Madi Araly said:
Hi there,
I was reading up on how planets are classified based on their size, but ran into a problem when I began to consider rogue celestial bodies.
If a body has significant enough mass to maintain a hydrostatic equilibrium, yet doesn't orbit a star, I understand that it's classified as a rogue planet. The information taught in my current Earth & Space Science course states that the only difference between a regular planet and a dwarf is that regular planets have cleared the neighborhood around their orbits, but how are the two differentiated between if not explicitly orbiting any stars?
Is there a standard mass that we use to do so? I can't imagine this would be the case since we can't accurately measure mass for those light years away, but I don't see another clear way that we could categorize a planet.
Planets are not classified based on their size. Which is part of the problem with the definition of a "planet."
Q:
What is the exact wording of the official IAU proposed definition of planet?
A: A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
Source: https://www.iau.org/public/themes/pluto/
Based upon the above definition, there is no maximum size. Furthermore, no other star can have planets since they do not meet the first definition. Lastly, none of the "planets" in our solar system meet the IAU definition since there are objects in the Lagrange Points 4 and 5 of every "planet" in our solar system. Hence, no "planet" has "cleared the neighbourbood around its orbit." This definition by the IAU is purely political and has absolutely nothing to do with science.
In order to properly define what a planet is, or what it is not, there needs to be a minimum and maximum size or mass. The minimum size has already been defined as having sufficient mass to achieve hydrostatic equilibrium. There must also be a maximum mass, which should be less than what is required to begin deuterium fusion, or less than ≈13 Jupiter masses.
Whether they orbit our sun, another star or stars (including brown dwarfs), or are not orbiting any other object, they should still be considered a "planet," as long as they fall within the minimum and maximum mass requirements. The definition of a "moon" can also be defined as any object that orbits a "planet," with no minimum size requirement. The maximum size requirement for a "moon" is automatically determined by the object it orbits. If it was larger than the "planet," then it cannot be a "moon" since the "planet" would be orbiting it, not the other way around.
However, that would make Ceres, Pluto, Eris, Makemake, 90377 Sedna, and several other objects "planets." That does not mean we cannot use additional qualifiers to better identify them. We can still call Ceres a "dwarf planet" or Pluto, Eris, Makemake, and 90377 Sedna "Trans-Neptunian Objects." All this definition does is allow us to define a group of objects that are big enough to achieve hydrostatic equilibrium, yet small enough so that there is no fusion taking place. Whether they orbit a star, more than one star, a brown dwarf, white dwarf, neutron star(s), black hole(s), orbit any other object more massive than a "planet," or not orbit anything at all, should make absolutely no difference. Nor should the definition change depending on whether these objects have "cleared the neighbourbood around its orbit."
