What a planet is, revisited. Also Pluto.

Plus Uranus, given a person with decent eyes and a dark sky.

Also, as has been pointed out, Uranus. Also 4 Vesta (even though it’s smaller than Ceres and not spherical), and occasionally some other asteroids. And Earth wouldn’t be a planet, but the Sun and Moon would be (according to the Ptolomeic system of planets).

Like continents, as far as the Solar system goes, “planet” is mainly a term that provides a convenient list of names to be memorized. For that reason, there can’t be too many. A dozen is probably the maximum for most people. So when new numerous classes of objects are discovered, like asteroids or Kuiper Belt objects, we redefine planet to keep the numbers down to a memorizable level.

Cool. Thanks. I did not know Uranus was naked eye visible even under the best conditions.
Agree that the real overarching point of the hullaballoo about “what is a planet” should be resolved by relaxing and admitting that, like “continent”, it’s a mostly arbitrary term appropriate for casual schoolkid use. Applying too much rigor just produces silliness.

I don’t see any particular use for terms like “dwarf planet,” “minor planet,” etc. As far as I’m concerned, eight planets is fine. We can use terms like asteroids, comets, or Kuiper belt objects to describe anything else. (However, something like “plutoid” would be better for the latter, which is unwieldy.) Maybe we need a general term for objects orbiting the Sun directly.

First strike, Pluto, as has been discussed, orbits on a different plane than the other celestial bodies that we commonly call planets. Second strike, it orbits elliptically to such a degree that it’s orbit crosses the orbit of Neptune. Planets, in my mind, don’t behave that way. Comets do though. In many ways Pluto has as much in common with a comet as it does with a planet.

Thus, not a planet.


I’m not aware that it has ever been adopted by any astronomy or planetology body, but the term “orbium copernicoid” (or just “copernicoid”) has been discussed to describe naturally occuring bodies orbiting the Sun in a not-too eccentric orbit with enough mass to draw them into a sphereoid shape. This would still encompass a larger number of bodies than what we would be comfortable as a colloqual usage of planet.

As we increase our observation of extrasolar systems we’re going to have to revise our definition of planet or come up with new categories of planet-like objects, especially when we find superjovian systems with planet-sized moons more amenible to lifelike conditions than most proper planets, or planets in periodic but stable shifting orbits due to being in multiple close orbiting star systems. The term “planet” will always be an arbitrary and shifting concept. Grey’s suggestion of the more generic “world” to indicate a body large enough for internal differentiation and geologic processes powered by internal thermal dynamics (as opposed to purely tidal forces) during some point in its evolution may be a better universal label.


Neil deGrasse Tyson reading “Mean Tweets” about Pluto


We do not *know *if Pluto actually has planetary differentiation, altho it now appears more likely than not.

Pluto even has a tail!

"The New Horizons Atmospheres team observed Pluto’s atmosphere as far as 1,000 miles (1,600 kilometers) above the surface, demonstrating that Pluto’s nitrogen-rich atmosphere is quite extended. This is the first observation of Pluto’s atmosphere at altitudes higher than 170 miles above the surface (270 kilometers).

The New Horizons Particles and Plasma team has discovered a region of cold, dense ionized gas tens of thousands of miles beyond Pluto – the planet’s atmosphere being stripped away by the solar wind and lost to space."

I wouldn’t count on it. When I was in elementary school in the 80’s, I was the bane of every teacher who had to teach astronomy/astrology subjects. As early as second grade, I had memorized a vast amount of astronomical trivia and delighted in correcting any facts that weren’t quite up to date. Many of our textbooks were using information that was twenty years old even when they had publication or revision dates recent enough to have caught some new discoveries.

[quote=“leftfield6, post:27, topic:725299”]

Neil deGrasse Tyson reading “Mean Tweets” about Pluto


And a hilarious bit with Colbert and Tyson regarding its planetude…

Neil deGrasse Tyson and Pluto do not like each other:

(Gestures NSFW)

I tend to agree with this. Why can’t any definition of a planet simply grandfather in Pluto to include it as a planet?

How many other things in astronomy interest the public than this debate?

Since the definition of planet is arbitrary, there’s no reason we can’t grandfather things in. We could call the moon a planet if we wanted.

However, if the word “planet” is going to have a useful meaning, then we either have to exclude Pluto from the meaning, or we have to start adding dozens of KBOs as well. Pluto is so different from so many planets that it’s always going to be an oddball.

For example, Pluto has already caused a great deal of planetary confusion just by the fact that it is sometimes closer to the sun than Neptune. Removing it from the list of planets seems much cleaner to me than trying to remember which thirty-year period has Pluto as the 8th planet instead of the 9th.

It is actually a twenty year period, and since we won’t have to worry about it again until the mid-twenty-third century the point is kind of moot. But both the orbital dynamics and evolution of Pluto is clearly much different from the major planets, all of which have orbits with very minor eccentricity and fall very close to the solar ecliptic and the invariable plane of the solar system, and also dominate their respective spheres of influence over all other local bodies.

Pluto, on the other hand, has clearly been thrown out of plane by exchange with some larger body (probably Neptune, or possibly Uranus) and forms a doublet with Charon (BTW pronounced “kAr-on” not “shAy-ron” or “chair-On” like the newsheads keep repeating). It may more may not have internal differentiation or active seismic processes but is not like any other body we would classify as planet. Making an exception for Pluto “just because” would be like classifying orcas and whales as fish just because the swim in the ocean.


These sorts of definitions rule out extra-solar planets, which inspire great interest and study. So according to that sort of scientific and journalistic usage, the shape of a planet’s orbital plane isn’t especially relevant. I’m detecting some Sol-centric bias which disturbs me. Ok not really.
Is planetary differentiation difficult to measure directly or indirectly? Is spheroid shape important to understanding celestial objects? Because if it isn’t, we might not want to make it a criteria for planet-hood. Following LSLGuy, we might want to make the criteria either a) easy to measure or b) important to measure. I’m comfortable with saying things like, “We don’t know whether Kuiper Belt object C is a planet, though it might be.”

Hydrostatic equilibrium: that might be better than planetary differentiation. Or not. Currently it distinguishes dwarf planets from smaller solar system objects. Wiki says that, “Hydrostatic equilibrium is the current distinguishing criterion between dwarf planets and small Solar System bodies, and has other roles in astrophysics and planetary geology. This qualification typically means that the object is symmetrically rounded into a spheroid or ellipsoid shape, where any irregular surface features are due to a relatively thin solid crust. There are 31 observationally confirmed such objects (apart from the Sun), sometimes called planemos,[2] in the Solar System, seven more[3] that are virtually certain, and a hundred or so more that are likely.[3]”

So I guess by my reckoning the planets comprise something like Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Eris, Pluto, Makemake, 2007OR10, Haumea, Quaoar, Sedna, Orcus, Ceres, 2002MS4, Salacia, 2003AZ84, 2013FY27, Varuna, 2002UX25, 2004GV9, 2005RN43, Varda, 2005UQ513, Ixion, 2002AW197, 2007UK126, Chaos, 2007JJ43, 2002TC302, 2012VP113, 2010KZ39, 2002XW93, 2004XA192, 2010RF43, 2013FZ27, 2002XV93, 2008ST291, 2003UZ413, 2004XR190, 2006QH181, 2010RE64, 2010FX86, 2014UM33, 2003VS2, 2005RM43, 2004NT33, 2001UR163, 2004TY364, 2008OG19, 2014FC69, 2010VK201, 2003QX113, 2007JH43, 2014FT71, 2004PF115, 2000YW134, 2004PG115, 2002WC19, 2007XV50, 2010EK139, 1999DE9, 1998SN165, 2005TB190, 2002KX14, 2003FY128, 2010TJ, 2010VZ98, 2010RF64, 2011FW62, Huya, 2002VR128, 2001YH140, 1999CD158, 2010EL139, 2003QX111, 1996GQ21, 2008QY40, 2010ET65, 2008AP129, 2005CA79, 2008NW4, 2008UA332, 2010HE79, 2011GM27, 2013FC28, 2013JW63, 2014FY71, 2001QF298, 2005QU182, 2004UX10, and 1999TC36. Though the actual list is probably longer than that.

Wiki tells me that a related scientific term is PMO or planemo: [INDENT][INDENT]A planetary-mass object (PMO), planemo…, or planetary body is a celestial object with a mass that falls within the range of the definition of a planet: massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain core fusion like a star. By definition, all planets are planetary-mass objects, but the purpose of this term is to refer to objects that do not conform to typical expectations for a planet. These include dwarf planets, the larger moons, and free-floating planemos, which may have been ejected from a system (rogue planets) or formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs). [/INDENT][/INDENT]

Speaking just for myself, I deliberately intended Sol-centricness. As a scientific term, “planet” is irretrievably broken. So treat it like what it is: a holdover from ancient Greek culture which referred to some specific thing they knew about. Sort of like the “7 wonders of the ancient world”. It means now what it meant then. The fact we have since built far more wondrous things in irrelevant.

We do need a word for things which orbit extra-solar stars. Whether it makes any sense to try to export to other systems a taxonomy we’ve developed over centuries based on our slowly growing insight into our sample of one is a valid question for the experts.

IME for extra-solar stuff (or even KBO stuff) we really need a definitions based on operational considerations, not ontological ones. We can only categorize based on the phenomena we can detect at a distance. Which, much like the situation in biology today with Linnaean nomenclature, leads pretty quickly into developing an elaborate system of classification that’s rendered largely bogus as our knowledge grows.

Maybe the smart money for real science is simply to have a nice term for “orbiting object”, e.g. copernicoid as **Stranger **mentioned above, and quit right there. Resist the temptation to get more organized this early in our understanding of the galaxy.

And most significantly, growing every day. The real reason to draw the line where the IAU did was to stop the never-ending growth.

Well I guess I have to like the idea; the the term however is absolute garbage.

Yes, world has a better ring to it than planemo.

Planets also have gravity. Not just gravity in astronomical scales, but gravity on a human scale. That means if I drop my cup of tea it falls to the ground in oh, say 1 minute. Not an hour. Not a year. Also, Arnold Schwarzenegger on planet Z weighs more than my cup of tea does on Earth.

Ok, here’s where I hit my limited understanding of physics. On Earth, acceleration is a really quite fast 9.8m/s^2, which I round to 10. Divide by 1000 and you get 1 cm/s^2, which is a lot lower but still readily perceivable. So our first cutoff is .001g.

What about the Arnie test? Schwarzenegger is 6 foot, 2 inches and weighs about 250 lbs, or 1/4 of a lb on Planet Z according to the first cutoff. My tea mug with beverage included weighs more than a pound. Fail. So let’s shift the cutoff by an order of magnitude: .01g.

That’s the sort of thing we can measure, right? What’s the list of planets by that criteria? Still fairly long. Ok, now shift again to .05g. By that definition planets include Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto (yay) and Eris. Ceres does not make the cut. I don’t think Makemake or Haumea would either. Huh. Move the line to .1g and Pluto/Eris drop out.

The average Brazilian weighs about 150 pounds or 68kg. At the .05 cutoff they would weigh 7.5lbs or 3.4kg. That’s light, but still entails a very perceivable amount of gravity. By these criteria we could rule Pluto and Eris in at .05g and out the door at .10g. Seems manageable, if Terra-centric.

If we accept a .05g cutoff and find a dozen more Pluto-sized spheroids, I can deal with that given the complexity of Pluto’s geology as revealed by the New Horizon’s probe. That scenario doesn’t sound likely, but IANAAstronomer. .01g is also defensible, depending upon how interesting we think 2003AZ84 et al might be. Though having our average Brazilian weigh in at 1.5lbs or .7 kg seems a little small to qualify as a planet.

That’s a nice analogy.