Once in a while I get so twisted up scientifically, I can't even formulate a question sensible enough for Google to answer.
Luckily I know a Real-Life Scientist, my old blogpal Matt of Long Burn, who has answered many of my queries that, if there weren't such things as dumb questions, would qualify as such.
Yesterday, after I wrote about spherical bodies [a couple posts down], I googled: "Why are planetary bodies round?"
Here's what Google said:
"1 AU = 1.49597870691·1011 m 1 year = 365.25 days = 31557600 s 1 ME = 5.9742·1024 kg
All orbital characteristics are specified with respect to the Solar system's barycenter and the J2000.0 ecliptic"
I see. Thanks, guys.
I turned to Matt.
Matt's answer: A sphere is the minimum-energy form for a planet to take.
Gravity is isotropic--it has no preferred direction. Take our own planet, and say it starts to get nonspherical. Before you know it, gravity is turning sharp new mountains (Himalayas, or the San Gabriels behind LA) into old mountains (the long roll of the Scandinavian peninsula), and in the end you have the Red River Valley of the North (seen it?).
Only small heavenly objects can keep a nonspherical form-- small mass, small gravitational forces.
Could there be nonspherical planets?
Maybe but probably not.
A spinning disk would work, but then where would you put your sun?
A spinning ring around the sun would also work, but it's highly improbable.
So, this sounds to me just like human behavior--we prefer to expend the least possible energy for the biggest possible result (the most bang for our buck).
Or, could we say, eating potato chips is our most efficient way to achieve an ideal spherical shape?
P.S. The book pictured here, The Hazards of Space Travel: A Tourist's Guide (2007) is, according to the NYT review, a serious but amusing book, by astrophysicist Neil F. Comins, not a Douglas Adams spin-off.
Comins discusses the coming space-tourism, warning that, speaking of round things in space, among other things:
"Space is littered with lithic debris, and a collision with a particle no bigger than a pebble could well be catastrophic. (Pockmarks from thousands of tiny impacts slowed the orbit of the Salyut 7 space station so much that it fell from the sky.)"
So, think twice before you hand over your $200,000 to Virgin Galactic for a space flight.