Should Pluto be reclassified as a planet?

I'm not an astronomer, and I'm glad that most people agree with the simple, standard name: it is a planet. This Mike Brown just wants to have his name on Pluto. Pay for a mission to SpaceX, and soon someone in the future will remember him.

Be a planet. I don't care if the moon is not. The moon is a satellite of the Earth, Pluto turns around the Sun

I think nostalgia should play a part. Sure it doesn’t meet all of the criteria (has Neptune cleared its orbit? Pluto is in it), but it could be used to inform why there aren’t 30 (or however many) other planets similar in size etc.

‘Pluto doesn’t technically meet the standards of what we now define as a planet, but out of respect for previous generations of astronomers it is included’

No.

There are two moons in the solar system larger that Mercury. It we find out something in the Kuiper Belt, or on the far side of it, bigger than Mercury (the so-called ninth planet), it probably won't be clearing a path for itself very efficiently, given its real slow orbit. If we find something even bigger, maybe in the Oort cloud, what would we call that? The Oort cloud doesn't revolve like something in or inside the Kuiper Belt. The Oort cloud extends halfway to Proxima Centauri. There are rogue planets outside any solar system. Is it OK to call those planets? Many of them are pretty big and don't move much dust or other matter out of their way. When a solar system is developing, planets interact with each other and the star. It's not unusual for really big planets to get kicked out of the system. What do you call them? We have renamed some asteroids "minor planets." There are asteroids that have moons.

A fundamental problem is that across the millennia humans have had this habit of labelling things and then somehow thinking we "know" a thing because we've put a tag on it. But nature is filled with grey scales / grandients.

Regarding Pluto, it was discovered about a hundred years ago with the telescope power we had available at the time, and it was large enough then to be discerned as a body orbiting the sun. We did not discover it to be revolving around a larger planet, so while it may be comparable to some of our solar system moons in size, it is Not a moon. It's revolving around the sun, and nothing else.

In terms of categorization, perhaps when we discover objects that are comparable in size to the moons of Jupiter, Saturn or Earth that are revolving solo around the sun, we could simply use the term "planetoid." That would to some degree be amenable to those who insist it's not big enough to be formally classified as a planet, as well as those who have regarded it as a planet since 1930.

A further complication is when we find moon-sized bodies floating among and alongside other cosmic debris, all of it revolving around the sun like the asteroid or Kuyper belts.

'Planetoid' seems like a reasonable classification we could assign to Pluto and other bodies of similar scale, not captured within the gravitational field of a larger local body, while revolving around the sun. But in order to do that, humankind would have to decide what diameter is "too small" to be considered a full-on planet. And when we truly think about deciding on that dimension, we may realize how truly subjective and even arbitrary these perceptions and classifications are. We're just hurtling through space, making stuff up, and every once in a while, finding aspects of nature upon which we can agree.

In no way does having Pluto classified as a planet imply or insist that the moon or anything else should be.

Similar to how the number 1 is defined as "not a prime", we can define planetary membership to be anything we want as long as it serves us or provides some utility. I'm a bit surprised that Mike has put this argument forward - it comes across as both sophomoric and dogmatic.

Brown is just plain wrong here, and his tone is extremely condescending. Most planetary scientists prefer the geophysical definition not because of nostalgia but because they see it as a better, more scientifically accurate definition. Unlike the IAU definition, which puts primacy on an object's location, the geophysical definition puts primacy on an object's intrinsic properties. Objects should be defined primarily by such properties rather than by location. Yes, this does view spherical moons as planets but acknowledges their dynamics by categorizing them as secondary or satellite planets. The Dawn mission at Ceres and the New Horizons mission at Pluto revealed these worlds to have the same complex geology and planetary processes as the terrestrial planets. The only difference is they are smaller. Furthermore, if Earth were in Pluto's orbit, it would not clear that orbit either, meaning the IAU definition could result in the same object being a planet in one location and not a planet in another location.

Brown's view is just one view in an ongoing debate. It is extremely arrogant of him to assume that planetary scientists who favor the geophysical definition do so becaue of nostalgia rather than due to scientific reasoning. Also, he refers to Ceres as an asteroid, which it is not because it is rounded by its own gravity. The geophysical definition is just as valid as the IAU definition, and there is no reason the IAU definition should be considered any more "official" than the geophysical one.

Notably, Brown co-discovered the 7 small planets he mentions; he did not discover them alone. Significantly, his partners in the discovery of Eris, David Rabinowitz and Chad Trujillo, reject the IAU definition.

Nostalgia has nothing to do with it.

The earth has a moon (if not the dozen or two that co-orbit). Pluto has a moon (or maybe five). Mars has two (even though they are probably captured asteroids). What is the problem? If it has a moon then it's a planet!!

Earth's moon is very round. Pluto's moon is very round. Mars' moons aren't that round, but they are still moons. If it's a round object orbiting something then it orbits around a planet!

Earth has mostly cleared out its orbit if you don't count the hundreds (or is it thousands) of trojans and horseshoe orbiters. The same more Pluto and the same for Mars. If it cleared out most of its orbit then it's a planet!

The problem with some scientists is that they think if they have a good enough argument for something that they are right. NO. Pluto should never have been demoted.

Truthfully, I'm fine calling it a minor planet or a planetoid, but it should still count as a planet. And, if that means that we need to add to the number of minor planets or planetoids, then so be it.

If Mike Brown wants to be known for something then I hope he finds Planet X before he is no longer here on Earth.

I still feel sorry for poor Pluto! I say it should be made an "honorary planet" - somewhat in the same way that great universities bestow honorary scientific degrees upon people who are not actually scientists.

We don’t need Pluto reclassified as a planet, we just don’t want it forgotten. It one time it was important enough to get into the books and our imaginations. Some of us, when we name our family members, include past and distant relatives. Some include closest friends, members of the household, or even pets. Some reject or don’t have blood relations but create nuclear family through other connections. Pluto doesn’t have to have planet status any more than the moon does, we just need to have a vocabulary that satisfies our desires to keep it in our address books and photo albums.

No! It's a TNO. There are too many Pluto like objects outside Neptune's orbit. Its orbit is way off-the- ecliptic. It hasn't cleared out It's orbital area.

In 1930 a new planet was discovered, and it was named Pluto. The object fit the definition of planet and was known worldwide as a planet. Then many years later 'science' redefined the criteria of what is a planet. As long as they keep redefining what things are they won't be out of a job. These 'scholars' with their PhD's (post hole digger is what it stands for) should actually perform activities in science and stop undermining what actually known. It's also true that most of what we know is wrong and maybe they should got out from behind their desks

I'm shocked that astronomer Mike Brown makes such argument which doesn't make sense to me.

A planet is a large, rounded astronomical body that orbits around a star, stellar remnant, or brown dwarf.

A moon (or natural satellite) is a celestial body that orbits a planet, dwarf planet, or asteroid.

Michael E. Brown is a 61 years old American astronomer, who has been professor of planetary astronomy at the California Institute of Technology since 2003. He is best known for discovering Eris, a massive dwarf planet in the outer solar system. I think that Mike Brown learned the above definitions 50 years ago or so. But now he feels so important that he can ignore the established definitions at his fancy.

It is somewhat disconcerting to see the primary argument of a respected scientist boils down to the binary conclusion that either dynamics matter or they don't. Clearly not all dynamics matter or we would also be discussing whether direction of rotation of a body determined if it was a planet or not, along with hundreds of other dynamic properties. It would not be surprising to have the number of planets increase as our ability to perceive them increases, so adding new planetary bodies should not be an argument for narrowing the definition. I also find the idea of a planet having to "clear its orbit" rather ironic considering the number of meteor showers we experience on earth every year... caused by debris left in our "cleared orbit".

Thanks for your insight, Mike. The comparison of Earth's moon to Pluto rings a bit hollow to me. Our moon could never be considered a planet, as it doesn't maintain an independent orbit around the sun. Likewise, Pluto could never be considered a moon, because it orbits the sun directly. Perhaps the conversation could be steered to address the status of dwarf planets in general and whether or not they all deserve the same degree of attention from which Pluto has historically so richly benefitted. We can't erase Pluto's legacy, planet or no. ...And no, the moon is not a planet...