Only Mercury and Venus have no moons. How come?

Not according to the IAU.

And t-bonham - I can’t answer the way I would like to since this isn’t the pit, so lets just say I did read the whole thread, and it was an honest mistake; I didn’t realize that “dwarf planet” was the current term for what Pluto is - I would point out that the IAU is still “searching for a name” for these objects.

Another reason why the IAU definition is dumb. Dwarf stars are low-mass stars. Dwarf galaxies are small galaxies. Dwarf planets… aren’t planets. :confused:

Well said :cool:

As long as the IAU was in a mood to screw around with definitions, they should have defined “planetoid” to mean any non-stellar body that has achieved hydrostatic equilibrium, regardless of what it does or does not orbit. This would allow us to draw distinctions between planetoid moons (like Earth’s moon, or Jupiter’s 4 major moons), and asteroid moons (like Mars’ moons, or Jupiters 18 gazillion minor moons), as well as allow us to conveniently talk about any “planet-like” object.

I don’t think the definition is dumb - I think your attempt to use grammar to derive what the definition “should” mean is at fault.

In computing, cf ATX/miniATX form factor motherboards.

Cymro

The definitional problem becomes even more complicated, once you step out of our own current Solar System. Presently, we’re still unable to detect anything we might call “moons” lightyears away, but some day, we will. Suppose, for instance, we found a system with a brown dwarf orbiting a main-sequence star, and smaller (but still world-sized) objects orbiting the brown dwarf. Are those objects planets or moons? Or what if you have an object in an unstable orbit, which sometimes primarily orbits a star, and sometimes a planet (whatever that is)? Does its status change as its position does, or is it, by virtue of being unstable, in none of the standard categories?

Does (or could) a moon to have it’s own “moons”? IOW, could one of Jupier’s larger moons have natural satellites of its own?

[/hijack]

See [post=8152339]post #14[/post] and subsequent posts above. Yes, it’s possible for a small moon to have its own satellites (depending on how you define a “satellite” it may be almost guaranteed–objects floating in the “Trojan” L4 and L5 libration points are in technically “in orbit” even though they’re not simultaneously in orbit of both bodies) but such orbits are unstable owing to the strong gravitational influence of the primary and resultant (likely chaotic) 3-body dynamics.

Stranger

[thread=404162]Here[/thread]'s another recent thread on the topic of moons and orbital objects.

Stranger