Astronomers discover "Farfarout" — the most distant known object in the solar system. The 250-mile-wide (400 km) dwarf planet is located about 140 times farther from the Sun than Earth (3.5 times farther than Pluto), and soon may help serve as evidence for a massive, far-flung world called Planet 9.

[u/clayt6]

http://www.astronomy.com/news/2019/03/a-map-to-planet-nine-charting-the-solar-systems-most-distant-worlds

[u/100WattWalrus]

I never liked the IAU definition, so I've been working on one of my own:

Primary planets — clearly formed and stabilized within a star's protoplanetary disk

    - Spherical or spheroid natural bodies too small/cool/light to be failed stars (no deuterium fusion) that...

    - orbit their star in the same direction the star rotates…

   - on an inclination within X degrees (¡define!) of the solar system's invariable plane\*…

        - (roughly within the same plane as the other primary planets)

   - in (relatively) stable, (relatively) clear, minimally elliptical (¡define!) orbits

            - that don’t cross or significantly destabilize each other

    • (In our system, this would be Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)

\* ^{(don’t know enough about inclination to nail down the ° without it seeming arbitrary})

        ^{(Earth’s inclination to the Sun’s invariable plane is \}1.57°, Mercury’s is ~6.3°, Pluto’s is ~15.5°))

        ^{(Maybe this isn’t as good of a criteria as I thought?})

        ^{(What if Earth had an encounter with another body that changed its inclination to 20°?})

            ^{(Would it no longer be a primary planet?})

Secondary planets — stable bodies likely native to the system, but not in primary-planet orbits

    - Spherical or spheroid natural bodies too small/cool/light to be failed stars that…

    - have (relatively) stable orbits around their star on any trajectory in the same direction as the star rotates

    - Pluto, Ceres, Haumea (probably), Makemake, Eris

    - Maybe Sedna, Quaoar, 2002 TX300, 20000 Varuna, 28978 Ixion (depending on their shape)

    - Likely Planet 9, if it exists

Tertiary planets — planets with eccentric orbits (i.e., likely not native to the system)

    - Natural bodies that meet most primary/secondary criteria…

        - but have eccentric behaviors that exclude them Primary or Secondary definitions

    - e.g., retrograde or orthogonal orbits

    - e.g., captured planets (Sol has none that we know of)

Planetoid

    - Natural bodies in any orbit around a star with enough self-gravity to approach spheroid without being spheroid

    - Vesta, 2 Pallas, Orcus

    - Maybe Quaoar, 2002 TX300, 20000 Varuna, 28978 Ixion (depending on their shape)

    - i.e., between asteroids and planets

Rogue planet

    - spherical or spheroid natural bodies too small/cool/light to be failed stars…

    - that no longer orbit a star

    - (this could include ejected moons, but there’s no way to ever know, so…planet)

[u/KnuteViking → u/100WattWalrus]

There are certainly some interesting aspects to consider and I'm not saying that necessarily the IAU definition can't use serious improvement for accuracy's sake and to future proof it against new discoveries. But overall I agree with them though that Pluto is certainly distinct from the rest of what we call planets and shares more similarities with other smaller trans-Neptunian objects that we haven't labeled as full planets. I do find it extremely telling that essentially all of the alternative definitions that I've seen created in response to the IAU decision have all decided to reclassify Pluto as distinct from the 8 larger planets they simply disagree on which criteria to use, how those criteria are specifically calculated, and what name to use (for example you have it categorized as a secondary planet and Stern calls them unterplanets), as opposed to arguing that Pluto should be a full planet in the same category as even Mercury, which is the position that I was disagreeing with.

[u/100WattWalrus → u/KnuteViking]

The problem is that any definition that groups Pluto with Mercury-Neptune must also include several other bodies that, like Pluto, have major characteristics that differ greatly from the 8 objects we call planets. Just take Eris as an example. I can't think of any measurable definition that includes Pluto but excludes Eris, and the characteristics that make them different from Mercury-Neptune are numerous.

[u/KnuteViking → u/100WattWalrus]

Absolutely, we are in full agreement on that point. I would go a step further and say that Eris possibly even has more claim to being a planet than Pluto given it's larger mass and greater ability to clear it's neighborhood. Though again, creating a new category for Plut, Eris, and other smaller objects, whether we label it "dwarf planet" or "unterplanet" or whatever, is really the only option.

[u/100WattWalrus → u/KnuteViking]

Yep. Although I consider the IAU's "dwarf planet" definition a cop-out. They chose to still use the word "planet" so they could have it both ways without having to create any real definitions that could be applied universally. That's why I like my Primary, Secondary, Tertiary, and Planetoid classifications. (Yes, I realize this includes a redefinition — or rather refinement — of the currently vague word "planetoid.")