NASA Scientists Propose New Definition of Planets, and Pluto Could Soon Be Back (sciencealert.com)

Rei writes: After several years of publicly complaining about the "bullshit" decision at the IAU redefining what comprises a planet, New Horizons program head Alan Stern and fellow planetary geologists have put forth a new definition which they seek to make official, basing planethood on hydrostatic equilibrium. Under this definition, in addition to Ceres, Pluto and other Kuiper Belt objects, large moons like Titan and Europa, as well as our own moon, would also become planets; "planet" would be a physical term, while "moon" would be an orbital term, and hence one can have a planetary moon, as well as planets that orbit other stars or no star at all (both prohibited under the current definition). The paper points out that planetary geologists already refer to such bodies as planets, citing examples such as a paper about Titan: "A planet-wide detached haze layer occurs between 300-350 km above the surface; the visible limb of the planet, where the vertical haze optical depth is 0.1, is about 220 km above the surface."

Comments

The definition is fine

[ceview]

The definition as proposed is prefaced as a 'geophysical definition of a planet' which already admits that it is using the definition based mostly on if the geophysics of the body is planet like. Saying pluto is a dwarf planet seems pretty good to me as it gives it a special place among planet like objects already. To increase the number of planets to over a 100 objects seems a bit silly. Astronomical bodies that orbit the sun include thousands of things, if the object is really big and clear most of the orbit and is dominant massive object that makes it a proper planet. If it is round but not a big mass then it's a dwarf planet, which still suggests it has planet like qualities.

Re:The definition is fine

[Rei]

Saying pluto is a dwarf planet seems pretty good to me as it gives it a special place among planet like objects already.

If they had simply stopped there, that wouldn't have been a problem. The problem is that they didn't. They declared that dwarf planets aren't planets at all - which is nonsense. Mars has far more in common with Pluto than, say, Jupiter. If anything should have been separated out, it's the gas and ice giants from the rocky/icy planets.

Hydrostatic equilibrium is a very meaningful dividing line to split groupings on. If a body is in hydrostatic equilibrium, it's experienced dramatic geologic change in its history - differentiation, tectonics, internal heating, generally fluids (particularly liquid water), and on and on. It's the sort of place you go if you want to learn about planetary evolution or search for life. If a body is not in hydrostatic equilibrium, it's made of primordial materials, preserved largely intact. It's the sort of place you go to learn about the formation of our solar system and its building blocks.

It's rare that nature gives you such clear dividing lines, but when it comes to planets, it has. It's not perfect - you can (and do) have bodies that straddle the border and are only partially or slightly differentiated. But in general, nature has drawn an obvious line in the sand, and we should respect that.

if the object is really big and clear

Is Earth's orbit clear? No, we have a huge massive object co-orbiting with us. Is Neptune's orbit clear? No, it has Pluto in it. They try their hardest to pretend that the IAU actually chose a "gravitationally dominant" standard, but that's not what they actually put in the definition. The standard in the definition is "cleared the neighborhood".

And it's based on a false premise - that each planet cleared its own neighborhood. Which is just pseudoscience. All of our models show that Jupiter, and to a lesser extent Saturn, cleared most of the solar system, including the vast majority of the clearing around Mars, and a good fraction around Earth (lesser around Venus). Mars did not clear its own neighborhood. Nor is it gravitationally dominant in its neighborhood; the vast majority of asteroids are in orbital resonance with Jupiter and not Mars.

And I've heard some people try to sneak around this by saying "Okay, maybe it isn't gravitationally dominant / cleared its neighbood now, but it has enough of a Stern-Levison parameter that it would have been had Jupiter not existed". First off, that's changing the definition yet again (to "would have cleared its neighborhood if no other planets were there"). But beyond that, it's abuse of the Stern-Levison parameter. The Stern-Levison parameter is built around a body's ability to clear asteroids - bodies with the current size and orbital distribution of our asteroid belt. Not protoplanets. In the early solar system it was the ability to clear protoplanets that caused neighborhoods to be cleared. Jupiter got rid of some really massive things that were forming in and near the inner solar system. There's a reason why our planetary system has such an unusual size distribution: the inner planets start getting bigger, the stop getting bigger, then get small, then debris, then something huge. That "something huge" stripped the building blocks out of the inner solar system, preventing it from becoming dominated by super-Earths. Saturn appears to have been our savior - its (delayed) formation appears to have stopped Jupiter's inward migration.

And even just going with the Stern-Levison parameter - Neptune has a Pluto-sized body in its "neighborhood". Now, Pluto may be small compared to Neptune, but compared to Mars it wouldn't be - yet Mars has a much lower Stern-Levison parameter than Neptune. Again: the only reason Mars doesn't experience stuff like this is because Jupiter cleared its neighborhood for it.

Re:Maybe

[Anonymous Coward]

Maybe stop changing arbitrary definitions.

Definitions like this will be arbitrary, and it just comes down to what makes it easier to write journal articles (where IAU has any authority). If Pluto was included in planets, there are quite a few orbital dynamics and evolution papers that would need to use the phrase, "The planets excluding Pluto....". There are plenty of papers on geology and atmospheric dynamics that wouldn't care about the orbit and would benefit from a definition like proposed here. There are others that would need to take a definition like proposed here, and modify it to say "All planets that aren't moons" or such.

Changing the definition of planet like this has zero effect on the science, it only changes the number of words needed to describe different groups.

It is like how you could define prime numbers such that one is a prime number. Math wouldn't be changed, just many, many theorems would require a few extra words due to saying "where p is any prime except one" instead of "where p is any prime."

Fuck you, NASA and shitty celebrity "scientists" like Neil Tyson.

You're putting too much importance on this. Even if you said "fuck you" to every rain drop that hits your car's windshield, saying fuck you to NASA or the IAU is putting too much importance on this. All of this drama seems to show a severe lack of understanding of the science and how disjoint the naming is from the actual science and more a matter of convenience.

Re:Maybe

[athmanb]

There's a lot of words but most of the points don't seem to make a lot of sense.

1. "Adjective nouns" need to have similarity to "noun" but aren't necessarily a subset. Gummy bears aren't a subset of bears either.

2. I'd like to see a citation on this. I highly doubt that you can simulate the formation of a solar system where multiple Mars analogues can coexist in the same orbit over billions of years without an accident happening to one of them.
Alone the fact that neither of the terrestrial planets have an orbital buddy tells us a lot about the chance of that happening.

3. In a geological sense yes. But the current definition of planets is based on orbital mechanics, after which Earth is a lot closer to Jupiter than to Ceres/Pluto.

4. Hydro-static equilibrium as a dividing line is way worse. There are roughly 100 TNOs where we don't really know whether they are elliptical. We'd have to visit each and every one of them with a probe just to put them in the proper category.
Meanwhile, it's completely clear which bodies qualify for the "clearing its orbit" rule. All currently qualifying planets have roughly 99% or more of the mass in their orbit in themselves. Ceres has 30%. Maybe there'll be an edge case eventually (Planet X or some Exoplanet) but that's a thing we can deal with in the future.

5. The definition should be mutable. Why should a planet that gets ejected keep counting as a planet?

6. I highly doubt life could form in a non-cleared orbit. There'd be late heavy bombardment style impacts all the time scouring the surface.
As for a life bearing celestial in orbit around another (gas giant) planet: I don't think anybody feels bad about calling that one a moon? As in "Yavin 4".

7. "Within each other's periapsis and apoapsis" seems like a reasonable enough definition that neither Ceres nor Pluto qualify for.

8. Yes that's silly but that'll probably be changed easily enough and has no effect on Pluto.

9. How are you planning to ascertain hydro-static equilibrium for an exoplanet if we can't even do it for Varuna.

etc etc the later points get even wordier and more politically minded.