New Planetary Definition Draft

[FarGetaNik]

There is a new paper on A GEOPHYSICAL PLANET DEFINITION. I was bothered a long time by the IAU definiton, less because Pluto was "demoted" but rather how impractical it is. This new proposol is surprisingly simple, intuitive and still usefull. I fully agree with their argumentation. "Dwarf planets" would be planets, as well as exoplanets, rougue planets and surprisingly major moons!

If someone is bothered by the thought of several planetary satellites being planets, we still can group objects by orbits. We would have planets that happen to be moons, or "dwarf planets" or rogue planets.

I'm courious about the part "has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid". It looks like the definition of hydrostatic equilibrium. But on closer examination, this could also mean that objects that once were in hydrostatic equilibrium are considered planets. This would include Iapetus and Miman (who are proven not to be in h.e.) and objects that can't be measured directly, the dozens of transneptunian objects of suitable size.

[john71]

The word "planet" comes from Ancient Greek πλανήτης ‎(planḗtēs, “wanderer”), from Ancient Greek πλανάω ‎(planáō, “wander about, stray”), of unknown origin. Perhaps from a Proto-Indo-European *pel- ‎(“to wander, roam”).

So I think the ancient meaning of the word does not exclude dwarf planets and moon planets.

A planet is massive enough to be rounded by its own gravity, and not massive enough to cause thermonuclear fusion...that's it...

[FarGetaNik]

The new proposal is closer to the original meaning of the word "planet", I agree on that. But the sun was also considered a planet in ancient times I think... so it is usefull to define the limits according to our current understanding of planetology.

Brown dwarfs are more similar to planets in that matter, they are not fusors (disregarding a brief period in their history when they would fuse Lithium). Stellar remnants in contrast are much more massive and can easily be excluded from being planets.

[john71]

I think two very important factors should be considered:

1.) the object must be massive enough to be rounded by its own gravity

and

2.) humans should be able to walk on it...

Maybe it is an anthropocentric definition but a practical one...

[FarGetaNik]

2.) humans should be able to walk on it...

A very vague statement. So Jupiter is not a planet? What about Venus, where you would die before taking the first step? By a strict interpretation only habitable planets would be planets. I think humanity made great progress in abandoning any antrhropocentric judgement.

About the classification of objects in Celestia, the only aspect that cares if something is a planet or something else is orbit and label rendering. So maybe the geophysical definition wouldn't affect Celestia, apart from the newly approved planets being defined as "dwarf planet" (we have to come up with a more suiting name) or major moons in Celesita.

[Fenerit]

we propose a geophysically based definition of “planet” that importantly emphasizes a body’s intrinsic physical properties over its extrinsic orbital properties.

bold point (mine) is more complicate: a true "planet" wouldn't be tidally locked, IMHO.

[FarGetaNik]

a true "planet" wouldn't be tidally locked, IMHO.

But what about the newly discovered Trappist-1 system? All of its planets are expected to be tidally locked. Some of them could even be habitable... And technically Pluto also is tidally locked as well and Earth is expected to reach this state in some billion years from now.

I think the authors have a valid point with their intrinsic definition. We just have to agree on a way to call such objects. The term "world" is used awkwardly for these objects. Calling them planets is certainly radical from a historic perspective, but I believe a scientifiv definiton should not care about such things.

[Fenerit]

The intrinsic propriety of hydrostatic equilibrium bring off the interior differentiation and then to the tectonics (if any) currently set forth in respect to the Earth. For tidally locked planet is unclear how such a tectonics is driven by tidal forces along the orbital path. If tectonics is not the same on Earth or there is not tectonics at all, still planets and moons shares the same "definition" once more.

EDIT LATER
Jupiter gas giant class excluded, I forgot. In short: TRAPPIST-1 are starmoons: if one of the starmoons is not tidally locked then is an exoplanet; if it has moon(s) those are exomoons.

[FarGetaNik]

The intrinsic propriety of hydrostatic equilibrium bring off the interior differentiation and then to the tectonics (if any) currently set forth in respect to the Earth. For tidally locked planet is unclear how such a tectonics is driven by tidal forces along the orbital path. If tectonics is not the same on Earth or there is not tectonics at all, still planets and moons shares the same "definition" once more.

I don't quite get this. How do tectonics come into play? Tectonics is a very rare phenomenon, that amoung the yet official planets only Earth exhibits. Hence, there are many objects in hydrostatic equilibrium without textonics, or even undifferenciated (some outer solar system moons).

TRAPPIST-1 are starmoons: if one of the starmoons is not tidally locked then is an exoplanet; if it has moon(s) those are exomoons.

So if you call planets stellar moons, I see no problem in calling planetary moons planets And for more examples: if Mercury's orbit wasn't that eccentric it would be tidally locked (instead of its 2:3 spin orbit resonance); if Venus' atmosphere hadn't such forcefull retrograde winds it also would be tidally locked.

[Fenerit]

I don't quite get this. How do tectonics come into play? Tectonics is a very rare phenomenon, that amoung the yet official planets only Earth exhibits. Hence, there are many objects in hydrostatic equilibrium without textonics, or even undifferenciated (some outer solar system moons).

It play a role as long as to this must be added this.