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.