Tuesday, August 22, 2006

Can o' Worms

As I'm sure you all know, the sixteen most massive objects in the solar system are:


"Xena" probably comes next, but its mass hasn't been pinned down yet. And then Pluto. And then "Santa" and "Easterbunny," most likely, followed by Titania, Oberon, and Sedna.

Note that I haven't said Charon or Ceres yet, which are the two objects the IAU listed right beside "Xena" in their definitely-planets-if-we-make-this-rule declaration. In fact, Charon is about half the mass of Sedna, and Ceres is less than a third the mass of Sedna. So why aren't Sedna, "Santa," and "Easterbunny" on the definitely-planets-if-we-make-this-rule list?

But then we run into another problem with the IAU's planet definition. They also state that the object should be almost spherical. This leaves off all but a couple asteriods (Ceres and Vesta). All asteroids ranked third and below are irregularly shaped.

But remember "Santa"? It's next in line behind Pluto and almost three times as massive as Charon (over four times as big as Ceres). But it's irregularly shaped. It's almost 2000 km long by less than 1000 wide. So is it a planet? Would the IAU really leave it off the list just because it's oblong?

. . . What would it be like to stand on such a planetoid as "Santa"? It would be as if the entire world were two huge mountains, with only a few level locations on the entire surface. At the top of the mountains (the ends of the long axis), the gravity would be 0.29 m/s2. At the bottom of the valley (the ends of the shortest axis), the gravity would be 1.1 m/s2. (Earth surface gravity is 9.8 m/s2.) You'd weigh over three times as much at the bottom than at the top! In fact, gravity at the low point would be almost twice as much as on Pluto, a more massive planet. Weird, huh?

Hm. "Santa" is oblong because it rotates so fast, they say. Its rotational period is 3.915 hours. Would you even be able to stand on top one of the mountains if it is spinning so fast? What's the escape velocity? 0.84 km/s. And at the top of the mountain, you'd be going 0.43 km/s standing still. So. . . you'd have to travel an extra 1452 km/h. Oh good.


Pedicularis said...

Following your logic, there can only be 8 major planets (Jupiter Saturn Neptune Uranus Earth Venus Mars Mercury). I have added the condition that a "major" planet must revolve around the sun and must be larger than any moons in the solar system. Perhaps being more precise, the sun must lie at the focal point of its elliptical orbit nearest perihelion.

The "minor" planets could include those smaller than the largest moon, but larger than the largest not-nearly-spherical object. Thus the minor planets are Xena and Pluto.

I suggest that Santa and all the smaller rocks (spherical or not) that have the sun at a focus of their orbit are asteroids or comets, no matter how elliptical the orbit or how far away the aphelion.

And of course, any rock that has a planet at its focal point in a moon.

Does that leave out the Trojan asteroids? No, I included them under asteroids/comets.

BTW, did you do the math yourself? Very nice. I did not know you were up to speed on gravitational physics.

Sotosoroto said...

What happens, though, at our next solar system? What if one of the gas giants has a moon larger than Earth? Would you then categorize all the terrestrial planets as asteriods?

Yes, I did the math myself. A couple formulas and a calculator do the trick every time.

Pedicularis said...

I see the boys at the International Astronomical Union are leaning towards a definition for planets that would reduce the number to 8 as well: Any object larger than 500 miles in diameter that orbits the sun, has a mass [that is at least?] roughly one-12,000th that of Earth, has enough self-gravity to pull itself into a round shape, and is the dominant object in its area. And they are using the terms "classical" and "dwarf" where I used "major" and "minor", but the results are very similar.

Their diameter and mass requirement solves your question about other solar systems.