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The BBC has a report that evidence has been found in lunar rock samples of a planetismal (called Theia) that was thought to have crashed into the Earth to form the Moon. The conclusion is based on a difference in oxygen isotope ratios detected in lunar rock samples returned from the Apollo space missions versus terrestrial samples.
The report is published in the journal Science [abstract]; report is paywalled.
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Evidence Of Planet That Formed Moon May have been Found In Rock Samples
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You are a fluke of the universe; you have no right to be here.
(Score: 4, Informative) by DrMag on Friday June 06 2014, @05:23PM
> Do you think there is a size threshold where under a certain size an object will just never round off?
Yes. Statement by Dr. Derek Sears in Scientific American [scientificamerican.com]:
> The statement assumes that there is only one mechanism for rounding and it applies to all planets. There is insufficient fact to rule out all other possibilities.
But that is the theory; there are no other theories I know of that are sound enough to explain it. Plus, it's in the IAU definition of a planet [wikipedia.org] (emphasis added):
> it would be like saying that mountains only come from volcanoes or only from plates crashing together
Again, that's the theory. I've not heard of any other mechanism for mountain formation. (Technically, volcanoes are very much related to plates crashing together (or pulling apart), so there's really only one mechanism, at least in the simplified sense of "mountains are created by ____".) Sediment deposit in the oceans doesn't create mountains; it creates stratified layers that are later pushed up into mountains by plate tectonics.
Point is, planet formation is a well-studied (albeit not perfectly understood) process; it is generally accepted that planets become round through self-gravitation. The moon isn't considered a planet because it doesn't fit the classification of "not a satellite", even though it is sufficiently large to undergo isostatic adjustment.
* Hopefully my tone is not condescending in all of this--just trying to share the knowledge that already exists. Do some reading; it really is a fascinating topic.
(Score: 3) by Alfred on Friday June 06 2014, @06:49PM
No condescension sensed. This is becoming quite the informative discussion. I appreciate your use of the word theory, most people take the first theory they hear as fact and run with it. And if I had mod points you would win for fantastic participation, or maybe informative I guess.
Self gravitation makes sense for bodies that are sufficiently fluid for their size. I am suspecting that an asteroid would go round if it were fluid enough even though it much smaller than a planet. The contrasting, really big and really solid, brings in interesting questions and the need to go draw some force-body diagrams.
If only I had time for reading enough. You are right this is a fascinating topic.
(Score: 3, Informative) by DrMag on Friday June 06 2014, @07:16PM
Keep in mind, though, what is actually meant by the word theory. Colloquially, it's usually interpreted as a synonym for hypothesis, but that's not the traditional definition. Scientifically, theory means a collection of concepts that brought together explain a particular phenomenon. Relativity, evolution, quantum mechanics, electrodynamics--these are all theories. That doesn't mean they're not well understood or are controversial (though it doesn't necessarily imply the antithesis either). Similarly, in science parlance, a "law" doesn't mean "that's the way it is", it just means we have a mathematical equation that describes a particular observation. "Newton's law of gravity" is wrong; very useful, but wrong. "Law" should never be conflated with truth, as that's not the intent of scientific law, just as "theory" should not be equated with "guess". A theory will be made of a number of hypothesis, observations, and laws. The three parts should be self-consistent, and also be testable.
The word theory is also used in many other fields, including art and music. I suspect the mix-up in definitions came from well-meaning people (many of whom were probably even scientists) who didn't really understand the scientific method. A quick dictionary look-up [reference.com] shows 4/5 definitions match what I'm describing--the other probably has become generally accepted because that's what happens in language; people start using a word wrong, and eventually the definition is adjusted to reflect common usage.
As for asteroids, in the absence of other bodies you're probably correct. It's surprising how much influence the various planets in a solar system can have on things, though, and with all the gravitational perturbations that are present, you might find that, at best, an asteroid becomes "round-ish" if sufficiently fluid, and not just in the oblate spheroid sense like the Earth is.
(Score: 3, Interesting) by TrumpetPower! on Saturday June 07 2014, @12:42AM
Liquidity is not required. Ever see a landslide? Even just a few pebbles rolling down a slope? That's the Earth's gravity causing the Earth to become even more spherical. Similar processes would work equally well to shape anything massive enough into something basically spherical.
b&
All but God can prove this sentence true.