We cannot see the Oort cloud of comets, why do we think it exists?

Answer 1

The idea of the Oort Cloud (also sometimes called the Oort-Opik Cloud) was developed as a hypothesis to explain where new, long-period comets originate from. New short-period comets are thought to come from the much closer in Kuiper Belt, but long-period comets arrive from all directions in the sky with highly eccentric orbits, so a shell of cometary nuclei very far away is thought to be where they come from.

Answer 2

Because it should exist. After the sun ignited the T-Tauri wind swept the inner system of most of the remaining material that hadn't been accreted into the sun or into the planets. A lot of this material was gathered by Jupiter and the other Jovian worlds. A lot was not. It continued outward but it did not have escape velocity. Therefore at a certain distance from the sun there HAS to be a globular region of left-over material from the accretion, and material swept from the inner system.

Answer 3

Because we can plot the orbits of non-periodic comets and they all come from places far out past Pluto but not from interstellar space.

Any object in an elliptical orbit moves faster when close to the Sun and slower when far from the Sun. Some comets are in elliptical orbits that bring them back every few years, but they spend most of their time moving slowly in the farthest part of their orbits. Halley's Comet spends about 70 years out past Jupiter and only a few years coming in past Earth each time around. But most are in nearly parabolic orbits. That means they spend a year or two in close to the Sun and thousands or millions of years or longer WAY out past Pluto. So there must be lots of them out there in the really slow parts of their orbits that we just haven't seen yet because we haven't been watching comets long enough. But no comet has been seen in a hyperbolic orbit. Any comet coming from interstellar space would be in a hyperbolic orbit. They all have orbits that come from far out beyond Pluto but not from interstellar space. We can't see most of that orbit, or even calculate the outer parts of it exactly from the short time they are visible near the Sun, but we can calculate it well enough to know that they will not escape from the Sun as they go back out and could not have come from interstellar space as they came in. So they must be kind of hanging around WAY out past Pluto most of the time, only falling in close to the Sun every million years or so.

Answer 4

Specifically the effect of their gravitational forces with the other heavenly bodies up there. That is why there is a Law of Universal Gravitational Force.

We dont need to see things to believe they are there. We cant see the air, the magnetic field & electric current but we know they exist. :)

Answer 5

We can determine the trajectory of comets and work out where they came from.

Answer 6

Because we can see and measure its effects.

Many things in science cannot be directly observed but are provably present. You cannot see the electrons in your computer, but it is not a mystery that it works.