How to improve net income of planet ?

After some research you will be able to build buildings that help wealth.

What ForesterSOF said (the techs are in the 4th category from the top -- sometimes labeled "Economics").

Additionally, for each planet go into the govern screen and move the slider up towards the top. This will negatively impact your production and research output, but increase your income.

Also, research tourism.

As with science and production, improving approval gives a bonus to income.

thanks for help

I find a good strategy is to up the population as much as possible, as they generate enough wealth to support all buildings and then some. If there is room for it, I add economy buildings. I also add space stations near every colony and add economy modules to those. After a while, the economy shoots through the roof!

A good tip for local space stations is to design a stripped down version of the constructor ship, as they get very expensive after a while as they automatically upgrade... But, with a stripped down version you can build one in 1 turn at even the least productive shipyard.