As a comet approaches the sun, its "hairy" coma and tails form. The gravitational attraction of a passing star can cause comets in the Oort Cloud to erupt. Some may be launched into the inner solar system. Others may be flung far beyond the orbit of Neptune.
Comets are composed of ice and dust. As they near the sun, the heat causes the ices to vaporize, forming a cloud around the comet's nucleus. This is why comets appear to come from nowhere in particular; they are actually from anywhere within an astronomical unit (AU) of the sun. At their most distant, comets reach as far as 50 AU from the sun.
Comets are also known as "astronomical visitors from outside our solar system." They are fragments of much larger objects that were once part of a planet or moon but were ejected due to gravity waves created by another body passing by. When this happens, parts of the crust and mantle are left behind. If the mass of the passing body is large enough, it will even create new planets or moons!
Comets were first theorized by Johannes Kepler in 1610. He proposed that stars move around clusters called "Oceans," which are made up of smaller bodies such as planets or moons.
When another star passes by the solar system, its gravity pushes the Oort cloud and/or Kuiper belt and forces comets to fall in a highly elliptical orbit around the sun, with the sun at one of the ellipse's foci. The reason for this is still not fully understood.
Some scientists believe that these objects are left over from the formation of the Solar System, while others think they were formed farther out and were brought in by collisions. Either way, they are important for understanding how our galaxy worked more than 4500 million years ago, when it was just forming.
Comets are very dirty snowballs composed of ice and dust. As they pass close to the Sun, the heat from the solar radiation melts some of the ice and turns it into vapor, which then floats up toward the top of the comet's atmosphere, or coma. When it reaches the edge, this vapor condenses into droplets that rain back down onto the comet's surface. This is why we see activity on comets: They are constantly being resurfaced by the falling water and gas molecules.
Comets were first discovered in the late 16th century by Dutch astronomers who saw spots on the Moon when viewing it through their telescopes. They called these new objects "comets" because they looked like long streaks of smoke flowing away from the Earth directionally following after the Moon and before the Sun.
Furthermore, the characteristics of this comet—the ion tail, the dust tail, the coma, and the nucleus—are shared by nearly all comets that approach our inner Solar System. When a comet warms up sufficiently, it produces an extended, gas-rich cloud known as a coma surrounding its nucleus. The coma can be very large, 10s of miles across, but only a small fraction of it is actually made up of cometary material. The rest is interstellar gas and dust trapped between the Sun and Earth when the Solar System was forming.
The nucleus is composed mainly of ice and rock. The ice may be pure water or possibly mixed with other substances such as ammonia or methane. It may also contain some organic molecules such as carbon dioxide, hydrogen cyanide, or acetic acid. The rock may be similar to that found on Earth, such as silicon dioxide (sand), iron oxide (rust), or calcium carbonate (shellrock). However, most of the mass of a comet's nucleus is made up of something called pyroxene, which is less common on Earth but many comets are rich in it.
Comets were first observed from Earth during the 17th and 18th centuries. At that time, they were seen as novas or new stars because they were thought to be extremely distant objects out in space. But later observations showed that comets have a very elliptical orbit, sometimes approaching within 100 million kilometers of the Sun but usually coming much farther away.