Comments on: A Dwarf By Any Other Name

By: genialityofevil

genialityofevil — Sun, 24 Jul 2016 22:56:07 +0000

Well Pluto wasn’t discoverd until the 1920s and the Kuiper Belt wasn’t discovered until the mid-90s (though it was predicted to exist). So there was no reason to apply the “clearing its orbit” requirement because no known planet “candidate” was in that situation.

By: Jean Tate

Jean Tate — Sun, 24 Jul 2016 21:02:48 +0000

That makes a lot of sense, if you regard astronomy as a purely observational science.

However, since at least the time of Newton, if not the discovery of Neptune, astronomy has been deeply influenced by physics. Ignoring physics other than gravity – so star/planet, rocky/ice/gas planet, etc – the orbital stability of systems of masses has been know for at least a century. So what, other than “tradition”, stopped the IAU from adopting a “clearing its orbit”-type criterion in the 19th century?

Yeah, lack of the ability to edit is a pain 😉

By: genialityofevil

genialityofevil — Sun, 24 Jul 2016 19:42:49 +0000

Oh for want of an edit button. Anyway, another part of the problem is sample size. We’ve discovered a good few thousand exoplanets by now but don’t know nearly enough about them to make judgements about their state, so the sample size of planets we can draw from is pretty much limited to the ones around us. Again, by the time the 2018 GA rolls around there might be enough collated data to make a clearer picture.

By: genialityofevil

genialityofevil — Sun, 24 Jul 2016 19:38:49 +0000

Definitions of ranges are hard to do. Neil DeGrasse Tyson has talked about it before, where we have to somehow draw a line between asteroids, satellites, rocky planets, ice giants, gas giants and stars. There’s a lot of grey area between all of those categories. For all the complaints people have made that the IAU’s definitions are arbitrary and unscientific, no-one has come up with more precise definitions.

By: Jean Tate

Jean Tate — Sun, 24 Jul 2016 19:02:17 +0000

Thanks. This detailed look into the history of how the IAU – the “official” body of all things astronomical – sure is interesting. And rather disturbing: as a field of study that is scientific, and likely more physics-driven than most, the ad hoc and arbitrary nature of its definitions is rather unsettling.

The stability of massive objects whose motions are dominated by gravity has been known for centuries, so why hasn’t the IAU cottoned-on to how inconsistent its definitions are?

What’s going to happen when a binary, composed of two objects with masses below that of a “star”, is discovered (perhaps a “brown dwarf binary”)? Will the more massive one be called, what, “the primary”? “the star”? And the less massive one a planet? Or will it become, by some semantic jujitsu, a binary planet?

How about when an exoplanet system is discovered, in which there’s something like super-Earth and an Earth-mass pair, kinda like a souped up Pluto-Charon?

Or even a very distant binary TNO, with each having ~the same mass, and well and truly in hydrostatic equilibrium?

How about massive objects which orbit an SMBH (super-massive black hole), such as SgrA*? It will surely be only a matter of time before one with a “planetary” mass is discovered (though discovering any binary planets may take a bit longer)!

Or will “planets”, “dwarf planets”, and “moons” (and “asteroids”, “comets”, etc) be limited to what orbits our homestar, the Sun (per the definition Brian posted), or its planets (etc)?

By: genialityofevil

genialityofevil — Sun, 24 Jul 2016 00:18:02 +0000

Well the IAU’s reasoning for why Pluto-Charon isn’t a binary planet system is that the definition of a satellite doesn’t involve its barycentre (an early draft of the 2006 GA resolution added it but was later rejected). The Moon and Charon still orbit around their parent planet, though since Charon orbits a point outside of Pluto’s surface, its classification as a moon has a small asterisk. In a few hundred million years the Moon will be in the same position. The Sun also has more influence over the Moon than Earth does, as calculated by Isaac Asimov. Not by much but it is technically enough to argue that the Moon actually orbits the Sun with Earth.

I don’t think it’s been raised at IAU general assemblies since 2006. Given the New Horizons data, it’ll probably come up in 2018.

By: Jean Tate

Jean Tate — Sat, 23 Jul 2016 14:44:56 +0000

Right. But why not?

By: genialityofevil

genialityofevil — Sat, 23 Jul 2016 14:41:45 +0000

Earth and the Moon, that is.

By: genialityofevil

genialityofevil — Sat, 23 Jul 2016 14:40:45 +0000

The IAU doesn’t have a definition of a binary planet system, otherwise Earth would probably be one too.

By: Jean Tate

Jean Tate — Sat, 23 Jul 2016 14:37:31 +0000

Question: is Charon a dwarf planet (apparently not)? Is Pluto-Charon a binary dwarf planet (also apparently not)?

If two stars, of quite unequal mass, can be called a binary (provided they orbit a common center of mass), why can’t Pluto-Charon be called a binary dwarf planet?