6.5. Comets

According to new definition, Neptune is the outermost planet of the solar system. However, it’s orbit does not mark the boundary of the solar system. The solar system extends much beyond at the edge of the solar system, there are billions of very small objects called ‘comets’ these comets were formed very early from the same gas cloud from which other members of the collar system were made. These comets are so far off that normally they cannot be seen. They keep on revolving around the Sun, unknown to the world.

Sometimes, however, the normal path of a comet is disturbed and the comet starts moving towards the sun. As the comet approaches the sun, it develops a long, glowing tail and becomes visible only when it approaches the sun because the sun’s rays make its gas glow which spreads out to form a tail millions of kilometres long. And it presents a spectacular sight. Thus, a comet is a collection of gas and dust, which appears as a bright ball of light in the sky with a long glowing tail. The tail of a comet always points away from the sun. Comets revolve around the sun like planets. The period of revolution of comets around the sun is, however, very large. For example, Halley’s Comet has a period of about 76 years. Halley's Comet last appeared in the inner Solar System in 1986 and will next appear in mid-2061.

Just like asteroids, comets are also of great interest to scientists. This is because they are made of the same material from which the whole solar system was made. The study of the tail of the comets has shown the existence of molecules of carbon, oxygen, hydrogen and nitrogen such as CO, CH4 and HCN on it. Since these simple molecules help to form complex molecules necessary for the origin of life, some scientist have suggested that the seeds of life on the earth were brought by comets from the outer space. Comets do not last forever. Each time a comet passes the sun, it loses some of its gas and ultimately only the dust particles are left in space. When these particle enter into the earth’s atmosphere, they burn up due to heat produced by air resistance and produce a shower of meteors or shooting stars.