2003 Ub 313: The Incredible Shrinking Planet?, No bigger than Pluto? Options

[Guest_BruceMoomaw_*]

(Thanks to Emily Lakdawalla):

http://sciencenow.sciencemag.org/cgi/content/full/2006/127/1

[Guest_BruceMoomaw_*]

Well, you've got to set the goalposts at SOME point. And the case for setting them at just a bit smaller than Pluto is pretty good, precisely because Pluto has been thought of as a planet for so long -- given the rate at which KBOs are being discovered now, it's unlikely that there will ever again be a gap so long between the discovery of KBOs more than 2000 km wide.

Of course, we should also very definitely be teaching kids that at this point the word "planet" doesn't have any scientific significance, that all it means is "object bigger than such-and-such a diameter", and that at this point we have no idea how many "planets" the Sun has even by that arbitrary criterion (and, for that matter, we never WILL know, thanks to the possibility of big Oort Cloud objects). If we're not going to admit that "planet" is strictly an arbitrary standard, we should stop using the word at all -- but I very much doubt that's going to happen. All we can do is make sure that people know what it actually signifies at this point.

[AndyG]

Well, you've got to set the goalposts at SOME point.

Why not set goalposts using the highest surface escape velocity? This neatly captures elements of diameter and mass, and avoids issues of how to define 'rotundity'.

Set limits something like:

asteroid < 750m/s
> 750m/s small planet < 2500m/s
> 2500 m/s medium planet <5000m/s
> 5000m/s large planet

Ceres has an escape velocity of ~500m/s. It's an asteroid, albeit a large one.

Pluto is ~1070m/s. Small planet.

Mercury & Mars are medium planets. Earth & Venus are large.

If the body orbits a planet, it's a moon. Change "planet" to "moon" and set the size qualifier one higher. "Asteroid" changes to "moonlet". So Titan and Ganymede (~2770m/s) become "large moons" and Europa (2040m/s), as with our Moon, become "medium moons". Miranda is a moonlet. And quite right too.

Andy G

[ugordan]

"Asteroid" changes to "moonlet". So Titan and Ganymede (~2770m/s) become "large moons" and Europa (2040m/s), as with our Moon, become "medium moons". Miranda is a moonlet. And quite right too.

Somehow calling a 500 km diameter object (such as Enceladus) a "moonlet" is a bit of a stretch for me. These moons are indeed small by standards of our own Moon, but they still merit being called regular moons. After all, they are round, not irregular. Are you also suggesting we put Enceladus in the same category as, for example, Daphnis, a recently discovered moonlet around Saturn, which is a couple of km, tops?