Off on a Tangent defines a planet as a “natural object orbiting the sun (Sol) that has a mean diameter of at least 2,000 kilometers,” and a large moon as a “natural planetary satellite that has a mean diameter of at least 1,000 kilometers.”
By these definitions, the solar system has ten known planets and sixteen known large moons:
- Mercury
- Venus
- Earth
- Luna (the Moon)
- Mars
- Jupiter
- Io
- Europa
- Ganymede
- Callisto
- Saturn
- Tethys
- Dione
- Rhea
- Titan
- Iapetus
- Uranus
- Ariel
- Umbriel
- Titania
- Oberon
- Neptune
- Triton
- Pluto
- Charon
- Eris
Compared to the IAU Definition
The International Astronomical Union (IAU), a semi-official organization that categorizes and names celestial objects, defines a planet as a “celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.”
When the IAU adopted this definition in 2006, it purported to “demote” Pluto, which had been considered a planet for seventy-six years, for allegedly not meeting condition (c). Critics argue that the IAU has not defined “clearing the neighborhood” and has not applied the term consistently. As planetary scientist Alan Stern wryly pointed out, “If Neptune had cleared its [neighborhood], Pluto wouldn’t be there.”
One year earlier, an IAU committee submitted three possible definitions of “planet” for consideration. According to chair Iwan Williams, the first of those definitions—that “anything with a radius greater than 1,000km would be a planet . . . ,”—had the most support (PDF link).
That definition is the basis of the Off on a Tangent definition, but with the size expressed as a mean diameter instead of a radius. It is superior to the IAU’s definition because:
- It is simpler and easier to understand (if equally arbitrary).
- It avoids the unnecessary and unpopular “demotion” of Pluto.
- It maintains the traditional order and numbering of the first nine planets.
- Future discoveries are unlikely to change the list, except, possibly, to add to its end.
Binary Systems
Some astronomers have suggested that Pluto and Charon are a binary system (regardless of whether Pluto is a planet).
Normally, a planet is so much more massive than its moons that the barycenter (center of mass) between them is inside the planet. Because Pluto and Charon are much closer in mass than others, their barycenter is in space between them. On this basis, some have argued that neither truly orbits the other.
For clarity on this point, Off on a Tangent defines binary planets as “a pair of natural objects orbiting the sun (Sol) that are in a mutual gravitational relationship when (1) the sum of their mean diameters is at least 2,000 kilometers, (2) their barycenter is in open space between them, and (3) the less massive object’s mass is at least two-thirds of the larger object’s mass.”
By this definition, Pluto and Charon are not binary planets because Charon’s mass is less than two-thirds of Pluto’s.
Adopted August 6, 2025