Halley's comet takes about 76 years to orbit the sun. why do we believe that halley's comet was not in its present orbit 4 billion years ago

Though too small to earn the distinction of "planet", asteroids and comets loom large in literature and folklore. The reason is clear: one of the chunky rocks or icy mud balls will eventually slam into Earth and change the planet irreversibly. Such an impact 65 million years ago is widely believed to have killed off the dinosaurs.

Asteroids and comets are considered remnants from the giant cloud of gas and dust that condensed to create the sun, planets, and moons some 4.5 billion years ago. Today, most asteroids orbit the sun in a tightly packed belt located between Mars and Jupiter. Comets are relegated to either a cloud or belt on the solar system fringe. Gravitational tugs, orbital collisions, and interstellar jostles occasionally perturb an asteroid or comet onto a wayward path.

d. they require a medium to travel through