Why is Pluto considered a dwarf planet?

The parameter Λ indicates the probability that a small object will have a close encounter with a larger object, resulting in a significant change in its orbit. The equation is complex: what is written between square brackets depends mainly on the orbit of the small body and not on the properties of the larger body. It can be found in the fraction before the arc. The parameter μ is the ratio of the planet's mass to the mass of the Sun A_p The semi-major axis of its orbit.

Desire to classify things

So, to see if you're dealing with an object that can regulate its own orbit, you can look at this first term right now. Because if you look at all the possible small objects, the average value of the expression in the parentheses doesn't change much. Corresponding estimates by Stern and Levison also show that Λ has a value larger than 1 for the eight planets in the solar system, but for objects such as Pluto or Ceres (the largest asteroid in the asteroid belt between Mars and Jupiter) Λ is smaller than 1. The astronomers refer to the first group as “The upper planets”, and the second is called “the lower planets”.

It may be reasonable to accept that the word “planet” existed long before any systematic scientific study of the solar system, and therefore it is difficult to use it forcefully to refer to Pluto. People in the United States may be rightly proud to have discovered this celestial body there. However, this discovery loses none of its value when one realizes that Pluto simply is not a planet.