The current definition of "Dwarf Planet" bothers me.
Was Jupiter a dwarf planet before it cleared its orbit?
Was Earth a dwarf planet shortly after its collision with Theia, when there was presumably debris left in orbit?
kelseydh•8mo ago
There really is a big problem with the new definition of a Planet, vs Dwarf Planet, in declaring that a Planet can "clear the neighbourhood around its orbit."
The further away a body orbits from its star, the more unreasonable it is to expect a planetary body to clear the neighbourhood around its orbit. Not even a large gas giant could hope to do it at far enough distances. What do we call those? Pluto could have easily cleared its orbital neighbourhood if it was close to the Sun like Mercury.
dmos62•8mo ago
Seems like the "dwarf" term is not about absolute size, but about its orbital dynamics.
kelseydh•8mo ago
The lack of scientific consistency reveals itself when talking about exoplanets outside our solar system.
The exoplanet 2MASS J21265040-8140293 is separated from its star by 7400 AU. Its unlikely that planet is clearing its orbital neighbourhood, yet we don't declare it an "exo-dwarf-planet".
bombcar•8mo ago
Isn’t it theoretically possible to put three Earths in the current orbit and have the balanced forever? That’s one of the stable positions.
Also Earth is no longer a planet because its orbit is filled with satellite junk.
throwawayffffas•8mo ago
No because of perturbations from the other planets.
throwawayffffas•8mo ago
The Greeks and the Trojans lead and follow Jupiter is its orbit clear?
layer8•8mo ago
From the Wikipedia page:
Over many orbital cycles, a large body will tend to cause small bodies either to accrete with it, or to be disturbed to another orbit, or to be captured either as a satellite or into a resonant orbit. As a consequence it does not then share its orbital region with other bodies of significant size, except for its own satellites, or other bodies governed by its own gravitational influence. This latter restriction excludes objects whose orbits may cross but that will never collide with each other due to orbital resonance, such as Jupiter and its trojans, Earth and 3753 Cruithne, or Neptune and the plutinos.
layer8•8mo ago
It's more about ability than completion. A better formulation may be that a planet is gravitationally dominant within its orbit. There have been proposals for how to quantify it: https://en.wikipedia.org/wiki/Clearing_the_neighbourhood
echelon•8mo ago
Was Jupiter a dwarf planet before it cleared its orbit?
Was Earth a dwarf planet shortly after its collision with Theia, when there was presumably debris left in orbit?
kelseydh•8mo ago
The further away a body orbits from its star, the more unreasonable it is to expect a planetary body to clear the neighbourhood around its orbit. Not even a large gas giant could hope to do it at far enough distances. What do we call those? Pluto could have easily cleared its orbital neighbourhood if it was close to the Sun like Mercury.
dmos62•8mo ago
kelseydh•8mo ago
The exoplanet 2MASS J21265040-8140293 is separated from its star by 7400 AU. Its unlikely that planet is clearing its orbital neighbourhood, yet we don't declare it an "exo-dwarf-planet".
bombcar•8mo ago
Also Earth is no longer a planet because its orbit is filled with satellite junk.
throwawayffffas•8mo ago
throwawayffffas•8mo ago
layer8•8mo ago
Over many orbital cycles, a large body will tend to cause small bodies either to accrete with it, or to be disturbed to another orbit, or to be captured either as a satellite or into a resonant orbit. As a consequence it does not then share its orbital region with other bodies of significant size, except for its own satellites, or other bodies governed by its own gravitational influence. This latter restriction excludes objects whose orbits may cross but that will never collide with each other due to orbital resonance, such as Jupiter and its trojans, Earth and 3753 Cruithne, or Neptune and the plutinos.
layer8•8mo ago