Sun With A Hundred Worlds

In Solar System by Brian Koberlein7 Comments

Ah, Pluto, for such a small and distant world you are able to stir controversy. 

In 2006 the International Astronomical Union (IAU) formally defined a planet as an object which 1) orbits the Sun, 2) is massive enough to be in hydrostatic equilibrium (basically that means it’s round), and 3) it has cleared the neighborhood around its orbit. Since Pluto didn’t satisfy the third criteria, it was tossed out of the official list of planets, much to the outcry of the general public. Pluto is still considered a dwarf planet, along with Ceres, Eris, Haumea, and Makemake, but that was small consolation. Then when New Horizons flew past Pluto in 2015, it found that Pluto was a rich geological world, with mountains and thin blue skies. The images captured by the probe showed that Pluto was very planet-like, and there were new calls to redefine Pluto as a planet.

Even some astronomers have issues with the current definition of a planet. To begin with, a planet must orbit the Sun, so the thousands of exoplanets orbiting other stars are not planets. That’s fine if you want to keep planets and exoplanets separate, but most people would figure that an Earth-like body orbiting any star would be a planet. Then there is the third criteria, the whole “cleared it’s neighborhood” business. If it weren’t for that, Pluto would still be a planet. The problem with this criteria is that the more distant a planet is, the more difficult it is to clear an orbit. Earth is a planet under the current definition, but if Earth were beyond Pluto in the Kuiper belt, it wouldn’t be a planet. That seems rather arbitrary.

So Alan Stern (principle investigator for the New Horizons mission) and others have proposed a new definition: A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters. In other words, if a body is basically round but too small to be a star, then it’s a planet.

Saturn’s moon Enceladus is under hydrostatic equilibrium. Credit: NASA/JPL-Caltech

This would broaden the definition of planet significantly. If this definition were adopted by the IAU Pluto would officially be a planet once once again. So would all the exoplanets we’ve discovered so far, and so would rogue planets that wander cold deep space all alone. Anything with a diameter larger than about 500 km, all the way up to bodies 15 times more massive than Jupiter would be considered planets.

But such a definition might too broad. Not only would Pluto be a planet, but so would its largest moon Charon. So would our Moon, making Earth a double planet. So would the largest moons of Jupiter and Saturn. The definition would shift our solar system from 8 planets to more than a hundred. Sure, Pluto would be a planet, but who cares at that point?

Personally, I think the proposed definition is too general. You could argue that Pluto should be a planet (I disagree), but Saturn’s small moon Enceladus shouldn’t be a planet. Even the common definition of a moon is that it orbits a larger body. In science fiction we have no problem with moons being large and Earth-like, even habitable. They are still moons. I think there is also something to be said for some kind of orbit-clearing condition. It is quite likely that there are thousands of objects larger than Enceladus in the outer edge of our solar system, and they aren’t the same as closer objects like Mars or Mercury.

All of this is worth discussing.  As we learn more about both our own solar system and others our definition of what a planet actually is will have to adapt. Whether we end up with hundreds of planets or only eight will depend on what we want the word “planet” to mean.


  1. How about gravity? Mars and Mercury have the lowest gravity of the planets yet still twice as high as any moon or dwarf planet.

  2. To the definition proposed by Alan Stern, one could add something like “… and it orbits around a (or its) star…”, thus avoiding the problem that if not so, our Moon and the other moons would be planets too.

    1. That would exclude the”rogue planets that wander cold deep space all alone”…

      1. Oh, you’re right, I didn’t think about them… Well, you could change the statement by something like “and it orbits around a star or doesn’t orbit nor describe any geometrical pattern around any star”.

  3. Gravitation of mercury in thermal conditions has been proven in open HS class instruction. Mars does go beyond Earth visuals. A left of out field is still in the ballpark of exchange. Variations of approach do retain a comparative.

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  4. The moon is the moon,stars are stars and pluto is a Dwarf planet. We years ago well almost two +talked Professor Brian Koberlein about Jenssen but it’s years or even decades away, now or at Princeton, the Researchers, and NASA/JPL and probably maryland university where your at got a glimpse of seven new exoplanets in the habital zone.arond a Dwarf Staras we map space we find the means to someday or millennium find the sweet zone and maybe start humanity once are Technology advances to a new level where Space Exploration will be like ridding a bike but at a much faster vilosity you know good job on PLUTO everyone is so sensitive but it’s always been the ninth planet from our sun so thank you My Good Friend and thank you for your Interpretations of quantum mechanics all the way back to Niels Bohr/ Einstein/Newton/Keplers third law says that if you take the Cube of the semi-major axis(a measure of the size of a ellipse)and divide it by square of its orbital period how long it takes to complete one orbit, then you always get the semi-major axis is measured in AU and then period in years,then that constant is mass of the central object.or you tought me let’s say a moon passes a planet and you see its shadow, by how fast that shadow is shown then disappears is the way we can find out the gravity of that planet . ` .your the best keep up the Brilliantly illustrated written and taught to the world oh ya you are the strongest link to Evolve.

  5. I think the issue here is that the word planet, along with other astronomical identifiers, has both a technical meaning and a public colloquial understanding. Being a chemist (i.e. not an astronomer), it seems to me that any definition that made the moon a planet would be absurd and would be unacceptable to the public. Look at the kerfuffle over the reclassification of Pluto and the creation of the crazy class “dwarf planet” (which contrary to any semantic logic is not a type of planet).

    I think that the notion of “planet” is quite (but not completely) clear now and this proposal will benefit no one.

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