Question

Why are planets with long orbital periods among the last to be discovered?

Answer 1

Answer: Because observers from Earth has to wait for a long time for the planet to repeat its orbit

Step-by-step explanation:

According to the Third Kepler Law of Planetary motion “The square of the orbital period of a planet is proportional to the cube of the semi-major axis (size) of its orbit”.

In other words, this law states a relation between the orbital period
`T` of a body (moon, planet, satellite) orbiting a greater body in space with the size
`a` of its orbit:

`T^(2) \propto a^(3)`

So, in the specific case of planets orbiting the Sun, the greater
`a` is (the distance between the planet and the Sun), the longer the period
`T`.

To understand it better:

This generally occurs with planets that are farther from the Sun. Since the period is longer, planets take longer than the internal planets to describe a complete orbit around the Sun. Therefore, from Earth, we must wait longer to verify this body has repeated its orbit.

That's why astronomers and scientists have to wait so long to verify the celestial body being watched is a planet.