Hubble finds Pluto's 5th "moon"

[gturner]

I assume they considered something simple like the gravitational attraction between a planet and the parent star (in Newtons).

If so, it was by a VERY small number of people.

My spreadsheet is currently busted (OpenOffice) and won't run when reinstalled (some Win7 owner/permissions issue where admin somehow lost some privileges on a recent update), but I was going to punch in some numbers to see if a single, arbitrary cut-point would've achieved the same result. Just a simple assumption of circular orbits and centripetal force, along with a table of masses and orbital periods, would probably suffice to answer that.

One thing I like about the total gravitational force approach is it might work for other star systems, perhaps with a tweak for the parent star's mass. Then "planetness" automatically grades itself by the induced observable wobble in the parent star, giving you a knew grading system (super-major planet, major planet, or what have you) that would automatically reflect various upgrades to the sensitivity of our planet-hunting instruments.

The only drawbacks I see with the adopted standard is the drawback you mentioned about clearing the orbit, which would raise the unwanted case that a non-planet body in an early solar system would become a planet over millions of years even if it didn't gain an ounce.

I don't think they had early/primordial solar systems in mind when they drew up this definition. Actually, it doesn't appear the definition really applies to exoplanets anyway; far from being comprehensive, it's really just a bare-bones guideline for how we deal with the dozen or so planets we know about in our own solar system right this minute.

And for that wouldn't a papal decree have worked just as well? There were great scientific reasons to make Pluto the flagship of a new class of body instead of planet, and it looks like they struggled to come up with a "sciency" justification with "official" definitions instead of just saying "This makes more sense, works better. So shall it be written."

Of course climatologists would've just linked Tombaugh to Exxon funding and been done with it.

 

[Christopher]

I assume they considered something simple like the gravitational attraction between a planet and the parent star (in Newtons). That has the advantage of giving a little more weight to closer, more brightly lit objects like Mercury and severely penalizes ice-dwarfs.

What? That's not a logical standard at all. What if there are actual full-size planets out beyond the Kuiper Belt, perhaps planets formed in the early days of the Solar System and ejected by Jovian migration, but remaining in cometary orbits rather than being expelled entirely to interstellar space? It would be arbitrary to say they weren't actual planets just because they were far enough away that the absolute value of their gravitational attraction is below some gratuitously selected number.

The goal is not to fudge the numbers so we can continue to justify our old assumptions. That's not how science works. The goal is to replace those imperfect old assumptions with a more logical, objective standard. If there's going to be a distinction drawn between planets and non-planets, it should be based on something that actually makes a meaningful physical difference rather than just reinforcing traditional definitions.

The only physically meaningful distinctions in terms of gravitational attraction, then, would be 1) whether an object is actually in orbit of the Sun rather than being a rogue interstellar body; and 2) whether an object is in orbit of the Sun directly rather than being a satellite of another body orbiting the Sun. Isaac Asimov argued that by the latter standard, the Earth's "Moon" was actually a companion planet, because the Sun's gravitational influence on Luna is twice as strong as the Earth's (due to the Sun's much greater mass). Others have disagreed. (And a debate over how to define a moon vs. a companion planet was a subtopic of the 2006 IAU debate, because there were questions about whether to count Charon as a moon of Pluto or a companion dwarf planet. Frankly I think they did let tradition win out there.)

Or where they thinking that the same body shouldn't become a planet just because of the size of its parent star (upping the gravitational force and orbital velocity in an otherwise unchanged situation)?

I'm sure that never entered their minds. For one thing, they weren't attempting to create a definition that encompassed exoplanets, but were only focusing on creating a definition that applied within the Solar System (which was itself one of the controversial aspects of the definition, because it's not very scientifically useful if it can't be generalized).

The only drawbacks I see with the adopted standard is the drawback you mentioned about clearing the orbit, which would raise the unwanted case that a non-planet body in an early solar system would become a planet over millions of years even if it didn't gain an ounce.

Indeed, and that's part of the same problem -- the definition is too narrowly focused on a particular place and time rather than being universally applicable.

There were great scientific reasons to make Pluto the flagship of a new class of body instead of planet, and it looks like they struggled to come up with a "sciency" justification with "official" definitions instead of just saying "This makes more sense, works better. So shall it be written."

Again, it's completely missing the point to think this is all about Pluto. It's the other way around: what prompted this whole thing was the discovery that Eris was bigger than Pluto (or equally big, we now think), which meant that Pluto wasn't the standout body of the group anymore; it was just part of the crowd of spheroidal trans-Neptunian objects. Since Pluto was no longer unique or exceptional within the class, it was necessary to come up with a definition that was not about Pluto specifically, but instead was about meaningful physical parameters that could be used to classify a whole range of objects.

 

[Crazy Eddie]

I assume they considered something simple like the gravitational attraction between a planet and the parent star (in Newtons).

If so, it was by a VERY small number of people.

My spreadsheet is currently busted (OpenOffice) and won't run when reinstalled (some Win7 owner/permissions issue where admin somehow lost some privileges on a recent update), but I was going to punch in some numbers to see if a single, arbitrary cut-point would've achieved the same result. Just a simple assumption of circular orbits and centripetal force, along with a table of masses and orbital periods, would probably suffice to answer that.

I'm sure it would, but why would you want to?

One thing I like about the total gravitational force approach is it might work for other star systems, perhaps with a tweak for the parent star's mass. Then "planetness" automatically grades itself by the induced observable wobble in the parent star, giving you a knew grading system (super-major planet, major planet, or what have you) that would automatically reflect various upgrades to the sensitivity of our planet-hunting instruments.

That would produce some rather screwy results for very small/tightly packed solar systems like Kepler-11, which probably has long-period comets that affect its star more than Earth does.

And for that wouldn't a papal decree have worked just as well? There were great scientific reasons to make Pluto the flagship of a new class of body instead of planet, and it looks like they struggled to come up with a "sciency" justification with "official" definitions instead of just saying "This makes more sense, works better. So shall it be written."

I'm pretty sure that the IAU basically did sit down and say "This makes more sense, works better, so shall it be written." That's essentially what all the pluto fans are whining about.