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Defining Planethood
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| steeljaw354 | Date: Friday, 17.06.2016, 10:18 | Message # 181 |
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| Well there are also systems like Orcus/Vanth, Quaoar/Weywot Eris/Dysnomia Salacia/Actaea , but I'm not really sure if there is a barycenter for these bodies. If these bodies are even round enough to be planets.
Edited by steeljaw354 - Friday, 17.06.2016, 10:27 |
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| midtskogen | Date: Friday, 17.06.2016, 11:38 | Message # 182 |
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| The purpose of a new set of criteria is not to find something matching the currently accepted planets of the solar system, but rather to find a clearer and possibly more meaningful definition. These criteria will include several trans-neptunian objects (Pluto for sure, likely Eris), possibly binary objects as well, as planets, and I'm fine with that.
An interesting question is whether Ceres will become a planet by these criteria. It would please Titius and Bode. 
NIL DIFFICILE VOLENTI
Edited by midtskogen - Friday, 17.06.2016, 11:44 |
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| steeljaw354 | Date: Friday, 17.06.2016, 19:21 | Message # 183 |
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| Ceres deserves it. I would be so happy if they accepted this instead, it makes much more sense and it so much simpler. And by this definition, eris and possibly other objects are planets and that warrants a mission to them, would be cool to see eris and dysnomia from New horizons 2.0.
Edited by steeljaw354 - Friday, 17.06.2016, 19:24 |
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| FaceDeer | Date: Friday, 17.06.2016, 22:44 | Message # 184 |
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| I don't see as strong a need for a "double planet" definition as I do for a planet, IMO, but my preferred criterion for double planethood is that the two planetary bodies be tidally locked with each other.
The reason I like tidal locking is that the formula for determining the timescale of tidal locking has a sixth-order term in it (semimajor axis is raised to the sixth power) so, much like the orbit-clearing issue earlier in this thread, you're very rarely going to see borderline cases - either an object will tide-lock very quickly (on the order of just millions of years) or very slowly (tens of billions of years, ie, not in the current lifespan of the universe).
This is actually well reflected in Space Engine, it's super rare to find a planet or moon that's almost-but-not-quite tide locked.
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| steeljaw354 | Date: Saturday, 18.06.2016, 00:14 | Message # 185 |
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| Super rare? I find them all the time.
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| Wicker1M | Date: Saturday, 18.06.2016, 18:26 | Message # 186 |
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| Quote steeljaw354 (  ) Would you like this instead?
1) Orbits the sun
2) Is in hydrostatic equilibrium
3) Is not man made
4) Doesn't undergo nuclear fusion or deuterium fusion currently or in the past
I don't like the "orbits the sun" part of the IAU definition. I think it should be modified to "currently orbits a star (or stellar remnant) or has orbited a star in the past". My modified criterion would include exoplanets, rogue planets that have been kicked out of their star systems, and planets that orbit stellar remnants such as pulsars and black holes.
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| midtskogen | Date: Sunday, 19.06.2016, 18:26 | Message # 187 |
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| Quote Wicker1M ( ) has orbited a star in the past
How do you determine whether something has orbited a star in the past?
NIL DIFFICILE VOLENTI
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| Wicker1M | Date: Tuesday, 21.06.2016, 22:39 | Message # 188 |
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| Quote midtskogen ( ) How do you determine whether something has orbited a star in the past?
Good question. There must be some method of determining whether a rogue planet has been kicked out of a stellar system.
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| steeljaw354 | Date: Tuesday, 21.06.2016, 22:54 | Message # 189 |
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| Well it makes 0 sense to have that in a definition, as rogue planets are considered "planemos" and not planets.
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| Watsisname | Date: Wednesday, 22.06.2016, 07:44 | Message # 190 |
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| Quote Wicker1M ( ) Good question. There must be some method of determining whether a rogue planet has been kicked out of a stellar system.
Well, we don't know how planets can be formed except in the context of disks -- whether that disk be around a newly forming star or brown dwarf, or maybe even the remnants of an exploded star. And we know that dynamical processes in young systems can easily eject planets into interstellar space. So it's probably safe to assume rogue planets we find were ejected from star systems, although it is basically impossible to prove it or determine exactly where one originated.
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| midtskogen | Date: Wednesday, 22.06.2016, 07:55 | Message # 191 |
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| What (detectable) then sets a (sub-)brown dwarf apart from a rogue planet?
NIL DIFFICILE VOLENTI
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| Watsisname | Date: Wednesday, 22.06.2016, 09:26 | Message # 192 |
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| What is meant by a sub brown dwarf? I'm aware of ~13 Jupiter masses being considered the separation of planets from brown dwarfs.
By direct imaging, the only real measurable for a rogue object is its flux, and its distance if we use parallax. Models can then be used to relate that to mass and temperature, which help distinguish between a brown dwarf or a planet.
If detected by gravitational microlensing, then we get its mass.
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| midtskogen | Date: Wednesday, 22.06.2016, 09:34 | Message # 193 |
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| Quote Watsisname ( ) What is meant by a sub brown dwarf?
Objects below 13 Jupiter masses formed by the collapse of a gas cloud.
NIL DIFFICILE VOLENTI
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| steeljaw354 | Date: Wednesday, 22.06.2016, 10:04 | Message # 194 |
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| Well they can also form like stars but with smaller disks.
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| Watsisname | Date: Wednesday, 22.06.2016, 11:03 | Message # 195 |
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| Oh, then I have no idea of an observable property that would determine that.
I think I'm in the camp that would just call such bodies free-floating planets rather than sub-brown dwarf. It's hard for me to think of the formation setting being important for what we call it if it's correct that we can't tell the difference.
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