How do planetary geologists learn about the geology of other planets? Since much of the available information is from spacecraft flying high above the planet, these scientists compare images taken of the planet with images taken on Earth. Look at the images above and see if you can find the similarities and differences. Which is from Earth and which from another planet?
It’s easy to figure out that the top image is Earth since there are plants. It is Devil’s Tower in Wyoming and the bottom image is Marte Vallis on Mars. The features that are common to the images are columnar basalts showing columnar jointing and talus slopes. Columnar basalts form from basalt lavas that flow in a thick layer along a cool surface. The vertical joints speed cooling. Weathering causes the columns to break and form talus slopes.
On Mars the columnar basalts are exposed on the rim of a 160 km diameter crater. Evidence of the lava flows covers more than 200 square km (77 square miles), similar to terrestrial flood basalts. A meteorite impact appears to have created a crater to expose the columnar basalts.
Devil’s Tower is a magma that cooled below the surface in sedimentary rock layers and formed columnar joints. The sedimentary rocks have since been eroded, exposing the tower. At the base are slopes of broken columns creating talus slopes.
Humans' view of the solar system has evolved as technology and scientific knowledge have increased. The ancient Greeks identified five of the planets and for many centuries they were the only planets known. Since then, scientists have discovered two more planets, many other solar-system objects and even planets found outside our solar system.
The ancient Greeks believed that Earth was at the center of the universe, as shown in Figure below . This view is called the geocentric model of the universe. Geocentric means "Earth-centered." In the geocentric model, the sky, or heavens, are a set of spheres layered on top of one another. Each object in the sky is attached to a sphere and moves around Earth as that sphere rotates. From Earth outward, these spheres contain the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. An outer sphere holds all the stars. Since the planets appear to move much faster than the stars, the Greeks placed them closer to Earth.
Figure 25.1
Model of a geocentric universe. This diagram of the universe from the Middle Ages shows Earth at the center, with the Moon, the Sun, and the planets orbiting Earth.
The geocentric model worked well, by explaining why all the stars appear to rotate around Earth once per day. The model also explained why the planets move differently from the stars and from each other.
One problem with the geocentric model is that some planets seem to move backwards (in retrograde) instead of in their usual forward motion around Earth.
A demonstration animation of retrograde motion of Mars as it appears to Earth can be found here:
http://projects.astro.illinois.edu/data/Retrograde/index.html
Around 150 A.D. the astronomer Ptolemy resolved this problem by using a system of circles to describe the motion of planets ( Figure below ). In Ptolemy’s system, a planet moves in a small circle, called an epicycle. This circle moves around Earth in a larger circle, called a deferent. Ptolemy’s version of the geocentric model worked so well that it remained the accepted model of the universe for more than a thousand years.
Figure 25.2
According to Ptolemy, a planet moves on a small circle (epicycle) that in turn moves on a larger circle (deferent) around Earth.
An animation of Ptolemy’s system is seen here:
http://www.youtube.com/watch?v=FHSWVLwbbNw&NR=1
Ptolemy’s geocentric model worked but it was not only complicated, it occasionally made errors in predicting the movement of planets. At the beginning of the 16th century A.D., Nicolaus Copernicus proposed that Earth and all the other planets orbit the Sun. With the Sun at the center, this model is called the heliocentric model or "sun-centered" model of the universe ( Figure below ). Copernicus’ model explained the motion of the planets as well as Ptolemy’s model did, but it did not require complicated additions like epicycles and deferents.
Figure 25.3
Unlike the geocentric model, the heliocentric model had the Sun at the center and did not require epicycles.
Although Copernicus’ model worked more simply than Ptolemy’s, it still did not perfectly describe the motion of the planets because, like Ptolemy, Copernicus thought planets moved in perfect circles. Not long after Copernicus, Johannes Kepler refined the heliocentric model so that the planets moved around the Sun in ellipses (ovals), not circles ( Figure below ). Kepler’s model matched observations perfectly.
Animation of Kepler’s Laws of Planetary Motion: http://projects.astro.illinois.edu/data/KeplersLaws/index.html
Figure 25.4
Because people were so used to thinking of Earth at the center of the universe, the heliocentric model was not widely accepted at first. However, when Galileo Galilei first turned a telescope to the heavens in 1610, he made several striking discoveries. Galileo discovered that the planet Jupiter has moons orbiting around it. This provided the first evidence that objects could orbit something besides Earth.
An animation of three of Jupiter’s moons orbiting the planet is seen here: http://upload.wikimedia.org/wikipedia/commons/e/e7/Galilean_moon_Laplace_resonance_animation_de.gif
Galileo also discovered that Venus has phases like the Moon ( Figure below ), which provides direct evidence that Venus orbits the Sun.
Figure 25.5
The phases of Venus.
Galileo’s discoveries caused many more people to accept the heliocentric model of the universe, although Galileo himself was found guilty of heresy for his ideas. The shift from an Earth-centered view to a Sun-centered view of the universe is referred to as the Copernican Revolution.
Watch this animation of the Ptolemaic and Copernican models of the solar system. Ptolemy made the best model he could with the assumption that Earth was the center of the universe, but by letting that assumption go, Copernicus came up with a much simpler model. Before people would accept that Copernicus was right, they needed to accept that the Sun was the center of the solar system
(1n - I&E Stand.)
:
http://www.youtube.com/watch?v=VyQ8Tb85HrU&feature=related
(0:47).
Today, we know that our solar system is just one tiny part of the universe as a whole. Neither Earth nor the Sun are at the center of the universe. However, the heliocentric model accurately describes the solar system. In our modern view of the solar system, the Sun is at the center, with the planets moving in elliptical orbits around the Sun. The planets do not emit their own light, but instead reflect light from the Sun.
Since the early 1990s, astronomers have discovered other solar systems, with planets orbiting stars other than our own Sun (called "extrasolar planets" or simply "exoplanets") ( Figure below ).
Figure 25.6
The extrasolar planet Fomalhaut is surrounded by a large disk of gas. The disk is not centered on the planet, suggesting that another planet may be pulling on the gas as well.
Some extrasolar planets have been directly imaged, but most have been discovered by indirect methods. One technique involves detecting the very slight motion of a star periodically moving toward and away from us along our line-of-sight (also known as a star’s "radial velocity"). This periodic motion can be attributed to the gravitational pull of a planet or, sometimes, another star orbiting the star.
An animation showing how the orbit of a smaller body, such as a planet or small star, can be identified by the effect it has on the orbit of a larger star is seen here from above: http://upload.wikimedia.org/wikipedia/commons/5/59/Orbit3.gif. This is in line with the plane of the system: http://en.wikipedia.org/wiki/File:Dopspec-inline.gif
A planet may also be identified by measuring a star’s brightness over time. A temporary, periodic decrease in light emitted from a star can occur when a planet crosses in front of (or "transits") the star it is orbiting, momentarily blocking out some of the starlight.
More than 500 extrasolar planets have been identified and the rate of discovery is increasing rapidly.
Extrasolar Planet
from the ESA discusses extrasolar planets and particularly a planetary system very similar to our solar system
(1g)
:
http://www.youtube.com/watch?v=ouJahDONTWc
http://www.youtube.com/watch?v=ouJahDONTWc
] (3:29).
An introduction to extrasolar planets from NASA is available at
(1g)
:
http://www.youtube.com/watch?feature=player_profilepage&v=oeeZCHDNTvQ
(3:14).
Since the time of Copernicus, Kepler, and Galileo, we have learned a lot more about our solar system. Astronomers have discovered two more planets (Uranus and Neptune), four dwarf planets (Ceres, Makemake, Pluto, and Eris), more than 150 moons, and many, many asteroids and other small objects.
( Figure below ) shows the Sun and the major objects that orbit the Sun. There are eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and the three known dwarf planets (Ceres, Pluto, and Eris).
Figure 25.7
Relative sizes of the Sun, planets and dwarf planets. The relative sizes are correct and their position relative to each other is correct, but the relative distances are not correct.
Although the Sun is just an average star compared to other stars, it is by far the largest object in the solar system. The Sun is more than 500 times the mass of everything else in the solar system combined! Table below gives data on the sizes of the Sun and planets relative to Earth.
Table 25.1
| Object | Mass (Relative to Earth) | Diameter of Planet (Relative to Earth) |
| Sun | 333,000 Earth's mass | 109.2 Earth's diameter |
| Mercury | 0.06 Earth's mass | 0.39 Earth's diameter |
| Venus | 0.82 Earth's mass | 0.95 Earth's diameter |
| Earth | 1.00 Earth's mass | 1.00 Earth's diameter |
| Mars | 0.11 Earth's mass | 0.53 Earth's diameter |
| Jupiter | 317.8 Earth's mass | 11.21 Earth's diameter |
| Saturn | 95.2 Earth's mass | 9.41 Earth's diameter |
| Uranus | 14.6 Earth's mass | 3.98 Earth's diameter |
| Neptune | 17.2 Earth's mass | 3.81 Earth's diameter |
Figure below shows the relative sizes of the orbits of the planets, asteroid belt, and Kuiper belt. In general, the farther away from the Sun, the greater the distance from one planet’s orbit to the next. The orbits of the planets are not circular but slightly elliptical with the Sun located at one of the foci ( Figure below ).
Figure 25.8
The relative sizes of the orbits of planets in the solar system. The inner solar system and asteroid belt is on the upper left. The upper right shows the outer planets and the Kuiper belt.
Figure 25.9
The planets orbit the Sun in regular paths.
While studying the solar system, Johannes Kepler discovered the relationship between the time it takes a planet to make one complete orbit around the Sun, its "orbital period," and the distance from the Sun to the planet. If the orbital period of a planet is known, then it is possible to determine the planet’s distance from the Sun. This is how astronomers without modern telescopes could determine the distances to other planets within the solar system.
Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as the distance from Earth to the Sun. 1 AU equals about 150 million km, or 93 million mi. Table below shows the distances to the planets (the average radius of orbits) in AU. The table also shows how long it takes each planet to spin on its axis (the length of a day) and how long it takes each planet to complete an orbit (the length of a year); in particular, notice how slowly Venus rotates relative to Earth.
Table 25.2
| Planet | Average Distance from Sun (AU) | Length of Day (In Earth Days) | Length of Year (In Earth Years) |
| Mercury | 0.39 AU | 56.84 days | 0.24 years |
| Venus | 0.72 | 243.02 | 0.62 |
| Earth | 1.00 | 1.00 | 1.00 |
| Mars | 1.52 | 1.03 | 1.88 |
| Jupiter | 5.20 | 0.41 | 11.86 |
| Saturn | 9.54 | 0.43 | 29.46 |
| Uranus | 19.22 | 0.72 | 84.01 |
| Neptune | 30.06 | 0.67 | 164.8 |
How old are you on Earth? How old would you be if you lived on Jupiter? How many days is it until your birthday on Earth? How many days until your birthday if you lived on Saturn?
Scaling the solar system creates a scale to measure all objects in solar system
(1i - I&E Stand.)
:
http://www.youtube.com/watch?feature=player_profilepage&v=-6szEDHMxP4
(4:44).
Isaac Newton was one of the first scientists to explore gravity. He understood that the Moon circles the Earth because a force is pulling the Moon toward Earth’s center. Without that force, the Moon would continue moving in a straight line off into space. Newton also came to understand that the same force that keeps the Moon in its orbit is the same force that causes objects on Earth to fall to the ground.
Newton defined the Universal Law of Gravitation, which states that a force of attraction, called gravity, exists between all objects in the universe ( Figure below ). The strength of the gravitational force depends on how much mass the objects have and how far apart they are from each other. The greater the objects’ mass, the greater the force of attraction; in addition, the greater the distance between the objects, the smaller the force of attraction.
Figure 25.10
The force of gravity exists between all objects in the universe; the strength of the force depends on the mass of the objects and the distance between them.
The distance between the Sun and each of its planets is very large, but the Sun and each of the planets are also very large. Gravity keeps each planet orbiting the Sun because the star and its planets are very large objects. The force of gravity also holds moons in orbit around planets.
There are two additional key features of the solar system:
1. All the planets lie in nearly the same plane, or flat disk like region.
2. All the planets orbit in the same direction around the Sun.
These two features are clues to how the solar system formed.
The most widely accepted explanation of how the solar system formed is called the nebular hypothesis . According to this hypothesis, the Sun and the planets of our solar system formed about 4.6 billion years ago from the collapse of a giant cloud of gas and dust, called a nebula .
The nebula was drawn together by gravity, which released gravitational potential energy. As small particles of dust and gas smashed together to create larger ones, they released kinetic energy. As the nebula collapsed, the gravity at the center increased and the cloud started to spin because of its angular momentum. As it collapsed further, the spinning got faster, much as an ice skater spins faster when he pulls his arms to his sides during a spin.
Much of the cloud’s mass migrated to its center but the rest of the material flattened out in an enormous disk, as shown in Figure below . The disk contained hydrogen and helium, along with heavier elements and even simple organic molecules.
Figure 25.11
An artist
As gravity pulled matter into the center of the disk, the density and pressure at the center became intense. When the pressure in the center of the disk was high enough, nuclear fusion began. A star was born—the Sun. The burning star stopped the disk from collapsing further.
Meanwhile, the outer parts of the disk were cooling off. Matter condensed from the cloud and small pieces of dust started clumping together. These clumps collided and combined with other clumps. Larger clumps, called planetesimals, attracted smaller clumps with their gravity. Gravity at the center of the disk attracted heavier particles, such as rock and metal and lighter particles remained further out in the disk. Eventually, the planetesimals formed protoplanets, which grew to become the planets and moons that we find in our solar system today.
Because of the gravitational sorting of material, the inner planets — Mercury, Venus, Earth, and Mars — formed from dense rock and metal. The outer planets — Jupiter, Saturn, Uranus and Neptune — condensed farther from the Sun from lighter materials such as hydrogen, helium, water, ammonia, and methane. Out by Jupiter and beyond, where it’s very cold, these materials form solid particles.
The nebular hypothesis was designed to explain some of the basic features of the solar system:
This video, from the ESA, discusses the Sun, planets, and other bodies in the Solar System and how they formed
(1a, 1d)
. The first part of the video explores the evolution of our view of the solar system starting with the early Greeks who reasoned that since some points of light - which they called planets - moved faster than the stars, they must be closer:
http://www.youtube.com/watch?v=-NxfBOhQ1CY&feature=player_profilepage
(8:34).
1. What does geocentric mean?
2. Describe the geocentric model and heliocentric model of the universe.
3. How was Kepler’s version of the heliocentric model different from Copernicus’?
4. Name the eight planets in order from the Sun outward. Which are the inner planets and which are the outer planets?
5. Compare and contrast the inner planets and the outer planets.
6. What object used to be considered a planet, but is now considered a dwarf planet? What are the other dwarf planets?
7. What keeps planets and moons in their orbits?
8. How old is the solar system? How old is Earth?
9. Use the nebular hypothesis to explain why the planets all orbit the Sun in the same direction.
What evidence do planetary geologists have to go on to determine the geology of the inner planets? On Earth, scientists can collect and analyze the chemistry of samples, do radiometric dating to determine their ages, and look at satellite images to see large-scale features. Rovers have landed on Mars and sent back enormous amounts of information but much of the rest of what is known about the inner planets is from satellite images.
The inner planets , or terrestrial planets , are the four planets closest to the Sun: Mercury, Venus, Earth, and Mars. Figure below shows the relative sizes of these four inner planets.
Figure 25.12
This composite shows the relative sizes of the four inner planets. From left to right, they are Mercury, Venus, Earth, and Mars.
Unlike the outer planets, which have many of satellites, Mercury and Venus do not have moons, Earth has one, and Mars has two. Of course, the inner planets have shorter orbits around the Sun, and they all spin more slowly. Geologically, the inner planets are all made of cooled igneous rock with iron cores, and all have been geologically active, at least early in their history. None of the inner planets has rings.
Although Earth is the third planet out from the Sun this lesson will start here. We know a lot more about Earth, so what we know can be used for comparison with the other planets.
Figure 25.13
What are Earth
As you can see in ( Figure above ), Earth has vast oceans of liquid water, large masses of exposed land, and a dynamic atmosphere with clouds of water vapor. Earth also has ice covering its polar regions. Earth’s average surface temperature is 14 o C (57 o F). Water is a liquid at this temperature, but the planet also has water in its other two states, solid and gas. The oceans and the atmosphere help keep Earth’s surface temperatures fairly steady.
As yet Earth is the only planet known to have life. The presence of liquid water, the ability of the atmosphere to filter out harmful radiation, and many other features make the planet uniquely suited to harbor life. Life and Earth now affect each other; for example, the evolution of plants allowed oxygen to enter the atmosphere in large enough quantities for animals to evolve. Although life has not been found elsewhere in the solar system, other planets or satellites may harbor primitive life forms. Life may also be found elsewhere in the universe.
The heat that remained from the planet’s accretion, gravitational compression, and radioactive decay allowed the Earth to melt, probably more than once. As it subsequently cooled, gravity pulled metal into the center to create the core. Heavier rocks formed the mantle and lighter rocks formed the crust.
Earth’s crust is divided into tectonic plates, which move around on the surface because of the convecting mantle below. Movement of the plates causes other geological activity, such as earthquakes, volcanoes, and the formation of mountains. The locations of these features are mostly related to current or former plate boundaries. Earth is the only planet known to have plate tectonics.
Earth rotates on its axis once per day , by definition. Earth orbits the Sun once every 365.24 days, which is defined as a year . Earth has one large moon, which orbits Earth once every 29.5 days, a period known as a month.
Earth’s moon is the only large moon orbiting a terrestrial planet in the solar system. The Moon is covered with craters; it also has large plains of lava. The huge number of craters suggests that Moon’s surface is ancient. There is evidence that the Moon formed when a large object — perhaps as large as the planet Mars — struck Earth in the distant past ( Figure below ).
Figure 25.14
Besides its Moon, Earth is orbited by a great deal of space debris, the remains of satellites, and rocket stages.
The smallest planet, Mercury, is the planet closest to the Sun. Because Mercury is so close to the Sun, it is difficult to observe from Earth, even with a telescope. However, the Mariner 10 spacecraft, shown in Figure below , visited Mercury from 1974 to 1975. The MESSENGER spacecraft has been studying Mercury in detail since 2005. The craft is currently in orbit around the planet, where it is creating detailed maps. MESSENGER stands for Mercury Surface, Space Environment, Geochemistry and Ranging.
Figure 25.15
(a) Mariner 10 made three flybys of Mercury in 1974 and 1975. (b) A 2008 image of compiled from a flyby by MESSENGER.
As Figure below shows, the surface of Mercury is covered with craters, like Earth’s moon. Ancient impact craters means that for billions of years Mercury hasn’t changed much geologically. Also, with very little atmosphere, the processes of weathering and erosion do not wear down structures on the planet.
Figure 25.16
Mercury is covered with craters, like Earth
There are many images, movies and activities on the MESSENGER site: http://messenger.jhuapl.edu/index.php
Mercury is named for the Roman messenger god, who could run extremely quickly, just as the planet moves very quickly in its orbit around the Sun. A year on Mercury — the length of time it takes to orbit the Sun — is just 88 Earth days.
Despite its very short years, Mercury has very long days. A day is defined as the time it takes a planet to turn on its axis. Mercury rotates slowly on its axis, turning exactly three times for every two times it orbits the Sun. Therefore, each day on Mercury is 57 Earth days long. In other words, on Mercury, a year is only a Mercury day and a half long!
Mercury is close to the Sun, so it can get very hot. However, Mercury has virtually no atmosphere, no water to insulate the surface, and it rotates very slowly. For these reasons, temperatures on the surface of Mercury vary widely. In direct sunlight, the surface can be as hot as 427 o C (801 o F). On the dark side, or in the shadows inside craters, the surface can be as cold as 183 o C (297 o F)! Although most of Mercury is extremely dry, scientists think there may be a small amount of water in the form of ice at the poles of Mercury, in areas that never receive direct sunlight.
Figure below shows a diagram of Mercury’s interior. Mercury is one of the densest planets. It’s relatively large, liquid core, made mostly of melted iron, takes up about 42% of the planet's volume.
Figure 25.17
Mercury contains a thin crust, a mantle, and a large, liquid core that is rich in iron.
Named after the Roman goddess of love, Venus is the only planet named after a female. Venus’ thick clouds reflect sunlight well so Venus is very bright. When it is visible, Venus is the brightest object in the sky besides the Sun and the Moon. Because the orbit of Venus is inside Earth’s orbit, Venus always appears close to the Sun. When Venus rises just before the Sun rises, the bright object is called the morning star. When it sets just after the Sun sets, it is the evening star.
Of the planets, Venus is most similar to Earth in size and density. Venus is also our nearest neighbor. The planet’s interior structure is similar to Earth’s with a large iron core and a silicate mantle ( Figure below ). But the resemblance between the two inner planets ends there.
Find out more about Venus at the following link: http://www.nasa.gov/worldbook/venus_worldbook.html
Figure 25.18
Venus rotates in a direction opposite the other planets and opposite to the direction it orbits the Sun. This rotation is extremely slow, only one turn every 243 days. This is longer than a year on Venus—it takes Venus only 224 days to orbit the Sun.
Venus is covered by a thick layer of clouds, as shown in pictures of Venus taken at ultraviolet wavelengths ( Figure below ).
Figure 25.19
This ultraviolet image from the Pioneer Venus Orbiter shows thick layers of clouds in the atmosphere of Venus.
Venus' clouds are not made of water vapor like Earth’s clouds. Clouds on Venus are made mostly of carbon dioxide with a bit of sulfur dioxide — and they also contain corrosive sulfuric acid. Because carbon dioxide is greenhouse gas, the atmosphere traps heat from the Sun and creates a powerful greenhouse effect. Even though Venus is further from the Sun than Mercury, the greenhouse effect makes Venus the hottest planet. Temperatures at the surface reach 465 o C (860 o F). That’s hot enough to melt lead.
The atmosphere of Venus is full of acid, its pressure is crushing, and the enormous amount of carbon dioxide causes runaway greenhouse effect
(4d)
:
http://www.youtube.com/watch?v=HqFVxWfVtoo
(2:05).
The atmosphere of Venus is so thick that the atmospheric pressure on the planet’s surface is 90 times greater than the atmospheric pressure on Earth’s surface. The dense atmosphere totally obscures the surface of Venus, even from spacecraft orbiting the planet.
Since spacecraft cannot see through the thick atmosphere, radar is used to map Venus’ surface. Many features found on the surface are similar to Earth and yet are very different. Figure below shows a topographical map of Venus produced by the Magellan probe using radar.
Figure 25.20
This false color image of Venus was made from radar data collected by the Magellan probe between 1990 and 1994. What features can you identify?
Orbiting spacecraft have used radar to reveal mountains, valleys, and canyons. Most of the surface has large areas of volcanoes surrounded by plains of lava. In fact, Venus has many more volcanoes than any other planet in the solar system and some of those volcanoes are very large.
Most of the volcanoes are no longer active, but scientists have found evidence that there is some active volcanism ( Figure below ). Think about what you know about the geology of Earth and what produces volcanoes. What does the presence of volcanoes suggest about the geology of Venus? What evidence would you look for to find the causes of volcanism on Venus?
Figure 25.21
This image of the Maat Mons volcano with lava beds in the foreground was generated by a computer from radar data. The reddish-orange color is close to what scientists think the color of sunlight would look like on the surface of Venus.
Venus also has very few impact craters compared with Mercury and the Moon. What is the significance of this? Earth has fewer impact craters than Mercury and the Moon too. Is this for the same reason that Venus has fewer impact craters?
It’s difficult for scientists to figure out the geological history of Venus. The environment is too harsh for a rover to go there. It is even more difficult for students to figure out the geological history of a distant planet based on the information given here. Still we can piece together a few things.
On Earth, volcanism is generated because the planet’s interior is hot. Much of the volcanic activity is caused by plate tectonic activity. But on Venus, there is no evidence of plate boundaries and volcanic features do not line up the way they do at plate boundaries.
Because the density of impact craters can be used to determine how old a planet’s surface is, the small number of impact craters means that Venus’ surface is young. Scientists think that there is frequent, planet-wide resurfacing of Venus with volcanism taking place in many locations. The cause is heat that builds up below the surface that has no escape until finally it destroys the crust and results in volcanoes.
Mars is the fourth planet from the Sun, and the first planet beyond Earth’s orbit ( Figure below ). Mars is a quite different from Earth and yet more similar than any other planet. Mars is smaller, colder, drier, and appears to have no life, but volcanoes are common to both planets and Mars has many.
Mars is easy to observe so Mars has been studied more thoroughly than any other extraterrestrial planet. Space probes, rovers, and orbiting satellites have all yielded information to planetary geologists. Although no humans have ever set foot on Mars, both NASA and the European Space Agency have set goals of sending people to Mars sometime between 2030 and 2040.
Find out all you want to know about Mars at http://mars.jpl.nasa.gov/extreme/ .
Figure 25.22
This image of Mars, taken by the Hubble Space Telescope in October, 2005, shows the planet
Viewed from Earth, Mars is reddish in color. The ancient Greeks and Romans named the planet after the god of war. But the surface is not red from blood but from large amounts of iron oxide in the soil.
The Martian atmosphere is very thin relative to Earth’s and has much lower atmospheric pressure. Although the atmosphere is made up mostly of carbon dioxide, the planet has only a weak greenhouse effect so temperatures are only slightly higher than if the planet had no atmosphere.
Mars has mountains, canyons, and other features similar to Earth. Some of these surface features are amazing for their size! Olympus Mons is a shield volcano, similar to the volcanoes that make up the Hawaiian Islands. But Olympus Mons is also the largest mountain in the solar system ( Figure below ).
Figure 25.23
Olympus Mons is about 27 km (16.7 miles/88,580 ft) above the Martian surface, more than three times taller than Mount Everest. The volcano
Mars also has the largest canyon in the solar system, Valles Marineris ( Figure below ).
Figure 25.24
Valles Marineris is 4,000 km (2,500 mi) long, as long as Europe is wide, and one-fifth the circumference of Mars. The canyon is 7 km (4.3 mi) deep. By comparison, the Grand Canyon on Earth is only 446 km (277 mi) long and about 2 km (1.2 mi) deep.
Mars has more impact craters than Earth, though fewer than the Moon. A video comparing geologic features on Mars and Earth is seen here: Mars tectonics video http://news.discovery.com/videos/space-3-questions-mars-tectonics.html
Water cannot stay in liquid form on Mars because the atmospheric pressure is too low. However, there is a lot of water in the form of ice and even prominent ice caps ( Figure below ). Scientists also think that there is a lot of water ice present just under the Martian surface. This ice can melt when volcanoes erupt, and water can flow across the surface temporarily.
Figure 25.25
The north polar ice cap on Mars.
Scientists think that water once flowed over the Martian surface because there are surface features that look like water-eroded canyons ( Figure below ). The presence of water on Mars, even though it is now frozen as ice, suggests that it might have been possible for life to exist on Mars in the past.
Figure 25.26
The Mars rover collected rounded clumps of crystals that, on Earth, are known to form in water.
A video of the top five Phoenix Lander sites on Mars is seen here: http://news.discovery.com/videos/space-top-5-mars-phoenix-lander-images.html
Mars has two very small moons that are irregular rocky bodies ( Figure below ). Phobos and Deimos are named after characters in Greek mythology — the two sons of Ares, who followed their father into war. Ares is equivalent to the Roman god Mars.
Figure 25.27
Mars has two small moons, Phobos (left) and Deimos (right). Both were discovered in 1877 and are thought to be captured asteroids.
An animation of the moons orbiting Mars is seen here: http://en.wikipedia.org/wiki/File:Orbits_of_Phobos_and_Deimos.gif
1. Name the inner planets from the Sun outward. Then name them from smallest to largest.
2. Why do the temperatures on some planets vary widely? Why are some temperatures much less variable?
3. Why does Venus have higher temperatures than Mercury?
4. How are maps of Venus made?
5. Name two major ways in which Earth is unlike any other planet.
6. Why is Mars red?
7. Suppose you are planning a mission to Mars. Identify two places where you might be able to get water on the planet. Why is this important?
The four outer planets are farther from the Sun as well as farther from Earth. They are much more difficult to learn about since they are very different from our home planet.
The four planets farthest from the Sun are the outer planets . Figure below shows the relative sizes of the outer planets and the Sun. These planets are much larger than the inner planets and are made primarily of gases and liquids, so they are also called gas giants .
Figure 25.28
This image shows the four outer planets and the Sun, with sizes to scale. From left to right, the outer planets are Jupiter, Saturn, Uranus, and Neptune.
The gas giants are made up primarily of hydrogen and helium, the same elements that make up most of the Sun. Astronomers think that hydrogen and helium gases comprised much of the solar system when it first formed. Since the inner planets didn’t have enough mass to hold on to these light gases, their hydrogen and helium floated away into space. The Sun and the massive outer planets had enough gravity to keep hydrogen and helium from drifting away.
All of the outer planets have numerous moons. They all also have planetary rings , composed of dust and other small particles that encircle the planet in a thin plane.
Because Jupiter is so large, it reflects a lot of sunlight. Jupiter is extremely bright in the night sky; only the Moon and Venus are brighter ( Figure below ). This brightness is all the more impressive because Jupiter is quite far from the Earth — 5.20 AUs away. It takes Jupiter about 12 Earth years to orbit once around the Sun.
Figure 25.29
This image of Jupiter was taken by Voyager 2 in 1979. The colors were later enhanced to bring out more details.
Jupiter is named for the king of the gods in Roman mythology. The planet is enormous, the largest object in the solar system besides the Sun. Although Jupiter is over 1,300 times Earth’s volume, it has only 318 times the mass of Earth. Like the other gas giants, it is much less dense than Earth.
Check out NASA's world book to learn more about Jupiter: http://www.nasa.gov/worldbook/jupiter_worldbook.html.
Astronauts trying to land a spaceship on the surface of Jupiter would find that there is no solid surface at all! Jupiter is made mostly of hydrogen, with some helium, and small amounts of other elements ( Figure below ).
Figure 25.30
Jupiter's atmosphere is composed of hydrogen and helium. Deeper within the planet, pressure compresses the gases into a liquid. Some evidence suggests that Jupiter may have a small rocky core of heavier elements at its center.
The upper layer of Jupiter’s atmosphere contains clouds of ammonia (NH 3 ) in bands of different colors. These bands rotate around the planet, but also swirl around in turbulent storms. The Great Red Spot ( Figure below ) is an enormous, oval-shaped storm found south of Jupiter’s equator. This storm is more than three times as wide as the entire Earth. Clouds in the storm rotate in a counterclockwise direction, making one complete turn every six days or so. The Great Red Spot has been on Jupiter for at least 300 years, since astronomers could first see the storm through telescopes. Do you think the Great Red Spot is a permanent feature on Jupiter? How could you know?
Figure 25.31
This image of Jupiter
Jupiter has a very large number of moons -- 63 have been discovered so far. Four are big enough and bright enough to be seen from Earth, using no more than a pair of binoculars. These moons — Io, Europa, Ganymede, and Callisto — were first discovered by Galileo in 1610, so they are sometimes referred to as the Galilean moons ( Figure below ). The Galilean moons are larger than the dwarf planets Pluto, Ceres, and Eris. Ganymede is not only the biggest moon in the solar system it is even larger than the planet Mercury!
Figure 25.32
This composite image shows the four Galilean moons and their sizes relative to the Great Red Spot. From top to bottom, the moons are Io, Europa, Ganymede, and Callisto. Jupiter
Scientists are particularly interested in Europa because it may be a place to find extraterrestrial life. What features might make a satellite so far from the Sun a candidate for life? Although the surface of Europa is a smooth layer of ice, there is evidence that there is an ocean of liquid water underneath ( Figure below ). Europa also has a continual source of energy — it is heated as it is stretched and squashed by tidal forces from Jupiter. Numerous missions have been planned to explore Europa, including plans to drill through the ice and send a probe into the ocean. However, no such mission has yet been attempted.
Figure 25.33
An enhanced color image of a portion of Europa
In 1979, two spacecrafts — Voyager 1 and Voyager 2 — visited Jupiter and its moons. Photos from the Voyager missions showed that Jupiter has a ring system. This ring system is very faint, so it is difficult to observe from Earth.
Saturn, shown in Figure below , is famous for its beautiful rings. Although all the gas giants have rings, only Saturn’s can be easily seen from Earth. In Roman mythology, Saturn was the father of Jupiter.
Saturn’s mass is about 95 times the mass of Earth, and its volume is 755 times Earth’s volume, making it the second largest planet in the solar system. Saturn is also the least dense planet in the solar system. It is less dense than water. What would happen if you had a large enough bathtub to put Saturn in? Saturn would float! Saturn orbits the Sun once about every 30 Earth years.
Figure 25.34
This image of Saturn and its rings is a composite of pictures taken by the Cassini orbiter in 2008
Like Jupiter, Saturn is made mostly of hydrogen and helium gases in the outer layers and liquids at greater depths. The upper atmosphere has clouds in bands of different colors. These rotate rapidly around the planet, but there seems to be less turbulence and fewer storms on Saturn than on Jupiter. One interesting phenomena that has been observed in the storms on Saturn is the presence of thunder and lightning (see video, below). The planet likely has a small rocky and metallic core.
Cassini scientists waited years for the right conditions to produce the first movie that shows lightning on another planet -- Saturn. http://saturn.jpl.nasa.gov/video/videodetails/?videoID=210 Lots more videos from the Cassini mission are indexed here: http://saturn.jpl.nasa.gov/video/
In 1610 Galileo first observed Saturn’s rings with his telescope, but he thought they might be two large moons, one on either side of the planet. In 1659, the Dutch astronomer Christian Huygens realized that the features were rings ( Figure below ).
Saturn’s rings circle the planet’s equator and appear tilted because Saturn itself is tilted about 27 degrees. The rings do not touch the planet.
Figure 25.35
A color-exaggerated mosaic of Saturn and its rings taken by Cassini as Saturn eclipses the Sun.
The Voyager 1 and 2 spacecraft in 1980 and 1981 sent back detailed pictures of Saturn, its rings, and some of its moons. Saturn’s rings are made of particles of water and ice, with some dust and rocks ( Figure below ). There are several gaps in the rings that scientists think have originated because (1) the material was cleared out by the gravitational pull within the rings or (2) by the gravitational forces of Saturn and of moons outside the rings.
Figure 25.36
A close-up of Saturn
The rings were likely formed by the breakup of one of Saturn’s moons or from material that never accreted into the planet when Saturn originally formed.
An animation of dark spokes in Saturn’s rings is seen here: http://en.wikipedia.org/wiki/File:Saturn_ring_spokes_PIA11144_300px_secs15.5to23_20080926.ogv. The spokes appear seasonally and their origin is as yet unknown.
Most of Saturn’s moons are very small and only seven are large enough for gravity to have made them spherical. Only Titan is larger than Earth’s Moon at about 1.5 times its size. Titan is even larger than the planet Mercury.
Scientists are interested in Titan because its atmosphere is similar to what Earth’s was like before life developed. Nitrogen is dominant and methane is the second most abundant gas. Titan may have a layer of liquid water and ammonia under a layer of surface ice. Lakes of liquid methane (CH 4 ) and ethane (C 2 H 6 ) are found on Titan’s surface. Although conditions are similar enough to those of early Earth for scientists to speculate that extremely primitive life may exist on Titan, the extreme cold and lack of carbon dioxide make it unlikely ( Figure below ).
Figure 25.37
This composite image compares Saturn
An incredible virtual tour of Titan as learned from Cassini-Huygens is in this video: http://saturn.jpl.nasa.gov/multimedia/flash/Titan/index.html; Saturn’s tiny moon, Enceladus, is also the subject of a video: http://saturn.jpl.nasa.gov/multimedia/flash/Enceladus/enceladus.html.
Uranus (YOOR-uh-nuhs) is named for the Greek god of the sky ( Figure below ). From Earth, Uranus is so faint that it was unnoticed by ancient observers. William Herschel first discovered the planet in 1781.
Figure 25.38
Uranus is an icy blue-green ball.
Although Uranus is very large, it is extremely far away, about 2.8 billion km (1.8 billion mi) from the Sun. Light from the Sun takes about 2 hours and 40 minutes to reach Uranus. Uranus orbits the Sun once about every 84 Earth years.
Uranus has a mass about 14 times the mass of Earth, but it is much less dense than Earth. Gravity at the surface of Uranus is weaker than on Earth’s surface so if you were at the top of the clouds on Uranus, you would weigh about 10% less than what you weigh on Earth.
Like Jupiter and Saturn, Uranus is composed mainly of hydrogen and helium, with an outer gas layer that gives way to liquid on the inside. Uranus has a higher percentage of icy materials, such as water, ammonia (NH 3 ), and methane (CH 4 ), than Jupiter and Saturn.
When sunlight reflects off Uranus, clouds of methane filter out red light, giving the planet a blue-green color. There are bands of clouds in the atmosphere of Uranus, but they are hard to see in normal light, so the planet looks like a plain blue ball.
Most of the planets in the solar system rotate on their axes in the same direction that they move around the Sun. Uranus, though, is tilted on its side so its axis is almost parallel to its orbit. In other words, it rotates like a top that was turned so that it was spinning parallel to the floor. Scientists think that Uranus was probably knocked over by a collision with another planet-sized object billions of years ago.
Uranus has a faint system of rings ( Figure below ). The rings circle the planet’s equator, but because Uranus is tilted on its side, the rings are almost perpendicular to the planet’s orbit.
Figure 25.39
This image from the Hubble Space Telescope shows the faint rings of Uranus. The planet is tilted on its side, so the rings are nearly vertical.
Uranus has 27 known moons and all but a few of them are named for characters from the plays of William Shakespeare. The five biggest moons of Uranus — Miranda, Ariel, Umbriel, Titania, and Oberon — are shown in Figure below .
Figure 25.40
These Voyager 2 photos have been resized to show the relative sizes of the five main moons of Uranus.
Neptune, shown in Figure below , is the only major planet that can’t be seen from Earth without a telescope. Scientists predicted the existence of Neptune before it was discovered because Uranus did not always appear exactly where it should appear. They knew that the gravitational pull of another planet beyond Uranus must be affecting Uranus’ orbit.
Neptune was discovered in 1846, in the position that had been predicted, and it was named Neptune for the Roman god of the sea because of its bluish color.
Figure 25.41
This image of Neptune was taken by Voyager 2 in 1989. The Great Dark Spot seen on the left center in the picture has since disappeared, but a similar dark spot has appeared on another part of the planet.
In many respects, Neptune is similar to Uranus ( Figure below ). Neptune has slightly more mass than Uranus, but it is slightly smaller in size. Neptune is much farther from the Sun at nearly 4.5 billion km (2.8 billion mi). The planet’s slow orbit means that it takes 165 Earth years to go once around the Sun.
Figure 25.42
Neptune’s blue color is mostly because of frozen methane (CH 4 ). When Voyager 2 visited Neptune in 1986, there was a large dark-blue spot that scientists named the Great Dark Spot, south of the equator. When the Hubble Space Telescope took pictures of Neptune in 1994, the Great Dark Spot had disappeared but another dark spot had appeared north of the equator. Astronomers think that both of these spots represent gaps in the methane clouds on Neptune.
The changing appearance of Neptune is caused by its turbulent atmosphere. The winds on Neptune are stronger than on any other planet in the solar system, reaching speeds of 1,100 km/h (700 mi/h), close to the speed of sound. This extreme weather surprised astronomers, since the planet receives little energy from the Sun to power weather systems. Neptune is also one of the coldest places in the solar system. Temperatures at the top of the clouds are about 218 o C (360 o F).
Neptune has faint rings of ice and dust that may change or disappear in fairly short time frames.
Neptune has 13 known moons. Triton, shown in Figure below , is the only one of them that has enough mass to be spherical in shape. Triton orbits in the direction opposite to the orbit of Neptune. Scientists think Triton did not form around Neptune, but instead was captured by Neptune’s gravity as it passed by.
Figure 25.43
This image Triton, Neptune
Fly by Neptune’s moon Triton by watching this video: http://www.space.com/common/media/video/player.php?videoRef=mm32_SunDeath#playerTop
1. Name the outer planets a) in order from the Sun outward, b) from largest to smallest by mass, and c) from largest to smallest by size.
2. Why are the outer planets called gas giants?
3. How do the Great Red Spot and Great Dark Spot differ?
4. Name the Galilean moons, and explain why they have that name.
5. Why might Europa be a likely place to find extraterrestrial life?
6. What causes gaps in Saturn’s rings?
7. Why are scientists interested in the atmosphere of Saturn’s moon Titan?
8. What liquid is found on the surface of Titan?
9. Why is Uranus blue-green in color?
10. What is the name of Neptune’s largest moon?
When the solar system formed, most of the matter ended up in the Sun. Material spinning in a disk around the Sun clumped together into larger and larger pieces to form the eight planets. But some of the smaller pieces of matter never joined one of these larger bodies and are still out there in space.
Asteroids are very small, rocky bodies that orbit the Sun. "Asteroid" means "star-like," and in a telescope, asteroids look like points of light, just like stars. Asteroids are irregularly shaped because they do not have enough gravity to become round. They are also too small to maintain an atmosphere and without internal heat they are not geologically active ( Figure below ). Collisions with other bodies may break up the asteroid or create craters on its surface.
Figure 25.44
In 1991, Asteroid 951 Gaspra was the first asteroid photographed at close range. Gaspra is a medium-sized asteroid, measuring about 19 by 12 by 11 km (12 by 7.5 by 7 mi).
Asteroid impacts have had dramatic impacts on the shaping of the planets, including Earth. Early impacts caused the planets to grow as they cleared their portions of space. An impact with an asteroid about the size of Mars caused fragments of Earth to fly into space and ultimately create the Moon. Asteroid impacts are linked to mass extinctions throughout Earth history.
Hundreds of thousands of asteroids have been discovered in our solar system. They are still being discovered at a rate of about 5,000 new asteroids per month. The majority of the asteroids are found in between the orbits of Mars and Jupiter, in a region called the asteroid belt , as shown in Figure below . Although there are many thousands of asteroids in the asteroid belt, their total mass adds up to only about 4% of Earth’s moon.
Figure 25.45
The white dots in the figure are asteroids in the main asteroid belt. Other groups of asteroids closer to Jupiter are called the Hildas (orange), the Trojans (green), and the Greeks (also green).
Scientists think that the bodies in the asteroid belt formed during the formation of the solar system. The asteroids might have come together to make a single planet, but they were pulled apart by the intense gravity of Jupiter.
More than 4,500 asteroids cross Earth’s orbit; they are near-Earth asteroids. Between 500 and 1,000 of these are over 1 km in diameter.
Any object whose orbit crosses Earth’s can collide with Earth and many asteroids do. On average, each year a rock about 5–10 m in diameter hits Earth ( Figure below ). Since past asteroid impacts have been implicated in mass extinctions, astronomers are always on the lookout for new asteroids, and follow the known near-Earth asteroids closely, so they can predict a possible collision as early as possible.
Figure 25.46
A painting of what an asteroid a few kilometers across might look like as it strikes Earth.
Scientists are interested in asteroids because they are representatives of the earliest solar system ( Figure below ). Eventually asteroids could be mined for rare minerals or for construction projects in space. A few missions have studied asteroids directly. NASA’s DAWN mission will be exploring asteroid Vesta in 2011 and 2012 and dwarf planet Ceres in 2015.
Figure 25.47
The NEAR Shoemaker probe took this photo as it was about to land on 433 Eros in 2001.
A meteor , such as in Figure below , is a streak of light across the sky. People call them shooting stars but they are actually small pieces of matter burning up as they enter Earth’s atmosphere from space.
Figure 25.48
A meteor streaks across the sky to the right of the Milky Way.
Meteors are called meteoroids before they reach Earth’s atmosphere. Meteoroids are smaller than asteroids and range from the size of boulders down to the size of tiny sand grains. Still smaller objects are called interplanetary dust. When Earth passes through a cluster of meteoroids, there is a meteor shower . These clusters are often remnants left behind by comet tails.
Although most meteors burn up in the atmosphere, larger meteoroids may strike the Earth’s surface to create a meteorite. Meteorites are valuable to scientists because they provide clues about our solar system. Many meteorites are from asteroids that formed when the solar system formed ( Figure below ). A few meteorites are made of rocky material that is thought to have come from Mars when an asteroid impact shot material off the Martian surface and into space.
Figure 25.49
A lunar meteorite originates on the Moon and strikes Earth
Comets are small, icy objects that have very elliptical orbits around the Sun. Their orbits carry them from the outer solar system to the inner solar system, close to the Sun ( Figure below ). Early in Earth’s history, comets may have brought water and other substances to Earth during collisions.
Figure 25.50
The highly elliptical orbit of Kohoutek (red) relative to Earth
Comet tails form the outer layers of ice melt and evaporate as the comet flies close to the Sun. The ice from the comet vaporizes and forms a glowing coma, which reflects light from the Sun. Radiation and particles streaming from the Sun push this gas and dust into a long tail that always points away from the Sun ( Figure below ). Comets appear for only a short time when they are near the Sun, then seem to disappear again as they move back to the outer solar system.
Figure 25.51
Comet Hale-Bopp, also called the Great Comet of 1997, shone brightly for several months in 1997. The comet has two visible tails: a bright, curved dust tail and a fainter, straight tail of ions (charged atoms) pointing directly away from the Sun.
The time between one appearance of a comet and the next is called the comet’s period. Halley’s comet, with a period of 75 years, will next be seen in 2061. The first mention of the comet in historical records may go back as much as two millennia.
Short-period comets, with periods of about 200 years or less, come from a region beyond the orbit of Neptune. The Kuiper belt (pronounced “KI-per”) contains not only comets, but asteroids, and at least two dwarf planets.
Comets with periods as long as thousands or even millions of years come from a very distant region of the solar system called the Oort cloud, about 50,000–100,000 AU from the Sun (50,000–100,000 times the distance from the Sun to Earth).
The dwarf planets of our solar system are exciting proof of how much we are learning about our solar system. With the discovery of many new objects in our solar system, in 2006, astronomers refined the definition of a planet. Their subsequent reclassification of Pluto to the new category dwarf planet stirred up a great deal of controversy. How the classification of Pluto has evolved is an interesting story in science. The question is: What is and is not a planet?
From the time it was discovered in 1930 until the early 2000s Pluto was considered the ninth planet. When astronomers first located Pluto, the telescopes were not as good so Pluto and its moon, Charon, were seen as one much larger object ( Figure below ). With better telescopes, astronomers realized that Pluto was much smaller than they had thought.
Figure 25.52
Pluto and its moon Charon are actually two objects.
Better technology also allowed astronomers to discover many smaller objects like Pluto that orbit the Sun. One of them, Eris, discovered in 2005, is even larger than Pluto ( Figure below ).
Figure 25.53
Eris and its moon Dysnomia.
Even when it was considered a planet, Pluto was an oddball. Unlike the other outer planets in the solar system, which are all gas giants, it is small, icy, and rocky. With a diameter of about 2,400 km, it is only about one-fifth the mass of Earth’s Moon. Pluto’s orbit is tilted relative to the other planets and is shaped like a long, narrow ellipse. Pluto’s orbit sometimes even passes inside Neptune’s orbit.
In 1992 Pluto’s orbit was recognized to be part of the Kuiper belt. With more than 200 million Kuiper belt objects, Pluto has failed the test of clearing other bodies out its orbit.
From what you’ve read above, do you think Pluto should be called a planet? Why are people hesitant to take away Pluto’s planetary status?
In 2006, the International Astronomical Union decided that there were too many questions surrounding what could be called a planet and so refined the definition of a planet.
According to the new definition, a planet must:
A dwarf planet is an object that meets items the first three items in the list above, but not but not the fourth. Pluto is now called a dwarf planet, along with the objects Ceres, Makemake, and Eris.
According to the IAU, a dwarf planet must:
A video showing why Pluto isn’t a planet any more: http://www.youtube.com/watch?v=FqX2YdnwtRc
Pluto has three moons of its own. The largest, Charon, is big enough that the Pluto-Charon system is sometimes considered to be a double dwarf planet ( Figure below ). Two smaller moons, Nix and Hydra, were discovered in 2005. But having moons is not enough to make an object a planet.
Ceres is the largest object in the asteroid belt ( Figure below ). Before 2006, Ceres was considered the largest of the asteroids, with only about 1.3% of the mass of the Earth’s Moon. But unlike the asteroids, Ceres has enough mass that its gravity causes it to be shaped like a sphere. Like Pluto, Ceres is rocky.
Is Ceres a planet? How does it match the criteria above? Ceres orbits the Sun, is round, and is not a moon. As part of the asteroid belt, its orbit is full of other smaller bodies, so Ceres fails the fourth criterion for being a planet.
Figure 25.54
This composite image compares the size of the dwarf planet Ceres to Earth and the Moon.
Makemake is the third largest and second brightest dwarf planet we have discovered so far ( Figure below ). With a diameter estimated to be between 1,300 and 1,900 km, it is about three-quarters the size of Pluto. Makemake orbits the Sun in 310 years at a distance between 38.5 to 53 AU. It is thought to be made of methane, ethane, and nitrogen ices.
Figure 25.55
Largest Known Trans-Neptunian Objects. Makemake is named after the deity that created humanity in the mythology of the people of Easter Island.
Eris is the largest known dwarf planet in the solar system — about 27% more massive than Pluto. The object was not discovered until 2003 because it is about three times farther from the Sun than Pluto, and almost 100 times farther from the Sun than Earth is. For a short time Eris was considered the “tenth planet” in the solar system, but its discovery helped to prompt astronomers to better define planets and dwarf planets in 2006. Eris also has a small moon, Dysnomia that orbits it once about every 16 days.
Astronomers know there may be other dwarf planets in the outer reaches of the solar system. Haumea was made a dwarf planet in 2008 and so now the total is five. Quaoar, Varuna and Orcus may be added to the list of dwarf planets in the future. We still have a lot to discover and explore.
1. Arrange the following from smallest to largest: asteroid, star, meteoroid, planet, dwarf planet.
2. Where are most asteroids found?
3. What is the difference between a meteor, a meteoroid, and a meteorite?
4. Why are meteorites extremely valuable to scientists?
5. What objects would scientists study to learn about the composition of the Oort cloud?
6. Why is Pluto no longer considered a planet?
7. Name the four known dwarf planets in our solar system.
Opening Mars image courtesy High Resolution Imaging Science Experiment, Arizona State University and Devil’s Tower image courtesy Wyoming Geographic Information Advisory Council, http://earthobservatory.nasa.gov/IOTD/view.php?id=38904 , and are in the public domain.
