Economy

Mouse over.

It's determined by your income rate.  There is a separate ranking for credits, metal, and crystal.  You can see your ranking by fressing the F1 key, and see you income rates by mousing over the display boxes on the upper right part of your hud.  That will show you what each planet is earning for tax income as well as trade income.  It will do the same for metal and crystal.  Plus it will show you how much is being taken away by fleet supply.

Income rate.

Credit income rate is influenced by planet population, alleigance, longest trade port chain, and number of trade ports.
Resource income rate is influenced by alleigance, number of constructed extractors, number of captured neutral extractors, number of refineries (or trade ports using Resource Focus and what % bonus to extraction rate RF provides for the Advent), and researched increases.

All income rate is decreased as your maximum fleet capacity increases (0%/9%/19%/28%/38% (?)/47% (?)/56%/66%/75% taxed).

Before this thread i thought it was how much stuff u had, thank you for clearing it up.

Now for the really newb question, whats a hud?

just kidding

LOL, had me worried for a sec.

In really large empires, a mouseover produces a large lookup wizard which goes off screen. In addition to this, I've noticed that the data for some planets vanishes altogether.

The Undying.

True, but it'll still show totals even if some of the planet info is off screen.  I do think it's annoying, though.