Is Pluto a planet or not? Who cares! Our love for the King of the Kuiper Belt is stronger than ever 95 years later

[Admin]

On the 95th anniversary of its discovery, Pluto remains one of the most beloved and enigmatic worlds in our solar system, whether you call it a planet or not.

Is Pluto a planet or not? Who cares! Our love for the King of the Kuiper Belt is stronger than ever 95 years later : Read more

 

[Witchy]

Obviously, many people do care about Pluto's planet status. It should be known that just 4% of the IAU voted on the controversial demotion, and most weren't planetary scientists but other types of astronomers. An equal number of professional planetary scientists, led by New Horizons principal investigator Alan Stern, signed a petition rejecting the demotion. To this day, Stern and most planetary scientists prefer the geophysical definition, an alternative to the IAU definition, which does not require orbit clearing for planethood and views dwarf planets as a subclass of full planets. Ironically, Stern is the person who first coined the term dwarf planet back in 1991, but he did so to designate a new subclass of planets, not to designate non-planets. The IAU members who voted in 2006 misused Stern's term.

 

[Helio]

Nice article!

The big reason Pluto's classification came into question was the discovery of Eris, originally estimated to be 10% larger than Pluto. [Modern figures put it at 97% to 99% that of Pluto.] It was discovered in 2005 by Michael Brown who could have chosen to argue he had discovered the 10th planet. Instead, he realized there would be many more large bodies out there, and having a very large number of "planets" would present problems. That's pretty humbling on his part, IMO. [ He has promised his daughter to find a "planet" out there, as noted in his book. ]

Today, there are almost 1000 KBOs, though none, so far, larger than Pluto. It's worth noting, perhaps, that even the HST has trouble looking at objects in the outer solar system. Put Jupiter out to about 10k AU (1/6th the way to the edge of the solar system), and it is too dim for the HST to detect.

The solution, IMO, is to use astrophysics more than astronomy. Margot published a paper that showed how massive an object must be in order to clear its orbit. The farther out an object is, the more massive it must be to clear the orbit. Ceres and Pluto are both too low in mass.

Oh, and the funniest thing I read about this was in Tyson's book where he stated that he never dreamed as an astrophysicist he would get "hate mail from 3rd graders".

 

[contrarian]

The farther out an object is, the more massive it must be to clear the orbit. Ceres and Pluto are both too low in mass.

According to the IAU, three criteria for a planet are:

It is in orbit around the Sun.
It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
It has “cleared the neighborhood” around its orbit.

Apparently it misses on the third item - it hasn't become gravitationally dominant. But what else in its orbit comes even close to Pluto's size in its "neighborhood"?

 

[Helio]

According to the IAU, three criteria for a planet are:

It is in orbit around the Sun.
It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
It has “cleared the neighborhood” around its orbit.

Apparently it misses on the third item - it hasn't become gravitationally dominant. But what else in its orbit comes even close to Pluto's size in its "neighborhood"?

These dwarf planets simply don’t have the physical ability to clear their unique orbits. It’s like, perhaps, in basketball where a child can’t quite reach the basket.

What’s lacking is that Pluto could clear its orbit given enough time. But Margot is using about 1 billion years, which seems reasonable, IMO.

 

[contrarian]

These dwarf planets simply don’t have the physical ability to clear their unique orbits.

I understand the rationale but am not aware of any significant bodies in its orbit, and there doesn't appear to be "recent" major impact events on Pluto or its moon to indicate an issue of this nature. How do they estimate this, or is it an assumption because it is a "KBO"?

 

[contrarian]

I now recall that Sputnik Planum is considered to result from a recent impact, but it only covers approximately 2.5% of Pluto's surface area. Not exactly what one would expect for these two objects if a lot of objects were in it orbit. But it is the largest "recent" impact on any body in the solar system that we know of, and is estimated at less than 10 million years.

 

[Helio]

I understand the rationale but am not aware of any significant bodies in its orbit, and there doesn't appear to be "recent" major impact events on Pluto or its moon to indicate an issue of this nature. How do they estimate this, or is it an assumption because it is a "KBO"?

This issue is why I like Margot's model since the question of clearing does not have to be based on what's there or not, but simply how physics works. Pluto likely has done some clearing in its, perhaps, 4 billion years in orbiting, but it lacks the muscle to be a true "tosser" of lesser objects.

Here's a web page on it....Here.

I'm mainly presenting what I think is the key argument for their definition. If Pluto gets re-labeled a planet, and there is some cool definition for this such as Stern's view, I would be okay with it.

But, if they do, I do have one favor to ask...Let's replace the ugly name of Uranus with the name of the planet originally given it by the one who discovered it (Herschel). He named it George ("Georgium Sidus" -- George's star).

 

[contrarian]

Pluto likely has done some clearing in its 4 billion years, perhaps, in orbiting, but it lacks the muscle to be a true "tosser" of lesser objects.

Agreed. I was just looking through impact history of Earth, and there is a mass extinction event every 30 million years or so, blamed on impacts. This would tend to suggest that Earth has not cleared its zone either. And Europa took a major hit 18 million years ago. And of course we are always looking for major impact threats, with one on the news right now. One is compelled to wonder what the definition of "clearing the orbit" really means......

 

[contrarian]

From Wiki: "In celestial mechanics, "clearing the neighbourhood" (or dynamical dominance) around a celestial body's orbit describes the body becoming gravitationally dominant such that there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence. "

It is the "no other bodies of comparable size" in its orbit that got me going on this. And that link to the article on revisiting Pluto's designation was quite informative. Seems like there are a number of planetary scientists who disagree with the downgrade. After looking at all those images from New Horizons, it certainly looks like a planet.

 

[Helio]

Agreed. I was just looking through impact history of Earth, and there is a mass extinction event every 30 million years or so, blamed on impacts. This would tend to suggest that Earth has not cleared its zone either. And Europa took a major hit 18 million years ago. And of course we are always looking for major impact threats, with one on the news right now. One is compelled to wonder what the definition of "clearing the orbit" really means......

The impact events are likely inbound objects and not orbital objects. Clearing the orbit would be for all the debris that is residual to the orbit.

 

[contrarian]

The impact events are likely inbound objects and not orbital objects. Clearing the orbit would be for all the debris that is residual to the orbit.

Clearly.

Just brought these up since there does not appear to be any large impact basins on Pluto except those over a billion years old. The "clearing the neighbourhood" definition is for " no other bodies of comparable size". If Pluto has not cleared its orbit, there should be larger, more recent impact basins, but there are not. And there are none in its orbit or we would have spotted them by now. These two observations indicate that Pluto has cleared its orbit, and is now just like the rest of the planets. It strongly suggests that it meets all three criteria for its designation as a planet.

 

[dava4444]

Hi

New here so please be gentle.

This whole question.. I have been having unshared thoughts for some time, so please bear with me.

I actually like the idea of Dwarf Planets.. we have Gas and Ice Giants.. so why not Dwarf Planets.. Pluto is still a *planet*.. just a Dwarf one. I'd go so far as being annoyed the opposite way and begin to defend Dwarf Planets..

Like I do not believe Haumea should be included as a 'Dwarf Planet'.. and I think the classification should be 'Asteroid Giant'.. I'd also like to see Deimos and Phobos be reclassified as this, as I believe to be a proper moon, it should be spherical.

Honestly I think Ceres is cool.. and one day, it maybe become, and probably will, Like Luna (one day hopefully soon) .. become the spaceport of which we springboard off to the inner solar system and beyond, Ceres will be our springboard to Jupiter and the outer solar system.

I do think calling The Asteroid Belt 'The Asteroid Belt' is dumb.. well no.. it's perfectly serviceable nickname, but IMHO I humbly suggest as an official name 'The Sol Belt' has much more meaning and gravitas. I mean Luna, Tera, Sol... The Moon, Earth, Sun.. all good and perfectly valid names. But once you learn there are more than one of something you can no long call it 'THE'.. as in the singular article.

We have also the Kuiper Belt now.. and one day we will reach the Oort Cloud.. and I *think* just like all the surprises we found in KB.. I think we will find many things to surprise us also, such as a Brown Dwarf with it's own mini solar system.. making us a quazi, dual star system, and answer that old question on what is tugging on Neptune?

peace

Dava