The “cleared its orbit” is a bad criterion for two reasons: 1) it’s often misunderstood, such as when people think Pluto makes Neptune fail that criterion, or for that matter, the Moon means Earth hasn’t cleared its orbit and 2) it’s a continual process so that you can never say that a specific planet has completely cleared its orbit.
Ultimately, in the 21st Century “planet” is an idea for elementary school kids regardless of their current ages. It’s not a simple list, it’s an egregiously simplistic list given our actual knowledge of what’s out there.
Back in Ancient Somewhereania (ref @wolfpup) “planet” was easy: Look in the sky: If you see something moving day by day vs the background stars, that’s a planet.
Had we stuck with that practical simple definition we’d still be good today: Mercury, Venus, Mars, Jupiter & Saturn. With Sun & Moon maybe included as a different sub-type of planet. Look up in the sky as the ancients did and see what the ancients saw. 5 moving dots or almost-dots, and two much larger circles.
Although for city dwellers the list of planets would be getting shorter as Saturn & Mercury are now hard / impossible to see vs light & air pollution, and even Jupiter can be challenging in some places.
TLDR: At bottom “planet” was a pre-scientific idea. We should restore it to that same simple & simple-minded state. At the same time astronomers can have all sorts of learned taxonomies of stellar satellite objects. But don’t try to force-fit one kind of list into the other world of learned discourse.
I would disagree : it was a definition based on observation: some things in the sky move! Science has moved on , but I’ll bet in the 9th century BC, that was a mind-blowing scientific concept.
No, it doesnt:Clearing the neighbourhood around [a celestial body’s] orbit" describes when the body has become gravitationally dominant, and there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence
Luna is both a natural satellite and under Terras gravitational influence. It’s true that the terra/luna system is close to being a double planet, but as the center of gravity is within Terras surface- not quite.
I certainly agree with your point about it being observational and factually based. Which are two of the main foundation stones of the scientific method. And I agree that as folks settled on this knowledge they thought it was significant. And rightly so.
I say “pre-scientific” in the sense that it’s just stamp collecting. They did not have a coherent “why” for any of it; neither were they seeking one. And as a result, their list isn’t scientifically defensible given modern knowledge.
Which leads to the kludges like IAU has had to make when new science discovers both new non-naked eye “planets” like Uranus & Neptune, and discovers all the rest of the zoo of satellites with all their attendant complications and exceptions.
IOW, the original work millennia ago was curiosity-driven and admirable for that. But the details they didn’t know and could not have known are an obstacle in our more knowledgeable era. Like the 4 elements of earth, wind, fire, and water, it was a darn fine first draft. And on the shoulders of such giants does our whole scientific edifice stand today. But we aren’t still trying to force-fit the periodic table of elements into the original 4 categories.
My objection to it is that it is so obviously a post hoc kludge just to exclude Pluto and other Kuiper Belt objects (although it conveniently excludes Ceres as well). Maybe astronomers considered this in the past, but I never heard of a criterion like that until Pluto became an issue after the discovery of Eris.
But “cleared its orbit” was implicit in the original demotion of Ceres. It was originally thought to be a small planet, and then as the nature of the asteroid belt became clear, they reconsidered it and held that it wasn’t a planet, because it was one of many objects in that orbit, unlike the other planets.
I chatted regularly with some astronomers when I was in physics grad school last century. There’s definitely been some discontent about Pluto for a while. It doesn’t fit well with the four terrestrial planets or the four gas giants. It’s origin has always been obviously different.
But until other large objects were found out there, Pluto was just a misfit. But then it became clear it wasn’t unique and the issue came to a head.
It’s interesting that Mercury does better than Mars on all those criteria - that seems to mean that being closer to the Sun makes it “easier” to be judged as “clearing” the orbit (I haven’t analyzed the equations to see if how that happens).
True. But Pluto and other Kuiper Belt objects could have been excluded from being planets on some other basis, such as origin, composition, or orbit. Whatever you use would be somewhat arbitrary, but the “hasn’t cleared its orbit” criterion is not only vague, it also ends up putting Ceres in the same class of objects, “dwarf planets,” as Pluto and Eris, which IMO is absurd. It doesn’t make conceptual sense.
But they tried to evolve answers to the why, such as the celestial spheres and the epicycles:
As the wiki article explains, the theory of epicycles did work, until displaced by further observations, namely the elliptical orbits:
It was scientific method in its infancy: observe, then try to work out answers to account for the observations. It took a lot longer for theories to be developed, given the technology and communications levels of the time, but it was the scientific method.
The fact that epicycle theory was displaced because better observations called it into question doesn’t mean it wasn’t a scientific attempt to explain the known observations.
That’s why I’ve always liked Asimov’s article, “The Relativity of Wrong”, which highlights that even though earlier attempts to explain the universe have been proven wrong doesn’t mean those earlier attempts should be treated as unscientific.
And by combining the epicycles with the celestial spheres, they then tried to calculate distances to different celestial objects:
They got the numbers wrong, but consider the magnitude of the numbers they came up with: Huge! The idea that the universe was that big must have been mind-blowing to the average ancient person: “Whoa! That big? Really, dude?”
It was early science based on observation, to explain the universe. The fact that it’s been displaced by subsequent observations is just the scientific method at work.
That #3 is horseshit. What does it mean? Jupiter has 2 swarms of asteroids ahead and behind it at its orbital Trojan points. So Jupiter hasn’t cleared its orbit neighborhood. I’m waiting for Jupiter’s demotion.
They don’t have to be of comparable size, just relatively large in aggregate.
In the article linked above, they give a criterion called Soter’s u
The figure for Ceres is only 0.33, while the smallest figure for a planet, Mars, is 5.1×10^3. Planets have the most mass in their orbits by orders of magnitude, while dwarf planets don’t.
Ceres is a member of the Gefion family, although it appears to be an interloper:
I assume that since Ceres is considered a dwarf planet within the definition, that the presence of these other asteroids means that Ceres has not cleared its orbit by bringing them into its gravitational dominance, unlike the way Jupiter has brought its Trojans into its dominance.