NASA official : ‘Pluto is a planet’

If it's round and in orbit around the Sun, it's close enough for me.

 

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Ceres is a planet! Yay! Now there's an even 10.

 

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But I work in Base 16.

 

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In that case, Pluto is 8. (zero-based numbering)

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OK. Ceres is A.

 

Finally, someone had the courage to speak the truth.

 

Bridenstine isn't just a 'NASA official', he's the NASA Administrator. (I like Jim Bridenstine. He's a space-geek and it's long past time that we had one of those in charge of NASA. Having grown up on Science Fiction should be a job requirement.)

I agree with his personal view of Pluto. (He's not challenging the IAU, he's just stating his opinion.) If dwarf stars can still be stars, then poor unloved, forsaken Pluto can damn well still be a planet! It wears its heart on its side... all it wants is to be loved.

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(Pluto is consistently childrens' favorite planet. They identify with it, smallest, furthest away, not entirely accepted...)

But James does make a legitimate point:

I'd say 'Yes'. I'm not convinced that the Solar System stops at Pluto. (New Horizons has already visited Ultima Thule, which admittedly was a pretty funky little thing.) There's probably lots more out there that we don't know about. Not just Pluto-like bodies like Eris and Sedna either. (These would be certifiable little worlds if you were standing on one.) There may be another big Neptune sized planet lurking out there as well. It's never been seen but some astronomers have deduced its possible presence from perturbations in the orbits of other known bodies. There are currently efforts underway to locate this mysterious 'Planet X'.

https://solarsystem.nasa.gov/planets/hypothetical-planet-x/in-depth/

https://iopscience.iop.org/article/10.3847/0004-6256/151/2/22

So... if Pluto is reinstated as a planet (if only honorary, if the IAU won't buy it), then it simultaneously has to surrender being the farthest planet.

Which is fine with me, since it leaves the Solar System a bigger place with things still left to discover out there in the Kuiper Belt, the Oort Cloud and wherever distant bodies may be found. We may currently only know about the innermost reaches of a vastly extended Solar System.

https://en.wikipedia.org/wiki/Trans-Neptunian_object

https://en.wikipedia.org/wiki/Planets_beyond_Neptune

 

I think the main objection to planet is its orbit

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It seems to me that planets can be treated the same way that stars are treated. There are a whole variety of different kinds of stars, yet they are all stars. Red giants, white dwarfs...

Similarly, we can just as easily say that there are different kinds of planets: Rocky Earthlike planets of varying sizes and characteristics, cold gas giants, hot gas giants (none of which is found in our Solar System but seem common elsewhere, and... dwarf planets. Each with its own defining characteristics, yet each one nevertheless a legitimate planet in the broad orbital sense that Michael alludes to - they all orbit their star.

Since as Janus58 says, 'planet' is something of an arbitrary label, it probably should conform as much as possible to traditional usage, to how people on the street were taught and how they use the word. (Bridenstine referred to that, saying Pluto being a planet is how he learned it and he's personally sticking with that.) People have an attachment for how they learned things and scientists shouldn't just go around changing everything unless they have some strong and overriding reason to do so. The public might not accept the change. (Nor must they accept it.)

Even among stars, there are varieties that do differ so dramatically from normal paradigmatic 'stars' that they probably shouldn't be considered 'stars' at all. Neutron stars for instance. They aren't composed of gas/plasma, they don't generate heat and light by nuclear fusion, they are something else entirely. Black holes differ even more dramatically, except that unlike neutron stars, nobody calls black holes 'stars'.

(In the 18th century when something like black holes/dark matter was first hypothesized they were called 'Dark Stars', stars so massive with so much gravity that all of the light particles they emit fall back in and can't escape. There were 18th century theorists who suggested using their Newtonian mechanics that most of the universe's mass, it's largest and most impressive stars, were hence invisible up there.)

So... in the case of dwarf planets, do they differ so dramatically from normal paradigmatic 'planets' that they don't really belong in the same broad 'planet' category at all? That's the question. Objects like Pluto certainly seem to me to be similar enough to Earthlike planets to belong to the same very general category. They are certainly more similar to Earth than Jupiter or Saturn are. (Pluto actually has a surface.) Of course there are differences that are probably important enough to assign it to a different subcategory of 'planet'. 'Dwarf planet' in this case. They are too small to exert the same kind of gravitational perturbations on other bodies near their orbit that larger planets exert, for instance.

I do think that there probably should be a dividing line between what we call 'planets' and things we don't. We might want to say that planets need to have enough gravity to make them spherical, so as to exclude all the irregular space-rocks, small asteroids and comets.