No. They form in the outer solar system where it is cold enough.
In the outer realms of the solar system.
I scientifically believe that comets are closer to the outer part of the solar system.
When they reach the inner solar system.
Occasionally they enter the realm of the inner solar system and become visible .five kilometers is a typical size -- but some are much larger.
No. Most comets are about the same age as the solar system as they formed with it.
Large planetesimals from the outer solar system when they are on a trajectory that leads them to the inner solar system. This keeps large enough asteroids and comets from coming into the inner solar system. This was made very aparent during the comet that crashed into Jupiter in the 90's.
Just got this from the internet. Comets are usually in the outermost regions of the Solar system (the Oort cloud), where it is extremely cold. Water ice can survive billions of years in the Oort cloud. However, the comets we observe -- those that come into the inner Solar system --- do lose a lot of volatiles. This is a part of the process that creates the tails, the signature we associate with comets. Comets that are trapped in the inner Solar system will soon (astronomically speaking) exhaust all their volatiles and become extinct (i.e., rocks with no cometary activities).
All the comets that have been seen in history were in the solar system and most still are.
The comets that orbit the sun in elongated ellipses are thrown into the inner Solar System and the Sun by gravitational changes of the Kuipler Belt planets from the Oort Cloud, consisting of debris left of from a solar nebula.
Nobody let them in. They formed with the solar system.
Comets contain various types of ice (water and also other frozen liquids or gases) which sublime to vapor when the comet approaches the inner solar system.
There are no visible comets at this time.