Nature of the Universe

Chapter 10

Small Objects in the Solar System

Apart from the Sun and the eight major planets, there are also many small objects in our solar system. Among them, the largest is Eris, discovered in 2005. It is classified as a dwarf planet. In this chapter, we will discuss the second largest dwarf planet, Pluto, and others small object including comets, asteroids and meteoroids.

Pluto

Courtesy STScI.
Pluto is not classified into either the terrestrial or Jovian planets due to its special features.

We do not understand much about Pluto partly because it is very far away from us and partly because Pluto has not been visited by a spacecraft. (In contrast, spacecraft have flown by comets and asteroids.)

The mass of Pluto is less than 1% of the Earth's, and its radius is about 1150km. It is even smaller than the Moon, as the radius of the Moon is about 1740km. Its density is roughly 2.3 times that of water. Usually, the Jovian planets have density less than 2, while terrestrial planets have density more than 5.

The orbit of Pluto is special. It is inclined 17.2° to the ecliptic. Moreover, part of its orbit lies inside Neptune's. As a result, Neptune was even farther away from the Sun from 1979 to 1999.

Pluto's surface and its weak atmosphere are made up of nitrogen. From its density, we know that it is rocky.

We have discovered three satellites of Pluto. The largest one is called Charon. Its diameter is about 1200km, which is relatively very large, over one half of Pluto's. Moreover, the time for Charon to go around Pluto is the same as the rotational period of Pluto. Therefore, Charon is a synchronous satellite. In other words, you will never see Charon on one half of the surface of Pluto, but you will always see it at a fixed position on the sky on the remaining half of Pluto. Last but not least, it is the only natural synchronous satellite in our solar system.

Comets

Courtesy NASA.
The photo at the right shows comet West, which visited the inner solar system in 1976.

Comets are usually modeled as "dirty snowballs." A comet consists of a tiny nucleus with diameter less than 10km. The nucleus is made up of frozen gases and dust. When it approaches the Sun, some gases will be vaporized and an extended coma will then be produced. When it moves even closer to the Sun, the solar wind (charged particles ejected from the Sun) and the Sun's radiation pressure (See Chapter 11.) push the dust and gases of the comets away, this will result in a beautiful long tail. From this, we know why the comet tail is always pointing away from the Sun.

Orbits of a comet may be open or close. A comet with an open orbit will only visit the Sun once. However, a comet with a closed orbit (actually it is elliptical) will visit the Sun again and again. Perhaps, the most famous one is the Comet Halley, it has a closed orbit with a period of 76 years. Unlike Comet Halley, the average distances from the Sun of most comets are greater than that of Pluto.

Asteroids

Another name for asteroid is minor planet. Most of them are found in the gap between Mars and Jupiter. There are over 10000 asteroids discovered so far, the largest one is about 1000km in diameter, and most of them are not round in shape. The following is the asteroid Gaspra.

Courtesy JPL/NASA.

This one is the asteroid Ida. Note its little moon (satellite), Dactyl, at the right.

Courtesy JPL/NASA.

When we take long exposure photos, asteroid may show up as a long trail. In fact, most of the asteroids are discovered in this way.

Courtesy STScI.

Kuiper Belt Objects

In addition to comets, many rocky objects are orbiting around the Sun beyond Pluto. We call them Kuiper Belt Objects. The first such object was discovered in 1992 and over 1000 of them have been discovered so far*. Owing to their great distance from the Sun, they are dim and move very slowly relative to background stars. Thus, they are found by comparing photos taken from the same region of sky separated by days or weeks.

Courtesy M. Brown, C. Trujullo and D. Rabinowitz .
The orbits of most Kuiper Belt Objects are less than 50 A.U. from the Sun. Although the diameter of most Kuiper Belt Objects are less than 100 km, a few big ones have been found. In particular, Eris is the largest and most massive discovered so far. Pluto is also considered as a big Kuiper Belt Object. Orcus is another one with diameter about 1600 km (and hence is even bigger than Charon). Sedna (also known as 2003 VB12) was the most distant Kuiper Belt Object at its time of discovery. It has a highly elliptical orbit, whose distance from the Sun varies from 76 to about 900 A.U.

Discovery of Kuiper Belt Objects challenges our view on major and minor planets. More importantly, in depth study of Kuiper Belt Objects may help us to further understand the origin of our Solar System.

* All Kuiper Belt Object data mentioned in this chapter are valid as in Aug 2007.

Meteoroids

Courtesy NASA.
Most meteoroids are small particles in space with size similar to grains of sand. When a meteoroid with a speed of tens of kilometers per second hits the Earth's atmosphere, the friction with air will heat up the meteoroid. The high temperature will cause the surrounding atmosphere to glow, creating a bright streak of light called a meteor. Meteors are usually called "shooting stars." They usually burnt up eventually. But for some massive ones, the remaining parts will hit the Earth surface, they are known as meteorites. If a meteorite is large enough, it could produce a crater on the Earth surface.

Courtesy NASA.

Courtesy NASA and D. Roddy of the US Geological Survey.

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