Actually, the earth/ moon system is more "binary planets" than primary and secondary. Isaac Asimov wrote a wonderful essay "Mooning around" which showed that (then) unique in the solar system the earth/ moon pair were held more each to the sun than to each other. (since, it has been shown that Pluto/Charon have a simlar … Read more

The "size" of the universe can be taken several ways. The way I choose is to say-- "how big is the matter/energy/ stucture that can influence us on earth?" This is something known as the "Hubble radius" (named after the same guy that NASA named the space telescope). Matter far from earth is travelling very … Read more

In my honest opinion, the word "planet" is obsolete. There is no meaningful collection of sun-orbiters that can be classified as "planets" without serious objection! There are five large, rocky inner orbits (Earth/moon is a binary system). There's a belt of small rocky asteroids. Four gas giants. An unknown number of icy Kuiper objects. Unknown … Read more