waves around the planet, causing volcanic activity on the opposite point of the impact.

We have written many stories about what is Mercury made out of here on Universe Today. Here’s an article about a the discovery that Mercury’s core is liquid. And how Mercury is actually less like the Moon than previously believed.

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.

Qué es el mercurio hecho

Posted on by Fraser Cain

Water on Mercury

Radar image of Mercury that shows water deposits.

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There’s water on Earth, obviously. And large quantities of water ice on Mars and in the outer Solar System. But astronomers have wondered, is there water on Mercury? There obviously aren’t any lakes and oceans of water on Mercury. We would have detected them from Earth, and definitely would have seen them when the various missions to Mercury skimmed past the planet, taking close up photographs. But does Mercury have water in any form?

The conditions on Mercury sure rule it out. As Mercury slowly rotates, the side facing the Sun experiences extremely high temperatures. At noon on the equator, the temperature rises to 700 Kelvin. And then dips down to just 100 Kelvin at night, since there’s no atmosphere to hold in the temperature. Any water on the surface of Mercury would boil away quickly and escape into space because of Mercury’s low gravity, and the constantly blowing solar wind.

But you might be surprised to know that astronomers have discovered water on Mercury. Not liquid water, but deposits of water ice at the planet’s poles. This is because there are craters at the north and south poles of Mercury which are eternally in shadow. The ice in these craters is never warmed by the Sun, and so it always remains frozen at the bottom of these craters.

When NASA’s MESSENGER spacecraft flew past Mercury in 2008, it also discovered the presence of water vapor in the thin atmosphere that surrounds Mercury. This atmosphere, or more precisely “exosphere”, is created when particles from the Sun’s solar wind bombard Mercury and kick up atoms into its atmosphere. It’s possible that the solar wind is kicking up water from the ice deposits at Mercury’s poles, or maybe it’s coming from cometary fragments. Or maybe the solar wind is depositing the oxygen and hydrogen atoms on Mercury’s surface in the first place.

So there is water on Mercury, just not very much.

We have written many stories about Mercury here on Universe Today. Here’s an article about a the discovery that Mercury’s core is liquid. And how Mercury is actually less like the Moon than previously believed.

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.

De agua sobre el mercurio

References:
NASA: Ice on Mercury
NASA Space Math: Is there Ice on Mercury?

Posted on by Stephanie Bender

Continental Crust

The crust is the top layer of the Earth’s Surface. Did you know that there are 2 types, though? One is called the Oceanic Crust, and the other, the Continental Crust. As its name suggests, the Oceanic Crust is the top layer of Earth that forms the ocean floor. The Continental Crust, however, will be our focus.

We walk on top of and dig down through the Continental Crust when we plant or drill. Even if there is an unstable surface at the very top, like sand, the deeper parts of the Crust are made of harder rocks. The large land masses, continents, have bases made from sedimentary, igneous, or metamorphic rocks, as well as any combination thereof. This shield rock is the oldest known; it’s been tested, dated, and found to have been here for 3,960,000,000 years!

Geologists, scientists who study the Earth, believe that shield rock was created when hot molten iron, known as magma cooled. If their math’s correct, it happened around the time these rocks formed, almost 4 billion years ago, right? Some of those rocks were so big it took a long time for them to cool. So, even if the rocks were formed 3.9 billion years ago, they might not have cooled for quite some time. Many estimate that the Continental Crust wasn’t completely hard for another 60,000,000 to 160,000,000 years.

The top portion of this rock has another name, platform rock. The oldest-known platform rocks are approximately 600,000,000 years old, and can be found in central North America. The sedimentary rock ranges from 1,000 to 2,000 meters thick; that is equivalent to more than a half mile to 1.25 miles. When we put the top and bottom portions of the Continental Crust together, we get what scientists call, a craton. Most cratons are stable and haven’t been damaged by earthquakes or volcanoes for hundreds of millions of years.

Around the edges are the continental margins, mostly created by sedimentary rock originally found in the oceans. How is that possible, you ask? Well, it’s due to earthquake and volcanic activity. In this case, it’s mainly due to a phenomenon called, subduction. You see, the Earth fits together like a puzzle; and, if you try to place the wrong piece into a spot where it fits, but isn’t quite right, what happens? Another piece might pop out of place. Sometimes, a continental margin works its way under the oceanic crust. When that occurs, the oceanic layer ends up on top of the continental margin. This is subduction. The most well-known place for this is along The Ring of Fire, an area that covers the edges along the Pacific Ocean. This is why so many and such violent earthquakes, volcanic eruptions, and tsunamis occur in that part of the world.

Universe Today has a wealth of information on this and other related topics. Here are just 2 of those available. The first is entitled,
Earth, Barely Habitable?.

The second is called, Interesting Facts About Planet Earth.

Universe Today also hosts Astronomy Cast, a science program that covers a variety of subjects. Episode 51: Earth, explains this subject in greater detail.

The Encyclopedia of Earth , by Michael Pidwirny has some excellent information, too.

Sources:
USGS
Science Daily

Posted on by Fraser Cain

Closest Planet to Mercury

A different side of Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

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What is the closest planet to Mercury? As you probably know, Mercury is the closest planet to the Sun, orbiting at an average distance of 57.9 million km. So then, what planet gets closest to Mercury in orbit.

The answer, of course is Venus, the 2nd closest planet to the Sun. Venus orbits at an average distance of 108.2 million km. So, how close can Mercury and Venus get? Well when I said that Mercury orbits at 57.9 million km, that’s actually an average. Mercury follows an elliptical orbit around the Sun. At its closest point, Mercury gets to within 46 million km, and then at its most distant point, it gets to be 69.8 million km.

And Venus also travels in an elliptical orbit, varying between 107.4 million km and 108.9 million km. So, when the two planets are closest to one another in orbit, the distance between them is about 37.6 million km. And at their most distant point, they can get to be 178.7 million km.

Can Earth ever be closer to Mercury than Venus? Earth’s orbit ranges between 147.1 and 152.1 million km. So when Earth and Mercury are at their closest point, they’re only separated by 77.3 million km. What this means is that there are times when Earth is actually closer to Mercury than Venus; although, most of the time, Venus is the closest planet.

We have written many stories about Mercury here on Universe Today. Here’s an article about a side of Mercury never before seen by spacecraft, and how Mercury is actually less like the Moon than previously believed.

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.

Más cercano al planeta Mercurio

Posted on by Abby Cessna

The Brightest Planet

Venus. From the Pioneer Venus Orbiter

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When you look up into the sky, if you are lucky, you see shining stars on a field of blue-black. Not all of these objects are stars though. Venus, which can be seen with the unaided eye from Earth, is the brightest planet in our Solar System. Venus was given the nickname evening star and morning star because of its bright, consistent presence. It is often called Earth’s twin because it is similar in shape and size to our own planet. The only objects brighter than Venus are the Moon and the Sun. Sirius, the brightest star that can be seen in the sky, is much dimmer than Venus is. People have known that Venus existed for centuries. The planet was named after the Roman goddess of love and this shining planet has long been associated with femininity.

A planet’s brightness is determined by how much light is reflected by the planet. The term albedo refers to the balance between how much light is absorbed and how much light is reflected by the planet. As the brightest planet, Venus has a very high albedo. Seventy percent of the light that hits Venus is reflected back into space. Why does Venus have such a high albedo though? There are droplets of sulfuric acid and acidic crystals in the atmosphere of Venus. The smooth surfaces of these droplets of sulfuric acid and crystals reflect light very well, which is one reason why Venus is so bright. There is a lot more to this planet than its shimmering appearance though.

The same atmosphere that causes Venus to shine like a beacon also prevents us from getting a look at the planet’s surface. Astronomers did not know what the planet looked like until probes checked out the surface in the last few decades. Because the surface of Venus could not be seen, scientists and writers imagined that it was a tropical, lush landscape. That was not the case though. In addition to hiding the surface and reflecting light, Venus’ atmosphere traps heat from the Sun, turning the planet into a raging furnace. Reaching temperatures of over 460°C, Venus is the hottest planet in the Solar System. Its landscape is similar to that of Mercury and the Moon – rocky, barren terrain with no sign of life. Venus has long enchanted viewers with its beauty and shining presence. Try looking for this planet in the sky; you may find yourself captivated by this shining beauty like many before you.

Universe Today has many articles on Venus including the atmosphere of Venus  and how to find Venus in the sky.

For more information check out these articles from NASA on an overview of Venus and from Nine Planets on the brightest planet.

Astronomy Cast has an episode on Venus you should take a look at.

Posted on by Fraser Cain

Does Mercury Have Rings?

Mercury's limb. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

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Think about the planets in the Solar System; Saturn has rings, so do Jupiter, Uranus and Neptune. So what about Mercury? Does planet Mercury have rings?

Sorry, Mercury doesn’t have rings right now. I say right now, because it’s possible that Mercury has had rings in the past, and it could have them again in the future.

There are two ways that scientists think a planet can get rings. The first is the icy rings around Saturn, Jupiter, Uranus and Neptune. Scientists think that Saturn’s rings come from an icy object or moon that was crushed under Saturn’s gravity, becoming its rings. It’s also possible that the ring material just formed in place back when the Solar System formed. We also know that some of Saturn’s moons, like Enceladus, are continuing to spew fresh material into the rings.

Unfortunately, Mercury could never get rings like this. That’s because it’s too close to the Sun. The powerful solar winds blast out from the Sun, and would melt and destroy any icy rings around Mercury. In fact, ice can’t exist closer to the Sun than the asteroid belt, unless it’s hidden in shadow, or protected by an atmosphere like we have on Earth.

The second way for a ring to form is for an asteroid or moon to get too close to a planet and be torn apart by gravity. For a few million years this crushed asteroid would appear as a ring around Mercury, until the chunks finally fell down onto the planet. Astronomers predict that the Martian moon Phobos will eventually suffer this fate, be torn apart by Mars’ gravity, and form a ring for a while.

Mercury doesn’t have any moons, and there aren’t many asteroids that might interact with it, so it might never get a ring – but maybe one day. One thing’s for sure, though, there are no Mercury rings today.

How many rings does Mercury have? Zero.

We have written many stories about rings here on Universe Today. Here’s an article about the Martian moon Phobos might only have 10 million years to live, and here’s an article about what Saturn’s rings are made of.

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.

Mercurio no tiene anillos?

Reference:
NASA Science for Kids