Posted on by Abby Cessna

Volume of the Planets

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

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There are a number of measurements that astronomers use, including mass, surface area, diameter, and radius, to determine the the size of the planets. Volume is one measurement of the size of a planet. It is a measurement of how much three-dimensional space an object occupies. The volumes of the planets, along with other measurements, help astronomers discover the physical composition of the planets in addition to other information about them.

Mercury is the littlest planet in our Solar System with the smallest volume of any planet. It has a volume of 6.083 x 1010 cubic kilometers, which is only 5.4% of Earth’s volume.

Venus is only slightly smaller than Earth with a volume of 9.38 x 1011 km3. That is 86% of the Earth’s volume. This may not seem like Venus is that close in size to our planet,  but Venus is closer in size to Earth than any other planet is.

Earth is the largest of the four inner planets, although it is nothing compared to the gas giants. Earth has a volume of 1.08 x 1012 cubic kilometers.

Mars is actually a rather small planet with a volume of 1.6 x 1011 cubic kilometers. While that is larger than Mercury’s volume and pretty big in general, it is only 15% of Earth’s volume. You could put over six planets the size of Mars inside the Earth.

The largest planet in our Solar System, Jupiter’s size is astounding. Jupiter has a volume of 1.43 x 1015 cubic kilometers. To show what this number means, you could fit 1321 Earths inside of Jupiter. It is hard to imagine how large that actually is.

Saturn is the second largest planet in the Solar System. It has a volume of 8.27 x 1014 cubic km. Although it is only a fraction of the size of Jupiter, you could fit 764 Earths inside of the gas giant.

Uranus is a large planet with a volume of 6.833 x 1013 cubic kilometers. You could fit a little more than 63 Earths inside of Uranus, but like the other gas giants, it is not very dense. Comprised mostly of gas, the planet is only about 14.5 times more massive than Earth is.

Neptune is the smallest gas giant in our Solar System, but it is still much larger than any of the inner planets. Neptune has a volume of 6.3 x 1013 cubic kilometers, which is equal to about 57 Earths. Even though Neptune’s volume is much greater than the Earth’s is, the gravity on Neptune is only about 14% greater than it is on Earth. This is due to the gas giant’s small mass.

Universe Today has articles on size of the planets and mass of the planets.

Check out an overview of the Solar System and all about the planets.

Astronomy Cast has an episode on Jupiter and episodes on all the planets.

Posted on by Abby Cessna

What are Planetoids?

Planetoid is another term for asteroids, which are also called minor planets. Planetoids are small celestial bodies that orbit the Sun. Planets are simply defined as asteroids, but the term asteroid is not well defined either. In 2006, The International Astronomical Union (IAU) defined it as  a “small Solar System body” (SSSB), which does not really tell us anything either. Webster’s Dictionary defines an asteroid as, “any of the thousands of small planets ranging from 1,000 km (621 mi) to less than one km (0.62 mi) in diameter, with orbits usually between those of Mars and Jupiter; minor planet; planetoid.”

Asteroids – planetoids – were first discovered in 1801, and many more have been discovered since then. Up until 1977, almost all the asteroids discovered were near Jupiter. However, then astronomers began to discover planetoids even farther out and started calling them centaurs and trans-Neptunian objects (TNOs). When a region of space in the outer Solar System filled with celestial bodies was discovered, it was called the Kuiper Belt and the objects in it were called Kuiper Belt Objects (KBOs). The large number of synonyms for planetoids is one reason why keeping these terms straight is so difficult.

Some of the largest planetoids are spherical and look like tiny versions of planets. The smaller ones are irregular in shape though. The objects range in size from around ten meters to hundreds of kilometers in diameter. Objects smaller than ten meters are called meteoroids. Unfortunately, astronomers do not know that much about the materials that make up planetoids. They are believed to be composed of various materials including ice, rock, and different metals.

Most planetoids are in a region called the asteroid belt, which is situated between Mars and Jupiter. There are millions of planetoids in this region. Despite the millions of objects, all of them combined are believed to have a mass of only about 4% of the Moon’s mass. After being discovered, the planetoids are given a temporary designation. If they are officially recognized, they are given a number and maybe a name. The first few planetoids were given symbols just like the planets. All except one of the first fifteen asteroids were given  extremely complex symbols. For example, one symbol was a star with a plant growing out of it. However, that soon ended when astronomers realized that there were many more planetoids. Planetoids, and other celestial bodies, are a subject of study by astronomers who hope to learn more about how the universe was formed from these ancient rocks.

Universe Today has articles on minor planets and planetesimals.

Check out articles on asteroids and planetoids beyond Pluto.

Astronomy Cast has an episode on asteroids.

References:
NASA StarChild: The Asteroid Belt
Planet-Like Body Discovered at Fringes of Our Solar System

Posted on by Abby Cessna

Mythology of the Planets

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

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Thousands of years ago, ancient civilizations turned to the heavens, marveling at their wonders. These ancient people worshipped various gods and often linked their gods with planets in the sky, which they considered to be “wandering stars.”

Mercury gets its name from the winged messenger of the gods. He was also the god of thievery, commerce, and travel. Most likely, the planet got its name from the rate at which it spins.

Venus was the Roman goddess of love and beauty, so it is a fitting name for this brightly shining planet. The only objects in our Solar System brighter than Venus are the Sun and the Moon. Ancient civilizations thought that Venus was two different objects – the Morning Star and the Evening Star. Other civilizations have also associated the planet with love. The Babylonians called the planet Ishtar after their goddess of womanhood and love.

Earth is the only planet not named after a Roman god or goddess, but it is associated with the goddess Terra Mater (Gaea to the Greeks). In mythology, she was the first goddess on Earth and the mother of Uranus. The name Earth comes from Old English and Germanic. It is derived from “eor(th)e” and “ertha,” which mean “ground.” Other civilizations all over the world also developed terms for our planet.

Mars is named after the Roman god of war. The planet got its name from the fact that it is the color of blood.  Other civilizations also named the planets for its red color.

Jupiter was the Roman king of the gods. Considering that Jupiter is the largest planet in our Solar System, it makes sense that the planet was named after the most important god.

Saturn was named after the Roman god of agriculture and harvest. While the planet may have gotten its name from its golden color, like a field of wheat, it also had to do with its position in the sky. According to mythology, the god Saturn stole the position of king of the gods from his father Uranus. The throne was then stolen by Jupiter.

Uranus was not discovered until the 1800’s, but the astronomers in that time period continued the tradition of naming planets after Roman gods. In mythology, Uranus was the father of Saturn and was at one time the king of the gods.

While Neptune almost ended up being named after one of the astronomers credited with discovering it – Verrier – that was greatly disputed, so it was named after the god of the sea. The name was probably inspired by its blue color.

Pluto is no longer a planet, but it used to be. The dark, cold, former planet was named after the god of the underworld. The first two letters of Pluto are also the initials of the man who predicted  its existence, Percival Lowell.

Universe Today has articles on names of the planets and all the planets.

For more information on the planets check out all about the planets and mythology of the planets.

Astronomy Cast has episodes on all the planets including Saturn.

Posted on by Abby Cessna

Surface of the Planets

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

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People have been intrigued for centuries by whether life could exist on other planets. While we now know that it is very unlikely that life as we know it could exist on other planets in our Solar System, many people do not know the surface conditions of these various planets.

Mercury resembles nothing so much as a larger version of the Moon. This planet is so close to the Sun that it is actually difficult to observe. The Hubble Space Telescope cannot look at it because it would permanently damage the lens.

Venus’ atmosphere of thick, toxic clouds hides the planet’s surface from view. Scientists and amateurs alike used to think that the planet was covered with thick forests and flora like tropical rainforests on Earth.  When they were finally able to send probes to the planet, they discovered that Venus’ surface was actually more like a vision of hell with a burning landscape that is dotted with volcanoes.

Mars has very diverse terrain. One of the planet’s most famous features is its canals, which early astronomers believed were “man”-made and contained water. These huge canyons were most likely formed by the planet’s crust splitting. Mars is also famous for its red color, which is iron oxide (rust) dust that covers the surface of the entire planet. The surface of Mars is covered with craters, volcanoes, and plains. The largest volcanoes of any planet are on Mars.

Jupiter is a gas giant, so it has no solid surface just a core of liquid metals. Astronomers have created a definition for the surface – the point at which the atmosphere’s pressure is one bar. This region is the lower part of the atmosphere where there are clouds of ammonia ice.

Saturn is also a gas giant so it has no solid surface only varying densities of gas. Like Jupiter, almost all of Saturn is composed of hydrogen with some helium and other elements in trace amounts.

Uranus and Neptune are also gas giants, but they belong to the subcategory of ice giants because of the “ices” in their atmospheres. Uranus’ surface gets its blue color from the methane in the atmosphere. Methane absorbs light that is red or similar to red on the color spectrum leaving only the light near the blue end of the spectrum visible.

Neptune is also blue due to the methane in its atmosphere. Its “surface” has the fastest winds of any planet in the Solar System at up to 2,100 kilometers per hour.

Universe Today has a number of articles including surface of Mars and surface of Mercury.

Check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.

Astronomy Cast has an episode on each planet including Earth.

Posted on by Abby Cessna

Protoplanets

Protoplanet by Moya

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Protoplanets are small celestial objects that are the size of a moon or a bit bigger. They are small planets, like an even smaller version of a dwarf planet. Astronomers believe that these objects form during the creation of a solar system.

The most popular theory of how a solar system is formed says that a giant cloud of molecular dust collapsed, forming one or more stars. Then a cloud of gas forms around the new star. As a result of gravity and other forces, the dust and other particles in this cloud collide and stick together forming larger masses. While some of these objects break apart on impact, a number of them continue to grow. Once they reach a certain size – around a kilometer  – these objects are large enough to attract particles and other small objects with their gravity. They continue to get larger until they form protoplanets. Some protoplanets continue colliding and growing until they form planets while others stay that size.

As the protoplanets grew to become planets, parts of them melted due to radioactivity, gravitational influences, and collisions. Where the objects had melted, the composition of the planets changed. Heavier elements sank, forming the cores of the planets, and lighter objects rose to the surface. This process is called planetary differentiation and explains why planets have heavy cores. Astronomers have discovered that even some asteroids have differentiated, so their cores are heavier than their surfaces.  

Protoplanets used to be highly radioactive due to how they were formed. However, over thousands of years, the radioactivity of these objects has greatly decreased because of radioactive decay. Astronomers are still discovering new protoplanets, and most likely, they will discover many more. With better technology, astronomers are now able to find protoplanets in other star systems. Last year, scientists discovered a protoplanet HL Tau b that will probably turn into an actual planet one day. Astronomers say that will not happen for about a million years though because the protoplanet’s star is also very young. In its final form, HL Tau b will look like Jupiter – a gas giant around the same size as that massive planet. It is hard to believe that thousands of years ago our planets were objects about the size of a moon, which were slowly evolving and growing. Astronomers continue to study protoplanets, the same way they study planetesimals, to find out more about how the Solar System was formed.

Universe Today has articles on Earth-sized planets and planetesimals.

You will also want to check out a new protoplanet and forming gas giants.

Astronomy Cast has an episode on how old the universe is.

References:
When is an Asteroid Not an Asteroid?
From Planetesimals to Terrestrial Planets: Habitable Planet Formation in Binary Star Systems

Posted on by Abby Cessna

Nine Planets

Planets in the Solar System. Image credit: NASA/JPL/IAU

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For seventy-six years, ever since Pluto was discovered in 1930, we had 9 planets in our Solar System. This all changed in 2006 when Pluto was demoted to the category of dwarf planets.

Mercury was only the second smallest planet back when there were 9. Mercury is closer to Earth than a number of other planets, but we cannot get a very good look at it because of its proximity to the Sun. Astronomers cannot use the Hubble Space Telescope to look at the planet because the Sun’s light would permanently damage the piece of equipment.

Venus is the brightest of all 9 planets. The only objects brighter in the Solar System are the Sun and the Moon. Venus is so bright that it can actually cast shadows. If the Moon is not out one night, you may be able to find some shadows thrown by the planet.

Earth is the densest of all planets in our Solar System. Our planet is composed mostly of iron, silicon, magnesium, and oxygen. Almost one-third of the planet (32.1%) is iron. There is nearly as much oxygen in the planet – 30.1%. There are lesser amounts of silicon (15.1%) and magnesium (13.9%). The materials are not spread equally throughout the planet. For example, most of the iron is in the core of the planet.

Ever since astronomers spotted what looked like canals on Mars, they have been searching for water and signs of life. While life has not been discovered yet, scientists have found deposits of water underneath the surface of the planet.

Not only is Jupiter the largest and most massive planet in our Solar System, but it is also the fastest spinning planet. Jupiter completes a full rotation in about 10 hours. The planet has actually flattened slightly at both ends due to the speed at which it spins.

Saturn does not have the most moons of any planet in our Solar System – that distinction goes to Jupiter with 63 moons – but Saturn comes in a close second with 60 moons. When Galileo first saw Saturn with a telescope, he thought that the planet’s rings were moons. Astronomers were not able to determine what the rings were until they developed better telescopes.

Uranus is the only planet to rotate on its side. This planet has the greatest axial tilt of any planet in our Solar System – 98°. As a result of this extreme tilt, the north pole is in darkness for 42 years then it gets 42 years of light before repeating the cycle.

Neptune is quite a bit larger than Earth, but its gravity is very similar. If you could stand on Neptune – you cannot because it does not actually have a surface – then you would only experience approximately 17% more gravity than you would standing on Earth.

Pluto, which was discovered in 1930, was the ninth planet. Pluto was by far the smallest planet. In fact, Pluto is even smaller than the Earth’s moon. This tiny planet was also the coldest one. Although its temperatures can drop to -240°C, the average temperatures on Pluto are -219°C.

Universe Today has articles on all 9 planets of the solar system including list of the planets and planets in the Solar System.

If you’d like more info on the nine planets, check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.

Astronomy Cast has episodes on all the 9 planets of the Solar System including Mercury.

Posted on by Abby Cessna

Giant Planets

Jupiter, seen by Cassini. Image credit: NASA/JPL

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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.

Universe Today has a number of articles to check out on gas giants and how big planets get.

You should also take a look at these articles on gas giants and British scientists discover giant planets hotter and heavier than Jupiter.

Astronomy Cast has an episode on extrasolar planets, hot Jupiters, and pulsar planets you should not miss.

Posted on by Abby Cessna

Beyond the Solar System

The Andromeda Galaxy Credit: Hubble

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You are probably somewhat familiar with our Solar System. At least you most likely know that there are eight planets in it, including the Earth, the Sun, moons, and a number of other objects like Pluto and asteroids. However, there is a lot more beyond the Solar System of which you may not be aware.

Our galaxy is the Milky Way Galaxy, but there are also other ones including the Andromeda Galaxy. Each galaxy is a system composed of different star systems, stellar remains, and interstellar medium. Although astronomers are not certain, they estimate that there are one hundred billion galaxies in the universe. Between the galaxies is intergalactic space, which has a thin gas in it. It is no wonder that the universe is considered to be infinite when you consider how large our Solar System is and that this Solar System is just one of many in our galaxy. This really puts into perspective exactly how small the Earth, and we, are in the big picture.

The Milky Way galaxy has many stars in it. Beyond our Solar System is interstellar medium and more stars along with their star systems. Interstellar medium is the vacuum of space between different star systems, although the space is not actually an empty vacuum. It has dust and other particles in it in addition to cosmic rays and magnetic fields.

Astronomers have already discovered many extrasolar planets – planets beyond our Solar System that orbit stars other than our own. The first extrasolar planet’s existence was not confirmed until 1995, because technology was not advanced enough to detect these distant planets. Since then, 357 extrasolar planets, also known as exoplanets, have been discovered. It is estimated that only a small percentage of stars have planets, and most of these stars are similar to our own Sun.

At first, the only extrasolar planets that astronomers could find were gas giants similar to Jupiter. However, in recent years, they have found planets similar to Neptune. This strengthened the hope of astronomers who were looking for Earth-like planets. In fact, some astronomers believe that they have found Earth-like planets in the past few years. Astronomers are still trying to find a way to determine whether there is life on these planets.

While there is still much more to learn in our own Solar System – the Moon is the only place besides Earth humans have actually set foot – there are also many things to discover beyond our Solar System. Not just other stars, but also other galaxies if we can reach them.

Universe Today has articles on finding a cyclops galaxy and astronomers could detect oceans on extrasolar planets.

For more information, try lightest exoplanet discovered and top 10 most intriguing extrasolar planets.

Astronomy Cast has an episode that deals with the extrasolar planets.

References:
NASA: Milky Way Galaxy
NASA Science

Posted on by Abby Cessna

All the 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.

Universe Today has many articles on the planets including order of the planets and planets in the Solar System.

If you are looking for more information, try all about the planets and an overview of the planets.

Astronomy Cast has episodes on all the planets including Jupiter.

Posted on by Abby Cessna

Planetesimals

Trojan asteroids sharing the orbits of Jupiter and Neptune. Image credit: Scott Sheppard.

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A planetesimal is an object formed from dust, rock, and other materials. The word has its roots in the concept infinitesimal, which indicates an object too small to see or measure. Planetesimals can be anywhere in size from several meters to hundreds of kilometers. The term refers to small celestial bodies formed during the creation of planets. One way to think of them is as small planets, but they are much more than that.

The planetesimal theory was suggested by the Russian astronomer Viktor Safronov. The planetesimal theory is a theory on how planets form. According to the planetesimal hypothesis, when a planetary system is forming, there is a protoplanetary disk with materials from the nebulae from which the system came. This material is gradually pulled together by gravity to form small chunks. These chunks get larger and larger until they form planetesimals. Many of the objects break apart when they collide, but some continue to grow. Some of these planetesimals go on to become planets and moons.  Since the gas giants are balls of gas with liquid cores, it may seem impossible that an asteroid-like object formed them. The planetesimals formed the core of these gaseous planets, which turned molten when it enough heat was created.

Other planetesimals turn into comets, Kuiper Belt Objects (KBOs), and trojan asteroids. There is some debate as to whether KBOs and asteroids can be called planetesimals. This is one reason why nomenclature of celestial objects is so difficult. The planetesimal theory is not universally accepted though. Like many theories, there are some observations that cannot be explained, but the planetesimal theory is still very popular.

Many people think that around 3.8 billion years ago, many of the planetesimals were thrown into far away regions, such as the Oort cloud or the Kuiper Belt. Other objects collided with other objects after being affected by gas giants. Phobos and Deimos are believed to be planetesimals that were captured by Mars’ gravity and became satellites. Many of Jupiter’s moons are believed to be planetesimals as well.

Planetesimals are very valuable to scientists because they can provide information about the creation of our Solar System. The exterior of planetesimals have been bombarded with solar radiation, which can change their chemistry, for billions of years. Inside though, there is material that has been untouched since the object was first formed. Using this material, astronomers hope to learn about the condition of the nebulae from which our Solar System was formed.

Universe Today has a number of articles to check out including formation of Mercury and hunting for meteors on Earth.

Check out NASA’s Solar System exploration page and NASA’s articles on formation of planetesimals in a nebula.

Astronomy Cast has an episode on how old the universe is.

Reference:
Wikipedia