Have you ever looked up at the night sky and seen a “shooting star?” You most likely saw a comet in action! Comets are leftover ice-rich planetesimals that orbit the Sun. You can think of a comet as being like a “dirty snowball,” made primarily of ice with some rocky dust mixed in. As a comet accelerates towards the Sun, its surface temperature increases. This causes the outer layers of ice to vaporize into gas, creating a huge, dusty atmosphere around the comet called the coma. As the coma grows, gas is pushed away from the Sun, creating the comet’s tail, which can be up to hundreds of millions of kilometers in length!
Comets have two tails: a plasma tail and a dust tail. The plasma tail consists of gas ionized by the Sun’s UV rays, and always points away from the Sun due to the solar wind. The dust tail consists of dust-sized particles that are unaffected by the solar wind, and are instead pushed away from the Sun due to radiation pressure. Comets actually also have a third, invisible tail that is made up of sand-sized particles. These particles are too big to be affected by the solar wind or radiation pressure, and so this tail is responsible for most meteors and meteor showers.
Comets exist in our solar system in two distinct regions: the Kuiper belt and the Oort cloud. The Kuiper belt is a donut-shaped region beyond the orbit of Neptune, and comets that live there tend to have a pattern to their orbits. The Oort cloud, where most comets come from, is a spherical region that is super far away from the Sun, and is home to about a trillion comets! Interestingly enough, Oort cloud comets originated closer to the Sun than Kuiper belt comets, even though they are now much farther from the Sun. Leftover planetesimals in the spaces between the jovian planets likely had close gravitational encounters with the planets, which caused them to be flung into space in all directions, forming the Oort cloud. Comets beyond the orbit of Neptune (in the Kuiper belt) were much less likely to have these gravitational encounters, causing their orbits to remain similar to those of planets.
Maddy Pollard Blog 