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From Rocks To Planets

Solar systems, including our own, include a wide variety of objects from microscopic dust and gas to enormous gas giant planets. On this page we'll talk about objects on the smaller end, up to the size of a terrestrial planet like Earth or Venus. On the next page we'll talk about larger objects, especially Neptune-like and Jupiter-like planets.

What is a planet?

The official definition of a planet, from the International Astronomical Union, states that a planet must fit all three of the following criteria:

This definition is not without its critics, but it is generally accepted at this time.

Terrestrial Planets

There are four "terrestrial" (or "rocky") planets in our solar system: Mercury, Venus, Earth, and Mars. Each of them is composed primarily of rock and metal, with a solid crust, thick sludge-like mantle, and a core of heavy metal under extremely high temperature and pressure.

Earth is the largest of our terrestrial planets, but we have found much larger terrestrial planets circling other stars.

Our solar system also has four gaseous planets. We'll talk more about them and other large objects on the next page. First, though, let's examine the smaller members of our solar system.

Plutoids and other Dwarf Planets

Objects that fail the third criterion - those that have not cleared other similar-sized bodies from their orbit - are considered dwarf planets. Pluto falls into this category, as do many other objects in the Kuiper belt, a vast area beyond Neptune. You can see artist renditions of several of them, to scale, in the image below.

Pluto is very far away; on average it is about forty times farther away from the Sun than Earth is. The planet is exceptionally cold, and the sun is very dim from that distance. Even our best telescopes cannot see it very clearly. Here is the best picture of Pluto that we have so far:

The composition of dwarf planets is still uncertain. Because no space probe has yet visited any of them, and they are very small and distant, the primary resource we have for describing them is physical modeling. Pluto and its relatives are likely to be made primarily of ice and rock. Pluto's density indicates that it is likely to be more rock than ice, with most of the rock in a central core. At these distances from the sun, the temperature is so low that the ice is composed of not just water, but solid carbon dioxide and even solid nitrogen.

Most dwarf planets do not have a substantial atmosphere. Heavier gases tend to become liquid or even solid at these temperatures, and the tiny dwarf planets do not have enough gravity to capture lighter gases.

Ceres, the largest object in the asteroid belt, is also considered a dwarf planet.

Asteroids and Comets

Below the size of dwarf planets, we have asteroids and comets. Also known as "minor planets" or Small Solar System Bodies (SSSB), these are pieces of rock, metal, or ice that are in orbit around our sun.

Some larger planets, like Jupiter and Saturn, have moons that are small and irregularly-shaped. These appear to be captured asteroids. Even the moons of Mars may once have been asteroids. Our own moon, however, is thought to be formed by a different process. So far no moon is suspected to be a captured comet.

Asteroids are classified by their composition. Most are made of carbonaceous rocks, some have a silicacious or "stony" composition, and some are more metallic. Asteroids range in size from tiny flecks of stone no larger than a pea to the impressive Ceres asteroid. At almost 1000 km in diameter, Ceres is the largest object in the asteroid belt (which is generally between the orbits of Mars and Jupiter) and was once counted as a planet in our solar system.

The asteroid Vesta is about a quarter the mass of Ceres. You can see pictures of them both below. NASA's Dawn spacecraft visited Vesta in 2011, and is scheduled to arrive at Ceres in 2015. You can see some nice animations of them spinning if you click on the pictures.

Comets have a very different composition. With highly elliptical orbits, they come from far out at the edge of our solar system, and are composed of a mixture of rock, water ice, frozen gases, and dust. Their passage into our solar system brings them closer to the sun, which heats and vaporizes the frozen gases on their surface. This produces the tail of the comet and the "coma" that surrounds their cores.

The smallest parts of the Solar System

Our solar system is not as empty as it might seem - the space between our planets is filled with interstellar dust, as well as the solar wind (particles sent out from our sun) and traces of hydrogen gas.

Though these pieces are small, they are important to our observations of the universe. Other solar systems also have these materials. In fact, some of them are still dominated by dust and gas, as planets slowly form around the central star. Dust clouds around other systems can obscure our view of newly-forming stars. The presence of a thick dust cloud may indicate a young star whose planets have not yet cleared the space around the star.

We will be moving on to focus on other topics. However, all of the bodies described here are still subjects of great research. If you would like to learn more about asteroids, comets, plutoids, or dust in our solar system, you may be interested in some of the links below.

Солнечная система и ее тайны