### Is Jupiter a dwarf planet?

• I know the question seems silly, but as far as I've understood, is this the definition of a dwarf planet:

• circling around a star, and not being a star itself

• having a round shape (otherwise every comet or asteroid could be a (dwarf) planet)

• not being able to clear its own orbit (otherwise it's a planet)

If we have a look at Jupiter, then we can easily confirm the two first points, but the third one is an issue: in Jupiter's orbit there are two sets of asteroids, called the Trojans, which are located, together with Jupiter, in some kind of regular triangle, turning around the sun. Following the laws of Kepler, Jupiter and the Trojans have the same speed, so Jupiter can't catch them and although Jupiter's gravity is very strong, it does not stretch to the other side of the sun, which means, following the definition, that Jupiter is a dwarf planet.

I know, Pluto is called a dwarf planet because of the many objects in its orbit, and Pluto's gravity is so small that it can't get rid of those objects. Nevertheless in a scientific you can't use vague wordt like "vicinity", "close", ..., so you can't say Pluto is a dwarf while Jupiter isn't because of the vicinity of the mentioned objects (being "too close").

Do I have a point here or am I missing one? :-)

Also see the graphs in this great answer which really helps visualize everything.

The Trojan regions are gravitational saddle points and the saddle points are a byproduct of a planet's strong gravity so, in no way should they be considered as not clearing out an orbit. Jupiter's vast number of Trojans is an example of it's gravitational dominance, not it's failure to clear out it's orbit. Planets can also affect other planet's Trojans, for example, Jupiter's significant mass is a primary reason Saturn has comparatively so few Trojans. If planet 9 exists, it could have lots of Trojans too, being so far away from other massive objects.

This is a great question and the answer is that you have to interpret "clear its own orbit" in the proper way.

I have been interested in this very same subject. I am currently writing a science research paper on the planet Pluto, and the definition of a dwarf planet came up. The book "Astronomy for Dummies" by Stephen P. Maran, PhD, mentioned this and how astronomers refuse to address this issue. I don't necessarily believe that Jupiter should be a dwarf planet, nor do I strongly feel that Pluto should regain its planet status. I simply believe that the IAU needs to come up with less vague, clearer definitions to the terms Planet and Dwarf Planet.

I take it you're asking this to point out the silliness of the IAU definition?

@MarkOlson: I wouldn't call it silliness, but indeed I don't like words like 'vicinity', 'neighbourhood', 'close' in a definition about astronomy. Although it does show how clear distinctions are sometimes so difficult to make.

@Dominique: When you're drawing lines to divide a continuum, you usually wind up with either vagueness or verbosity. Devising a classification scheme for planets makes sense, but not if you basically ignore the physical characteristics of the planets themselves. Basing it on dynamics alone is, well, silly. Hence your point that Jupiter is a dwarf planet.

• I suggest taking a look to https://en.wikipedia.org/wiki/Clearing_the_neighbourhood .

Quoting it,

In the end stages of planet formation, a planet (as so defined) will have "cleared the neighbourhood" of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its satellites or those otherwise under its gravitational influence.

The Trojans are textbook example of objects under the influence of a planet (Jupiter).

For another example, Pluto is said to not have cleared its neighbourhood because Neptune is in it. Neptune has cleared its neighbourhood although Pluto is in it because Pluto is a lot smaller and Pluto orbit is governed by resonance with Neptune.

Up to the fact that what constitutes "clearing the neighborhood" is not clearly defined, Pluto's ability to clear it's neighborhood ignores Neptune, and vice versa. Their orbital resonance is generally considered too large for them to count against each other: they basically can't hope to ever clear each other out because they basically don't interact.

This is a good way to look at the third rule. +1

• Do I have a point here or am I missing one?

You are missing several points.

Point 1: "Clearing the neighborhood" does not mean getting rid of every last spec of mass in the vicinity of a planet. The concept instead addresses the ability to somehow "clear" the vast majority of the mass in the neighborhood of a planet. The total mass of all of Jupiter's Trojans are less than a millionth of Jupiter's mass. By whatever mathematical expression one uses (there are three), that qualifies as clearing the neighborhood.

Point 2: "Clearing the neighborhood" is as much about the future as it is the present or past. Pluto is not a planet because not only has it not cleared its neighborhood, it will not do so while the Sun is still shining. Jupiter is a planet because by whatever objective measure one uses, it cleared its neighborhood shortly after the solar system formed.

Point 3: There are three competing objective definitions of what qualifies as "clearing the neighborhood" as opposed to the unrealistic concept of having eliminated ever last spec of mass: Alan Stern's and Harold Levison's $\Gamma$, Steven Soter's $\mu$, and Jean-Luc Margot's $\Pi$. For an overview, read the wikipedia article on clearing the neighborhood, and follow the references therein.

Point 4: There are several ways a planet can clear its neighborhood. The most obvious are collisions and ejections from the star system. Less obvious are capturing them as moons or stable pseudo moons. Stable Trojans are an example of the latter.

I don't know if I'd call the Trojans "stable pseudo moons". They don't orbit the planet, they orbit the Sun. They are in a stable Lagrange point though meaning they don't have to be cleared for the orbit to count as cleared since their Lagrange orbit counts as Jupiter dominating them.

@zephyr -- Pseudo moons aren't moons in the conventional sense. They are objects that orbit the primary of some secondary but are somehow caught in a 1:1 orbital resonance. For example, trojans, but also horseshoe orbits and tadpole orbits fall into this category. A more technically correct but more boring name for such objects is "co-orbital objects."