High School Earth Science/Other Objects in the Solar System

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When our solar system formed, most of the matter ended up in the Sun, the star at the center of the system. Material spinning in a disk around the Sun clumped together into larger and larger pieces, forming the eight planets and their moons. But some of the smaller pieces of matter in the solar system never joined one of these larger bodies. In this lesson, we will talk about some of these other objects in the solar system.

Lesson Objectives[edit]

  • Locate and describe the asteroid belt.
  • Explain where comets come from and what causes their tails.
  • Differentiate between meteors, meteoroids, and meteorites.


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 also sometimes called planetoids or minor planets, because in some ways they are similar to miniature planets. Unlike planets, though, asteroids are irregularly shaped because they do not have enough gravity to become round like planets. They do not have atmospheres, and they are not geologically active. The only geological changes to an asteroid are due to collisions, which may break up the asteroid or create craters on the asteroid's surface. Figure 25.31 shows a typical asteroid.

Figure 25.31: Asteroid 951 Gaspra was the first asteroid photographed at close range. This picture was taken in 1991 by the Galileo probe on its way to Jupiter. 951 Gaspra is a medium-sized asteroid, measuring about 19 by 12 by 11 kilometers (12 by 7.5 by 7 miles).

The Asteroid Belt[edit]

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 25.32. 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 25.32: The asteroid belt is a ring of many asteroids between the orbits of Mars and Jupiter. 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 believe that the bodies in the asteroid belt formed there during the formation of the solar system. However, they have never been able to form into a single planet because the gravity of Jupiter, which is very massive, continually disrupts the asteroids.

Near-Earth Asteroids[edit]

Near-Earth asteroids are asteroids whose orbits cross Earth's orbit. Any object whose orbit crosses Earth can collide with Earth. There are over 4,500 known near-Earth asteroids; between 500 and 1,000 of these are over 1 kilometer in diameter. Small asteroids do in fact collide with Earth on a regular basis—asteroids 5–10 m in diameter hit Earth on average about once per year. Evidence suggests that large asteroids hitting Earth in the past have caused many organisms to die and many species to go extinct. 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.

Asteroid Missions[edit]

Scientists are interested in asteroids in part because knowing more about what they are made of can tell us about our solar system and how it might have formed. They may also eventually be mined for rare minerals or for construction projects in space. Some asteroids have been photographed as spacecraft have flown by on their way to the outer planets. A few missions have been sent out to study asteroids directly. In 1997, the NEAR Shoemaker probe went into orbit around an asteroid called 433 Eros, and finally landed on its surface in 2001. The Japanese Hayabusa probe is currently studying an asteroid and may return samples of its surface to Earth. In 2007, NASA launched the Dawn mission, which is scheduled to visit some of the largest asteroids in 2011-2015.


If you have spent much time looking at the sky on a dark night, you have probably seen a "shooting star", like in Figure 25.33. A shooting star is a streak of light across the sky. The proper scientific name for a shooting star is a meteor. Meteors are not stars at all. Rather, they are small pieces of matter burning up as they enter Earth's atmosphere from space.

Figure 25.33: This photo captures a meteor - also called a "shooting star", streaking across the sky to the right of the Milky Way.


Before these small pieces of matter enter Earth's atmosphere, they are called meteoroids. Meteoroids are like asteroids, only smaller. Meteoroids range from the size of boulders down to the size of tiny sand grains. Objects larger than meteoroids are considered asteroids, and objects smaller than meteoroids are considered interplanetary dust. Meteoroids are sometimes found clustered together in long trails. These are remnants left behind by comets. When Earth passes through one of these clusters, there is a meteor shower, an increase in the number of bright meteors in a particular region of the sky for a period of time.


Figure 25.34: A lunar meteorite.

Suppose a small rocky object—a meteoroid—enters Earth's atmosphere. Friction in the atmosphere heats the object quickly so it starts to vaporize, leaving a trail of glowing gases. At this point, it has become a meteor. Most meteoroids vaporize completely before they ever reach Earth's surface, but larger meteoroids may have a small core of material that travels all the way through the atmosphere and hits the Earth's surface. The solid remains of a meteoroid found on Earth's surface is called a meteorite.

Meteorites provide clues about our solar system. Many meteorites come from meteoroids that formed when the solar system formed (Figure 25.34). Some are from the insides of asteroids that have split apart. A few meteorites are made of materials more like the rocks on Mars. Scientists believe the material in these meteorites was actually knocked off the surface of Mars by an asteroid impact, and then entered Earth's atmosphere as a meteor.


Comets are small, icy objects that orbit the Sun in very elliptical orbits. Their orbits carry them from the outer solar system to the inner solar system, close to the Sun. When a comet gets close to the Sun, the outer layers of ice melt and evaporate. The gas and dust released in this way forms an atmosphere—called a coma—around the comet. Radiation and particles streaming from the Sun also push some of this gas and dust into a long tail, which always points away from the Sun no matter which way the comet is moving. Figure 25.35 shows Comet Hale-Bopp, which shone brightly for several months in 1997.

Figure 25.35: 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.

Gases in the coma and tail of a comet glow, and also reflect light from the Sun. Comets are very hard to see except when they have their comas and tails. For this reason, 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. The time between one appearance of a comet and the next is called the comet's period. For example, the first comet whose period was known, Halley's comet, has a period of 75 years. It last traveled through the inner solar system in 1986, and will appear again in 2061.

Where Comets Come From[edit]

Comets that have periods of about 200 years or less are considered short period comets. Most short-period comets come from a region beyond the orbit of Neptune. This area, which contains not only comets but also asteroids and at least two dwarf planets, is called the Kuiper belt. (Kuiper is pronounced "KI-per", rhyming with "viper".)

Some comets have much longer periods, as long as thousands or even millions of years. Most long-period comets come from a very distant region of the solar system called the Oort cloud, which is about 50,000–100,000 AU from the Sun (50,000–100,000 times the distance from the Sun to Earth). Comets carry materials from the outer solar system to the inner solar system. Comets may have brought water and other substances to Earth during collisions early in Earth's history.

Dwarf Planets[edit]

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. According to the new definition, a planet must:

  1. orbit a star
  2. be big enough that its own gravity causes it to be shaped like a sphere
  3. be small enough that it isn't a star itself
  4. have cleared the area of its orbit of smaller objects

At the same time, astronomers defined a new type of object: dwarf planets. A dwarf planet is an object that meets numbers 1–3 above, but not number 4. There are four dwarf planets in our solar system: Ceres, Pluto, Makemake, and Eris.

Figure 25.36 shows Ceres, a rocky, spherical body that is by far the largest object in the asteroid belt. Before 2006, Ceres was considered the largest of the asteroids. Ceres has enough mass that its gravity causes it to be shaped like a sphere. Still, Ceres only has about 1.3% of the mass of the Earth's Moon. Ceres orbits the Sun, is round and is not a star but the area of its orbit is full of other smaller bodies, so Ceres fails the fourth criterion for being a planet, and is now considered a dwarf planet.

Figure 25.36: This composite image compares the size of the dwarf planet Ceres to Earth and the Moon.


From the time it was discovered in 1930 until 2006, Pluto was considered the ninth planet of the solar system. However, it was always thought of as an oddball planet. Unlike the other outer planets in the solar system, which are all gas giants, it is small, icy and rocky. Pluto is roughly 2400 kilometers in diameter. It is only about 1/5 the mass of Earth's Moon. Its orbit is tilted relative to the other planets and is shaped like a long, narrow ellipse, sometimes even passing inside the orbit of Neptune.

Starting in 1992, many objects have been discovered in the same area as Pluto’s orbit, an area now known as the Kuiper belt. The Kuiper belt begins outside the orbit of Neptune and continues out at least 500 AU. We have discovered more than 200 million Kuiper belt objects. Pluto orbits within this region. When the definition of a planet was changed in 2006, Pluto failed the test of clearing out its orbit of other bodies, so it is now considered a dwarf planet.

Pluto has 5 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 25.37). Two smaller moons, Nix and Hydra, were discovered in 2005. Two more moons, Kerberos and Styx, were also discovered, in 2011 and 2012 respectively.

Figure 25.37: Largest known Trans-Neptunian objects.


Makemake is the third largest and second brightest dwarf planet we have discovered so far (Figure 25.37). It is about three quarters the size of Pluto. Its diameter is estimated to be between 1300 and 1900 kilometers. Makemake is named after the deity that created humanity in the mythology of the people of Easter Island. It orbits the Sun in 310 years at a distance between 38.5 to 53 AU. It is believed to be made of methane, ethane and nitrogen ices.


Eris is the largest known dwarf planet in the solar system—about 27% more massive than Pluto (Figure 25.37). It was not discovered until 2003 because it is extremely far away from the Sun. Although Pluto, Makemake, and Eris are in the Kuiper belt, Eris is about 3 times farther from the Sun than Pluto is, and almost 100 times farther from the Sun than Earth is. When it was first discovered, it was considered for a short time to be the "tenth planet" in the solar system. The discovery of Eris helped prompt the new definition of planets and dwarf planets in 2006. Eris also has a small moon, Dysnomia, that orbits it once about every 16 days.

Astronomers already know there may be other dwarf planets in the outer reaches of the solar system. Look for Haumea, Quaoar, Sedna, and Orcus to be possibly added to the list of dwarf planets in the future. We still have a lot to discover and explore!

Lesson Summary[edit]

  • Asteroids are irregularly-shaped, rocky bodies that orbit the Sun. Most of them are found in the asteroid belt, between the orbits of Mars and Jupiter.
  • Meteoroids are smaller than asteroids, ranging from the size of boulders to the size of sand grains. When meteoroids enter Earth's atmosphere, they vaporize, creating a trail of glowing gas called a meteor. If any of the meteoroid reaches Earth, the remaining object is called a meteorite.
  • Comets are small, icy objects that orbit the Sun in very elliptical orbits. When they are close to the Sun, they form comas and tails, which glow and make the comet more visible.
  • Short-period comets come from the Kuiper belt, beyond Neptune. Long-period comets come from the very distant Oort cloud.
  • Dwarf planets are spherical bodies that orbit the Sun, but that have not cleared their orbit of smaller bodies. Ceres is a dwarf planet in the asteroid belt. Pluto, Makemake, and Eris are dwarf planets in the Kuiper belt.

Review Questions[edit]

  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. What kind of objects would scientists study to learn about the composition of the Oort cloud?
  5. Why is Pluto no longer considered a planet?
  6. Name the four known dwarf planets in our solar system.


Rocky objects larger than a few hundred meters that orbit the Sun in the region known as the asteroid belt.
asteroid belt
Region between the orbits of Mars and Jupiter where many asteroids are found.
A small, icy, dusty object in orbit around the Sun.
dwarf planet
Around celestial object orbiting the Sun that has not cleared its orbit of other objects.
Kuiper belt
A region of space around the Sun beyond the orbit of Neptune that contains millions of frozen objects.
Material from outer space that burns up as it enters Earth's atmosphere.
The solid portion of a meteor that hits Earth's surface.
A small rock in interplanetary space that has not yet entered Earth's atmosphere.
meteor shower
An area of frequent meteors appearing to originate in a particular part of the sky.

Points to Consider[edit]

  • In 2006, astronomers changed the definition of a planet and created a new category of dwarf planets. Do you think planets, dwarf planets, moons, asteroids, and meteoroids are clearly separate groups?
  • What defines each of these groups, and what do objects in these different groups have in common? Could an object change from being in one group to another? How?
  • We have learned about many different kinds of objects that are found within our solar system. What objects or systems of objects can you think of that are found outside our solar system?

Outer Planets · Stars, Galaxies, and the Universe