What a planet is, revisited. Also Pluto.

Nice discussion of Sedna, which is 3 times as far from the sun as Neptune and ~42-45% the diameter of Pluto. It is outside of the Kuiper belt and in the inner part of the Oort cloud. They think it’s round though. Discussion of what a planet is and how it could be defined follows. http://web.gps.caltech.edu/~mbrown/sedna/index.html

Possible ways of defining planet: historic, “Historic-plus”, gravitational rounding (the author prefers something like planetoid or sphereoid), and population classification. That last one is tied to the “Cleared its neighborhood” criteria and is currently accepted scientific usage. “Historic-plus” is the conventional definition: planets are defined as bodies that are equal to or bigger than Pluto.

Great article about Sedna. Thanks.

If you pronounce “planemo” kind of like “dynamo” (PLAN-uh-Moh) it doesn’t sound like anything. It’s pretty blah.

But if you pronounce it (pluh-NEE-Moh) it sounds pretty cool, a bit like “neutrino”. As a coinage for a generic star orbiting body it sounds kinda neat and high tech sciency when said that way.

Apparently not. Wikipedia: [INDENT]There is not an established lower limit on what is considered a “moon”, as natural satellites will be referred to in this section. Every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been identified as a moon, though objects a tenth that size within Saturn’s rings, which have not been directly observed, have been called moonlets. …

The upper limit is also vague. Two orbiting bodies are sometimes described as a double body rather than primary and satellite. [/INDENT] Natural satellite - Wikipedia

If it’s bigger than the Moon and orbits a sun, it’s a planet. If it’s smaller than the Moon, or if it orbits a planet, then it’s not a planet.

Pluto is not a planet, and neither is Ceres, and neither is Quaoar, and neither is Eris.

Yes, the Moon is an arbitrary standard. So what?

I object. No right thinking person would include 2000YW134 in that list. Think of the children.

I think planets should be round things that big enough according to some arbitrary criteria, no matter what they are in orbit around and whether they have cleared their orbit.

This size threshold would be conveniently set above the size of the earth moon, thus it is not a planet and neither is Pluto. Some of Jupiters natural satellites would be planets though.

This makes sense to me. In science fiction, that’s what a planet is. They rarely if ever talk about landing on moons, even though that’s what they would have to be sometimes (for instance in Firefly).

As I see it, the orbiting the sun requirement was a mistake we made due to the specific configuration in our solar system, that we are now making awkward definitory gymnastics in order to try and keep. Such as clearing the orbit. Would earth have cleared Pluto’s orbit if it had been out there - I don’t think so. And this shows how arbitrary that definition is. The dwarf planet definition also makes more sense if it’s (strictly) a criteria about size.

I would also prefer the sun(s) to be the closest star(s) with the rest called stars, and the moon(s) to be the closest natural satellite(s) with the rest called satellites.

Well strictly speaking, Earth is not a Planet as it has not cleared the neighbourhood either and neither are Venus, Jupiter, Mars and Neptune and the justifications given, like in the article are frankly weak.

The largest body orbiting the sun is a Dwarf Planet.

Your cite states

“Earth is indeed in a very crowded orbit, surrounded by tens of thousands of asteroids and other objects. As it turns out, that clutter still isn’t remotely enough to cost Earth its planet status, but it’s worth understanding just what makes Earth’s case different from Pluto’s. And to do that, we have to go back to the definition of a planet”

“…consider those objects relative to the planets themselves. Pluto, for instance, is just .077 times the mass of all the other objects in its orbit, meaning it makes up roughly 8% of the mass found in its orbit. Earth, on the other hand, is 1.7 million times the mass of all the other objects in its orbit…”

Bolding is mine

So if I walk around at night with a lit flashlight, I am a planet.

Which is why I stated that justification such as ststed in the article are weak. Basically they are saying that Pluto is not fat enough to be a Planet

You did read the whole article right?

“The IAU didn’t include a strict cut-off for how clean a neighborhood has to be for an object to be considered a planet. Obviously, when the gap between the least clean planet and the cleanest dwarf planet is separated by a factor of more than 72,000, there isn’t much need for one.”

At the risk of falling for a whoosh …

No. Not at all.

What I meant was that the word “planet” meant 6 specific things to the Greeks: Mercury through Saturn. (Although as **ludovich **& **Colibri **pointed out there were a couple others known to the Greeks).

My point is the word today ought to mean exactly those things it meant to the Greeks, no more and no less. It’s *not *shorthand for a rule which can be applied to include or exclude any given astronomical body. It never was such a thing.

It’s instead the name of a particular list written millennia ago. That’s what it meant then and that’s what it means now.
Would it be useful to create a taxonomy of orbiting stuff? Sure. Just don’t include the obsolete word “planet” anywhere in there when inventing the names of your classifications.

That is absolutely, positively not what planet meant to the Greeks. They had no concept that Earth was a planet. None, nil, cipher, naught. The Earth was the center of the universe.

Also, they considered the Sun and Moon to be planets. A planet, to the Greeks, was an object that moved in relation to background stars.

Any definition of a planet that involves orbiting a star starts from a point unknown to the ancient Greeks.

Ok, ok you win: …2004PF115, 2000YW134, 2004PG115, 2002WC19, 2007XV50, 2010EK139, 1999DE9, 1998SN165, 2005TB190, 2002KX14, 2003FY128, Hari Seldon with a flashlight, 2010TJ… :smiley:

Yeah that article could have been better. I understand though that if Earth was transported to Pluto’s orbit, that the neighborhood wouldn’t be cleared either. Don’t blame Pluto, blame its environment.
I see that there are only 5 official dwarf planets though there are hundreds of likely dwarves. One of them is Haumea, which is believed to be in hydrostatic equilibrium, but also longer than it is round by a factor of 2. Another dwarf named Makemake lacks a substantial atmosphere, unlike Pluto. Maybe it will gain one as it gets closer to the sun. It also has no satellites again unlike Pluto. It is about 5% longer than it is wide.

Those aren’t planets! This is a planet.

Languages evolve, and languages have evolved a lot since ancient Greece. As have our knowledge about astronomy. There is absolutely no reason to think that words today should mean exactly the same as they did back then.

Pluto is better off as king of the Kepler Belt then the lonely outsider of the planets.

I don’t see it as a demotion anymore but a far better place for Pluto to be.

Yeah the article isn’t great. That said I’m not sure you can simply drop Earth out at 30 AU and claim “Nope. Place is a mess - not cleared”. To be fair, you’d have to wait and see what kind of clearing takes place over a few million years.

Consider the following Pluto vs. Earth at 30 AU
Fg[sub]pluto[/sub] vs. Fg[sub]sun[/sub] on an object 0.1 AU further out → 4.5x10[sup]-6[/sup]

Fg[sub]earth[/sub] vs. Fg[sub]sun[/sub] on an object 0.1 AU further out → 2.7x10[sup]-1[/sup]

Or more directly Fg[sub]earth[/sub] vs. Fg[sub]pluto[/sub] on an object 0.1 AU further out → 60000

I’d hazard a guess that the inner solar system would become a whole lot more interesting while we did this experiment.

You never know when Disney might mount an astroturf campaign to return Pluto to among the “real” planets.

Remember the reason Clyde Tombaugh was looking for Pluto in the first place - it was the perturbations in Neptune’s orbit that was puzzling everyone, just as Neptune was found by examining Uranus’ orbit. If something is significantly affecting another planet’s orbit, it seems that it is itself a planet. Or something like that…

Although Pluto was gound during a search for a hypothetcal ninth planet to explain then inexplicable devistions in the orbits of Uranus and Neptune, astronimers were looking for a much more massive planet out well past the orbit of Pluto. As it turns out, the deviations were due largely to measurement errors in radial velocity and Pluto really has no discernable effect on the orbits of the other planets. Finding Pluto and misidentifying it as a major planet was essentially an accident.