Planet X? There's not even a Planet IX.

I guess dtilque figured out what was up here. The asteroid named Hera is 103rd on the list. The other three, plus 10 Hygeia, are the four that are bigger than 400 km in diameter–all the rest are less than 350km.

How about this:

  1. Your brown dwarf criterion
  2. Primary gravitational influence is a star or brown dwarf
  3. Approx. spherical
  4. Center of gravity of two-body system is not inside another planet (otherwise a moon of a Jupiter size planet near the sun would qualify)
  5. Larger than 200km radius

I admit that 5 is somewhat arbitrary, but then so is 3, in that we have to agree on a limit for non-sphericity. (Those links in my previous post are to pictures that extablish non-sphericity for Ceres and Vesta.) Anything smaller than 200km radius would fit in Wyoming, right?


rocks

Yeah, that’s what happened: I thought that it ought to be Latin, to be consistent with the other Solar System bodies, but I knew that there was a Hera, and I didn’t think that they’d have two bodies named after (essentially) the same goddess. I guess, though, that if you’re going to try to name ALL of the asteroids after goddesses, you’re gonna run out of names. For that matter, too, Pallas isn’t Latin, either… She would be Minerva.

RM, I don’t have a problem with your criteria, except that 3 and 5 are redundant. Anything solid that’s approximately spherical is going to have to be larger than 200 km. Admittedly, an icy body might liquefy long enough to get into shape, and then re-freeze, but it’s not going to stay spherical for long-- ice is rather easily deformed by collisions etc. Seeing as those two are our most arbitrary criteria, we ought to eliminate one of them.

Minerva is the name of 93. But they’ve long since run out of goddesses’ names for asteroids. Here is a list of all named asteroids.

There’s thousands of named asteroids in that list (warning to those with slow feeds). The highest number in the list is in the 14000’s, although most of those above 10000 haven’t been named yet (and a fair number below 10000 haven’t been named either). One of the cleverest names is 11111 Repunit.

I hate to suggest even a slight hijack, but I asked this on GQ this March and got no real answer. I hear some good answers here, so I’ll re-ask. At what size/definition does a chunk of rock become a “moon”. I mean, by some definitions, Saturn would have at least hundreds. And they keep finding smaller ones. If the guys in the Space shuttle tossed out a 100# boulder, would Earth have 2 moons?

The reason you got no answer is that there isn’t one. Up until about 20 years ago, the objects we could see in space were either fairly large or collections of dust/gas. Intermediate sized objects were too small to detect, since they didn’t hang around in bunches, although there were theories (now found to be wrong) that the rings of Saturn were made of boulders. So the issue didn’t come up and there was no pressing need to set an arbitrary division.

Since then, the size of asteroids and moons discovered has been steadily getting smaller, often to only a couple hundred meters or so. At the extreme, there’ve been a few bodies found that measure 10 meters or less across. These were mostly found by radar and then only when they make a close approach to Earth. So the gap is narrowing, and some day they are going to have to make an arbitrary cut-off point between notable (nameable) bodies and space junk.

But as far as objects tossed out of spacecraft, they would be considered artificial, even if it were just a hunk of rock (not that we’re likely to waste money on lofting such an object).

Chronos

I don’t follow how Luna is not excluded by this condition. Isn’t the Earth’s gravity stronger on the moon than the Sun’s gravity? If not, then why doesn’t the moon break free of the Earth and have it’s own independant orbit around the Sun?

The earth’s gravity is not stronger than the sun’s, on the moon. The forces are balanced though–and some people have claimed that the moon does sorta have an independent orbit. For instance, the path of the moon around the sun has no places where it is outwardly concave, so the earth doesn’t affect it that much.

Another point I’ve seen arguing that Luna is a planet, not a satellite, (due, I believe, to Asimov), is that Luna’s orbit relative to the Earth is essentially in the plane of the ecliptic, not in the plane of Earth’s equator, as is the case for all major satellites in the System, and most of the minor ones (that is to say, they’re in the plane of their primary’s equator, not Earth’s).

Is there any mathematical data to back up that the sun exerts more force on the moon than Terra (gotta use those proper names) does? The sun’s certainly much, much larger, but we’re much, much closer, and I would find it rather a stretch to believe that the gravitational exertions of both bodies on the moon precisely cancel each other out.

This bothers me too. I know that Luna’s tidal force on Terra is stronger than Sol’s. If its gravitational were as well (how do we define that?) Earth would not be a planet by Chronos’s & Mentock’s definition.

Of course, Rescind, I should have posted the calculations way back when I meantioned it. The relevant numbers are:
M[sub]s[/sub] = 210[sup]30[/sup] kg : Mass of Sun
M[sub]e[/sub] = 6
10[sup]24[/sup] kg : Mass of Earth
R[sub]e[/sub] = 1.510[sup]11[/sup] m : Distance from Sun to Earth and Moon
R[sub]m[/sub] = 3.8
10[sup]8[/sup] m : Distance from Earth to Moon

The formula for gravitational force is:

F = GMm
        r[sup]2[/sup]

so F[sub]s[/sub]/F[sub]e[/sub] = (M[sub]s[/sub]*R[sub]m[/sub][sup]2[/sup])/(M[sub]e[/sub]*R[sub]e[/sub][sup]2[/sup])

which works out to about 2.1, so the force of the Sun on the Moon is over twice that of the Earth on the Moon.

That’s true, but the sun’s gravitational effect on the earth is two hundred times greater than the gravitational effect of the moon.

The reason that Chronos’s calculation comes up with 2.1 is because the mass of the earth is about a hundred times larger than the mass of the moon.

Simulpost; I didn’t notice this before:

That’s because tidal forces go as the inverse cube of the distance, wheras gravitational force is only the inverse square: This means that the tidal force is more sensitive to distance. Since Luna is closer than Sol, its tides can dominate, but it’s not close enough for the gravity itself to dominate.

Chronos, how do you define “tidal force”? I would think it would be pretty much the same as gravitational force.

On another note: we all know that the moon does not really revolve around the earth but rather they both revolve around the common center of gravity. Now, suppose the moon gets larger in relation to the earth, (or the earth smaller) at waht point or ratio would they both be planets?

The tidal force is the effect that causes the tides. The gravitational force of the sun on the earth is 200 times the gravitational force of the moon on the earth, but the tides due to the moon are twice those of the sun.

Which definition of planet do you want to use?

I don’t get it. OK, my calculations show the Sun’s attraction to be about 178 times that of the moon. So why are the tides caused by the moon larger?

The tidal effect is a result of how much the gravitational force changes from one side of the earth to the other. Since the gravitational force varies as the inverse square, the tide-raising force varies as the derivative of that, or as the inverse cube. Since the sun is around 400 times farther away, you have to divide 200 by 400–in other words, the sun tide is about half the moon tide.

Ah, OK, now i get it. It’s funny how i was thinking about it for a while and I didn’t get it and suddenly, like the light went on, I got it and now I cannot go back to “not getting it”. No it is clear. It is like when you cant make out a drawing and suddenly you make it out and after that it’s automatic. it was a funny experience.