The History of Pluto
People started looking for a 9
th
planet because of perturbations on Neptune's orbit.
Implied that this planet was 3 to 10x mass of Earth.
When Pluto was discovered in 1929, it fit expectations because it was 'bright'.
However, an object can be bright because:
It is low albedo (% of light reflected) and large.
It has high albedo and small.
Most Solar System objects are low albedo - The Moon only reflects a few percent of sunlight.
So, people thought Pluto was big.
It was not until Pluto's satellites was discovered that people knew how small Pluto is.
0.002 X Mass of Earth.
From Hubble Space Telescope
