Pluto is still a planet - it's official classification has just been slightly altered to reflect its small mass and eccentric orbit. The public outcry from the original story was so significant and strident that even had they wanted to (and most didn't) the members of the international astronmers union would never have made such a change.
Also, fwiw, most planetary scientists who care about such matters classify
the Earth-Luna pair as a double planet. Luna has about 1/81 the mass of
Earth, which is a huge amount for a moon in this solar system. Whether Luna
was captured or blown off of the Earth is still up for debate.
Points in favor of capture include an apparently significantly different
makeup, and the difficulty of modeling an impact that could throw off so
much mass, without ejecting it completely from co-orbit, or having it fall
Points in favor of capture include an apparently significantly different makeup, and the difficulty of modeling an impact that could throw off so much mass, without ejecting it completely from co-orbit, or having it fall back.And the moon's impact history doesn't have a lot of evidence of any ancient "sweeping up lots of little junk in near orbit" period, which one would expect from such a genisis. Later impacts and volcanic activity might have completely obliterated any such signs, of course.
On the other hand, models for stable capture of such a massive planetoid are damn difficult to come by as well. And we don't really know enough about the moon's actual composition to say for sure that it's not earth-like.
It's damn convenient that it's *just* big enough at this point in our planet's history to give us great solar eclipses. Seems almost too perfect. But of course, any alien patrons that wanted to help our little species out with figuring out the whole planet-solar system-galaxy thang, could have cut us a real break by letting the moon hang on to some of it's rotational momentum just a few tens of millions of years longer. A spinning moon face would have made it pretty obvious to our ancestors that we were surrounded by globes (and probably lived on one ourselves).
Disclaimer - although I was a course 12 (aka Earth Air and Planetary Science) u-grad at MIT in my earlier days (circa late 80s), I now work as a software developer and technology liason. I am still an interested amateur astronomer, but may not have the latest skinny on the Earht-Luna debate). Be gentle with any counter-rants.
| Jeffrey Fabijanic, Designer The Future exists,
| Primordial Software first in Imagination,
| "Software of the First Order" then in Will,
| Boston, MA * (617) 983-1369 and finally in Reality.