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).

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).

A painting of what an asteroid a few kilometers across might look like as it strikes Earth.

The NEAR Shoemaker probe took this photo as it was about to land on 433 Eros in 2001.

Meteors

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.

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.

Meteorites

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.

Comets

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.

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.

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.

Where Comets Come From

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).

Dwarf Planets

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?

Pluto

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.

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).

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:

• Orbit a star.
• Be big enough that its own gravity causes it to be shaped as a sphere.
• Be small enough that it isn’t a star itself.
• Have cleared the area of its orbit of smaller objects.

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:

• Orbit a star.
• Have enough mass to be nearly spherical.
• Not have cleared the area around its orbit of smaller objects.
• Not be a moon.

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

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.

Makemake

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.

Eris

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.

Lesson Summary

• Asteroids are irregularly shaped, rocky bodies that orbit the Sun. Most 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, it is a meteorite.
• Comets are small, icy objects that have 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

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.

Points to Consider

• 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?