Gravity.
All the comets that have been seen in history were in the solar system and most still are.
Comets are made up of remnants from the early solar system, so their age is generally around the same as our solar system, approximately 4.6 billion years old. While some comets may contain even older material from beyond our solar system, as a whole, comets are not older than our solar system.
I scientifically believe that comets are closer to the outer part of the solar system.
Nobody let them in. They formed with the solar system.
There are no visible comets at this time.
The sun.
Comets Time Capsules of the Solar System - 1981 was released on: USA: 1981
Yes. The solar system is everything that orbits the sun.
It has been suggested that comets originate in the Oort cloud and then travel in long elliptical orbits around the Sun.
the members of the solar system are sun, comets, planets, asteroids,
comets can be found in the oort cloud to the south of Pluto
Unfortunately, you neglected to include the list of allowable choices that "follows" the question. Still, I know the answer. The force that keeps comets in closed orbits around the sun is the one that has the same effect on the planets; gravity. The only difference is that the orbits of the long-term comets are much more 'eccentric' (long and skinny) than those of the planets, so they have much shorter minimum distances from the sun, and much longer maximum distances, than planetary orbits have. So, the correct awnser is Gravity.